Wanganui Wastewater Treatment Plant

Transcription

1 Wanganui Wastewater Treatment Plant Evaluation of Long-Term Improvements for Consent Compliance Prepared for Wanganui District Council 24 April 2013

2 Document Information Prepared for Wanganui District Council Project Name File Reference 375- P-01-V2-Long-Term Report Job Reference 375 Date 24 April 2013 Contact Information Cardno BTO 55 Cuba Street Petone 5046 New Zealand Telephone: Document Control Version Author Initials D1 Internal Review AS, LS, SL, MMc D2 Internal Review 1 st Draft AS, LS, SL, MMc D3 Internal Review 2 nd Draft AS, LS, SL, MMc R1 For client review AS, LS, SL, MMc V1 Final AS, LS, SL, MMc V2 Revised Final AS, LS, SL, MMc Reviewer Mike McCoy Mike McCoy Mike McCoy Mike McCoy Mike McCoy Mike McCoy Reviewer Initials MMc MMc MMc MMc MMc MMc Cardno Copyright in the whole and every part of this document belongs to Cardno and may not be used, sold, transferred, copied or reproduced in whole or in part in any manner or form or in or on any media to any person other than by agreement with Cardno. This document is produced by Cardno solely for the benefit and use by the client in accordance with the terms of the engagement. Cardno does not and shall not assume any responsibility or liability whatsoever to any third party arising out of any use or reliance by any third party on the content of this document P-01-V2-Long-Term Report Cardno BTO ii

3 Executive Summary The Wanganui Wastewater Treatment Plant (WWTP) has experienced various problems since it was opened in 2007, including failure to meet discharge effluent consent standards and seasonal odour events. Most recently, a significant and prolonged odour event occurred over the 2012/13 summer. Cardno BTO were engaged by Wanganui District Council (WDC) to assist with short-term odour mitigation measures and longterm strategies to achieve consent compliance. Specifically, the WDC set forth the terms of reference for the long-term options study in a WDC Resolution: o To determine whether it is viable to continue with (the present) plant; Cardno s assessment is that continuing with the plant in its current configuration is not viable. The original design concept is flawed and capital improvements are required in order to reliably meet the effluent consent and minimise the risk of odours. o If viable, which options to achieve satisfactory performance are available for completion over the next two years; Continued operation of the plant in its current configuration is not viable. Despite this, consideration has been given within this evaluation to make best use of existing infrastructure when possible. o If not viable, then establish which type of plant would be suitable for Wanganui s waste and what size that would need to be. A facility that consists of a biological treatment process that produces a settleable floc and continuous sludge handling is critical. This evaluation concludes that overall most cost effective improvement scenario includes: - Anaerobic pond to provide primary treatment - Contact stabilisation with new secondary clarifiers for secondary treatment or sequencing batch reactors (SBRs) - UV disinfection Co-wasting of primary sludge and WAS from the anaerobic pond to solids handling Given the plant s history, there is no opportunity to trial experimental, high-risk solutions. This report evaluates long-term options that can confidently meet the resource consent and minimise the risk of odour events. Cardno BTO recognise that process options outside of those presented herein exist; however, only those solutions evaluated in detail in this report are suitable to meet the strict objectives set forth by WDC. A number of significant issues affect the Wanganui WWTP; these are discussed in detail in the appendices of this report. These issues include a long-standing history of consent non-compliance, historical odour events, the need to address removal and disposal of sludge in existing ponds, historical surface aerator mechanical issues, impacts of trade waste, and the potential for beneficial use of plant effluent in the future. Evaluation of the original design concept is a necessary component of this study. The original design for the Wanganui WWTP was based on a deep aerated lagoon in which settlement, sludge storage, and aerobic biological treatment are performed within a single lagoon. The design flaws associated with this design concept include: o Suspended solids settle in the parts of the lagoon not fully mixed by the surface aerators Incomplete mixing to allow for solids to settle means that maintaining a mixed liquor in the aerobic zone is difficult. Maintaining a mixed liquor is essential for treatment in order to provide time for biomass growth, BOD consumption, and generation of a settleable floc. o Storage, compaction, and anaerobic digestion of settled sludge in the base of the lagoon Anaerobic digestion intended to take place in the base of the lagoon is impacted by the aerobic process occurring in the lagoon. Anaerobic digestion occurs in the absence of oxygen; supplying 375- P-01-V2-Long-Term Report Cardno BTO iii

4 oxygen to the upper layer of the pond has likely partially inhibited the anaerobic digestion process in the lower layer of the pond. It is likely that inhibition of anaerobic digestion has been occurring since the plant was originally opened and that the most recent odour event was exacerbated when surface aerators were turned off. When the bottom layer of the pond was no longer inhibited by the oxygen being introduced by the aerators, the rate of digestion accelerated, releasing odourous compounds that were previously bound in undigested solids. o Aerobic biological treatment in the upper layer of the lagoon The upper layer of the pond is not fully mixed and growth of a biomass is difficult. Mixing in the aerated lagoon is only provided by the surface-mounted aeration equipment. Once solids settle past the aeration equipment area of influence, it cannot be resuspended. o Storm flow storage in the 2 metres of depth above the dry weather operating level of the lagoon Storm flows routed to the lagoon facilitate washout of any aerobic biomass that may be in the upper layer of the pond. This design is unconventional and enhancing the original design concept will not meet the resource consent suspended solids standard and will present an unacceptable odour risk. Since these two primary objectives cannot reliably be met, options that maintain the original design concept are not considered. The figure below summarises the improvement options that have been identified for the Wanganui WWTP based on what is physically practical at the existing site. The options have been broken down into the main treatment areas: primary treatment, secondary treatment, sludge management, storm flow management and disinfection. Each shortlisted alternative, highlighted in yellow in the matrix, is evaluated in detail in the report P-01-V2-Long-Term Report Cardno BTO iv

5 Increasing Cost Maintain Original Concept New Concepts Primary Treatment - Reduce secondary treatment size & energy - Remove compounds that may inhibit secondary process Partial Primary Settlement in Aerated Lagoon No Primary Treatment Anaerobic Pond New Primary Clarifiers & Anaerobic Digesters Secondary Treatment - Achieve Effluent TSS Standards: New Surface Aerators - 40 mg/l (median) Chemical Dosing for - 90 mg/l (95 %ile) Enhanced Settling in Settling Pond In-Pond Aeration Zone with Settling Zone (Biolac-type system) In-Pond Batch System (SBRs) Contact Stabilisation with External Secondary Clarifiers Sludge Management - Storage In-Pond Storage and Digestion - Stabilisation (odour reduction) Periodic Desludging - Volume reduction prior to disposal and Disposal by Contractor Waste Primary Sludge Waste and WAS Primary from Sludge Anaerobic and WAS Pond from Anaerobic Pond Waste Primary Sludge From Anaerobic Pond and Process WAS Separately Anaerobic Co-digestion Storm Flow Management - Reduce downstream unit size - Avoid overload of downstream units Wet Weather Storage in Aerated Lagoon Wet Weather Bypass of Primary & Secondary Treatment Disinfection - Meet resource consent microbe standards: - Enterococci - Faecal coliforms UV Disinfection Chlorine Disinfection Figure ES-1: Wanganui WWTP Long-Term Options Matrix It is critical that the upgraded treatment plant meets the resource consent standards; therefore as with any design, a margin of safety must be provided between the plant s design effluent standards and the resource consent standards. The size of the safety margin selected is a balance between risk of non-compliance and cost. The design flows and loads for the upgraded WWTP were developed based on historical flows and loads to the WWTP. Maximum flow to UV disinfection and the WWTP is limited to 650 l/sec (56,000 m 3 /day) and 1,120 l/sec (97,000 m 3 /day), respectively. For the purposes of this report a 20% contingency has been added to the historical flows and loads to ensure there is sufficient capacity to treat the current consented trade waste loads and to allow some spare capacity. All designs are flexible and can be adjusted in future to accommodate any changes to the design flows and loads as the design progresses. In order to assess the relative pros and cons of the improvement options in a structured fashion, each option is evaluated against a set of criteria. Meeting the required effluent quality and odour standards is not optional, therefore these are not included as assessment criteria but form part of the design basis for the improvements. Criteria include constructability, process risk, future upgrade complexity, completion time, and cost P-01-V2-Long-Term Report Cardno BTO v

6 Detailed assessments of the options are presented in the report and include sizing criteria, performance against stated criteria, and costs. The overall most cost effective improvement scenario includes: o Anaerobic pond to provide primary treatment o Secondary treatment with contact stabilisation with new secondary clarifiers or sequencing batch reactors (SBRs) o UV disinfection o Co-wasting of primary sludge and WAS from the anaerobic pond to solids handling The total capital cost of this option is estimated to be in the range of $17-$19 million. Table ES-1 presents a summary of cost estimates for the preferred options P-01-V2-Long-Term Report Cardno BTO vi

7 Table ES-1: Summary of Costs for Preferred Options Option 1 Option 3 Primary Treatment Anaerobic Pond $2,343,000 $2,343,000 Primary Clarifiers Secondary Treatment SBRs $14,707,000 Contact Stabilisation with New Secondary Clarifiers $13,551,000 UV Disinfection $0 $0 Sludge Management Storage in Anaerobic Pond $1,738,000 $1,738,000 Primary Sludge Storage in Anaerobic Pond, Separate WAS Treatment Anaerobic Codigestion Capital Cost ($) $18,788,000 $17,632,000 Primary Treatment Anaerobic Pond $838,000 $838,000 Primary Clarifiers Secondary Treatment SBRs $10,221,000 Contact Stabilisation with New Secondary Clarifiers $5,041,000 UV Disinfection $3,829,000 $3,829,000 Sludge Management Storage in Anaerobic Pond $13,644,000 $13,644,000 Primary Sludge Storage in Anaerobic Pond, Separate WAS Treatment Anaerobic Codigestion 20-year NPV of Operating Cost ($) $28,532,000 $23,352,000 Primary Treatment Anaerobic Pond $3,181,000 $3,181,000 Primary Clarifiers Secondary Treatment SBRs $24,928,000 Contact Stabilisation with New Secondary Clarifiers $18,592,000 UV Disinfection $3,829,000 $3,829,000 Sludge Management Capital Cost 20-year NPV of Operating Cost 20-year Net Present Value Storage in Anaerobic Pond $15,382,000 $15,382,000 Primary Sludge Storage in Anaerobic Pond, Separate WAS Treatment Anaerobic Codigestion 20-year Net Present Value ($) $ 47,320,000 $ 40,984, P-01-V2-Long-Term Report Cardno BTO vii

8 There are a number of recommendations that stem from this evaluation: o Special sampling should be conducted over a one-month period in order to gain an understanding of the composition of the wastewater such that the process design of any long-term improvements is appropriate. o Discussions with Horizons Regional Council regarding potential consenting requirements are recommended. o It is recommended that WDC continue to take a collaborative approach to engage industry, as an open consultation process where industry and WDC can work together to address trade waste discharges. This will result in a positive outcome. Desludging of the existing ponds, discussed in Appendix C, is an activity that must take place prior to construction of a facility upgrade. The timing of a desludging operation is flexible but there are a number of factors that influence when the activity should commence. 1. Time of year. Commencement of desludging prior to summer will help to decrease odours generated in the warm months. 2. Timing of construction. It is desirable that desludging take place at a time such that maximum sludge can be removed. If the desludging operation takes place too far in advance of construction there is a possibility that additional desludging may be required when the ponds must be emptied for construction. The timing presented in Table ES-2 presents a preliminary suggested programme that can be modified dependent upon budgets and priorities. Task Special Sampling Design of WWTP Improvements Table ES-2: Programme Timing Immediate Must begin immediately Sludge Survey June 2013 Resource Consents Coordination with Horizons Regional Council September 2013 Trade Waste Bylaw Update To be initiated within 9 months Project Tendering December 2013 Completion of Construction December 2014 Table ES-3 presents a summary of costs that are required over the next year in order to meet the proposed programme. Task Table ES-3: Total Costs $ million Capital Improvements at WWTP $18.79 Annual Operating Cost for WWTP* $1.93 Sludge Removal and Disposal (mechanical dewatering to 20%) $3.90 Trade Waste Monitoring $1.00 Wastewater Characterisation $0.05 Medium-term Odour Control $1.00 Total $26.67 *assumes operation of option with SBRs in 2013 dollars 375- P-01-V2-Long-Term Report Cardno BTO viii

9 Table of Contents Executive Summary iii 1 Introduction Background Terms of Reference Previous Studies 14 2 Description of Existing Plant Overview Inlet Works Aerated Lagoon Transfer Station Settling Pond Ultraviolet (UV) Disinfection System General Comments on WWTP Process Design 16 3 Issues Resource Consent Non-Compliance Odour Sludge Management Surface Aerator Mechanical Issues Trade Waste 21 4 Improvement Assessment Criteria Constructability Process Risk Future Upgrade Complexity Completion Time Cost 22 5 Design Basis for Improvements Effluent Quality Standards Standards Required to Meet Existing Resource Consent 24 Margin of Safety Odour Influent Flows and Loads Measured Influent Flows and Loads Design Flows and Loads 25 Other Influent Parameters 25 6 Improvement Options Overview Options Matrix Enhancement of Original Concept Primary Treatment No Primary Treatment 28 Primary Clarifiers with Anaerobic Digestion 29 Anaerobic Pond Secondary Treatment In-Pond Aeration Zone with In-Pond Clarifier 30 Contact Stabilisation with New Secondary Clarifiers 30 Sequencing Batch Reactor P-01-V2-Long-Term Report Cardno BTO ix

10 6.5 Disinfection UV Disinfection 31 Chlorine Disinfection Sludge Management Storm Flow Management Shortlisted Options 32 7 Primary Treatment Options Anaerobic Pond Description 33 Process Design 34 Discussion 34 Cost Primary Clarifiers with Anaerobic Digestion Description 36 Process Design 38 Discussion Cost Summary of Primary Treatment Options 39 8 Secondary Treatment Options Contact Stabilisation Description 40 Process Design Parameters 40 Discussion 41 Cost Sequencing Batch Reactors (SBRs) Description 42 Process Design Parameters 42 Discussion 43 Cost Comparison of Secondary Treatment Options 44 9 Disinfection Use of Ultraviolet Disinfection System Process Design Discussion Cost Disinfection Summary Sludge Management Options Storage of Sludge in Anaerobic Pond Description 49 Process Design 50 Discussion 51 Cost Storage of Primary Sludge in Anaerobic Pond & Separate WAS Processing Description 51 Process Design 52 Discussion 53 Cost Anaerobic Co-digestion of Sludge P-01-V2-Long-Term Report Cardno BTO x

11 Description 53 Process Design 54 Discussion 55 Cost Sludge Dewatering Description 55 Discussion Cost Summary of Sludge Treatment Options Summary of Evaluation Recommendations Upgrade Option Sampling Regional Council Engagement Trade Waste Interim Site Maintenance Programme References 63 Appendices Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Historical Resource Consent Non-Compliance Historical Odour Discussion Sludge Management Trade Waste Discussion Resource Consents Site Layouts Tables Table 2-1 Table 2-2 Table 2-3 Table 3-1 Table 3-2 Table 3-3 Table 3-4 Table 3-5 Table 4-1 Table 5-1 Table 5-2 Table 5-3 Table 5-4 Wanganui WWTP Aerated Lagoon Volumes Taken from As-Built Drawings 15 Wanganui WWTP Settling Pond Volumes Taken from As-Built Drawings 16 Wanganui WWTP versus Conventional Process Design Parameters 17 Wanganui WWTP Resource Consent Effluent Monitoring Requirements 18 Wanganui WWTP Effluent Monitoring Results Summary ( ) 18 Total costs for Pond Dewatering 19 Aerated Lagoon Surface Aeration Timeline 20 Consented Loads for Major Trade Waste Dischargers 21 Cost Assumptions 23 Wanganui WWTP Consent and Design Effluent Standards to Ensure Consent Compliance 24 Measured Influent Flows and Loads Design Influent Flows and Loads 25 Influent Parameter Assumptions Used for this Report P-01-V2-Long-Term Report Cardno BTO xi

12 Table 6-1 Advantages and Disadvantages of No Primary Treatment 29 Table 6-2 SBR Operating Steps 30 Table 6-3 Advantages and Disadvantages of Sequencing Batch Reactors 31 Table 7-1 Pond Based Anaerobic Pond Average Sludge Production 33 Table 7-2 Anaerobic Pond Process Design Summary 34 Table 7-3 Primary Clarifiers and Anaerobic Digestion Average Sludge Production 38 Table 7-4 Lamella Settlers and Digesters Anaerobic Pond Process Design Summary 38 Table 7-5 Comparison of Primary Treatment Options 39 Table 8-1 Contact stabilisation Process Design Summary 41 Table 8-2 SBR Process Design Summary 43 Table 8-3 Comparison of Secondary Treatment Options 44 Table 9-1 Summary of Faecal Contaminant Limits 45 Table 9-2 Existing UV disinfection System Design Basis 45 Table 9-3 Cost Estimates for UV Disinfection Options 47 Table 9-4 Existing UV System Assessment Summary 48 Table 10-1 Solids Handling Requirements Based on Primary Treatment Process 49 Table 10-2 Storage in Anaerobic Pond Average Sludge Production 50 Table 10-3 Design Parameters for Anaerobic Pond with Continuous Dewatering 50 Table 10-4 Separate WAS Thickening Average Sludge Production 52 Table 10-5 Design Parameters for Sludge Thickening with Continuous Dewatering 52 Table 10-6 Comparison of Sludge Thickening Equipment 53 Table 10-7 Anaerobic Co-digestion Average Sludge Production 54 Table 10-8 Design Parameters for Anaerobic Co-digestion 55 Table 10-9 Comparison of Sludge Dewatering Equipment 56 Table Comparison of Secondary Treatment Options 57 Table 11-1 Cost Comparison of Treatment Options 58 Table 11-2 Comparison Summary of Treatment Options 59 Table 12-1 Programme 61 Table 12-2 Programme Costs 61 Table C-3 New Zealand Biosolids Guidelines for Chromium in Biosolids 78 Table C-4 Wanganui WWTP Aerated Lagoon Sludge Chromium Sampling Results 78 Table C-5 Dewatered Sludge Transport and Disposal Costs 79 Table C-6 Dredging and Dewatering Costs (excluding transport & disposal) 80 Table C-7 Total costs for Desludging Wanganui WWTP Ponds 81 Figures Figure 6-2: Wanganui WWTP Shortlisted Improvement Options 32 Figure 7-1 Process Schematic of a Pond-Based Anaerobic Lagoon 33 Figure 7-2 Process Schematic of Primary Clarifiers with Anaerobic Digestion 36 Figure 7-3 Diagram of Primary Clarifiers with Tube Settlers 37 Figure 7-4 Process Schematic of Solids Handling Scheme: Primary Clarifiers with Anaerobic Digestion 37 Figure 8-1 Process Schematic of Contact Stabilisation Process P-01-V2-Long-Term Report Cardno BTO xii

13 Figure 8-2 Process Schematic of the Sequencing Batch Reactor (SBR) System 42 Figure 10-1 Process Schematic of Sludge Treatment from Co-settled in Anaerobic Lagoon 50 Figure 10-2 Process Schematic of Storage of Primary Sludge in Anaerobic Pond & Separate WAS Processing 52 Figure 10-3 Process Schematic of Anaerobic Co-digestion of Sludge 54 Figure A-2 Wanganui WWTP Effluent Total Suspended Solids Concentrations 66 Figure A-3 Wanganui WWTP Effluent Faecal Coliform Concentrations 67 Figure A-4 Wanganui WWTP Effluent Enterococci Concentrations 67 Figure A-5 Wanganui WWTP UV Performance August December Figure B-1 Photograph of gas bubbles on lagoon surface 73 Figure D-1 Breakdown of BOD Load 85 Figure D-2 Breakdown of TSS Load 86 Figure D-3 Breakdown of FOG Load P-01-V2-Long-Term Report Cardno BTO xiii

