Foxton Wastewater Treatment Plant Design Parameter Summary

Transcription

1 Foxton Wastewater Treatment Plant Design Parameter Summary (LEI, 2015:B6) Prepared for Horowhenua District Council Prepared by September 2015

2 Foxton Wastewater Treatment Plant Design Parameter Summary (LEI, 2015:B6) Horowhenua District Council This report has been prepared for the Horowhenua District Council by Lowe Environmental Impact (LEI). No liability is accepted by this company or any employee or sub-consultant of this company with respect to its use by any other parties. Quality Assurance Statement Task Responsibility Signature Project Manager: Prepared by: Reviewed by: Approved for Issue by: Status: Hamish Lowe Philip Lake Hamish Lowe Hamish Lowe Final Prepared by: Lowe Environmental Impact P O Box 4467 Palmerston North 4462 Ref: Foxton_WWTP-B6-Design_Parameter_Summary- FINAL.docx T [+64] Job No.: E office@lei.co.nz W Date: September 2015 Revision Status Version Date Author What Changed and Why 4 14/09/15 PL Editing based on client feedback. 3 04/08/15 PL Clarification of data and text with some additional data. 2 12/07/15 HL Internal review 1 08/07/15 PL Original internal draft

3 TABLE OF CONTENTS 1 EXECUTIVE SUMMARY INTRODUCTION Background Purpose Scope DATA SOURCES DATA ASSESSMENT METHODOLOGIES DATA INTEGRITY Climate Data Wastewater Inflow Data Wastewater Outflow Data Trade Waste Data DATA INTERPRETATION WWTP Physical Description Population Climate Data Trade Waste Flow Rates Wastewater Flow Rates FWWTP Total Wastewater Inflow and Outflow Correlation Trade Waste Composition and Loads FWWTP Raw Wastewater Composition and Loads FWWTP Treatment Performance CONCLUSIONS AND RECOMMENDATIONS General WWTP Design... 25

4 7.3 Sludge Flows Loads REFERENCES... 28

5 1 EXECUTIVE SUMMARY Horowhenua District Council (HDC) operate the Foxton municipal wastewater treatment plant (FWWTP) on Matakarapa which is surrounded by Foxton Loop and the Whirokino Cut of the Manawatu River. The FWWTP discharge requires re-consenting and a possible treatment system and discharge upgrade. In order to progress the review of the current FWWTP design and performance, HDC require a summary of the current FWWTP design parameters; this report fulfils that requirement. The design parameters for FWWTP presented in this report have been developed using historical flow and performance data. These design parameters are recommended to be used for describing the FWWTP design and performance, as baseline data for future design reviews and upgrade decisions, and as baseline data for resource consent applications. The values presented in this report for each of these parameters are valid for minimal changes to the wastewater sources and population base of Foxton, but will require adjustment for forecasting and design review purposes if significant changes to trade wastes or Foxton s population are to be accounted for. The FWWTP consists of a three stage oxidation pond system. Stage one consists of a 4.6 ha pond with stages two and three being 0.8 ha maturation ponds respectively. The ponds have a basic earthen liner and are approximately 2 m deep with a normal operating depth of 1.5 m. FWWTP has a theoretical working volume of 69,000 m 3 for the primary pond, and 12,000 m 3 for each of the two maturation ponds. No screening is provided and the ponds are aerated naturally without mechanical assistance. A sludge survey in August 2013 estimated that the three ponds contained a total sludge volume of some 14,400 m 3 ; the primary pond contained nearly 12,000 m 3 of this sludge volume. HDC intend to de-sludge the existing ponds during 2017/18 in order to return the WWTP ponds to their optimum performance levels. The de-sludging process is likely to cause a short term change to the historic range of wastewater discharge quality due to disturbance of the deposited sludge, but this will not alter the parameters used to complete the FWWTP design review and consent applications. The daily total raw wastewater inflow averages about 1,300 m 3 /d and increases from summer flows of about 1,100 m 3 /d to winter flows of about 1,550 m 3 /d. Foxton s trade waste customers generate average flow rates of about 420 m 3 /d during weekdays and some Saturdays. When averaged across full weeks, the daily flow contributions from trade wastes are about 300 m 3 /d and represent about 23% of the average total daily FWWTP flows. After subtracting trade wastes from the total flows, dry weather domestic flows average 837 m 3 /d and wet weather domestic flows average about 1,045 m 3 /d. The hydraulic residence times for the FWWTP are 72 days for the original pond dimensions, and 61 days for their reduced volumes when accounting for accumulated sludge volumes. Comparisons of wastewater inflows and outflows indicated an overall loss of 12% (about 200 m 3 /d) from losses due to combined surface evaporation and leakage through the base of the ponds. Due to the large area of the ponds, this equates to an average loss of about 2-3 mm/d. The flow data indicates that pond leakage is around 150 m 3 /d during October when net zero gains from combined rainfall and evaporation occur. The accuracy of this loss rate is to be confirmed as a part of a separate investigation and report (LEI, 2015:A6). The raw wastewater load statistics for FWWTP are summarised in Table 1.1 below. Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 1