14 1 Introduction 1.1 Background The Wanganui Wastewater Treatment Plant (WWTP) has experienced various problems since it was opened in 2007, including failure to meet discharge effluent consent standards and seasonal odour events. Most recently, a significant and prolonged odour event occurred over the 2012/13 summer. Cardno BTO were engaged by Wanganui District Council (WDC) to assist with short-term odour mitigation measures and longterm strategies to achieve consent compliance. This report presents the results of an investigation into the improvement options for achieving reliable long-term resource consent compliance. Given the plant s history, there is no opportunity to trial experimental, high-risk solutions. The scope of work for long-term improvements requires that only those options identified as being capable of reliably meeting the resource consent and minimising the risk of odour events be evaluated. Cardno BTO recognise that process options outside of those presented herein exist; however, only those solutions evaluated in detail in this report are suitable to meet the strict objectives set forth by WDC. 1.2 Terms of Reference Cardno BTO staff met with WDC on 18 January 2013 during which the scope of services for the short- and long-term improvement works were established. The terms of reference for the long-term options study were discussed and summarised subsequently in a WDC Resolution: o To determine whether it is viable to continue with (the present) plant; o If viable, which options to achieve satisfactory performance are available for completion over the next two years; o If not viable, then establish which type of plant would be suitable for Wanganui s waste and what size that would need to be. This report presents the evaluation of long-term upgrade options and recommendations for the way forward. 1.3 Previous Studies Cardno BTO conducted an investigation into the Wanganui WWTP in November 2011 (BTO, 2011). The objectives of the 2011 investigation were to compare the actual performance and capacity of the WWTP to that of its original design and to identify options to improve plant performance. The focus of the 2011 study was to identify low-cost measures to quickly improve the performance of the existing ponds. Whilst there is some overlap with the previous study in terms of review of current and historic performance, the objective of the present study is to identify options to achieve long-term consent compliance, preferably utilising the existing assets but not exclusively so. Cardno BTO also developed a Plant Odour Mitigation Discussion Report (BTO, 2012) for the WDC resulting from odours events in Summer 2011/2012. The objective of this report was to provide a review of the odour issues experienced within the reticulation network and at the WWTP P-01-V2-Long-Term Report Cardno BTO 14

15 2 Description of Existing Plant 2.1 Overview The existing treatment process comprises screening and grit removal at the Beach Road Pump Station, followed by pumping to the WWTP south of the Wanganui River. The WWTP consists of a partially mixed, deep aerated lagoon, a settling pond, and UV disinfection before disposal via ocean outfall. The WWTP was designed for the consent term of 25 years. The Wanganui WWTP services an estimated population of 40,000 (based on the 2006 Census) and a number of industrial trade waste dischargers. The historical average municipal load is approximately 15% of the overall influent load to the WWTP. The industrial trade waste discharges contribute a large load to the plant, the historical average industrial pollutant load in 2012 being equivalent to a domestic population of approximately 250,000. The 95 th percentile industrial load is equivalent to a population of 350,000. The substantial industrial contribution means that the wastewater is more concentrated than typical domestic wastewater. A discussion on trade waste discharges is presented in Appendix D. 2.2 Inlet Works All wastewater collected in the Wanganui sewer network is pumped to the Beach Road Pump Station on the north side of the Wanganui River. The inlet works is located at the pump station immediately upstream of the wet well. Here the wastewater is screened in 3mm step screens before grit removal by a vortex grit removal system. The pump station contains four pumps which can transfer a flow rate of up to 1,300 l/s to the WWTP across the Wanganui River. A surge chamber is provided to protect against surge conditions in the rising main. In the event of an emergency, a diversion valve provides the capability to discharge screened wastewater flows directly to the ocean outfall. This is initiated automatically when the wet well level is high, on high pressure in the rising main, or on high level in the aerated pond. It can also be manually opened if required. 2.3 Aerated Lagoon Screened, degritted wastewater is pumped via a rising main to the aerated lagoon. The aerated lagoon is kidney-shaped to fit the existing contours of the site, has a total liquid volume of 255,000 m 3 and is 10 metres deep. The aerated lagoon is only partially mixed, and is designed to allow various treatment functions to occur within separate zones of the lagoon. The treatment functions which the aerated lagoon is designed to perform are: o Settlement of suspended solids in the parts of the lagoon not fully mixed by the surface aerators o Storage, compaction and anaerobic digestion of settled sludge in the base of the lagoon o Aerobic biological treatment in the upper layer of the lagoon o Storm flow storage in the 2 metres of depth above the dry weather operating level of the lagoon Table 2-1 Wanganui WWTP Aerated Lagoon Volumes Taken from As-Built Drawings Section Depth Range (m) Volume (m 3 ) Sludge Storage Layer ,000 Aerobic Layer ,000 Stormflow Storage Layer ,000 Like conventional aerated lagoons, the biological treatment layer of the Wanganui aerated lagoon has a hydraulic retention time (HRT) of 3.5 days at average daily flows, and relies solely on mechanical aeration for oxygen, with no significant algae growth occurring in the lagoon. The design differs, however, from a 375- P-01-V2-Long-Term Report Cardno BTO 15

16 conventional aerated lagoon in that it is partially mixed and is bottomless, as it sits on top of the underlying sludge storage layer. 2.4 Transfer Station The transfer station is designed to buffer storm flows by controlling the forward flow rate to the settling pond via an automated valve and a mag flow meter. Outflows from the aerated lagoon are allowed to gradually increase to a maximum discharge of 650 L/s, as the aerated lagoon level rises. An overflow pipe from the aerated pond to the settling pond provides a safety mechanism for overflows, should the storm buffer capacity be exceeded. 2.5 Settling Pond The settling pond is fed at a controlled feed rate (apart from overflows) by the transfer station. The settling pond is designed with a one day HRT at average dry weather flow to separate the solids in the wastewater so that they accumulate on the pond floor. The four inlets to the settling pond are designed to spread the flow and solids evenly across the pond. Table 2-2 Wanganui WWTP Settling Pond Volumes Taken from As-Built Drawings Section Depth Range (m) Volume (m 3 ) Sludge Storage Layer ,000 Aerobic Layer , Ultraviolet (UV) Disinfection System Treated effluent from the settling pond passes to the ultraviolet (UV) system for disinfection of the treated effluent in order to meet consent bacteriological standards. The UV disinfection system is a Trojan UV3000 plus open channel system, using low pressure, high output lamps. The lamps are in a single channel, configured in two banks of 104 lamps. There is space provided in the channel to increase the total number of lamps from 208 to 272 in the future, if required. The UV system uses variable output lamps and is designed for automatic dose pacing based on the measured flow rate to the UV and the UV intensity within the lamp array. The performance guarantee for the UV system was conditional on the treatment plant achieving a minimum UV transmission (UVT) of 20% and a 90 th percentile total suspended solids concentration of 75 mg/l. After UV treatment, the effluent is discharged by gravity to the existing 1.8 km ocean outfall off the coast of Wanganui. 2.7 General Comments on WWTP Process Design The design of the Wanganui aerated lagoon is unconventional in that it combines an aerated lagoon-type process on top of a 4 metre deep anaerobic sludge layer. The settlement and storage of suspended solids underneath the aerated layer requires that the aerated layer is not fully mixed. The combination of partial mix conditions and bottomless configuration prevents a suitable settleable biomass developing in the biological treatment layer, as suspended flocs spend only a fraction of the required time in the aerated layer before sinking to the anaerobic sludge layer. Partial mix lagoons containing an anaerobic sludge layer are a commonly used treatment process in New Zealand for domestic wastewater treatment, and are more commonly termed facultative ponds or aerated facultative ponds. However conventional facultative ponds, whether aerated or unaerated, are slow-rate systems, with hydraulic retention times of days which allows enough time for biological treatment in partial mix conditions. In addition, conventional facultative ponds are usually no more than 2 metres deep due to the requirement for algae to provide most of the oxygen needed for biological treatment. As a result, the BOD loading rate on the ponds is low and the resulting sludge layer is usually no more than 1 metre deep P-01-V2-Long-Term Report Cardno BTO 16

17 To highlight the key points of difference with the Wanganui WWTP design, Table 2-3 compares some of the key design parameters of the Wanganui WWTP with corresponding typical values for conventional facultative ponds taken from standard design wastewater design text books (Metcalf and Eddy, 2003 and IWA, 2005). Table 2-3 Wanganui WWTP versus Conventional Process Design Parameters Parameter Units Wanganui WWTP Conventional Facultative Pond Parameters Hydraulic Retention Time days Influent BOD Loading Rate kg/ha/day 6, *Twister data sheet 375- P-01-V2-Long-Term Report Cardno BTO 17

18 3 Issues There are a number of significant issues affecting the Wanganui WWTP, which are discussed in detail in Appendix A through Appendix D. These issues include: o A long-standing history of consent non-compliance o Odours o Need to address removal and disposal of sludge in existing ponds o Surface aerator mechanical issues o Trade waste o Potential for beneficial use of plant effluent in the future 3.1 Resource Consent Non-Compliance The Wanganui WWTP discharges effluent to the ocean, an activity permitted by Coastal Permit granted by the Manawatu-Wanganui (Horizons) Regional Council. The existing consent expires 30 June 2026 and is provided in Appendix E of this report. The concentrations of various pollutants are limited under conditions in the resource consent, and the final effluent must be sampled and analysed for the pollutants covered by the consent. Table 3-1 summarises the resource consent effluent monitoring requirements. Table 3-1 Wanganui WWTP Resource Consent Effluent Monitoring Requirements Parameter Units Sample Type Frequency TSS & total grease g/ m 3 24-hour composite 3 x per week Faecal coliforms & enterococci cfu/100ml Grab sample 3 x per week Sulphides & metals g/ m 3 24-hour composite Four monthly Table 3-2 shows a summary of the consent monitoring results and the required standards for the key contaminants of concern. Although effluent BOD concentration is not covered by the resource consent, it is measured by WDC as it provides an indication of the general level of treatment. Total grease, sulphides and metals are not listed in Table 3-2 as these contaminants have always fallen well within the resource consent limits. Figures of the final effluent concentrations of these pollutants over the past 4 years are presented in Appendix A. Table 3-2 Wanganui WWTP Effluent Monitoring Results Summary ( ) Parameter Units Basis Resource Consent Standard BOD g/m 3 median n/a TSS g/ m 3 95th %ile median n/a Enterococci Faecal Coliforms Median : 95th %ile ratio n/a cfu/100ml cfu/100ml median 3,900 20,000 24,000 51,000 4,000 maximum 160,000 2,100,000 1,600, ,000 12,000 median 7,500 22,000 86, ,000 10,000 90th %ile 56,600 91, , ,000 25, P-01-V2-Long-Term Report Cardno BTO 18

19 Although the resource consent standard is written as a 95th percentile concentration, working with median effluent concentrations is more useful for WWTP design purposes. Using a median to 95th percentile ratio of 0.40 the plant must produce a median effluent TSS concentration of 40 g/m 3 in order to remain within the 95th percentile limit of 100 g/m 3. Although a median effluent TSS concentration of 40 g/m 3 is achievable from an aerated lagoon/settling pond system, it is at the lower end of the achievable range for this type of process and is dependent upon many factors. To put this value into perspective, activated sludge treatment plants, which feature purpose-designed clarifiers and sludge recycle systems, would normally not be guaranteed to achieve lower than 30 g/m 3 median TSS in the final effluent. It is also important to note that the maximum historical loads to the WWTP are extreme and reinforce the need for a conservative design basis in order to consistently meet the resource consent. A pond system would not be common for this load, population equivalent, and close proximity to a city and airport. A more appropriate design is an activated sludge process that incorporates continuous sludge handling. Whilst the poor historical final effluent faecal coliform and enterococci results could suggest substandard performance of the UV disinfection system, the alternative explanation is that the effluent UV transmissivity and TSS concentrations were outside the limits that the UV system was designed for. 3.2 Odour Odour nuisance has been a major issue at the WWTP, especially during the summer months. Due to the site location and prevailing winds, there was always a high odour nuisance risk with a pond based process. The WWTP site is largely flat with little or no shelter belt planting to shield odours from the ponds to surrounding properties. The nearest neighbours to the WWTP are the Wanganui airport maintenance workshops around 350 metres west of the ponds and the rifle range 300 metres to east of the ponds. The prevailing wind is north westerly; however the wind often reverses to south southwest which takes odours into the city, 1.2 km north of the ponds across the river. Prior to the recent 2012/13 odour event, odour complaints have been a fairly regular occurrence since the WWTP opened. Wanganui Airport staff has regularly complained about odours since the plant was opened; airport staff also expressed a concern about birds attracted to the site being a hazard to aircraft. Odour complaint logs from Horizons Regional Council indicate that over 60 households complained of objectionable odours from the WWTP since mid-december A discussion on the sources of odour and factors contributing to odour are presented in Appendix B. 3.3 Sludge Management Management of solids within the existing ponds at the Wanganui WWTP is an issue that must be appropriately managed regardless of the long-term approach for WWTP compliance. The primary issue with sludge currently stored in the ponds is the odours caused by the sludge digestion. A detailed discussion on options for management of sludge in the existing ponds is presented in Appendix C. Table 3-3 presents a summary of costs associated with sludge dredging, dewatering, and disposal for four different scenarios. The total cost is estimated assuming a total sludge inventory of 4,500 tonnes of dry solids in both ponds, a number close to the tonnes of dry solids estimated from the mass balance. Total variable charges are dependent upon the landfill receiving WWTP sludge and include dredging, dewatering, transport, and disposal costs. Item Table 3-3 Total costs for Pond Dewatering Units Mechanical Mechanical Geobags Geobags 15% dry solids 20% dry solids 20% dry solids 30% dry solids Total variable charges $/t dry solids Fixed costs $000 s Total cost for 4,500 t dry solids $000 s 4,400-4,700 3,700-3,900 3,200-3,400 2,500-2,600 There are a number of risks associated with sludge removal, dewatering, and disposal. The key risks include: o Potential to damage the pond liner during desludging operation 375- P-01-V2-Long-Term Report Cardno BTO 19

20 o Odour generation o Potential cost inaccuracies It is recommended that mechanical dewatering of WWTP sludge take place in order to eliminate any potential odours associated with geobag dewatering. Furthermore, in order to reduce the risks associated with sludge removal and disposal, a sludge survey should be conducted in order to better quantify the solids in the existing ponds and refine costs associated with the operation. 3.4 Surface Aerator Mechanical Issues There have been various mechanical problems with the surface aerators since the WWTP was opened in 2007 and whilst the poor effluent quality and odour nuisance cannot be attributed to the aerator issues, they have exacerbated the problems. The aerated lagoon was originally provided with pontoon-mounted Aeromix Tornado aspirating aerators. Aspirating aerators are high speed aerators with an angled shaft and impeller rotating at 1,500 rpm, which sucks air into the water and propels the water/air mixture below the water surface in the direction of the shaft. A number of problems occurred with the Tornado aerators during start-up-, resulting in the aerator supplier having to replace the Tornado aerators with Aeromix Twister aerators after approximately 18 months of troubleshooting. The motors from the Tornado aerators were reused and retrofitted onto the Twisters. The Twister aerators are a slow-speed surface aerator, which use a vertical shaft and rotor rotating at around 70 rpm. A gearbox is required to achieve the necessary rotation speed. Aeration is achieved by throwing the water outwards from the centre in all directions to encourage water/air contact; in contrast to the aspirating aerators there is no directional mixing with these types of aerators. Figure 3-1 shows the two types of aerators used. Figure 3-1 Tornado Aerator (left) and Twister Aerator (right) The Twister aerators suffered from mechanical problems resulting from the retrofit of the Tornado motors onto the aerators and as result a significant number of aerators were out of service for prolonged periods. Table 3-4 summarises the timeline of the aerators used on the aerated lagoon. Prior to the Twister aerators being turned off by WDC in December 2012, there were nine operating Twister aerators in the aerobic lagoon. Table 3-4 Aerated Lagoon Surface Aeration Timeline Dates Aerator Type Power No. Total Power July 2007 (plant opening) Tornado (Aspirating) 45 kw 23 1,035 Late 2008 Twister (Slow-Speed) 45 kw December 2012 Twister (Slow-Speed) 45 kw P-01-V2-Long-Term Report Cardno BTO 20

21 The plant is currently operating multiple types of short-term aeration equipment within the aerated lagoon due to the immediate need for additional aeration capacity to alleviate odours whilst minimising disturbance of the pond surface. 3.5 Trade Waste There are several major industries in Wanganui that discharge their trade wastes to the WDC wastewater system. The WDC regulates these industrial trade wastes through the 2008 Trade Waste Bylaw (Bylaw). There are minor trade waste dischargers to the WDC wastewater system in addition to the individuallyconsented major trade waste dischargers. A detailed discussion on trade waste dischargers is presented in Appendix D. Table 3-5 presents a summary of consented loads for major trade waste dischargers. Industry Table 3-5 Consented Loads for Major Trade Waste Dischargers BOD Load (kg/day) TSS Load (kg/day) Oil and Grease Load (kg/day) AFFCO Imlay 10,100 5,200 1,512 Tasman Tanning 14,700 13,840 7,545 Land Meat 4,910 7,500 2,889 Cavalier Spinners Open Country Dairy 1, MARS Petcare Total 32,280 27,760 12,236.5 It is known that some existing industries have plans for expansion and some new industries are currently exploring Wanganui as a location for facilities in the future. Developing a strategy for managing additional trade waste flows in the future is critical. It is difficult at this point in time to estimate what these loads could be; however, consideration must be given as part of the long-term strategy as to whether additional plant capacity should be allocated to trade waste or whether management of trade waste at the source is preferred. It was the uncertainty of future trade waste discharges that was the primary driver for increasing the historical flows and loads by 20% to develop the design flows and loads. WDC is currently working with industry to develop the best approach for trade waste monitoring and charging. At a minimum, it is recommended that each large trade industry should have a composite sampler on site, as well as sufficient equipment to additionally measure ph, temperature, and flow. An S:CAN, an online wastewater quality instrument, should also be procured by WDC and can be deployed to various industries as required P-01-V2-Long-Term Report Cardno BTO 21

22 4 Improvement Assessment Criteria In order to assess the relative pros and cons of the improvement options for the Wanganui WWTP in a structured fashion, each option will be evaluated against a set of criteria. The assessment criteria used have been chosen to ensure that the objectives and drivers of the WWTP upgrade are paramount in the evaluation and selection process. Meeting the required effluent quality and odour standards is not optional, therefore these are not included as assessment criteria but form part of the design basis for the improvements (Section 5). Similarly, options that cannot suitably meet the odour standard and required effluent quality are not evaluated in detail. 4.1 Constructability One of the main objectives of this investigation is to determine if the existing ponds can be retained and upgraded to meet the required effluent quality and odour standards. The priority is to use the existing ponds if at all possible. Alternatively, if new treatment tanks or structures are required they must fit within the existing site boundary and be practical to construct given the constraints of the site. 4.2 Process Risk There is a risk of effluent quality or odour non-compliance associated with all treatment processes. The excursion risk is a combination of influent flow or load risk, inherent process variability, degree of operator control, and mechanical failure risk. The level of excursion risk generally reduces with increasing cost. 4.3 Future Upgrade Complexity The existing treatment process is simple and does not require a high degree of monitoring and adjustment by the plant operators. This is appropriate given the effluent quality requirements of a long ocean outfall with suspended solids removal the only issue. Ideally any improvements should maintain this philosophy whilst also providing the appropriate level of non-compliance risk. 4.4 Completion Time Ultimately, WDC desires an improvement strategy to meet the required standards that can be implemented as quickly as possible. The time required for design, construction, and commissioning of any improvement works is therefore an important consideration. Factors that impact completion time such as long equipment lead times, and opportunities to shorten schedules, will be identified. Consenting is a specific area that has the potential to significantly impact project timing. The existing discharge consent for the WWTP expires 1 July However, the Horizons Regional Council has identified four dates over the course of the consent where the consent conditions can be reviewed. The first review date is 1 July In addition to this, the consent explicitly states that any change from the location, design concepts and parameters, implementation and/or operation may require a new resource consent or a change of consent conditions pursuant to section 127 of the Resource Management Act Discussions with Horizons Regional Council regarding potential consenting requirements are recommended. 4.5 Cost The costs of any improvement works comprise consenting, capital, and operation and maintenance (O&M) costs. All costs will be combined into a single net present value (NPV) cost to allow a fair comparison of all options. Power to the WWTP is provided by Contact Energy. The cost paid by WDC for power is determined by Contact Energy s pricing model which is influenced by factors such as temperature, time of day, season, and retailer. A power cost is generated for every 4-hour period. The pricing schedule for May 2010 to April 2015 has been set in a contract. However, pricing beyond April 2015 is not negotiated and cannot be predicted at this point in time. Due to the complexity of power pricing, an average unit cost for power of 14.5 cents per 375- P-01-V2-Long-Term Report Cardno BTO 22

23 kilowatt-hour has been used in development of 20-year life cycle costs. Cost assumptions are presented in Table 4-1. Table 4-1 Item Capital Costs Cost Assumptions Value Mechanical install & commissioning 15% Electrical & I/C costs 20% Contingency 15% Preliminary & general 15% Engineering & project management fees 15% Electricity Costs Fixed monthly electricity tariff (per month) $9,500 Electricity rate ($/kwh) Financial Cost Assumptions Discount Rate 7.0% Inflation Rate 2.5% 375- P-01-V2-Long-Term Report Cardno BTO 23

24 5 Design Basis for Improvements 5.1 Effluent Quality Standards Standards Required to Meet Existing Resource Consent The effluent standards in the existing resource consent that are relevant to the design of the treatment plant are the total suspended solids and microbiological standards; there are no BOD or nutrient standards in the consent. In addition, the resource consent total suspended solids standard is a 95 th percentile standard which is not sufficient to design the treatment plant. A median effluent suspended solids concentration is required for design. There are other effluent quality parameters limited by the resource consent conditions (grease, sulphide, metals). However effluent monitoring data shows that providing the TSS standards are met, these parameters will be met also. For the purposes of this report the other parameters need not be discussed further however during subsequent stages of design these parameters will need to be considered Margin of Safety It is critical that the upgraded treatment plant meets the resource consent standards; therefore as with any design, a margin of safety must be provided between the plant s design effluent standards and the resource consent standards. The size of the safety margin selected is a balance between risk of non-compliance and cost. The table below shows the consent and design standards that have been adopted for the design of the treatment options. The design standards shown are considered to provide the minimum prudent safety margin to ensure compliance. Table 5-1 Wanganui WWTP Consent and Design Effluent Standards to Ensure Consent Compliance Parameter Units Basis Consent Value Design Value TSS concentration g/ m 3 Median n/a 40 95th percentile Faecal coliform concentration cfu/100ml Median 10,000 1,000 90th percentile 25,000 10,000 Enterococci concentration cfu/100ml Median 4, Odour 90th percentile 25,000 4,000 The design of any plant improvements must meet the requirement of no objectionable odours at the site boundary at all times. This requirement will require that all odour sources are evaluated and it may be necessary to cover and provide forced ventilation and odour treatment. 5.3 Influent Flows and Loads Measured Influent Flows and Loads The design influent flows and loads for improvements are based on the available flow and load monitoring data over the past two years. 24-hour composite samples of the influent are taken three times per week at the Beach Rd pump station and analysed for the following parameters: o BOD o COD o Total suspended solids 375- P-01-V2-Long-Term Report Cardno BTO 24

25 o Ammonia nitrogen o Oil and grease o Sulphide o ph o Flow Loads were calculated using the 24-hour composite concentrations and multiplying by the 24-hour volume recorded at the Beach Rd pump station. Dry weather flows were obtained by selecting flow data only on days where there was no rainfall for the previous three days. Table 5-2 shows the measured influent flows and loads. Table 5-2 Measured Influent Flows and Loads Parameter Units Average 95 th percentile Maximum Dry Weather Flow m 3 /day 25,000 32,048 94,500* BOD load kg/day 12,100 25,800 56,700 COD load kg/day 27,300 49, ,400 TSS load kg/day 12,700 37,300 68,800 NH 3-N load kg/day 1,400 2,100 2,200 * maximum wet weather value Design Flows and Loads For the purposes of this report a 20% contingency has been added to the measured flows and loads to ensure there is sufficient capacity to treat the current consented trade waste loads and to allow some spare capacity. All designs are flexible and can be adjusted in future to accommodate any changes to the design flows and loads as the design progresses. The design flows and loads for the upgraded WWTP are shown below. Maximum flow to UV disinfection and the WWTP is limited to 650 l/sec (56,000 m 3 /day) and 1,120 l/sec (97,000 m 3 /day), respectively. Table 5-3 Design Influent Flows and Loads Parameter Units Average 95th percentile Maximum Dry Weather Flow m 3 /day 30,000 38, ,400* BOD load kg/day 14,520 30,960 68,040 COD load kg/day 32,760 58, ,880 TSS load kg/day 15,240 44,760 82,560 TKN load kg/day 2,330 3,990 3,660 NH3-N load kg/day 1,680 2,520 2,640 * maximum wet weather value Other Influent Parameters Due to the lack of detailed data, for the purposes of this report some key parameters will be assumed using typical literature figures for wastewater. Before the project moves into the design stages, more detailed testing of the influent will be required in order to refine the design. This is especially important due to the substantial industrial wastewater contribution. For the purposes of this options assessment, the use of assumed values is considered reasonable P-01-V2-Long-Term Report Cardno BTO 25

26 Table 5-4 Influent Parameter Assumptions Used for this Report Parameter Units Value Comment / Reference VSS : TSS ratio n/a 0.85 Typical value for de-gritted influent COD of volatile solids gcod/gvss 1.60 Typical value Non-biodegradable particulate COD fraction % 10.0 Typical value Non-biodegradable soluble COD fraction % 5.0 Typical value N : COD ratio % 2.5 Typical value 375- P-01-V2-Long-Term Report Cardno BTO 26

27 6 Improvement Options Overview 6.1 Options Matrix Figure 6.1 below summarises the improvement options that have been identified for the Wanganui WWTP based on what is physically practical at the existing site. The options have been broken down into the main treatment areas: primary treatment, secondary treatment, sludge management, storm flow management and disinfection. All individual options are discussed conceptually in this section; options shortlisted for further evaluation are presented in later sections. Increasing Cost Maintain Original Concept New Concepts Primary Treatment - Reduce secondary treatment size & energy - Remove compounds that may inhibit secondary process Partial Primary Settlement in Aerated Lagoon No Primary Treatment Anaerobic Pond New Primary Clarifiers & Anaerobic Digesters Secondary Treatment - Achieve Effluent TSS Standards: - 40 mg/l (median) - 90 mg/l (95 %ile) New Surface Aerators Chemical Dosing for Enhanced Settling in Settling Pond In-Pond Aeration Zone with Settling Zone (Biolac-type system) In-Pond Batch System (SBRs) Contact Stabilisation Process with External Secondary Clarifiers Sludge Management - Storage - Stabilisation (odour reduction) - Volume reduction prior to disposal In-Pond Storage and Digestion Periodic Desludging and Disposal by Contractor Waste Primary Sludge and WAS from Anaerobic Pond Waste Primary Sludge From Anaerobic Pond and Process WAS Separately Anaerobic Co-digestion Storm Flow Management - Reduce downstream unit size - Avoid overload of downstream units Wet Weather Storage in Aerated Lagoon Wet Weather Bypass of Primary & Secondary Treatment Disinfection - Meet resource consent microbe standards: UV Disinfection Chlorine Disinfection - Enterococci - Faecal coliforms Figure 6-1: Options Matrix It can be observed in the figure that project costs will increase as the scope of the improvements moves away from the original design concept. Costs associated with long-term improvements are presented later in the report where options are discussed in greater detail. As a reference, the cost of a greenfield wastewater treatment plant to meet the consent requirements is expected to be $60 million to $80 million. This estimate is based on historical NZ construction costs for WWTPs that must achieve a similar effluent quality, adjusted for equivalent population. 6.2 Enhancement of Original Concept The original design for the Wanganui WWTP was based on a deep aerated lagoon in which settlement, sludge storage, and aerobic biological treatment are performed within a single lagoon (Section 2.3). This design is unconventional and the facility has regularly failed to achieve the required effluent quality and been a source of odour nuisance as discussed in previous sections. There are several modifications that could be made to improve the existing plant whilst maintaining the basic process design concept, such as replacing the existing surface aerators or using coagulant or polymer 375- P-01-V2-Long-Term Report Cardno BTO 27

28 dosing to enhance settlement. However, enhancing the original design concept will not meet the resource consent suspended solids standard and will present an unacceptable odour risk. Since resource consent compliance and minimisation of odour risk are the two primary objectives for capital improvements at the facility, options that maintain the original design concept are not further considered. The aerated lagoon was designed to achieve a number of objectives. The design flaws associated with this design include: o Settlement of suspended solids in the parts of the lagoon not fully mixed by the surface aerators Incomplete mixing to allow for solids to settle means that maintaining a mixed liquor in the aerobic zone is difficult. Maintaining a mixed liquor is essential for treatment in order to provide time for biomass growth, BOD consumption, and generation of a settleable floc. o Storage, compaction and anaerobic digestion of settled sludge in the base of the lagoon Anaerobic digestion intended to take place in the base of the lagoon is impacted by the aerobic process occurring in the lagoon. Anaerobic digestion occurs in the absence of oxygen; supplying oxygen to the upper layer of the pond has likely partially inhibited the anaerobic digestion process in the lower layer of the pond. It is likely that inhibition of anaerobic digestion has been occurring since the plant was originally opened and that the most recent odour event was exacerbated when surface aerators were turned off. When the bottom layer of the pond was no longer inhibited by the oxygen being introduced by the aerators, the rate of digestion accelerated, releasing odourous compounds that were previously bound in undigested solids. o Aerobic biological treatment in the upper layer of the lagoon The upper layer of the pond is not fully mixed and growth of a biomass is difficult. Mixing in the aerated lagoon is only provided by the surface-mounted aeration equipment. Once solids settle past the aeration equipment area of influence, it cannot be resuspended. o Storm flow storage in the 2 metres of depth above the dry weather operating level of the lagoon Storm flows routed to the lagoon facilitate washout of any aerobic biomass that may be in the upper layer of the pond. 6.3 Primary Treatment The objective of primary treatment is to remove a portion of suspended solids and organic matter from the wastewater. Primary treatment generally involves the physical separation of suspended solids from raw wastewater, usually using some form of settling process. It can also involve an anaerobic treatment process such as an anaerobic pond. This section presents preliminary primary treatment options for implementation at the Wanganui WWTP No Primary Treatment Primary treatment is almost always economic for larger plants as shown by the fact that primary treatment is provided for all but one of the 10 largest cities in New Zealand. In addition, primary treatment provides a barrier of protection for the secondary treatment process by removing potentially toxic compounds such as heavy metals which could be present in the influent solids. This is particularly important for influent with large industrial components such as Wanganui. The primary driver for not providing primary treatment is when biological nutrient removal is required and the carbon content of the influent is needed in the secondary treatment process. Table 6-1 presents the advantages and disadvantages associated with no primary treatment. The 'no primary treatment' option is not selected for further consideration since the associated process risk and lifecycle cost is unreasonably high P-01-V2-Long-Term Report Cardno BTO 28

29 Table 6-1 Advantages and Disadvantages of No Primary Treatment Advantages Disadvantages Lower CAPEX No primary sludge treatment train Increased process risk Increased size of secondary treatment process Higher OPEX of secondary treatment process Variability in WAS characteristics No biogas production Primary Clarifiers with Anaerobic Digestion There are several common primary clarifier configurations including circular and rectangular primary clarifiers which provide for continuous sludge collection and removal. For larger plants (including those of Wanganui size), primary clarifiers are almost always provided with anaerobic digesters to stabilise the primary sludge, reduce the sludge volumes for disposal, and produce energy. The digesters therefore provide a financial payback due to reduced sludge loads and energy production. Both preliminary primary clarification options considered for the Wanganui WWTP are coupled with anaerobic digestion Rectangular Lamella Clarifiers Lamella clarification is a high-rate clarification process that uses inclined plates or tubes for solids removal. Wastewater enters an inlet chamber where it flows horizontally through ports at the bottom of the chamber into a rectangular settling tank. As flows travel upwards, solids settle on the inclined plates or tubes and drop into a hopper at the bottom of the clarifier. This process can be chemically enhanced with flocculant addition and flash mixing upstream of entry into the inlet chamber. Surface overflow rates for lamella clarification under average flow conditions are in the range of 1200 m 3 /m 2.d. Removal efficiencies of this technology average 35 to 40 percent of BOD and 65 to 75 percent of TSS, respectively. This technology is commonly utilised in industrial wastewater treatment. The higher loading rates and associated smaller facility footprint relative to conventional circular primary clarifiers make it an attractive option for detailed evaluation Circular Primary Clarifiers In circular clarifiers, wastewater flows up through a circular well in the middle of the tank before being distributed radially. A raking mechanism scrapes solids from the bottom of the clarifier into a hopper at the center of the tank. The hydraulic retention time in conventional primary clarifiers is commonly in the range of 1.5 to 2.5 hours. Circular primary clarifiers are generally designed with surface overflow rates under average flow conditions of 30 to 50 m 3 /m 2.d. Removal efficiencies of this conventional technology average 25 to 40 percent of BOD and 50 to 70 percent of TSS. The following considerations must also be made in the design of circular primary clarifiers: o Potential for hydraulic short circuiting o Effects of temperature changes and the potential to form density currents o Effects of wind and the ability to decrease performance through circulation cells Due to the industrial nature of the influent wastewater to the Wanganui WWTP and the need to minimise the footprint of a primary clarification process, circular primary clarifiers are not evaluated in detail Anaerobic Pond An alternative form of primary treatment that is most commonly used in high strength industrial wastewaters is an anaerobic pond. The Wanganui influent has a significant industrial composition, which highlights the potential suitability of anaerobic treatment upstream of secondary treatment. Anaerobic ponds capture solids and digest them in the base of the pond, and provide partial treatment of the liquid phase. Anaerobic ponds produce H 2 S and other odourous compounds and must be covered to prevent odour release. The anaerobic digestion of biosolids also produces biogas which can be conditioned (if required) and beneficially used or flared. Examples of municipal anaerobic ponds near Wanganui are the Marton and Fielding WWTPs P-01-V2-Long-Term Report Cardno BTO 29

30 6.4 Secondary Treatment The purpose of secondary treatment is to achieve the design effluent quality standards. For the Wanganui WWTP the only requirement is to meet the TSS standard; there are no BOD or nutrient standards. Compliance with the effluent TSS standard implies compliance with other effluent requirements, namely total grease total sulphide, total chromium, zinc, nickel, copper, lead, and mercury. This is a reasonable assumption since evaluation of historical data indicates that other effluent requirements are met when effluent TSS requirements are met. Therefore the purpose of secondary treatment is to form a settleable biomass that can be removed in a downstream settling pond or clarifier In-Pond Aeration Zone with In-Pond Clarifier Solids capture and recycle can be achieved via an in-pond clarifier or using external clarifiers. An in-pond clarifier can be created by baffling to create a quiescent settling zone downstream of the aeration zone. Solids would be allowed to settle in this zone and a scraping mechanism to collect solids from the bottom of the zone would be required; solids must be collected and pumped back to the aeration zone. Solids will be wasted as required to the solids handling process. There is at least one known proprietary system that is specifically designed to retrofit an activated sludge process into a pond system; this system Biolac, manufactured by Parkson Corporation, includes an in-pond clarifier. The Biolac process is to our knowledge untried in New Zealand however according to Parkson there are currently around 1,200 Biolac plants in operation worldwide. The largest facility is in Kent County in Delaware, USA, designed for 68,000 m 3 /day, however most installations are for smaller communities. There are two Biolac plants in Australia at Teys meat processing facilities in Wagga Wagga, and Tamworth, NSW. The main disadvantage with the Biolac system is the greater process risk compared with the other options, in particular the risk associated with the integral in-pond clarifier which is not as robust and lacks the controllability of conventional secondary clarifiers or SBR decanters. The polishing pond will settle solids carried over from the clarifier however the sludge capture and return to the aeration zone may not be effective. The high risk associated with the in-pond clarifier eliminates this option from detailed evaluation Contact Stabilisation with New Secondary Clarifiers Contact stabilisation is an efficient and effective high rate activated sludge process where nitrification is not required. Contact stabilisation is a type of high rate activated sludge process where return activated sludge is aerated in a stabilisation zone prior to mixing with influent wastewater in the contact zone. Due to the short sludge age, nitrification is avoided which saves on aeration costs. Creating a separate aeration zone for the return activated sludge only reduces the total aeration volume Sequencing Batch Reactor The sequencing batch reactor (SBR) is a fill-and-draw reactor in which aeration and clarification take place in the same tank. SBRs operate through the cycling of the following steps: fill, react, settle, decant, and idle. In order for a facility to continually process wastewater, at least two SBR tanks must be provided. Flow balancing upstream of a single SBR must otherwise be provided. A description of the SBR operational steps are presented in Table 6-2 (Metcalf and Eddy, 2003). Table 6-2 Operational Step Fill React SBR Operating Steps Description During the fill operation, volume and substrate (raw wastewater or primary effluent) are added to the reactor. The fill process typically allows the liquid level in the reactor to rise from 75% of the capacity (at the end of the idle period) to 100%. When two tanks are used, the fill process may last about 50% of the full cycle time. During fill, the reactor may be mixed only or mixed and aerator to promote biological reactions with the influent wastewater. During the react period, the biomass consumes the substrate under controlled environmental conditions P-01-V2-Long-Term Report Cardno BTO 30

31 Operational Step Settle Decant Idle *Metcalf and Eddy, 2003 Description Solids are allowed to separate from the liquid under quiescent conditions, resulting in a clarified supernatant than can be discharged as effluent. Clarified effluent is removed during the decant period. Many types of decanting mechanisms can be used, with the most popular being floating or adjustable weirs. An idle period is used in a multitank system to provide time for one reactor to complete its fill phase before switching to another unit. Because idle is not a necessary phase, it is sometimes omitted. Advantages and disadvantages of SBRs are presented in Table 6-3. Advantages Table 6-3 Advantages and Disadvantages of Sequencing Batch Reactors Disadvantages Smaller footprint than a conventional activated sludge process Adjustable cycle timing provides increased process flexibility Potential cost savings with no need for external clarifiers and RAS system Downstream flow balancing is required to control flows to the UV disinfection system Operation of continuous-flow SBRs during wet weather events can be challenging Higher level of sophistication on timing and controls relative to conventional systems SBRs are best characterised by their operational flexibility and robustness. The potential ease of retrofit to the existing aerated lagoon to accommodate SBRs makes this an option that should be evaluated in detail. 6.5 Disinfection UV Disinfection The WWTP currently contains a Trojan UV3000Plus open channel system. It is understood that disinfection using ultraviolet light was viewed by tangata whenua as the most favourable disinfection method during the original facility design. The present UV disinfection system has space for additional lamps if required, and following a performance testing program, the need to upgrade the existing UV disinfection system can be determined Chlorine Disinfection Chlorine disinfection is not normally consented for wastewater discharges in New Zealand and would only be considered if UV disinfection were considered to be not feasible. 6.6 Sludge Management Regardless of the individual solids handling technologies available, continuous processing of sludge at the Wanganui WWTP will be required with any improvement to the liquid treatment train. Any solids handling process will include the following elements: storage, thickening, dewatering, and disposal. It is evident that the sludge management options are driven by the primary treatment process. If primary clarifiers are implemented, they will be coupled with anaerobic digestion. Similarly, anaerobic ponds inherently provide for sludge storage and may eliminate the need for some sludge storage. Three options will be evaluated in detail: o Storage of sludge in anaerobic pond o Storage of primary sludge in anaerobic pond and separate WAS processing o Anaerobic co-digestion 6.7 Storm Flow Management Storm flow management is required in order to restrict the flow rate passed forward to downstream treatment units such as the settling pond or UV disinfection system in order to prevent overloading of these units P-01-V2-Long-Term Report Cardno BTO 31