6 Table 1.1: Statistics for Wastewater Loads Entering FWWTP ( ) Parameter Units Range Mean Median 95 th Percentile ScBOD 5 kgo 2/d , ,130 TSS kg/d , Total Kjeldahl kg/d Nitrogen Ammoniacal N kg/d Nitrate N kg/d Nitrite N kg/d Total Nitrogen kg/d DRP kg/d TP kg/d Table 1.2 indicates the percentage load contributions of trade wastes to the total wastewater load of the FWWTP for some parameters. Most parameters monitored at each location do not match, so direct comparisons are only possible for two parameters. Table 1.2: Trade Waste Contributions to FWWTP Median Loads for Parameter Units Trade Waste FWWTP Influent Trade Waste Contribution 1 ScBOD 5 kgo 2/d Not monitored 393 BOD 5 kgo 2/d 279 Not monitored COD kgo 2/d 494 Not monitored TSS kg/d % Total Kjeldahl kg/d % Nitrogen Note: 1 The trade waste contributions to the FWWTP influent loads were calculated using direct daily comparisons of trade waste and FWWTP influent loads when sampling at both locations coincided. The FWWTP s median influent and effluent quality and its median treatment performance during are summarised in Table 1.3 below. Table 1.3: Foxton Wastewater Treatment Performance Median Values Parameter Parameter Units Trade Waste FWWTP Influent FWWTP Pond 3 Parameter Removal Rate Effluent ScBOD 5 go 2/m % BOD 5 go 2/m COD go 2/m 3 1,385 TSS g/m % Total Kjeldahl N g/m % Ammoniacal N g/m % Total Oxidised N g/m Total Nitrogen g/m % DRP g/m % Total P g/m % E. coli cfu/100 ml 4.55 x log Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 2

7 2 INTRODUCTION 2.1 Background Horowhenua District Council (HDC) operate the Foxton municipal wastewater treatment plant (FWWTP) on Matakarapa which is surrounded by Foxton Loop and the Whirokino Cut of the Manawatu River. A location figure is presented in Figure 2.1. The FWWTP was originally constructed at this location in 1974, and it currently consists of three oxidation ponds with no mechanical assistance. The treated wastewater is currently discharged via a surface drain into the Foxton Loop to the west of the FWWTP. Figure 2.1: Location Plan for FWWTP The FWWTP discharge requires re-consenting and a possible treatment system upgrade. In order to progress the review of the current FWWTP design and performance, HDC require a summary of the current FWWTP design parameters. The outcomes of the design review (LEI, 2015:C8) and other investigations will assist HDC with its selection of a design for the future FWWTP and its discharge system. This design parameters summary will become fundamental background information for the resource consent applications that will be required for the construction and operation of the future FWWTP and its discharges. Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 3