32 Currently dry weather flows range from 15 MLD to 40 MLD with an average dry weather flow of 25 MLD. The maximum wet weather flow through the plant is 97 MLD (1,120 l/s) after which the plant may be bypassed. The UV disinfection system is sized for a wet weather flow rate of 650 l/s (56 MLD). Flows in excess of this value are stored in the aerated lagoon. There are two options for management of storm flows at the Wanganui WWTP: storage of wet weather flows in the aerated lagoon or bypass of wet weather flows. It is more effective to bypass storm flows around primary or secondary treatment units rather than provide in-pond storage. This approach eliminates the risk of biological washout by not allowing wet weather flows to enter the biological treatment process. UV disinfection of all flows will likely still be required. For purposes of this evaluation, it is assumed that flows over 56,000 m 3 /day will be bypassed around primary and secondary treatment to flow balancing upstream of UV disinfection. 6.8 Shortlisted Options The options for the treatment functions described above are summarised in Figure 6-2. Options highlighted in the figure are discussed in detail in following sections. Options that are evaluated in detail are more complex than the original concept. More operators and a higher level of training and process understanding will be required for successful operation. Management and maintenance of assets will become more important in time with the addition of more advanced, complex equipment. There will also be an increased need for monitoring and control. Addition of a sludge processing stream is the largest paradigm shift. Increasing Cost Maintain Original Concept New Concepts Primary Treatment - Reduce secondary treatment size & energy - Remove compounds that may inhibit secondary process Partial Primary Settlement in Aerated Lagoon No Primary Treatment Anaerobic Pond New Primary Clarifiers & Anaerobic Digesters Secondary Treatment - Achieve Effluent TSS Standards: New Surface Aerators - 40 mg/l (median) Chemical Dosing for - 90 mg/l (95 %ile) Enhanced Settling in Settling Pond In-Pond Aeration Zone with Settling Zone (Biolac-type system) In-Pond Batch System (SBRs) Contact Stabilisation with External Secondary Clarifiers Sludge Management - Storage In-Pond Storage and Digestion - Stabilisation (odour reduction) Periodic Desludging - Volume reduction prior to disposal and Disposal by Contractor Waste Primary Sludge Waste and WAS Primary from Sludge Anaerobic and WAS Pond from Anaerobic Pond Waste Primary Sludge From Anaerobic Pond and Process WAS Separately Anaerobic Co-digestion Storm Flow Management - Reduce downstream unit size - Avoid overload of downstream units Wet Weather Storage in Aerated Lagoon Wet Weather Bypass of Primary & Secondary Treatment Disinfection - Meet resource consent microbe standards: - Enterococci - Faecal coliforms UV Disinfection Chlorine Disinfection Figure 6-2: Wanganui WWTP Shortlisted Improvement Options 375- P-01-V2-Long-Term Report Cardno BTO 32

33 7 Primary Treatment Options 7.1 Anaerobic Pond Description Anaerobic treatment is a biological treatment process operating in the absence of oxygen. Anaerobic pond systems are typically used for high strength, industrial wastewater as a means to remove a substantial portion of the influent organic load prior to secondary treatment. The Wanganui wastewater has a significant industrial composition, which highlights the potential suitability of anaerobic treatment. It effectively treats BOD and provides solids destruction, reducing the sludge volume and load to the secondary treatment process. Figure 7-1 presents a schematic of this option. Figure 7-1 Process Schematic of a Pond-Based Anaerobic Lagoon Table 7-1 presents a summary of solids production and percentage dry solids at various stages through the solids handling process. The daily solids production from the digested anaerobic sludge is estimated at 2.4 tonnes per day. Table 7-1 Pond Based Anaerobic Pond Average Sludge Production Process Daily Mass (kg/d) %DS (%DS) Comment Anaerobic sludge Cold digestion + continuous withdrawal from sludge layer Secondary waste activated sludge Thickened WAS sludge Two options for treating the waste activated sludge (WAS) from the secondary process are returning it to cosettle and thicken in the anaerobic zone or separate thickening before dewatering of the combined sludge. The two options are detailed in Section 10. The covered anaerobic zone would be created by separating off a section of the existing lagoon with a wall, constructed using a baffle curtain, concrete or timber wall or earthen bund wall. The feed to the anaerobic pond will flow from the inlet pipe at the base of the pond to maximise the solids capture and minimise shortcircuiting P-01-V2-Long-Term Report Cardno BTO 33

34 The sludge layer will accumulate over a period to ensure a minimum sludge retention time before the continual withdrawal of digested sludge through a grid of sludge pipe laid on the zone floor. The main liquid outflow shall be a number of pipes just below the water surface, preventing any scum (fats, oils and grease) that float to the surface from travelling to the downstream process. A gas flare system will be required as part of the anaerobic pond option. The flare system, in addition to providing for biogas destruction, will also provide for treatment of any odourous air coming from the anaerobic pond. Due to the strict need to minimise the potential for odours extending beyond the WWTP boundary, planning for a redundant odour control system is conservative. It is assumed that the backup odour control system will be an odour scrubber Process Design The required anaerobic pond volume is based on the biological load of the influent. An estimate of the level of treatment is dependent on the minimum daily temperature for the coldest month, as a lower temperature reduces the activity of the anaerobic microorganisms. The process design of the anaerobic pond is shown in Table 7-2. Table 7-2 Anaerobic Pond Process Design Summary Parameter Units Basis Value Comment / Reference BOD loading rate g/m 3. d average 160 Average hydraulic retention time days average 2.7 BOD removal % average 50 Estimated based on historical minimum temperature of 13 C TSS removal % average 50 Conservative estimate based on common design values Solids retention time in sludge layer years 25% Vol 7 VSS destruction in sludge layer % average 60 Solids concentration in sludge layer % dry solids average 6.0 Ideal total volume m³ 80,000 Sludge storage volume m³ 20,000 Total depth m 10 Gas production rate Nm³/h average 50 Based on VSS destruction, with conservative methane content Discussion An anaerobic pond can be either constructed within the existing aerated lagoon using an internal wall, or a separate pond, or converting the existing settling pond with a pipe connection to the secondary process. The separate pond was eliminated as an option due to the high capital cost and the existing settling pond volume was eliminated because it was too small. The installation of a wall within the existing pond for separation may be difficult, and may require a full draindown of the aerated lagoon. The practical mechanisms for the separation within the existing aerated lagoon are: o Plastic baffle curtain o Baffle wall with piled structural supports o Earth bund and lining connecting to existing lining A high density plastic baffle curtain could be used to create a separate zone, suspended from a cable across the pond. The plastic welding of the curtain to form an air tight seal with the pond cover means this is not a realistic option. The stability of the curtain allows minimal change in water depth and no difference in water level into the secondary treatment. Walls or earth bunds are the main options likely to be viable and provide more robust structures P-01-V2-Long-Term Report Cardno BTO 34

35 A timber wall is possible but would require major piling through the existing liner for support and resealing of the liner before construction of the main wall. The wall would have a flow path for the liquid to minimise shortcircuiting and provide a steady flow to the secondary process. The wall would be robust but the inability to make a connection between the wall and the liner watertight, coupled with the connection to the pond cover will increase the difficulty and make this option less viable. A concrete wall will require major piling, as well as bracing for support. This will would be robust and provide a water tight seal. The wall would require careful engineering during detailed design to ensure stability, particularly if there is a difference in water level. An earth bund allows a robust system and the most viable option for water retention. The option would also include a section of new liner with seals to the existing liner. The bund would be built up from external materials and build-up with a batter from the top to the base. A drawback of the bund is the loss of volume from within the lagoon due to the slope required for the batter. Anaerobic treatment is a biological treatment process which occurs in the absence of oxygen to break down wastewater, the main by-products of which are CO 2 and methane gas. However other products are created in anaerobic conditions that are odourous, the most likely is hydrogen sulphide. Odours will be contained by the pond cover and removed with a gas system which will have a blower to maintain a pressure to the treatment process. Methods to treat the odours are: o Flare o Biological odour filter (Biofilter) o Odour scrubber The methane within the gas produced can be destroyed in a flare, which will also provide destruction of any odourous compounds. The methane gas could be used for energy recovery which may be an option for consideration, however the temperature will have a major effect on the digestion process and gas production is uncertain. It is important that there is a proper seal between the pond liner and the cover to minimise the risk of methane and air mixing to create an explosive atmosphere. The cover will provide a portion of gas pressure with pressure relief valves as protection but the main flow will be provided by the blower for the flare. The system is common for industrial sites, meatworks waste treatment and farm sites such as piggeries. Injecting the odour through the aeration treatment process will use the air to oxidise the odours, but this will cause an increase in aeration demand. The biofilter uses biological activity to oxidise the odour. The odour scrubber is the best solution because it is a mechanical solution, using chemicals to treat the odour and will have a quick response when required. The base of the anaerobic pond is flat, which means the collection of sludge may be difficult. Sludge will settle at the bottom of the anaerobic pond and be removed from the anaerobic pond by an array of pipes on the base of the pond. A sludge layer of 1.5m depth is expected to take 6-8 months to build-up. This will allow a steady pumped withdrawal of the accumulated sludge to further processing, similar to collection systems used by sequencing batch reactors Cost The costs have been estimated for the anaerobic lagoon with continual desludging and dewatering. Capital costs for this option are estimated at $2.34 million. The primary components of this option include: o Dividing earth wall with attached pond liner o Sludge collection system o Anaerobic pond cover o Gas collection system and flare o Odour scrubber system for flare bypass 375- P-01-V2-Long-Term Report Cardno BTO 35

36 Annual operating costs are estimated at approximately $55,000. Total net present value over a 20-year life cycle is estimated at $3.18 million. This does not include beneficial use of the biogas, which may be viable in the future and could offset the NPV. A periodic desludge was considered but not investigated fully due to the fact the option would disrupt the continuity of the pond cover and operation of the plant. If the anaerobic pond is constructed it is possible for WAS to be sent to the anaerobic pond for short-term storage, prior to being further processed along with primary sludge. 7.2 Primary Clarifiers with Anaerobic Digestion Description An alternative for anaerobic primary treatment is primary settlement, which uses gravity to remove a portion of solids. This option evaluates lamella clarifiers and anaerobic digestion. As previously mentioned, primary clarifiers are almost always provided with anaerobic digesters to stabilise the primary sludge. Stabilisation of primary sludge is likely required at the Wanganui WWTP for disposal and anaerobic digestion reduces the sludge volumes for disposal and produces energy. Figure 7-2 presents a schematic of this option. Figure 7-2 Process Schematic of Primary Clarifiers with Anaerobic Digestion Under this configuration, flow will enter the lamella clarifier through a quiescent zone before traveling upward through the lamella plates or tubes and over a weir. The configuration of a lamella settling process can be seen in Figure P-01-V2-Long-Term Report Cardno BTO 36

37 Figure: Brentwood Industries, 2013 Figure 7-3 Diagram of Primary Clarifiers with Tube Settlers The required surface area for the lamella settlers is based on the maximum flow rate that is expected through the clarifiers. The lamella settler provides a greater settling area for the flow relative to circular clarifiers, reducing the footprint of the clarifiers. The solids handling process associated with anaerobic digestion is presented in Figure 7-4. Effluent from the lamella clarifiers will flow downstream to the secondary treatment process. Solids intercepted by the tubes and plates will be collected from the bottom of the clarifier and be blended with thickened WAS prior to anaerobic digestion. Secondary sludge will be thickened prior to blending and introduction to the anaerobic digesters. Dissolved air flotation (DAF) or mechanical thickening removes excess water from the sludge prior to blending with primary sludge. The benefits of WAS thickening include reduction of digester volume, and a reduction of the energy required to heat the sludge to the digestion operating temperature. A blend tank is required upstream of the digesters. Complete mixing of the WAS and primary sludge will provide a stable feed to the digesters and provide the best conditions for gas production. The sludge will be heated to the operating temperature of the digesters to ensure steady gas production, improve the sludge stability and reduce the final disposal volume. Figure 7-4 Digestion Process Schematic of Solids Handling Scheme: Primary Clarifiers with Anaerobic Digested sludge will be dewatered prior to disposal. Options for dewatering are discussed in Section 10. Table 7-3 presents solids production estimates and concentration through the solids treatment train for this option P-01-V2-Long-Term Report Cardno BTO 37

38 Table 7-3 Primary Clarifiers and Anaerobic Digestion Average Sludge Production Process Daily Mass (kg/d) % Dry Solids (%DS) Daily Volume (m 3 /d) Comment Primary Sludge (lamella settlers) 8, % TSS removal Secondary waste activated sludge 5, ,830 Thickened WAS sludge 5, Blended sludge 13, Digested sludge 7, Sludge converted to gas Dewatered sludge 7, Improved dewatering Process Design The process design of the rectangular lamella settler clarifiers and anaerobic digestion is shown in Table 7-4. Further evaluation to determine which lamella clarifier type tube or plate- is most appropriate can be determined in the future. Tube settlers have been evaluated and costed for purposes of this report. The digesters are sized to provide a sufficient sludge retention time under average solids loading conditions. An increase in the solids loading rate will decrease the sludge retention time, solids destruction, and gas production. Table 7-4 Lamella Settlers and Digesters Anaerobic Pond Process Design Summary Parameter Units Basis Value Comment / Reference Lamella Settlers Number of clarifiers 3 2 duty, 1 standby under design flow conditions Surface area (each) m² peak day 200 maximum flow of 650L/s Water Depth m average 3 Total clarifier volume m³ 1500 tank maintenance performed at low flow Tube settlers specific area m²/m³ 21 Maximum overflow rate m/d 21 TSS removal % average 70 Anaerobic Digesters Number of Digesters 2 each with 50% capacity Digester diameter (each) m 15 Total digester volume m³ average 6,100 includes thickened secondary sludge Sludge retention time days average 19 VSS destruction in digesters % average 60 Based on 85% and 75% VSS content in primary sludge and WAS, respectively Gas production rate Nm³/h average 180 Potential Energy Production (Combined Heat and Power Unit) kw average 500 CHP main heat source all year Discussion There are a number of items that should be pointed out. First of all, the primary clarifiers offer a good level of control for the process and remove a significant portion of the solids and BOD before the secondary process, thus reducing the aeration requirement. Overall this option is significantly more complex than the existing WWTP configuration; implementation of this option will require highly trained operations staff P-01-V2-Long-Term Report Cardno BTO 38

39 Siting and construction of the clarifiers will be challenging but possible. Ideally, the clarifiers will be constructed as close as possible to the influent end of the existing aerated lagoon to minimise the need for buried piping; consideration should also be given to allow for gravity flow of wastewater to secondary treatment. The details involved in the anaerobic digestion process are discussed in section Cost Capital costs for this option are estimated at $23.66 million. The capital cost of the lamella clarifiers alone is $3.67 million. Major components of this alternative include: o Lamella clarifiers o Anaerobic digesters o Digester heating system o Gas handling system o WAS thickening system o Pre-digestion blending tank Annual operating costs are expected to be negative due to cost savings that can be achieved through energy generation. Annual operating cost savings are estimated to be $265,000. Total net present value over a 20- year life cycle is estimated at $20.33 million. 7.3 Summary of Primary Treatment Options Table 7-5 presents a comparison of the primary treatment options. Table 7-5 Comparison of Primary Treatment Options Criteria Anaerobic Pond Primary Clarifiers + Anaerobic Digestion Constructability Constructability risk of large earthworks bund wall inside existing aerated lagoon. Can be built off line. Process Risk Future Upgradability Higher risk due to variable primary effluent and sludge quality. Sludge withdrawal system more risky than primary clarifiers. Expansion not possible but increased loading of anaerobic pond possible. Piling will be required. Limited site space available. Complexity Simple operation, but maintenance of pond will be difficult. Lowest risk primary treatment option Completion Time Cost CAPEX OPEX (NPV) NPV Construction of associated solids handling facilities will require long construction period Construction time dependent upon baffle type selected $2.34M $0.84M $3.18M The clarifiers are large structures requiring long construction period The digester and associated equipment are complex structures requiring long construction periods $23.66M ($3.97M) $19.69M 375- P-01-V2-Long-Term Report Cardno BTO 39

40 8 Secondary Treatment Options 8.1 Contact Stabilisation Description This option involves constructing a fully mixed aeration zone inside the existing aerated lagoon and external secondary clarifiers to capture and return mixed liquor solids to the aeration zone. The objective of the contact stabilisation process is to create settleable flocs which can be easily separated from the treated effluent to meet the required TSS standards. Contact stabilisation is a type of high rate activated sludge process where return activated sludge is aerated in a stabilisation zone prior to mixing with influent wastewater in the contact zone. Due to the short sludge age, nitrification is avoided which saves on aeration costs. Creating a separate aeration zone for the return activated sludge only reduces the total aeration volume. Because the aeration zones are very small in comparison with the current aerated lagoon volume, biological solids do not have sufficient time to grow in a single pass through the zones and solids must be captured and returned to the process in order to avoid washout. Figure 8-1 presents a conceptual process flow diagram for this option. Figure 8-1 Process Schematic of Contact Stabilisation Process Process Design Parameters The key process design parameters for the contact stabilisation option are presented in Table P-01-V2-Long-Term Report Cardno BTO 40

41 Table 8-1 Contact stabilisation Process Design Summary Parameter Units Basis Value Comment / Reference Solids retention time days average 2.0 High rate process MLSS concentration g/m 3 average 1,500 Waste activated sludge yield kg/kg BOD removed average 1.00 Total aerated volume m 3 6,000 Contact plus stabilisation zones Waste activated sludge production tonnes/day dry solids average 5.19 m 3 /day average 1,730 Aeration zone depth m 6.0 SOTR : AOTR ratio Aeration peak hour : average day 3.50 Aeration peaking factor SOTR kg/h average 540 No nitrification allowed for SOTR kg/h maximum 1,890 No nitrification allowed for SOTE % 30% Aeration air flow rate Nm 3 /h average 6,470 Nm 3 /h maximum 22,645 Blower power draw kw average 180 maximum 630 Number of clarifiers 3 Clarifier diameter m 23 Side water depth m 3.5 Surface overflow rate m/h average 1.0 Surface overflow rate m/h maximum Discussion The main advantage of the contact stabilisation process is that is a well proven and reliable process and minimises aeration costs. The contact stabilisation process is a high rate process with a short solids retention time (sludge age) of two days. This is because the objective of the process is solely to develop settleable biological flocs. Nitrogen removal is not required and therefore the system is not designed for nitrification (the first step in nitrogen removal). By avoiding nitrification, significant capital and operational cost savings can be achieved as nitrification increases the aeration energy requirement significantly. The main disadvantage is the high capital cost involved with constructing the three external clarifiers. Due to the ground conditions on site, piling of the clarifiers will be needed to provide a secure foundation for these structures. Fine bubble diffused aeration (FBDA) has been included in the design. FBDA systems are typically around three times more efficient than mechanical aerators such as venturis, and the resulting energy savings provide a lower total net present value (NPV) cost compared with mechanical aerators. Construction of the aeration zone inside the existing aerated lagoon would be achieved using earthworks walls. Concrete walls would provide a tidier solution and would enable easier access to equipment inside the aeration zone however would be more expensive than earthworks Cost The main capital cost items for the contact stabilisation option are: o Construction of lined earthworks walls inside the existing aerated lagoon o Baffle curtain to create stabilisation zone 375- P-01-V2-Long-Term Report Cardno BTO 41

42 o Construction of 3 x 23 m diameter secondary clarifiers inside to the existing aerated lagoon o Fine bubble diffused aeration system o Aeration blowers o Return activated sludge pump station and pipework The estimated capital cost of this option is $13.55 million. The main operating costs with the contact stabilisation option are the aeration energy costs. The estimated annual operating costs of this option are $340, Sequencing Batch Reactors (SBRs) Description This option involves constructing two sequencing batch reactors (SBRs) inside the existing aerated lagoon. In contrast to conventional continuous flow processes, SBRs process the wastewater in batches, each batch consisting of fill, aerate, settle and decant steps, as described in Section By providing two SBR reactors, one SBR can be undergoing the settle and decant steps (which require quiescent conditions) while the other SBR receives and aerates incoming primary treated wastewater. The SBRs then alternate in a repeated, automatic cycle. Because the decant step occurs only intermittently, downstream flow balancing is required to avoid overloading the UV disinfection system. Flow balancing will be provided in the remainder of the aerated lagoon. Figure 8-2 presents a conceptual process flow diagram for this option. Figure 8-2 Process Schematic of the Sequencing Batch Reactor (SBR) System Process Design Parameters The main process design parameters for the SBR option are presented in Table 8-2: 375- P-01-V2-Long-Term Report Cardno BTO 42

43 Table 8-2 SBR Process Design Summary Parameter Units Basis Value Comment / Reference Solids retention time days average 2.5 Aerated SRT MLSS concentration g/m 3 average 1,500 Waste activated sludge yield kg/kg BOD removed average 0.94 Volume per SBR at bottom WL m 3 7,500 Waste activated sludge production tonnes/day dry solids average 5.13 m 3 /day average 1,030 Water depth at bottom WL m 4.00 Water depth at top WL m 6.00 Total cycle time hours 6.0 Fill / aerate time hours per cycle 4.0 Settle / decant time hours per cycle 2.0 SOTR : AOTR ratio Aeration peak hour : average day 3.50 SOTR per SBR kg/h average 940 SOTR per SBR kg/h maximum 3,290 Standard O 2 transfer efficiency % 30.00% Aeration air flow rate per SBR Nm 3 /h average 11,250 Nm 3 /h maximum 40,000 Blower power draw per SBR kw average Discussion kw maximum 1,100 The main advantages of the SBR process are that is a robust and reliable process, and avoids the need to construct secondary clarifiers and return activated sludge facilities. It is also highly adjustable and easily upgradable in future (by adding additional SBRs). Also there is built in redundancy as one SBR can be taken off line and the plant operated with limited capacity on one SBR. The main disadvantage of SBRs is that they are best suited to high sludge ages and for nitrogen removal, which is not required for the Wanganui case. In fact, nitrification (the first step in nitrogen removal) is undesirable for Wanganui as this adds significantly to the aeration demand and associated cost. Nevertheless, as SBRs are difficult to operate at low sludge ages, an aerated sludge age of 2.5 days and full nitrification with some credit for partial denitrification has been allowed for in the design and cost estimates. Some aerations savings may be possible with anoxic periods however for design purposes no allowance for denitrification has been made. Two SBRs have been provided for in the design to allow operational flexibility. Single SBRs with continuous feed and intermittent decant, known as intermittent decant aerated lagoons (IDAL s), are commonly used in Australia; however for the large scale of the Wanganui treatment plant, two SBRs are justified. Fine bubble diffused aeration (FBDA) has been included in the SBR design. FBDA systems are typically around three times more efficient than mechanical aerators such as those currently in use at Wanganui, and the resulting energy savings provide a lower total net present value (NPV) cost than mechanical aerators. Construction of the SBRs inside the existing aerated lagoon can be achieved using earthworks bund walls or concrete walls. Concrete walls would provide a tidier solution by creating a rectangular concrete box inside the aerated lagoon. The vertical concrete walls would enable easier access to equipment inside the SBR and provide for more efficient and effective mixing when compared with sloped earthworks walls. Piling of the concrete walls will be required due to the nature of the underlying material. However, earthworks walls will be cheaper and so have been selected for costing purposes, along with walkways to allow access to all 375- P-01-V2-Long-Term Report Cardno BTO 43

44 equipment. The floor level of the SBRs will need to be raised in order to keep the maximum operating depth in the SBRs to within 6.0 metres Cost The main capital cost items for the SBR option are: o Construction of earthworks walls and floor inside the existing aerated lagoon o Fine bubble diffused aeration system o Aeration blowers o Decanters The estimated capital cost of this option is $14.71 million. The main operating costs with the SBR option are the aeration energy costs. The estimated annual operating cost of the SBR option is $690, Comparison of Secondary Treatment Options Table 8-3 presents a comparison of the three secondary treatment options for consideration at the Wanganui WWTP. Table 8-3 Comparison of Secondary Treatment Options Criteria Contact Stabilisation SBRs Constructability Process risk Concrete tank construction inside existing lagoon. Clarifiers constructed at base of existing aerated lagoon. Well proven process for short sludge age applications where nutrient removal not needed. Constructability risk of earthworks SBR s inside existing aerated lagoon. More suited to longer sludge ages and nutrient removal. Good at handling variable loads. Future upgradability Difficult to upgrade effluent quality. Can upgrade for nutrient removal. Operational complexity Reasonably simple process. More complex than contact stabilisation due to cycle control. Completion time Piled concrete tank construction inside existing aerated lagoon. Earthworks SBR construction inside existing lagoon. Cost CAPEX OPEX (NPV) 20-year NPV $13.55M $5.04M $18.59M $14.71M $10.22M $24.93M 375- P-01-V2-Long-Term Report Cardno BTO 44

45 9 Disinfection The resource consent for the WWTP requires that limits for faecal coliforms and enterococci be met. Under the current plant configuration, effluent from the settling pond flows to the ultraviolet (UV) disinfection system. After UV treatment, the effluent is discharged by gravity to the existing 1.8km sea outfall off the coast of Wanganui. The limits specified in Table 9-1 are absolute limits, meaning that the limits cannot be exceeded for the stated basis. Table 9-1 Summary of Faecal Contaminant Limits Parameter Units Basis Option 1 Existing consent Option 2 Sand washing Faecal coliform concentration cfu/100ml Median 1,000 Max 10,000 Enterococci concentration cfu/100ml Median % less than 4, Use of Ultraviolet Disinfection System This option evaluates the continued use of the existing UV system for effluent disinfection. The design basis for the existing UV disinfection system is presented in Table 9-2. This information comes from a tender recommendation prepared for WDC by the original WWTP designers. Table 9-2 Existing UV disinfection System Design Basis Parameter Units Basis Design Value UV Influent Parameters: Flow rate l/s Maximum 650 UV transmittance % Minimum 20 TSS concentration g/m 3 90 th percentile 75 Pre-UV faecal coliform concentration cfu/100ml Maximum 650,000 Pre-UV enterococci concentration cfu/100ml Maximum 160,000 Guaranteed Faecal Coliform Reduction*: Post-UV faecal coliform concentration cfu/100ml 90 th percentile 10,000 Log reduction n/a 90 th percentile 1.81 Guaranteed Enterococci Reduction*: Post-UV enterococci concentration cfu/100ml 90 th percentile 4,000 Log reduction n/a 90 th percentile 1.60 UV Dose Provided: Calculated (theoretical) UV dose mj/cm 2 minimum Process Design The most important factor influencing the ability to continue to use the existing UV disinfection system is the expected water quality from the secondary treatment system. Secondary treatment options must meet a TSS limit of 40 mg/l as presented in Section 5.1 in order for the effluent resource consent limit to reliably be met. That inherently implies that all secondary treatment options that meet this requirement will provide an influent to the UV system that meets the system design basis for suspended solids P-01-V2-Long-Term Report Cardno BTO 45

46 There are a number of factors that should be considered when designing a UV disinfection system. They include: o UV Dose o The UV dose dictates the effectiveness of the UV disinfection process and is calculated based on UV intensity (mw/cm 2 ) and exposure time (seconds). There are multiple methods to calculate the UV dose, the most common being the bioassay method. Ultimately, the UV dose is determined by the UV system manufacturer based on a design water quality to the system. o Impact of Particles o The presence of suspended solids in the wastewater impacts the ability of the UV light to come into contact with targeted microorganisms. This is problematic for the removal of coliform bacteria, as they can associate with particles to such a degree that they are completely shielded from UV light resulting in a residual coliform bacteria concentration (Metcalf and Eddy, 2003). Improved microorganism removal will be observed with decreasing suspended solids concentration. o Reactor Design and Hydraulics o A well-designed UV reactor will allow for a uniform flow that maximises UV exposure. Considerations should be given such that short-circuiting of flows and the presence of dead zones are limited. Also, reactor design should facilitate easy operator access for maintenance. o Microorganism Characteristics o Disinfection efficiency is impacted by physical characteristics of microorganisms. UV doses required for pathogen inactivation are being regularly refined as methods of analysis are improving. Current literature should be consulted to identify the UV dose required for inactivation of specific microorganisms. o Chemical Constituents in Wastewater o Chemical constituents in wastewater can inhibit the UV disinfection process by reducing the UV transmissivity. Targeted removal of these constituents, if identified and deemed to have a significant impact on disinfection efficiency, should take place. 9.3 Discussion The existing UV disinfection system is a Trojan UV3000Plus open channel system. This system uses low pressure, high output lamps and is designed for automatic dose pacing based on the measured flow rate to the UV and the UV intensity within the lamp array. The lamps are installed in a single channel, configured in two banks. Each bank contains 13 racks, each rack containing 8 UV lamps (a total of 104 lamps per bank). There is space provided in the channel to increase the total number of lamps from 208 to 272 if required. This would increase the UV dose from 225 J/m 2 to 295 J/m 2. The existing UV disinfection system has been problematic for the WDC. First of all, as observed in figures presented in Appendix A, the UV disinfection system has failed to meet effluent limits due to out of specification influent to the system. Data contained in the February 2010 WWTP Commissioning Report (MWH, 2010) shows that even when UV transmissivity was over 20% (minimum required transmissivity), the system failed to meet its effluent limit. The WDC never conducted a commissioning test on the UV system after installation because the water quality upstream of the system never consistently met the design specification; the defects liability period has since lapsed. WDC has also experienced a number of mechanical issues with the UV system. A plastic fiber weed mat was laid on the WWTP site after construction; over time pieces of this mat have been degraded by the sun, blown into the ponds, and have become clogged in the UV system. WDC staff reports that this mat ends up lodged between lamp tubes and cleaning system wipers causing tubes to break and condensate to form in ballasts. It is recommended that the remaining weed mat on site be removed to prevent UV equipment failure and decrease maintenance costs P-01-V2-Long-Term Report Cardno BTO 46

47 The ability to continue use of the existing system hinges upon industrial trade waste point source management and improved performance of the upstream treatment process. The industrial trade wastes contributed to the Wanganui WWTP have likely historically had an impact on the performance of the UV disinfection system. Spectroscopic analysis was conducted on a sample of settling pond effluent; UVT of the raw sample was measured to be 1.8% at 254 nm. This sample was then filtered through a 0.45 µm filter and transmissivity was measured to be 51%, indicating the presence of very fine particulate matter. Online measurement of UVT would be a beneficial addition to the system. Gas chromatography/mass spectroscopy was conducted on the same sample. The compound Furfural, an aromatic aldehyde absorbent in the UV range, was identified through this analysis. It is recommended that further characterisation of industrial trade wastes be conducted in order to identify any other chemical compounds that could negatively impact UV transmissivity. The following investigations/actions are recommended in order for successful use of UV disinfection in the long-term: o Evaluation of trade waste discharges to identify any chemical constituents inhibitory to UV disinfection (and subsequent point source control) o Removal of plastic fiber weed mat on site o Addition of online UV transmissivity monitoring o Reassessment of UV dose o Installation of additional lamps to provide appropriate dose, if required, or addition of a new UV channel The general conclusion is that UV can be continued to be used for disinfection in the long-term. If flows entering the UV system can meet the original design specification, then it is unlikely that any changes will be required. If, however, the flows to the UV system do not meet the original design specification, then a new bioassay will be required and modifications to the system will be required. 9.4 Cost WDC annual equipment costs for the UV system are approximately $120,000. This includes the cost to replace lamps and failed ballasts. This estimate is exclusive of the power required to operate the system and the labour cost associated with maintaining the system. WDC estimates that with existing challenges, 16 man-hours are required per week to maintain operation of the system. The worst-case scenario is that an additional UV channel and lamps is required. Table 9-2 presents the costs associated with the three potential UV disinfection options. Table 9-3 Cost Estimates for UV Disinfection Options Option CAPEX OPEX (NPV) NPV Existing System $0 $3.83 M $3.83 M Expansion of Existing System $0.1 M $3.90 M $4.00 M Expansion of Existing System + New UV Channel + New Bypass Channel 9.5 Disinfection Summary $2.14 M $3.83 M $5.97 M A summary of criteria associated with maintaining the existing UV disinfection system is presented in Table P-01-V2-Long-Term Report Cardno BTO 47

48 Table 9-4 Existing UV System Assessment Summary Criteria Constructability Comment Civil works may not be required Process Risk Future Upgradability No bypass channel No generation of disinfection byproducts Ongoing issues with weed mat clogging poses a risk; failure of tubes and ballasts reduces UV system capacity Industrial trade wastes and upstream process performance impact UV system performance No online UV transmissivity meter Current opportunity to increase UV dose through lamp addition Potential in the future to add another UV channel, but not without significant capital expenditure Complexity Complexity of operation would not change Complexity of maintenance would improve with weed mat removal Completion Time Likely no modification required under this alternative Cost Existing System Expansion of Existing System Expansion of Existing System + New UV Channel + New Bypass Channel CAPEX OPEX (NPV) 20-year NPV $0M $3.83M $3.83M $0.1M $3.90M $4.00M $2.14M $3.83M $5.97M 375- P-01-V2-Long-Term Report Cardno BTO 48

49 10 Sludge Management Options This section evaluates the various options for sludge management at the Wanganui WWTP. The solids handling process is driven by the selected method of primary treatment. Specifically, implementation of primary clarifiers in lieu of an anaerobic pond is coupled with anaerobic digestion. Processing of sludge prior to and following digestion is dictated by the digestion process. The overall sludge management process consists of the following elements: storage, thickening, dewatering, and disposal. Table 10-1 shows these different elements for the different options. Table 10-1 Solids Handling Requirements Based on Primary Treatment Process Anaerobic Pond Primary Clarifiers and Anaerobic Digestion Primary Sludge Secondary Sludge Primary Sludge Secondary Sludge in anaerobic pond in anaerobic pond Storage storage in aerobic digestion not required thickened WAS storage combined thickened sludge storage Thickening in anaerobic pond in anaerobic pond DAF or mechanical thickening in primary clarifier DAF or mechanical thickening dewatering of combined sludge from anaerobic pond Dewatering dewatering of digested sludge dewatering of combined sludge from thickened sludge storage Disposal dewatered sludge storage prior to disposal dewatered sludge storage prior to disposal It is common that dewatering processes are operated on a set schedule, such as 8 hours per day, 5 days per week. It is recommended that, regardless of which solids handling option is the most appropriate for implementation at the Wanganui WWTP, identification of a regular operating schedule be determined and considered in the detailed design Storage of Sludge in Anaerobic Pond Description This option evaluates the solids handling process required at the Wanganui WWTP if activated sludge is wasted to the anaerobic pond. The anaerobic pond is detailed in Section 7.1. Figure 10-1 presents this sludge handling option P-01-V2-Long-Term Report Cardno BTO 49

50 Figure 10-1 Process Schematic of Sludge Treatment from Co-settled in Anaerobic Lagoon Under this option, WAS will be pumped from the secondary process into the base of the anaerobic pond. Considerations must be made such that WAS is introduced into the lagoon a safe distance from the overflow. Sludge will accumulate with the primary sludge in the base of the anaerobic pond for withdrawal via a grid of pipes on the base of the pond prior to sludge dewatering and disposal. The sludge will be pumped to the dewatering facility, where the sludge will be dewatered to 20% for off-site disposal. The centrate from the solids dewatering process will be returned to the liquid treatment train. Table 10-2 presents a summary of solids production dry solids concentrations at various stages through this solids handling process. The daily solids production from the blended co-settled sludge is estimated at 5.5 tonnes per day. Table 10-2 Storage in Anaerobic Pond Average Sludge Production Process Daily Mass (kg/d) %DS (%DS) Daily Volume (m 3 /d) Comment Anaerobic sludge 2, % destruction Secondary waste activated sludge 3, ,030 40% destruction Blended sludge 5, Cold digestion + continuous withdrawal from sludge layer Dewatered sludge 5, Improved dewatering if WAS is stabilised Process Design The waste activated sludge will be wasted to maintain the level of treatment in the secondary process. This option uses the anaerobic lagoon as storage and thickening of the WAS, thickening it from 0.3 to 6%. The design parameters are shown in Table The anaerobic process destroys a large proportion of the volatile solids within the sludge, significantly reducing the total volume of sludge that must be disposed. Table 10-3 Design Parameters for Anaerobic Pond with Continuous Dewatering Parameter Units Design Basis Design Value Total storage volume m³ Average 20,000 Sludge layer depth m Average 2-3 VSS destruction in sludge layer % Average 60 Primary 40 WAS Daily sludge volume for dewatering m³/d Average 95 Solids concentration in sludge layer % dry solids Average 6.0 Weekly operating basis d/wk 4 Daily operating basis h/d 8 Dewatering flow rate m³/h Average P-01-V2-Long-Term Report Cardno BTO 50

51 Discussion The storage of the sludge within the anaerobic pond is a low energy option. The anaerobic pond will reduce the solids volume to be dewatered through VSS destruction and the water depth will provide thickening for the WAS. The gas system will treat the odours from the pond and potentially to convert the gas for energy recovery in the future. This process is expected to be easy to operate relative to anaerobic digestion and will require relatively little operator management. The risks associated with this option include: o WAS destabilisation of the anaerobic pond and reduced treatment of the influent o WAS (approximately 10% of the sludge layer) may displace primary sludge o Long recovery time after a process failure o Few variables can be manipulated to control process performance o Anaerobic digestion will vary considerably with temperature, with potentially no activity in winter It is possible for WAS to be handled separately; doing so would lessen the process risks but increase the capital and operational cost of this option. This option is evaluated in Section Cost Capital costs for this option are estimated at $1.74 million. The primary components of this option include: o Sludge dewatering unit o Sludge pumping equipment o Polymer storage and feed equipment o Conveyors o Dewatering building o Odour control o Associated civil works, including access road Annual operating costs are estimated at $920,000. Total net present value over a 20-year life cycle is estimated at $15.38 million Storage of Primary Sludge in Anaerobic Pond & Separate WAS Processing Description This option evaluates separate WAS treatment prior to co-dewatering of sludge. Under this configuration, primary sludge will be wasted from the anaerobic pond and WAS will be wasted from secondary treatment. Instead of WAS being sent to the anaerobic pond for storage and treatment as evaluated in Section10.2, the WAS will be sent to an aerobic digester where it can be conditioned and stored prior to thickening to increase the solids content of sludge by removing a portion of the liquid fraction. This will allow the sludge to be more easily handled and reduce the total volume for transport or storage. The sludge from the aeration ponds, at 0.3% - 0.5%DS, is not concentrated enough for disposal without additional processing; the transport costs associated with unthickened and undewatered WAS is significant. Figure 10-2 shows a schematic of this option P-01-V2-Long-Term Report Cardno BTO 51

52 Figure 10-2 Process Schematic of Storage of Primary Sludge in Anaerobic Pond & Separate WAS Processing The WAS will be continuously wasted from the secondary treatment process and aerobic digestion and WAS thickening will have to operate seven days a week to maintain control of the secondary process. The liquid sidestreams from the thickening and dewatering processes will be returned to the front of the plant for treatment. Table 10-4 presents a summary of solids production and percentage dry solids at various stages through the solids handling process. The daily solids production from the blended anaerobic sludge and thickened sludge is estimated at 7.8 tonnes per day. Process Table 10-4 Separate WAS Thickening Average Sludge Production Daily Mass (kg/d) %DS (%DS) Daily Volume (m 3 /d) Comment Primary sludge 2, Post cold digestion + continuous withdrawal from sludge layer Secondary waste activated sludge 5, ,700 Aerobically digested sludge 4, Thickened WAS sludge 4, Blended sludge 6, Dewatered sludge 6, Improved dewatering if WAS is stabilised Process Design There are four primary elements of this option that must be designed. These include a WAS aerobic digester, WAS thickener, blended sludge storage tank, and dewatering process. Table 10-5 presents the relevant design parameters for these elements. Table 10-5 Design Parameters for Sludge Thickening with Continuous Dewatering Parameter Units Design Basis Design Value Daily WAS volume m³/d Average 1700 Number of aerobic digesters qty New tank 1 Aerobic digester volume m³ 7 days storage 45,000 Number of dewatering units qty 2 Dewatering flow rate m³/h Operates 4 days/week 34 Solids handling building area m² P-01-V2-Long-Term Report Cardno BTO 52