8 2.2 Purpose The purpose of this report is to summarise the current design parameters of the FWWTP. This report assesses available data from monitoring of trade waste, the FWWTP inlet and outlet wastewater quality parameters, and daily flow rates in order to determine the most appropriate design data statistics that best describe the current FWWTP design features and ranges of operating parameters. The outcomes of this summary are required to enable an assessment of whether the current design parameters of the FWWTP are appropriate for its loads, whether upgrades are required, and what criteria future designs need to account for in order to attain appropriate treatment performance. This report informs the Design Review (LEI, 2015:C8) and Conceptual Design (LEI, 2015:C7) reports and processes for the future upgrade and this report ultimately supports the resource consent applications for the future the FWWTP and its discharge systems. 2.3 Scope This report reviews and summarises data on the following aspects of the FWWTP: Pond dimensions and operation; Climate and population; Trade wastewater quality and daily flow rates; Daily flow rates of incoming wastewater and effluent discharges; and Quality of wastewater influent and effluent. Based on these dataset reviews, the following design parameters have been calculated and summarised for FWWTP: Hydraulic retention times; Wastewater parameter loads requiring treatment by FWWTP; and Inflows and infiltration (I & I) increases of wastewater flow rates due to rainfall events and seasonal increases in groundwater levels. This report does not include any assessments of treatment performance or consent compliance, as these aspects are specifically addressed in the separate Design Review (LEI, 2015:C8) and Compliance Summary (LEI, 2015:B2) reports. Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 4

9 3 DATA SOURCES HDC records were the primary source of data. Trade waste daily flow records were supplied to HDC by the trade waste customers and HRC provided a set of FWWTP effluent flow rate data. The FWWTP pond dimensions and design features have historically been reported in a number of different documents for consenting or design review purposes, but have been checked for their accuracy against recent survey data and original design documents during the preparation of this report. A sludge survey undertaken in August 2013 has been relied upon for an indication of the sludge volume and depth retained in the FWWTP; HDC intend removing this sludge during 2017/18. Daily climate data (rainfall, soil moisture deficit, runoff, and evapotranspiration) for the Levin meteorological station was obtained from NIWA s on-line climate database. Levin was the closest current NIWA weather station to Foxton. Daily rainfall data was obtained from NIWA s on-line climate database for their Waitarere Forest station between 1980 and 1989, and for their Moutoa station between 1980 and Daily rainfall data was obtained from HRC for their Moutoa and Waitarere Forest stations since 2009 (which is the date that HRC took over these meteorological monitoring stations from NIWA). Population data for Foxton was sourced from Statistics New Zealand s census database which contains the 2001, 2006, and 2013 census figures. Table 3.1 summarises the extent and key parameters of the various datasets that were relied upon in this report for describing the design parameters and performance of FWWTP. Data Type Climate data for Levin Climate data for Waitarere Forest Climate data for Waitarere Forest Wastewater inflow rate to FWWTP Wastewater inflow rate to FWWTP Wastewater effluent outflow rate from FWWTP Wastewater effluent outflow rate from FWWTP Wastewater effluent outflow rate from FWWTP FWWTP influent composition Dataset Duration 10/01/80 31/12/89 and 01/01/93 30/04/15 01/01/80 31/12/89 18/11/09 31/05/15 01/01/00 31/03/15 03/05/12 28/04/15 9/11/05 31/03/15 24/02/12 28/04/15 20/07/13 13/05/15 27/04/10 07/04/15 Table 3.1: Dataset Summary Data Frequency Daily Daily Parameters Total rainfall, soil moisture deficit, runoff, Priestley-Taylor and Penman evapotranspiration, and open pan evaporation Total rainfall, soil moisture deficit, and runoff NIWA NIWA Daily Total rainfall HRC Source Daily Total daily inflow HDC telemetry Weekly Total influent volume since last manual meter reading HDC (Downer) manual readings of flow meter Daily Total daily effluent outflow HDC telemetry Weekly Total effluent volume since last manual meter reading HDC (Downer) manual readings of flow meter Daily Total daily effluent outflow HRC telemetry Monthly Ammoniacal-N, nitrate-n, nitrite- N, total Kjeldahl nitrogen, total nitrogen, CBOD5, dissolved reactive phosphorus, total HDC (Downer) Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 5