53 Discussion There are a number of different technologies that can be used to thicken WAS. Table 10-6 presents a summary of these technologies. For purposes of this report, it has been assumed that DAF units will be used. If thickeners are required as part of a long-term solution, the various technology options can be evaluated in greater detail. Table 10-6 Thickening Technology Comparison of Sludge Thickening Equipment Package Plant Suits WAS Proven Gravity Thickening N/A Med Med Low Gravity Belt Thickening 4% Low Low Med Dissolved Air Flotation 5% Low High Med Disc Thickener 4% Low Low Med Rotary Drum Thickener N/A Med Med Low Centrifuge 4% Low Low Med There are also various dewatering technologies. Since each solids handling option requires dewatering, a discussion on dewatering is presented in Section Cost Capital costs for this option are estimated at $6.55 million. The primary components of this option include: o Aerobic digesters o Thickener (DAF) units o Blended sludge storage o Dewatering facility o Associated civil works, including access road Annual operating costs are estimated at $1.31 million. Total net present value over a 20-year life cycle is estimated at $25.94 million Anaerobic Co-digestion of Sludge Description This option involves co-digestion of WAS and primary sludge. This option can only be implemented if primary lamella settling tanks are selected for implementation. Under this option, WAS would be thickened prior to blending with primary sludge in a storage tank. Blended sludge would be fed to the anaerobic digesters; digested sludge would be dewatered and sent to disposal. The solids handling process associated with anaerobic digestion is presented in Figure DS Achieved NZ Example Poly required Odour CAPEX OPEX 375- P-01-V2-Long-Term Report Cardno BTO 53

54 Figure 10-3 Process Schematic of Anaerobic Co-digestion of Sludge The blended primary and thickened WAS is mixed prior to anaerobic digestion for consistent feed. The benefits of WAS thickening include reduction of digester volume, and a reduction of the energy required to heat the sludge to the digestion operating temperature. The sludge will be heated to the operating temperature of the digesters to ensure steady gas production, improve the sludge stability and reduce the final disposal volume. The gas will be collected and transported to the gas system for use in a Combined Heat and Power (CHP) plant or boiler. The CHP provides both heat and energy to maintain the digestion process. The digested sludge will be stored in a tank before dewatering, removing more water from the sludge for off-site disposal. Table 10-7 presents a summary of sludge quantity and concentration. Table 10-7 Anaerobic Co-digestion Average Sludge Production Process Daily Mass (kg/d) %DS (%DS) Daily Volume (m 3 /d) Comment Primary sludge 7, % capture in primary clarifiers Secondary waste activated sludge 5, ,830 Thickened WAS 5, Blended sludge 13, Digested sludge 7, Dewatered sludge 7, Process Design Table 10-8 presents design parameters for this option. Some of the parameters are detailed in Section 7.2, but the digesters are designed for a solids loading rate, and to maintain a minimum sludge retention time in the digesters to maximise the gas production for energy conversion. The digestion process control will affect the gas production, hence the energy available, also affecting the solids reduction and the amount the sludge can be dewatered. The digested sludge will be pumped to the dewatering facility P-01-V2-Long-Term Report Cardno BTO 54

55 Table 10-8 Design Parameters for Anaerobic Co-digestion Parameter Units Design Basis Design Value Daily feed of blended sludge m³/d Average 300 Number of anaerobic digesters qty 2 Total anaerobic digester volume m³ 19 days 6,100 Number of dewatering units qty 3 Dewatering flow rate m³/h 34 Solids handling building area m² Discussion The primary clarifiers are required for digesters and can be used to reduce the operating costs for the site, with beneficial use of the gas produced. Carbon dioxide and methane gas are the main byproducts of anaerobic digestion; however, other products are created in anaerobic conditions that are odourous. These odourous products will be incinerated in the boiler, combined heat and power (CHP) plant or the emergency flare. The risk of odours from the digestion process is minimal due to the level of control and the incineration within the boiler or CHP. It is possible that conditioning of the digester gas may be required; common equipment required for conditioning includes chemical scrubbers, siloxane filters, and condensate traps. Equipment in the digester facility must be rated for a hazardous environment due to the presence of methane gas; restricted access and tight control of any work to be carried out in the area will also be required Cost Capital costs for this option are estimated at $22.56 million. Major components of this alternative include: o Lamella clarifiers o Anaerobic digesters o Digester heating system o Gas handling system o WAS thickening system o Pre-digestion blending tank o Digester tanks o Digester heating equipment o Digester gas system o Sludge mixing system o Sludge recirculation system o Dewatering and disposal costs Annual operating costs are estimated at $910,000. Total net present value over a 20-year life cycle is estimated at $36.10 million Sludge Dewatering Description Dewatering, a component of all the solids handling options presented in this report, is used to remove additional liquid from the sludge to produce a sludge cake which can be easily transported. Dewatering is most effective when carried out on a thickened sludge stream. The liquid extracted from the sludge is returned to the front of the plant for reprocessing. The dewatered sludge cake concentration is typically in the order of 18-30% dry solids (DS) P-01-V2-Long-Term Report Cardno BTO 55

56 There is no requirement that a dewatered cake product be produced at the Wanganui WWTP. Regardless, sludge cake has significant advantages because the volume of sludge to be stored and transported is dramatically reduced. For example a 20% DS cake has 13 times less volume than an unthickened sludge of 1.5% DS. Added to this is the fact that a 20% DS sludge is much easier to handle, transport and could be stored as stacks rather than requiring containment within a lagoon or tank. It is also important to note that many landfills require sludge to be cake with a dry solids content of at least 20%. There are a number of different dewatering technologies shown in Table A comparison of the different methods is used, including the OPEX and CAPEX which is ranked from Low, Medium and High. Table 10-9 Dewatering Technology Comparison of Sludge Dewatering Equipment Package Plant Proven NZ Example Belt Filter Press 18% Low Med Low Low Vacuum Belt Filter 20% Med Low Low Low Centrifuge 20% High Low Med High Rotary Press >20% Low Low Med Low Screw Press 20% Med Med Med Low Discussion Since dewatering is a component of all long-term solids handling options, procurement of this equipment and construction of its necessary housing structure may be fast-tracked. There is some financial benefit that can be realised through fast-tracking of a dewatering facility. Specifically, Appendix C presents sludge management options for the dredging, dewatering, and disposal of sludge in the existing ponds. Mechanical dewatering is evaluated as an option; Contractors can bring in temporary mechanical dewatering units during their sludge removal and disposal operation. However, if WDC elected to construct a dewatering facility prior to pond desludging operations, WDC dewatering equipment could be used in lieu of Contractor-owned equipment. Under that scenario, WDC would save on dewatering equipment mobilisation and rental fees Cost The cost of dewatering and disposal has been included in each sludge management option. The primary elements of the dewatering facility include: o Dewatering units o Sludge pumping equipment o Polymer storage and feed equipment o Conveyors o Dewatering building o Dewatered sludge storage bins (covered and odour controlled) o Odour control o Associated civil works, including access road DS Achieved Noise Odour CAPEX OPEX 375- P-01-V2-Long-Term Report Cardno BTO 56

57 10.6 Summary of Sludge Treatment Options Table presents a summary of solids handling options for the Wanganui WWTP. Table 10-10Comparison of Secondary Treatment Options Criteria Storage in Anaerobic Pond Primary Sludge Storage in Anaerobic Pond with Separate WAS Treatment Anaerobic Co-Digestion Constructability See primary treatment Piling of aerobic digesters and blending tank required. Piling of anaerobic digesters required. Process Risk Higher dewatering risk due to more variable sludge quality from anaerobic pond. Lower dewatering risk due to more consistent sludge quality. Lowest risk, good controllability of process. Future upgradability Expansion not possible but increased loading of anaerobic pond possible. Can be accommodated in design. Can be accommodated in design. Complexity Simple operation, but maintenance of pond will be difficult. More complex, requires separate storage and thickening of WAS. Most complex of the three options. Completion Time Same as primary treatment anaerobic pond. Piled concrete tank construction outside lagoon (aerobic digesters & blending tank). Piled concrete tank construction outside lagoon (anaerobic digesters). Cost CAPEX $1.74M $6.55M $22.56M OPEX (NPV) $13.64M $19.39M $13.54M NPV $15.38M $25.94M $36.10M 375- P-01-V2-Long-Term Report Cardno BTO 57

58 11 Summary of Evaluation A summary of costs for the options is presented in Table 11-1 and a summary of the evaluation criteria is provided in Table Table 11-1 Cost Comparison of Treatment Options Option 1 Option 2 Option 3 Option 4 Option 5 Option 6 Primary Treatment Anaerobic Pond $2,343,000 $2,343,000 $2,343,000 $2,343,000 Primary Clarifiers $3,670,000 $3,670,000 Secondary Treatment SBRs $14,707,000 $14,707,000 $14,707,000 Contact Stabilisation with New $13,551,000 $13,551,000 $13,551,000 Secondary Clarifiers UV Disinfection $0 $0 $0 $0 $0 $0 Sludge Management Storage in Anaerobic Pond $1,738,000 $1,738,000 Primary Sludge Storage in Anaerobic $6,554,000 $6,554,000 Pond, Separate WAS Treatment Anaerobic Codigestion $22,556,000 $22,556,000 Capital Cost ($) $18,788,000 $23,604,000 $17,632,000 $22,448,000 $40,933,000 $39,777,000 Primary Treatment Anaerobic Pond $838,000 $838,000 $838,000 $838,000 Primary Clarifiers $1,099,000 $1,099,000 Secondary Treatment SBRs $10,221,000 $10,221,000 $10,221,000 Contact Stabilisation with New $5,041,000 $5,041,000 $5,041,000 Secondary Clarifiers UV Disinfection $3,829,000 $3,829,000 $3,829,000 $3,829,000 $3,829,000 $3,829,000 Sludge Management Storage in Anaerobic Pond $13,644,000 $13,644,000 Primary Sludge Storage in Anaerobic $19,385,000 $19,385,000 Pond, Separate WAS Treatment Anaerobic Codigestion $13,539,000 $13,539, year NPV of Operating Cost ($) $28,532,000 $34,273,000 $23,352,000 $29,093,000 $28,688,000 $23,508,000 Primary Treatment Anaerobic Pond $3,181,000 $3,181,000 $3,181,000 $3,181,000 Primary Clarifiers $4,769,000 $4,769,000 Secondary Treatment SBRs $24,928,000 $24,928,000 $24,928,000 Contact Stabilisation with New $18,592,000 $18,592,000 $18,592,000 Secondary Clarifiers UV Disinfection $3,829,000 $3,829,000 $3,829,000 $3,829,000 $3,829,000 $3,829,000 Sludge Management Capital Cost Storage in Anaerobic Pond $15,382,000 $15,382,000 Primary Sludge Storage in Anaerobic $25,939,000 $25,939,000 Pond, Separate WAS Treatment 20-year NPV of Operating Cost 20-year Net Present Value Anaerobic Codigestion $36,095,000 $36,095, year Net Present Value ($) $ 47,320,000 $ 57,877,000 $ 40,984,000 $ 51,541,000 $ 69,621,000 $ 63,285, P-01-V2-Long-Term Report Cardno BTO 58

59 Table 11-2 Comparison Summary of Treatment Options 1. Primary Treatment 2. Secondary Treatment 3. Sludge Management Primary Clarifiers Anaerobic Pond Contact Stabilisation Sequencing Batch Reactors Combined Primary and WAS Storage in Anaerobic Pond Primary Sludge in Anaerobic Pond + Separate WAS Processing Anaerobic Digesters Constructability risks Can be built off line. Piling will be required. Limited site space available. Constructability risk of large earthworks bund wall inside existing aerated lagoon. Concrete tank construction inside existing lagoon. Clarifiers constructed at base of existing aerated lagoon. Constructability risk of earthworks SBR s inside existing aerated lagoon. See primary treatment Piling of aerobic digesters and blending tank required. Piling of anaerobic digesters required. Process risks Lowest risk primary treatment option More consistent sludge quality Biogas system and CHP engines are complex Higher risk due to variable primary effluent and sludge quality. Sludge withdrawal system more risky than primary clarifiers. Well proven process for short sludge age applications where nutrient removal not needed. More suited to longer sludge ages and nutrient removal. Good at handling variable loads. Higher dewatering risk due to more variable sludge quality from anaerobic pond. Lower dewatering risk due to more consistent sludge quality. Lowest risk, good controllability of process. Future upgradability Opportunity for expansion in the future can be accommodated in design Expansion not possible but increased loading of anaerobic pond possible Difficult to upgrade effluent quality. Can upgrade for nutrient removal. Expansion not possible but increased loading of anaerobic pond possible. Can be accommodated in design. Can be accommodated in design. Operational complexity More complex and labour intensive than anaerobic pond. Simple operation, but maintenance of pond will be difficult. Reasonably simple process. More complex than contact stabilisation due to cycle control. Simple operation, but maintenance of pond will be difficult. More complex, requires separate storage and thickening of WAS. Most complex of the three options. Completion time Piled concrete tank construction outside lagoon. Large earthworks bund wall inside existing aerated lagoon. Piled concrete tank construction inside existing aerated lagoon. Earthworks SBR construction inside existing lagoon. Same as primary treatment anaerobic pond. Piled concrete tank construction outside lagoon (aerobic digesters & blending tank). Piled concrete tank construction outside lagoon (anaerobic digesters). CAPEX $3.67M $2.34M $13.55M $14.71M $1.74M $6.55M $22.56M OPEX (NPV) $1.10M $0.84M $5.04M $10.22M $13.64M $19.39M $13.54M 20-year NPV $4.77M $3.18M $18.59M $24.93M $15.38M $25.94M $36.10M 375- P-01-V2-Long-Term Report Cardno BTO 59

60 12 Recommendations 12.1 Upgrade Option All options evaluated in detail in this report are able to meet the two primary objectives, that the WWTP be able to reliably meet the existing resource consent conditions and minimise the risk of odours. The lowest cost option would be the following: o Anaerobic pond to provide primary treatment o Secondary treatment with contact stabilisation with new secondary clarifiers or sequencing batch reactors (SBRs) o UV disinfection o Co-wasting of primary sludge and WAS from the anaerobic pond to solids handling The total capital cost of this option is estimated to be in the range of $17-$19 million. The capital costs of the secondary treatment options are very close, within the level of accuracy of the estimate. Considering this, more detailed evaluation of these options can be part of the next phase of work since each option may be more suitable depending upon, for example, any variations to the effluent consent. In the case that nutrient removal is required in the future, the recommended secondary treatment process would be the SBR. Nutrient removal cannot be achieved through contact stabilisation due to the short solids retention time. Also, an approximate additional $5 million capital investment is required if the desire to decrease process risk associated with wasting WAS to the anaerobic pond is desired. Wasting RAS to the anaerobic pond has the potential to impact the consistency of waste sludge and require adjustment to the solids handling process. The volume of WAS to the anaerobic pond is a small fraction of the average daily flow to the plant and the risk posed to the anaerobic pond process is expected to be minimal Sampling At a minimum, the following special sampling should be conducted over a one-month period. Understanding the composition of the wastewater is critical to ensure that the process design of any long-term improvements is appropriate. Daily influent composite samples shall be analysed for the following parameters, at a minimum: o BOD o Soluble BOD o COD o Soluble COD o Total Phosphorus o Orthophosphate o Total Nitrogen o Ammonia o Total Suspended Solids o Volatile Suspended Solids Influent fractionation of BOD should also take place during the special sampling period. It is also recommended that further characterisation of industrial trade wastes be conducted in order to identify any other chemical compounds that could negatively impact UV transmissivity. A new sludge survey should be conducted to better quantify the solids content in the existing ponds. Analysis should also be done to identify the concentration of solids, metals, and other constituents in the sludge P-01-V2-Long-Term Report Cardno BTO 60

61 12.3 Regional Council Engagement Discussions with Horizons Regional Council regarding potential consenting requirements are recommended Trade Waste It is recommended that WDC take a collaborative approach to engage industry, as an open consultation process where industry and WDC can work together to address trade waste discharges. This will result in a positive outcome. It is recommended that WDC engage a non-partisan facilitator to lead this effort. Efforts to engage industries are required as part of the Enforcement Order Application submitted by Horizons Regional Council to the Environmental Court and WDC has made progress in the way of meeting industry leaders to begin discussions Interim Site Maintenance It is recommended that the remaining weed mat on site be removed to prevent UV equipment failure and decrease maintenance costs Programme A high-level programme of tasks required in order to achieve consistent effluent compliance. The timing presented in Table 12-1 presents a preliminary suggested programme that can be modified dependent upon budgets and priorities. Table 12-1 Programme Task Special Sampling Design of WWTP Improvements Timing Immediate Must begin immediately Sludge Survey June 2013 Resource Consents Coordination with Horizons Regional Council September 2013 Trade Waste Bylaw Update To be initiated within 9 months Project Tendering December 2013 Completion of Construction December 2014 Table 12-2 presents a summary of costs that are required over the next year in order to meet the proposed programme. Table 12-2 Programme Costs Task $ million Capital Improvements at WWTP $18.79 Annual Operating Cost for WWTP* $1.93 Sludge Removal and Disposal (mechanical dewatering to 20%) $3.90 Trade Waste Monitoring $1.00 Wastewater Characterisation $0.05 Medium-term Odour Control $1.00 Total $26.67 *assumes operation of option with SBRs in 2013 dollars 375- P-01-V2-Long-Term Report Cardno BTO 61

62 Desludging of the existing ponds, discussed in Appendix C, is an activity that must take place prior to construction of a facility upgrade. The timing of a desludging operation is flexible but there are a number of factors that influence when the activity should commence. 3. Time of year. Commencement of desludging prior to summer will help to decrease odours generated in the warm months. 4. Timing of construction. It is desirable that desludging take place at a time such that maximum sludge can be removed. If the desludging operation takes place too far in advance of construction there is a possibility that additional desludging may be required when the ponds must be emptied for construction P-01-V2-Long-Term Report Cardno BTO 62

63 13 References BTO. Wanganui District Council WWTP Process Capacity Review and Optimisation Draft Report. November BTO. Wanganui Wastewater Reticulation and Treatment Plant Odour Mitigation Discussion Report. March Horizons Regional Council. Coastal Permits & /2, September International Water Association (IWA), Pond Treatment Technology (Andy Shilton ed), Metcalf and Eddy, Wastewater Engineering: Treatment and Reuse, 4 th ed, MWH. New Wanganui Wastewater Treatment Plant Confirmed Process Design, Report No 11, Draft for Client, August MWH. Wanganui WWTP Operation and Maintenance Manual. MWH. Wanganui Wastewater Treatment Plant Commissioning Report, Prepared for Wanganui District Council, February Water Environment Federation, Design of Wastewater Treatment Plants: WEF Manual of Practice 8, 4 th ed P-01-V2-Long-Term Report Cardno BTO 63

64 Evaluation of Long-Term Improvements for Consent Compliance APPENDIX A HISTORICAL RESOURCE CONSENT NON-COMPLIANCE 375- P-01-V2-Long-Term Report Cardno BTO 64

65 A. Resource Consent Non-Compliance Effluent Quality Monitoring Results The Wanganui WWTP discharges effluent to the ocean, an activity permitted by Coastal Permit granted by the Horizons Regional Council. The existing consent expires 30 June 2026 and is provided in Appendix E of this report. The concentrations of various pollutants are limited under conditions in the resource consent, and the final effluent must be sampled and analysed for the pollutants covered by the consent. Table A-1 summarises the resource consent effluent monitoring requirements. Table A-1 Wanganui WWTP Resource Consent Effluent Monitoring Requirements Parameter Units Sample Type Frequency TSS & total grease g/ m 3 24-hour composite 3 x per week Faecal coliforms & enterococci cfu/100ml Grab sample 3 x per week Sulphides & metals g/ m 3 24-hour composite Four monthly Table A-2 shows a summary of the consent monitoring results and the required standards for the key contaminants of concern. Although effluent BOD concentration is not covered by the resource consent, it is measured by WDC as it provides an indication of the general level of treatment. Total grease, sulphide and metals are not listed in Table A-2 as these contaminants have always fallen well within the resource consent limits. Figure A-1 through to Figure A-4 show graphs of the final effluent concentrations of these pollutants over the past 4 years. Table A-2 Wanganui WWTP Effluent Monitoring Results Summary ( ) Parameter Units Basis Resource Consent Standard BOD g/ m 3 median n/a TSS g/ m 3 95th %ile median n/a Median : 95th %ile ratio n/a Enterococci Faecal Coliforms cfu/100ml cfu/100ml median 3,900 20,000 24,000 51,000 4,000 maximum 160,000 2,100,000 1,600, ,000 12,000 median 7,500 22,000 86, ,000 10,000 90th %ile 56,600 91, , ,000 25, P-01-V2-Long-Term Report Cardno BTO 65

66 BOD Concentration (g/m 3 ) Oct 08 Jan 09 Apr 09 Jul 09 Oct 09 Jan 10 Apr 10 Jul 10 Oct 10 Jan 11 Apr 11 Jul 11 Oct 11 Jan 12 Apr 12 Jul 12 Oct 12 Jan 13 Influent Effluent Figure A-1 Wanganui WWTP Influent and Effluent BOD Concentrations TSS Concentration (g/m 3 ) Oct 08 Jan 09 Apr 09 Jul 09 Oct 09 Jan 10 Apr 10 Jul 10 Oct 10 Jan 11 Apr 11 Jul 11 Oct 11 Jan 12 Apr 12 Jul 12 Oct 12 Jan 13 Monthly Median Annual 95 %ile Consent 95 %ile Limit Figure A-2 Wanganui WWTP Effluent Total Suspended Solids Concentrations 375- P-01-V2-Long-Term Report Cardno BTO 66

67 10,000,000 1,000,000 Faecal Coliform Concentration (cfu/100ml) 100,000 10,000 1, Oct 08 Jan 09 Apr 09 Jul 09 Oct 09 Jan 10 Apr 10 Jul 10 Oct 10 Jan 11 Apr 11 Jul 11 Oct 11 Jan 12 Apr 12 Jul 12 Oct 12 Jan 13 Monthly Median Consent median liimit Annual 90 %ile Consent 90 %ile Limit Figure A-3 Wanganui WWTP Effluent Faecal Coliform Concentrations 10,000,000 1,000,000 Enterococci Concentration (cfu/100 ml) 100,000 10,000 1, Oct 08 Jan 09 Apr 09 Jul 09 Oct 09 Jan 10 Apr 10 Jul 10 Oct 10 Jan 11 Apr 11 Jul 11 Oct 11 Jan 12 Apr 12 Jul 12 Oct 12 Jan 13 Monthly Median Consent 50 %ile Limit Annual maximum Consent Maximum Limit Figure A-4 Wanganui WWTP Effluent Enterococci Concentrations 375- P-01-V2-Long-Term Report Cardno BTO 67

68 Comment on Aerated Lagoon/Settling Pond Performance The 95th percentile total suspended solids concentration has exceeded the resource consent standard of 100 g/m 3 for each of the past four calendar years (Table A-2 and Figure A-2). Although the resource consent standard is written as a 95th percentile concentration, working with median effluent concentrations is more useful for WWTP design purposes. Using a median to 95th percentile ratio of 0.40, to ensure consent compliance the plant must produce a median effluent TSS concentration of 40 g/m 3 in order to guarantee remaining within the 95th percentile limit of 100 g/m 3. Although a median effluent TSS concentration of 40 g/m 3 is achievable from an aerated lagoon/ settling pond, it is at the lower end of the achievable range for this type of process. To put this value into perspective, activated sludge treatment plants, which feature purpose-designed clarifiers and sludge recycle systems, would normally not be guaranteed to achieve lower than 30 g/m 3 median TSS in the final effluent. Table A-3 shows median and 95th percentile effluent TSS concentration data from other aerated lagoon wastewater treatment plants in New Zealand. The Wanganui WWTP TSS results for the year 2010, when the effluent quality was at its best, are shown as a comparison. Table A-3 Plant Effluent Quality from Aerated Lagoons in New Zealand Population Equivalent Type of Influent Treatment Process Effluent TSS (g/m 3 ) Median 95th percentile Otaki 10,000 Domestic Aerated lagoon / clarifier Feilding 10,000 Domestic / industrial Aerated lagoon / clarifier Woodend 10,000 Domestic Aerated lagoon / settling pond Wanganui 300,000 Domestic / industrial Aerated lagoon / settling pond 89* 212* * 2010 results only It is not possible to directly compare the results from different treatment plants, due to the differing treatment objectives, process designs, and influent loading rates of the plants. Nevertheless, the above table demonstrates that effluent TSS concentrations from aerated lagoon systems are generally well in excess of activated sludge effluent quality and that a median TSS of 40 g/m 3 would not be considered a comfortably achievable standard for this process. The design of the Wanganui aerated lagoon is unconventional in that it combines an aerated lagoon-type process on top of a 4 metre deep anaerobic sludge layer. The settlement and storage of suspended solids underneath the aerated layer requires that the aerated layer is not fully mixed. The combination of partial mix conditions and bottomless configuration prevents a suitable settleable biomass developing in the biological treatment layer, as suspended flocs spend only a fraction of the required time in the aerated layer before sinking to the anaerobic sludge layer. Given the unconventional design of the Wanganui aerated lagoon there was always a high level of risk that this process would fail to meet the TSS standard required by the resource consent, even without the high industrial loads and the mechanical problems that occurred with the aerators. Comment on UV Performance The 95th percentile total faecal coliform and enterococci concentrations have exceeded the resource consent standards for each of the past four calendar years, whilst the median concentrations were within the consent standards for 2009 but have exceeded them since then. In addition, there is a trend of increasing effluent concentrations. Whilst the poor final effluent faecal coliform and enterococci results could suggest substandard performance of the UV disinfection system, the alternative explanation is that the effluent UV transmission and TSS concentrations were outside the limits that the UV system was designed for P-01-V2-Long-Term Report Cardno BTO 68

69 It is understood that performance testing of the UV disinfection system was not undertaken by WDC, and until such time as a UV performance testing program is undertaken, the adequacy of the UV system cannot be finally determined. Nevertheless, some idea of UV performance can be gained by considering the available data on the reduction of faecal coliform and enterococci bacteria versus the UV performance guarantee figures, as well as comparing the effluent UV transmission and TSS concentration against the values that the UV system was designed for. Table A-4 summarises the design parameters for the UV system. Table A-4 UV Disinfection System Design Summary Parameter Units Basis Design Value UV Influent Parameters: Flow rate l/s Maximum 650 UV transmittance % Minimum 20 TSS concentration g/m 3 90th percentile 75 Pre-UV faecal coliform concentration cfu/100ml Maximum 650,000 Pre-UV enterococci concentration cfu/100ml Maximum 160,000* Guaranteed Faecal Coliform Reduction**: Post-UV faecal coliform concentration cfu/100ml 90th percentile 10,000 Log reduction n/a 90th percentile 1.81 Guaranteed Enterococci Reduction*: Post-UV enterococci concentration cfu/100ml 90th percentile 4,000 Log reduction n/a 90th percentile 1.60 UV Dose Provided: Calculated (theoretical) UV dose mj/cm 2 minimum 22.5 * The MWH tender recommendation describes this value to be a pre-uv E Coli concentration but this is believed to be a typographical error. ** Providing the UV influent complies with the parameters shown in the table The guaranteed 90th percentile post-uv faecal coliform and enterococci concentrations are the same as the resource consent median concentrations providing an appropriate safety factor. The log reductions shown in the table above are not explicitly stated in the UV contract however are implied by the UV influent and effluent bacteria concentrations. When assessing UV system performance, log reduction is a more accurate parameter to use than UV effluent concentration, as the UV effluent concentration will vary depending on the UV influent concentration. Using log reduction removes this variable and allows results to be compared on an equal basis. UV transmissivity of the final effluent is not routinely monitored, and the most recent UV performance data including UV transmissivity is from August December 2009, contained in the February 2010 WWTP Commissioning Report (MWH, 2010). This data is shown in Figure A-5 below: 375- P-01-V2-Long-Term Report Cardno BTO 69

70 Log Reduction Effluent UV Transmission (%) /08/09 15/08/09 29/08/09 12/09/09 26/09/09 10/10/09 24/10/09 07/11/09 21/11/09 05/12/09 19/12/09 0 Faecal Coliforms Enterococci Design log red. FC Design log red. Ent. UVT Design UVT Figure A-5 Wanganui WWTP UV Performance August December 2009 To meet the UV performance guarantee, 90% of the log reduction results must be above their respective guaranteed values (providing that the UV transmission is 20% or higher), indicated by horizontal lines in the graph. The results show that although the UV transmission was consistently higher than 20% over the period 31 August 1 December 2009, the measured log reductions fell well short of the UV performance guarantee. The average influent TSS to the UV disinfection system was over 200 mg/l during this period. Regardless, there could be an issue with UV performance and testing is required to determine system performance. There is space provided in the UV channel for an additional 64 lamps (a 31% increase in UV power). This might be required as part of the upgraded WWTP. Given that the lamps are variable output lamps, the energy cost of adding more lamps could be minimised by controlling the lamp output if possible P-01-V2-Long-Term Report Cardno BTO 70

71 Evaluation of Long-Term Improvements for Consent Compliance APPENDIX B HISTORICAL ODOUR DISCUSSION 375- P-01-V2-Long-Term Report Cardno BTO 71

72 B. Historical Odour Discussion Odour nuisance has been a major issue at the WWTP, especially during the summer months. Prior to the recent 2012/13 odour event, odour complaints have been a fairly regular occurrence since the WWTP was opened. Wanganui Airport staff has regularly complained about odours since the plant was opened; airport staff also expressed a concern about birds attracted to the site being a hazard to aircraft. Odour complaint logs from Horizons Regional Council indicate that over 60 households complained of objectionable odours from the WWTP since mid-december The WWTP site is largely flat with little or no shelter belt planting to shield odours from the ponds to surrounding properties. The nearest neighbours to the WWTP are the Wanganui airport maintenance workshops around 350 metres west of the ponds and the rifle range 300 metres to east of the ponds. The prevailing wind is north-westerly; however the wind often reverses to south southwest which takes odours into the city, 1.2 km north of the ponds across the river. Odour Sources The main sources of odour discharges from the WWTP are discussed below. Industrial Sulphide Discharges into Sewer System Sulphide (S 2- ) is used in industrial processes (in particular the tanning industry) and is also formed by the anaerobic decomposition of sulphate and organic matter. The main sulphide dischargers into the Wanganui sewer system are the Tasman Tannery and AFFCO meat processing plant. In water, some of the sulphide ion forms hydrogen sulphide (H 2 S); the proportion of sulphide converted to H 2 S is a function of the ph of the water. H 2 S gas has a distinctive rotten egg odour and a very low detection threshold concentration of around ppm in air. H 2 S concentrations as high as 5 ppm were routinely detected inside the Beach Rd inlet works room prior to covering the channels and installing a new forced air ventilation system in Domestic sewage typically has low levels of sulphide, providing that the retention time in the sewer network is not long. The average sulphide concentration measured in the influent wastewater at the Beach Rd pump station is approximately 3.0 mg/l, which reflects the industrial nature of the wastewater. In addition to the high average concentration, there are occasional sulphide spikes of mg/l from time to time. Hydrogen Sulphide Generated in WWTP Pond Sludge Layers Another source of hydrogen sulphide is the sludge layer in both the aerated lagoon and the settling pond. The breakdown of sludge in these layers by anaerobic bacteria releases carbon dioxide, methane and H 2 S gas. The anaerobic bacteria do not function in temperatures below 15 C so H 2 S release from the sludge layers usually only occurs in the spring and summer months. The warm temperatures experienced during the summer of 2012/13 undoubtedly contributed to the recent odour event by increasing anaerobic activity in the pond sludge layers. At times gas bubbles could be seen rising from the bottom of the ponds over much of pond area, indicating a high level of anaerobic activity. The surface activity can be observed in Figure B P-01-V2-Long-Term Report Cardno BTO 72

73 Figure B-1 Photograph of gas bubbles on lagoon surface Other Volatile Components in Influent Wastewater In addition to H 2 S, other volatile components in untreated wastewater cause offensive odours, such as ammonia, organic sulphides and volatile fatty acids. These compounds are detected as raw sewage type odours. Meat-processing type odours have also been detected at the ponds, reflecting the AFFCO wastewater component in the influent. In a properly functioning aerobic treatment process these compounds are rapidly removed through biological oxidation, and there should be no offensive odours from an aerobic treatment process. Odour Removal Mechanism at WWTP The primary odour mitigation measure at the wastewater treatment plant is the aerobic biological treatment layer in the aerated lagoon, providing that there is a positive dissolved oxygen concentration and a sufficient population of suspended aerobic bacteria in the upper layer. Under these conditions, odourous compounds, whether generated from within the sludge layer or introduced with the influent, will be either adsorbed onto biological flocs or oxidised to non-odourous compounds. Properly functioning aerated wastewater treatment ponds or tanks generate an earthy musty smell however this odour is generally not considered offensive and does not tend to travel as far as the offensive type odours. Causes of Historic Odour Problems There are a number of issues that have contributed to the historic and ongoing odour problems at the plant. These are discussed below. Industrial Sulphide Discharges As discussed above there are two major industrial sulphide discharges into the sewer system, AFFCO and Tasman Tannery. Even with a properly functioning aerobic wastewater treatment plant, odours may still be released at the inlet of the plant given the high sulphide loads in the influent. In the future it is important that industrial sulphide discharges are controlled at source through the trade waste consenting system, in order to ensure that sulphide loads do not exceed the treatment capacity of the WWTP P-01-V2-Long-Term Report Cardno BTO 73

74 Aerated Lagoon Design As discussed in Section 2.7, the aerated lagoon design at the Wanganui WWTP is unconventional and aspects of the design have also contributed to the odour problems. Firstly, the mixing intensity within the upper layer of the aerated lagoon is lower than in aerated lagoons, due to the need to settle solids and store sludge in the base of the lagoon. Therefore, biomass will not remain in suspension for the full 3.5 days required to grow and treat the wastewater. The result is an insufficient biomass concentration in the upper layer of the aerated lagoon to adequately treat odourous compounds. Secondly, the deep anaerobic sludge layer at the base of the aerated lagoon is a source of H 2 S during the summer. Facultative ponds also have anaerobic sludge layers however these ponds do not always suffer from odour problems. This is due to the fact that facultative ponds are larger so the sludge is spread over a wider surface area and is less deep. This means that the H 2 S generated per m 2 surface area puts less of a load on the upper aerobic layer. Further to this, the anaerobic digestion intended to take place at the base of the lagoon is impacted by the aerobic process occurring in the upper layer of the lagoon. Anaerobic digestion occurs in the absence of oxygen; supplying oxygen to the upper layer of the pond has likely inhibited the anaerobic digestion process in the lower layer of the pond. It is likely that inhibition of anaerobic digestion has been occurring since the plant was originally opened and that the most recent odour event was exacerbated when surface aerators were turned off. When the bottom layer of the pond was no longer inhibited by the oxygen being introduced by the aerators, the rate of digestion accelerated, releasing odourous compounds that were previously bound in undigested solids. Thirdly, conventional facultative ponds utilise algae for most or all of the oxygen needed for treatment. Algae use energy from the sun to strip CO 2 from the wastewater for cell growth, which raises the ph of the pond. During the day when photosynthesis occurs, the ph of ponds typically reaches 8. This reduces the H 2 S concentration in the liquid. Aerator Mechanical Problems Aeration is required in order to provide the oxygen needed for suspended bacteria to assimilate and oxidise pollutants including odourous compounds. If there is insufficient oxygen supply to meet the biological oxygen demand, then little or no biomass growth or pollutant removal will occur in the upper layer of the aerated lagoon. As discussed in the previous section, in a properly functioning aerobic treatment process there is little if any odour. Due to mechanical problems with the aerators, the total aeration power installed on the ponds is now less than half of the original aeration power (see Section A for further discussion of aerator issues). The reduced aeration capacity is undoubtedly contributing to the odour problems as biological treatment is not occurring to its full potential. Future aeration systems must be robust, reliable and designed with sufficient redundancy to ensure full capacity is available at all times, allowing for maintenance outages. Agitation of Pond Surface by Slow Speed Surface Aerators Slow speed surface aerators transfer oxygen from the air into the pond by vigorously agitating the pond surface thereby creating a large contact surface area between air and water. During this process, volatile compounds contained in the liquid phase are released into the air. Therefore, slow speed aerators have a higher odour risk compared with aspirating aerators or diffused aeration systems. All surface aerators were switched off in November 2012, and once turned off could not be turned back on as the vigorous agitation of the pond aggravated the odours. In the future, diffused aeration systems should be considered in order to minimise the risk of odours, due to the sensitivity of the odour issue and the high sulphide loads to the plant. Odour Event of 2012/13 As of mid-december 2012 the number of odour complaints from the Wanganui community increased considerably. This is not the first summer which odour problems have presented from the plant and while we have not been provided with records of the odour complaints from previous summer s the summer of 2012/13 has clearly been exceptional in terms of the level of odour generated from the plant. HRC has 375- P-01-V2-Long-Term Report Cardno BTO 74

75 received odour complaints from more than 60 different households, many of which have complained more than once P-01-V2-Long-Term Report Cardno BTO 75

76 Evaluation of Long-Term Improvements for Consent Compliance APPENDIX C SLUDGE MANAGEMENT 375- P-01-V2-Long-Term Report Cardno BTO 76

77 C. Sludge Management Management of solids within the existing ponds at the Wanganui WWTP is an issue that must be appropriately managed regardless of the long-term approach for WWTP compliance. The primary issue with sludge currently stored in the ponds is the odours caused by the sludge digestion. This discussion presents an estimate on sludge quantity and costs associated with sludge removal and disposal options. Current Sludge Inventory in Ponds The total sludge storage capacity in the aerated lagoon was estimated by MWH to be 81,440 m 3 assuming a sludge depth of 4 m over the entire pond (MWH, 2005). An estimate of sludge volume in the aerated lagoon was conducted via two methods using the information from the sludge surveys conducted by Parklink Ltd and using historical TSS and flow data in a mass balance. Two separate surveys of aerated lagoon sludge levels were conducted by Parklink Ltd, one in December 2011 and one in January The surveys consisted of 50 spot sludge level measurements taken across 10 transects of the aerobic lagoon. Sludge volumes were not calculated as part of the surveys; however volumes have been estimated using the Parklink data and are presented below. No sludge surveys of the settling pond have been undertaken. Table C-1 Estimated Sludge Volumes from Parklink Aerated Lagoon Sludge Survey Results Parameter Units December 2011 January 2013 Average sludge depth in aerated lagoon m Total volume of sludge in aerated lagoon m³ 32,800 38,200 The January 2013 survey indicates a total sludge volume of approximately 40,000 m 3 or nearly 50% of the sludge storage capacity estimated by MWH. The average dry solids concentration measured in sludge samples taken by WDC at the time of the sludge survey in January 2013 was 4.5% dry solids giving a total dry solids inventory in the aerated lagoon of 1,800 tonnes. The samples were collected at a depth estimated at 0.5 m into the sludge layer. The sludge layer was identified using a sludge interface detector. Total dry solids concentrations are expected to be higher than 4.5% at the bottom of the pond. MWH estimated that over time, the solids concentration at the bottom of the pond would approach 12%. An estimate of solids accumulation in the aerobic lagoon and settling pond was developed based on average solids loads into and out of both ponds over the historical period 2011 to A solids concentration of 7% was assumed in the mass balance calculations as it is a conservative value between the 12% solids concentration predicted by MWH for the bottom of the lagoon and the 4.5% solids measured 0.5 m into the sludge layer (MWH, 2005). Table C-2 Wanganui WWTP Sludge Mass Balance Parameter Units Calculated Aerated lagoon influent TSS load tonnes per annum 3,610 Aerated lagoon effluent TSS load tonnes per annum 2,340 Settling pond effluent TSS load tonnes per annum 1,720 Assumed solids destruction in sludge layer % 60% Sludge accumulation in aerated lagoon in 5.5 years tonnes dry solids 2,810 Sludge accumulation in both ponds in 5.5 years tonnes dry solids 4,180 Assumed average solids concentration in sludge % dry solids 7.0% Sludge accumulation in aerated lagoon in 5.5 years m³ 40,200 Sludge accumulation in settling pond in 5.5 years m³ 19,500 The sludge volume was estimated to be 40,000 m 3 using both methods, despite the difference in assumed solids concentration. It is recommended that a new sludge survey be to confirm the sludge inventory in both 375- P-01-V2-Long-Term Report Cardno BTO 77