10 Data Type Pond 3 Effluent composition FWWTP Effluent composition Dataset Duration 13/09/10 07/04/15 06/10/09 07/04/15 Trade waste flow rate 28/11/09 16/04/15 Trade waste 07/02/12 composition 14/04/15 Data Frequency Parameters Source phosphorus, ph, suspended solids, and E. coli. Weekly Ammoniacal-N, nitrate-n, nitrite- HDC (Downer) N, total Kjeldahl nitrogen, total nitrogen, CBOD5, dissolved reactive phosphorus, total phosphorus, suspended solids, and E. coli. Monthly (plus Temperature, conductivity, ph, HDC (Downer) weekly for E. ammoniacal-n, total oxidised coli during nitrogen, SCBOD5, dissolved Nov-Mar) reactive phosphorus, suspended solids, and E. coli. Daily Total daily flow Trade waste telemetry Weekly Temperature, ph, COD, BOD5, suspended solids, and total Kjeldahl nitrogen Trade waste customers The sampling locations and sewer main route for the FWWTP monitoring that generated each of the wastewater flow rate and composition datasets are shown on Figure 3.1 below. Pump Station Final Effluent Pond 3 Effluent FWWTP Inlet Sewer Main Figure 3.1: FWWTP Flow and Water Quality Monitoring Locations (Base Image Sourced from HDC s On-line Maps) The FWWTP inflow meter is a mag meter design that is mounted on the rising main pipeline near the Stewart Street pump station and designed for an accuracy of +5%. The effluent flow meter is located at the outlet weir of the third pond and designed for an accuracy of +5%. Flow meter calibration is checked annually. Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 6

11 4 DATA ASSESSMENT METHODOLOGIES All datasets were assessed for any gaps or errors (extreme outliers were scrutinised for their likelihood of being errors instead of true outliers). Where data gaps were found, they were filled where possible by estimating likely values from adjacent days of data within the datasets or from corresponding data in related datasets. The various datasets were also correlated and integrated with each other as far as possible. The daily flow data for 2010 to 2015 was compared for correlations between trade waste, total FWWTP influent, and effluent. This was intended as a check of the flow meter calibrations as well as an indication of FWWTP pond leakage. The trade waste and FWWTP flows were assessed for weekly and seasonal cycles. Monthly and annual flow statistics were also generated for the Turks trade waste contributions and total FWWTP wastewater inflows. After deducting the daily trade waste flows from the daily total FWWTP wastewater inflows, the residual (domestic) flows were calculated. These were then categorised into wet weather flow (WWF) when the previous three days received any rainfall according to NIWA s daily rainfall records for Levin, or by default as dry weather flow (DWF). Statistics were then generated for the WWF and DWF inflows for 2010 to The trade waste, total FWWTP influent and FWWTP effluent loads were each calculated from the wastewater quality concentrations and corresponding daily flow rate data. The percentage contributions to FWWTP contaminant loads from trade wastes were then calculated on a monthly basis. The individual daily percentage reductions in total influent and effluent concentrations were calculated for each matching contaminant when the dates of these results coincided. Statistics were also calculated for the ranges of contaminant reductions achieved by the treatment processes during the years of data. Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 7

12 5 DATA INTEGRITY 5.1 Climate Data LEI s environmental data summary report found that HRC s Moutoa rainfall data was unreliable when compared with NIWA s concurrent data for Moutoa during prior to NIWA s transfer of the site to HRC in As a result of this poor correlation, the two sets of Moutoa data were not able to be merged into a single continuous record, and it was therefore discounted as a reliable and up to date climate dataset. NIWA s rainfall data for Levin and Moutoa were found to be highly correlated, so the Levin data was selected as the best representation of Foxton s likely rainfall out of the available datasets. Levin continues to be monitored on a daily basis, so it is the best continuous up to date climate dataset for the Levin-Foxton area. There were no instances of missing daily rainfall, soil moisture deficit, or runoff data within the NIWA climate dataset for Levin from 1 January 1980 to 31 December 1989 and from 1 January 1993 to 16 April The Levin evapotranspiration data had a number of missing days of data, sometimes for continuous periods of up to a month, but generally only 1-4 days at a time. The Penman ET dataset was the most complete, with 100 days of missing data since 1 January The Priestley-Taylor ET dataset was the least complete with 204 missing days. The Open Water ET dataset was missing 136 days of data. All three of these evapotranspiration data series were missing 11 days after 1 January These datasets are complete since 20 July 2011 except for 7 December 2011 and 25 January NIWA s daily rainfall data for their Waitarere Forest climate station during 1 January 1980 to 31 December 1989 was complete except for a period of no records from 1 May to 1 October 1987 inclusive. There were no instances of missing daily rainfall data from HRC s Waitarere Forest climate station dataset for 18 November 2009 to 31 May 2015, but the daily rainfall totals during 25 March to 7 April 2015 were erroneously static at 0.2 mm; the Levin rainfall data during this period indicated rainfall of up to 12.2 mm and about half of these days had no rainfall at all. 5.2 Wastewater Inflow Data A number of gaps were apparent in the HDC total wastewater inflow data where no daily total flow rate records were available. Some flow measurements immediately before or after these gaps were also clearly erroneous as they were well below 400 m 3 /day (the minimum was otherwise about 550 m 3 /day). In total, 301 days of total wastewater inflow data were blank or unrealistically low since 1 January 2000, but this reduced to 118 days after 11 October These gaps were therefore excluded from the datasets used for this project. The date ranges for these wastewater inflow data gaps were as follows: 31 December 2002 to 12 January February 2003 to 24 February April 2003 to 21 April January 2004 to 7 February May 2004 to 11 May March 2005 to 8 April June 2005 to 11 October March 2010 to 28 March September 2010 to 1 November October 2011 to 17 November 2011 Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 8