78 ponds, in terms of volume, dry solids content, and tonnes dry solids. The estimate of sludge mass in the aerated lagoon of 2,800 tonnes using the mass balance approach is significantly higher than the 1,800 tonnes estimated using the Parklink information. Accurate characterisation and quantification of solids within the aerated lagoon and settling pond is critical since it significantly impacts sludge removal and disposal costs. For the purposes of this report, a total sludge inventory of 4,500 tonnes of dry solids in both ponds is assumed. This is close to the tonnes of dry solids estimated from the mass balance. Sludge Chromium Concentration It is known that some industries in Wanganui use metal-containing chemicals, specifically chromiumcontaining agents, in their processes. Chromium is a regulated constituent for one of the major trade waste dischargers. Regardless, the chromium present in the sludge impacts potential disposal routes. As a reference, the New Zealand Biosolids Guidelines (NZWWA, 2003) defines grades of stabilisation (A or B) and contaminant concentration (a or b). The rating of biosolids determines whether unrestricted or restricted reuse can occur. Sludges that do not meet any of the reuse grades must be disposed to landfill. Table C-3 presents the NZ Biosolids Guidelines for chromium. Table C-3 Biosolids Grade New Zealand Biosolids Guidelines for Chromium in Biosolids Maximum Concentration Grade a Grade b 600 mg/kg dry weight 1,500 mg/kg dry weight Sludge sampling was undertaken in January 2013 by Wanganui District Council and analysed for chromium. The results of the sampling are presented in Table C-4. Table C-4 Sample Location Wanganui WWTP Aerated Lagoon Sludge Chromium Sampling Results As shown in the above table, the chromium content of the sludge is extremely high at 18 times the Grade b limits. Additional testing to confirm metals concentrations is recommended in conjunction with an updated sludge survey. The high chromium concentration precludes beneficial reuse of the sludge and therefore the only disposal route is to a secure landfill. Biosolids Disposal Bonny Glenn Landfill Total Chromium (mg/kg wet weight) Percent Solids (%) Total Chromium (mg/kg dry weight) 1 1, % 24, , % 28, , % 27, , % 27, , % 26, , % 25, , % 28,043 Average 27,013 The nearest landfill that accepts municipal sludge is the Bonny Glen Landfill, operated by Midwest Disposals Ltd. It is located near Marton, 40km from Wanganui. Discussions with Bonny Glen staff have indicated the likely disposal fee would be $80 per wet tonne for WDC or $85 per wet tonne for a sludge contractor. The advantage of Bonny Glen is its proximity to Wanganui and hence low transport cost of $7 per wet tonne; however a major drawback is the current daily sludge disposal limit of 40 wet tonnes per day, which would require storage of the dewatered sludge on site at the WWTP for an extended period P-01-V2-Long-Term Report Cardno BTO 78

79 Hampton Downs Landfill Envirowaste (which owns a 50% stake in Midwest Disposals Ltd) operates a landfill facility at Hampton Downs in the North Waikato, approximately 400km from the Wanganui WWTP. The disposal fee for Hampton Downs is $55 per wet tonne. In addition, Envirowaste has indicated the landfill could accept up to 150 wet tonnes sludge per day. Envirowaste has quoted a transport cost of $40 per wet tonne from Wanganui to Hampton Downs. Table C-5 summarises the transport and disposal costs for each landfill. Table C-5 Dewatered Sludge Transport and Disposal Costs Item Units Bonny Glen Hampton Downs Transport charge $ / wet tonne 7 40 Disposal charge $ / wet tonne Total (transport + disposal) $ / wet tonne Daily limit wet tonne Time needed to dispose of 4,500 tonnes dry solids: At 15% dry solids weeks At 20% dry solids weeks At 30% dry solids weeks Total cost to dispose of 4,500 tonnes dry solids: At 15% dry solids $000 s 2,610 2,850 At 20% dry solids $000 s 1,957 2,137 At 30% dry solids $000 s 1,566 1,710 As transport and disposal costs are on a wet tonne basis, the cost decreases with increasing solids content due to the lower water content of the sludge. Hence it is important to remove as much water from the sludge as possible prior to transport off site. Other Landfills The following landfills were contacted but were found to be unsuitable: o Ruapehu District Landfill o Waiouru Landfill o Colson Road Landfill o Levin Landfill o Waitomo District Landfill o Broadlands Road Landfill Biosolids Dewatering There are two main on-site dewatering options available for the pond sludge: Mechanical dewatering One option is to mechanically dewater the sludge. Dredged material would be dosed with polymer and pumped to a mechanical dewatering unit, likely a centrifuge or belt filter press. The centrate or filtrate would be returned to the pond. The advantage of mechanical dewatering is that it is quicker and has a smaller footprint than the alternative dewatering option. Geobags A common dewatering method is to use geotextile dewatering bags. In this process, sludge is dewatered over time through consolidation of sludge in large woven plastic bags. A lined and bunded dewatering area 375- P-01-V2-Long-Term Report Cardno BTO 79

80 would need to be constructed on site, where the geobags can be located and filtrate from the bags can be collected and pumped back to the lagoon. Sludge is retained in the bags for around four months, during which time the sludge dries to 20 to 30% dried solids. The dewatered solids can then be sent to landfill. Disposal of geobag-dewatered sludge requires that the geobags be cut open and solids removed from the bags using a digger. The rate of geobag mining and solids disposal can be controlled. Geobags can achieve a higher solids content than mechanical dewatering but require a long on-site storage time and a bunded dewatering area. If installed properly, geobags may be a good option for WDC. The odour potential for geobag installations, especially if poorly done, can be great. There is also odour potential associated with sludge removal from the geobags. Four sludge dredging and dewatering contractors were contacted in order to develop budgetary pricing for sludge removal and dewatering. The contractors are as follows: Conhur Conhur Ltd is a civil contracting company, established in 1999, which provides services to the water, wastewater and construction sectors. Conhur propose to use a purpose built auger-fed sludge dredge, and can provide mechanical (centrifuge) or geobag dewatering. Dredging Solutions Dredging Solutions NZ Ltd s core business is the dredging and dewatering of ponds, waterways and marina sludge using Geotubes. They own their own dredges, pumps, dosing units, pipework and associated gear. Dredging Solutions only offer geobag dewatering. Envirowaste Envirowaste are a privately owned waste management company of more than 400 employees. Their services range from waste collection, composting, recycling, hazardous waste management and landfill operation. Envirowaste offer both mechanical (centrifuge) and geobag dewatering. AquaClear AquaClear are an Auckland-based company that specialise in industrial and municipal sludge dewatering. They offer a belt filter press dewatering process. Budget dredging and dewatering prices from the contracting firms are presented in Table C-6. Table C-6 Dredging and Dewatering Costs (excluding transport & disposal) Item Units Conhur Dredging Solutions Mechanical Dewatering EnviroWaste AquaClear Dredging + dewatering charge $/t dry solids 610 n/a Mobilisation & other fixed costs $000 s 128 n/a Total cost for 4,500 t dry solids $000 s 2,871 n/a 1, Geobag Dewatering Dredging + dewatering charge $/t dry solids n/a Establishment & other fixed costs $000 s 1,908 1, n/a Total cost for 4,500 t dry solids $000 s 3,101 2,167 1,488 n/a *includes cost of geobags, dewatering area, filtrate return piping (no pumping), and installation Table C-7 presents total costs for Wanganui WWTP for four options, based on the above costs: 1. Mechanical dewatering to achieve 15% dry solids 2. Mechanical dewatering to achieve 20% dry solids 3. Geobag dewatering to 20% dry solids 4. Geobags dewatering to 30% dry solids 375- P-01-V2-Long-Term Report Cardno BTO 80

81 Transport and landfill disposal costs will vary depending upon percent dry solids and landfill used. Also, depending upon the total daily mass to be disposed, multiple landfills can be used. For example, tipping at the closer Bonny Glen Landfill can take place first and if the daily mass limit is met, disposal to the farther Hampton Downs Landfill can take place. Table C-7 Item Total costs for Desludging Wanganui WWTP Ponds Units Mechanical Mechanical Geobags Geobags 15% dry solids 20% dry solids 20% dry solids 30% dry solids Dredging + dewatering charge $/t dry solids Transport charge $/t dry solids Landfill disposal charge $/t dry solids Total variable charges $/t dry solids Fixed costs $000 s Total cost for 4,500 t dry solids $000 s 4,400-4,700 3,700-3,900 3,200-3,400 2,500-2,600 Risks There are a number of risks associated with sludge removal, dewatering, and disposal. The key risks include: o Potential to damage the pond liner during desludging operation. The liner is thin and could easily be torn by mechanical equipment during dredging. Contractors have indicated that they can fit protection equipment to cover the moving sharp mechanical parts of their dredging equipment. Further discussions will need to be held with contractors about this risk prior to the commencement of any desludging operation. o Odour generation. There is a risk of increased odour associated with geobag dewatering due to having the sludge stored in geotextile bags on the ground surface. The contractors offering geotextile bag dewatering have stated that they have not experienced unreasonable amounts of odour escaping; however it is possible that they are bias in this assessment. Further discussions will need to be held with contractors about this risk when WDC decides to empty the ponds of solids. o Cost inaccuracies It is important to note that the following factors influence the accuracy of the above estimates: o Aerator cable responsibility o A dredge deployed in the aerated lagoon must be navigated around aerator cables or the aerators and aerator cables must be removed from the pond. If it is the responsibility of the Contractor to navigate around the aerator cables or remove cables and aerators from the ponds during desludging mobilisation and fixed costs will increase. o Total mass of sludge in the ponds and existing solids concentration o There is uncertainty regarding the total mass of dried solids in the ponds. All contractor s prices have included, at least in part, a charge rate which is in the unit of $/tonne dried solids. It is recommended that WDC commission a new sludge survey to get an accurate assessment of the total mass and concentration of solids in the ponds. o Solids concentration of dewatered sludge 375- P-01-V2-Long-Term Report Cardno BTO 81

82 Evaluation of Long-Term Improvements for Consent Compliance APPENDIX D TRADE WASTE DISCUSSION 375- P-01-V2-Long-Term Report Cardno BTO 82

83 D. Trade Waste Impacts There are several major industries in Wanganui that discharge their trade wastes to the WDC wastewater system. The WDC regulates these industrial trade wastes through the 2008 Trade Waste Bylaw (Bylaw). The Council has issued trade waste consents to the following large industries: o AFFCO Imlay o Tasman Tanning Heads Road and Todd Street Facilities o Land Meat o Cavalier Spinners o Open Country Dairy o MARS Petcare Table D-1 presents a summary of consented loads for BOD, TSS, and Oil & Grease loads for major industries in Wanganui. Other constituents are consented for different industries and are not presented in this table. The original basis for trade waste consents was that industries accounted for 90% of the total load to the Wanganui WWTP. Subsequent trade waste consent limits were arbitrarily assigned based on industrial loads that were contributed by each industry. Table D-1 Summary of Consented BOD, TSS, and Oil & Grease Loads for Major Industries Industry BOD Load (kg/day) TSS Load (kg/day) Oil and Grease Load (kg/day) AFFCO Imlay 10,100 5,200 1,512 Tasman Tanning 14,700 13,840 7,545 Land Meat 4,910 7,500 2,889 Cavalier Spinners Open Country Dairy 1, MARS Petcare Total 32,280 27,760 12,236.5 There are minor trade waste dischargers to the WDC wastewater system in addition to those major trade waste dischargers identified above. While the flows and loads contributed by minor trade waste dischargers may not be significant, chemical constituents contributed by any of these industries has the potential to impact the efficiency of ultraviolet disinfection. Gas chromatography of a settling pond effluent sample identified furfural in the wastewater. This compound absorbs in the UV range and negatively impacts effectiveness of UV disinfection. This compound, a chemical used in adhesives, is one example of a constituent likely contributed by a minor trade waste discharger that has the potential to impact WDC s ability to reliably achieve resource consent compliance. WDC is currently working with industry to develop the best approach for trade waste monitoring and charging. At a minimum, it is recommended that each large trade industry should have a composite sampler on site, as well as sufficient equipment to additionally measure ph, temperature, and flow. An S:CAN, an online wastewater quality instrument, should also be procured by WDC and can be deployed to various industries as required. Major Trade Waste Dischargers Under the Bylaw, major trade waste dischargers must monitor flows and constituent concentrations as stated in individual consents, generally including BOD 5, suspended solids, ph, and oil and grease. Additional monitoring requirements are dependent upon the nature of the industry. Sulphide is commonly used in two of the major industries within the WDC service area, AFFCO Imlay and Tasman Tanning. Sulphide in trade waste has a direct and significant impact on the odours experienced at the WWTP P-01-V2-Long-Term Report Cardno BTO 83

84 The most recent trade waste consents expired in June However, under the Bylaw, they remain in force indefinitely except under explicit conditions, none of which currently apply for any industry. The WDC conducts monitoring over a 10-day period four times each year. Historical data for the major industries was evaluated and is presented later in this section. WDC is currently conducting random sampling of trade waste and is working alongside industry to revise the trade waste policy. It is expected that a revised trade waste policy will be in effect before construction of a long-term upgrade at the WWTP. AFFCO Imlay AFFCO Imlay is an ovine/calf processing facility, which also has an on-site rendering plant. Typical constituent loads from a meat processing plant are elevated levels BOD 5, TSS, fats, oil and grease (FOG), ammonia (NH 3 ), and sulphides (S 2- ). The ph of the waste stream can also widely vary. It is known that the facility contains an onsite pre-treatment system that is consented to be continuously operated but the configuration and condition of the system is unknown. AFFCO Imlay is one of two industries in Wanganui that has a sulphide concentration limit. Tasman Tanning Tasman Tanning is a bovine tannery making finished leather. Tasman Tanning operates two facilities in Wanganui, one on Heads Road and one on Todds Street. In addition to producing finished leather, Tasman Tanning processed hides to the wet-blue stage, named for the colour derived from the chromium used in the process. The typical tannery trade waste is characterised by elevated BOD 5, fats, oil and grease (FOG), sulphates (SO 4 ), sulphides (S 2- ), and chromium. The ph of the waste stream can also widely vary. Tasman Tanning trade waste consented flow is the largest of all of the trade waste consents in the Wanganui wastewater reticulation network (9,550 m 3 /day). It is also the only industry in Wanganui that must meet concentration limits for chromium (III), sulphate, and ammonia. Tasman Tanning, like AFFCO Imlay, must also meet a sulphide concentration limit. Tasman Tanning has onsite treatment of its wastewater, the details of which are currently unknown. Continued maintenance and operation of this system is a requirement of their trade waste consent. Land Meat Land Meat is a meat processing plant located in Castlecliff. Typical constituent loads from a meat processing plant are elevated levels BOD 5, TSS, fats, oil and grease (FOG), ammonia (NH 3 ), and sulphides (S 2- ). The ph of the waste stream can also widely vary. It is known that the facility contains an onsite pre-treatment system that is consented to be continuously operated but the configuration and condition of the system is unknown. Cavalier Spinners Cavalier Spinners is located on Leamington Street, Gonville. The plant produces yarn for Cavalier/Bremworth for the manufacture of carpet. It is believed that this plant receives scoured wool as its raw material, and dyes the wool onsite prior to producing yarn. Cavalier Spinners has historically contributed flows and loads significantly lower than their consented amount and WDC no longer conducts quarterly monitoring of Cavalier Spinners trade waste. Cavalier Spinners has a concentration limit for Bifenthrin, a synthetic pyrethroid insecticide. This chemical is used for insect-proofing of the wool yarn. Most of the insecticide (98% to 99%) is absorbed by the wool; however, a minute amount remains in soluble form in trade waste. Bifenthrin is toxic to many aquatic animal species and control of bifenthrin released to water bodies is required to protect the aquatic environment. Open Country Dairy Open Country Dairy is located on Imlay Place, Wanganui, and is a whole milk powder processing plant. Typical trade waste from a milk powder processing plant is characterised by elevated BOD 5, TSS, fats, oil and grease (FOG), and a varying ph. The facility contains an onsite pre-treatment system that is consented to be continuously operated and maintained. The configuration of the pre-treatment system is not known. Since the start-up of the facility in August 2009, construction of a new milk powder dryer has been planned. The existing whole milk powder dryer is rated for 8 ½ tonnes; an additional 18 ½ tonne dryer will be added to the facility at some point in the future P-01-V2-Long-Term Report Cardno BTO 84

85 MARS Petcare MARS Petcare is a pet food processing plant that produces canned, chilled, dry and shelf-stable cat and dog folds. The processing plant is located on Bryce St, Wanganui. Typical constituent loads from a pet food processing plant are similar to a meat processing plant which are characterised by elevated levels of BOD 5, TSS, fats, oil and grease (FOG), ammonia (NH 3 ), and sulphides (S 2- ). The ph of the waste stream can also widely vary. There is an onsite pre-treatment system that is consented to be continuously maintained and cleaned out at necessary frequency. Information about the onsite pre-treatment is not known. Major Trade Waste Dischargers Historical Data Recent historical data for the major trade waste dischargers was provided by WDC. Cavalier Spinners, despite being characterised a major trade waste discharger, is not currently monitored by the WDC since historical flow, BOD, and TSS contributions indicate that their trade waste no longer warrants quarterly monitoring. It is recommended, however, that monitoring continue and that characterisation of this wastewater to identify any constituents that absorb in the UV range take place. Figure D-1 through Figure D-3 present breakdowns of BOD, TSS, and FOG loads by contributor for The domestic load is included in each graph. Loads were calculated based on concentrations measured from grab samples taken during the sampling period and the annual average historical flow for each industry. It is important to note that the domestic component also includes the flows contributed by small, non-consented trade waste dischargers. It can be observed in Figure D-1 that in 2012, the largest BOD discharger during the sampling periods was AFFCO Imlay. The domestic BOD load during sampling was only 13% of the total BOD load to the plant. This reinforces previous observations that while the Wanganui WWTP is owned and operated by the WDC, the flows to the WWTP are industrial in nature. Figure D-1 Breakdown of BOD Load Figure D-2 presents TSS loads to the Wanganui WWTP during industrial sampling periods in AFFCO Imlay contributed more than a third of the total TSS load, followed by the domestic load which accounted for 22% P-01-V2-Long-Term Report Cardno BTO 85

86 Figure D-2 Breakdown of TSS Load Figure D-3 presents oil and grease loads to the Wanganui WWTP during the historical monitoring periods in AFFCO Imlay was the most significant oil and grease discharger, followed by Land Meat. Figure D-3 Breakdown of FOG Load The previous figures present only three parameters that impact the operation of the WWTP. Other parameters, such as chromium, and sulphate, are also very important to the performance of the WWTP. Future Trade Waste Considerations The viability of any long-term strategy is contingent upon management of trade waste contributions. A program to review the trade waste consent conditions based on each industry s specific discharges should be implemented. This program would involve a discussion with each industry in order to discuss their trade waste (discharge volume, characteristics, frequency). It is recommended that WDC take a collaborative approach to engage industry, as an open consultation process where industry and WDC can work together to address trade waste discharges will result in a positive outcome. It is recommended that WDC engage a non-partisan facilitator to lead this effort. The trade waste charging system should be reviewed as part of this process in order to ensure that it is methodical, fair and appropriate P-01-V2-Long-Term Report Cardno BTO 86

87 Also, it is known that some existing industries have plans for expansion and some new industries are currently exploring Wanganui as a location for facilities in the future. Developing a strategy for managing additional trade waste flows in the future is critical. It is difficult at this point in time to estimate what these loads could be; however, consideration must be given as part of the long-term strategy as to whether additional plant capacity should be allocated to trade waste or whether management of trade waste at the source is preferred. It was the uncertainty of future trade waste discharges that was the primary driver for increasing the historical flows and loads by 20% to develop the design flows and loads P-01-V2-Long-Term Report Cardno BTO 87

88 Evaluation of Long-Term Improvements for Consent Compliance APPENDIX E RESOURCE CONSENTS 375- P-01-V2-Long-Term Report Cardno BTO 88

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