13 15 May 2012 to 7 June December 2012 to 2 January 2013 HDC s weekly manual readings of the influent flow meter were unable to be used to fill the gaps in HDC s daily effluent flow data since May 2012 because the manual readings proved that the flow meter stopped working on each occasion. The monthly influent composition data was complete from 13 September 2009 except for a gap in CBOD 5 data from 25 July to 15 August Wastewater Outflow Data HDC Telemetry Analysis of daily total wastewater inflow and daily total wastewater outflow data for 2009 to 2015 indicated that HDC s outflow records suddenly increased to unrealistically high and randomly variable values when compared with the corresponding daily wastewater inflow volumes. The timing of this change occurred immediately following the installation of HRC s SCADA telemetry system on 15 February HDC s records indicate a recognition that HRC s new telemetry system was causing randomly variable wastewater outflow pulse counter errors for HDC s existing telemetry system, and this was never satisfactorily resolved. The annual meter calibrations and manual weekly readings of the flow meters confirmed that the meters are reliably accurate, but the telemetry data could not be corrected with a series of constant or gradually drifting correction factors; it was randomly variable. Consequently, the daily effluent flow data generated by HDC s telemetry system after 15 February 2010 was excluded from the dataset used for this project. A number of gaps were also apparent in the HDC daily total effluent outflow data where no daily total flow rate records were available. Some flow measurements immediately before or after these gaps were also clearly erroneous as they were well below 300 m 3 /day (the minimum was otherwise about 350 m 3 /day). In total, 519 days of data were blank or unrealistically low since 9 November 2005, but this reduced to 323 days after 1 January These gaps were therefore excluded from the dataset used for this project. The date ranges for these effluent outflow data gaps were as follows: 4 June to 22 August to 18 February May to 25 June 2008 unrealistically low 26 June to 18 October January to 16 July June to 21 July October to 14 December June to 19 July 2013 HDC Manual The weekly manual readings of HDC s effluent meter were useful as the only reliable record for indicating average daily effluent flow rates during each week for about 17 months prior to the commencement of HRC s daily effluent flow data records. HDC s weekly manual readings of the effluent flow meter also filled some of the gaps in HDC s daily effluent flow data since 24 February There were no gaps in the weekly total flow records, but the intervals between readings were sometimes variable. However, the flow meter stopped working on or just before 3 July 2013 and was rectified by 17 July The manually recorded weekly effluent meter records are Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 9

14 considered to be reliable, as they were directly read from the flow meter and mostly matched the HRC effluent flow data. HRC Telemetry There were no gaps in the HRC daily total effluent flow data throughout its duration of 20 July 2013 to 13 May Although the HRC SCADA telemetry system was installed in February 2010, daily data was unable to be retrieved between the date of installation and 20 July The weekly manual readings of HDC s effluent meter correlated well with the HRC data, which confirmed that HRC s telemetry system did not generate any significant errors. Composition The monthly effluent composition dataset was complete from 6 October 2009 except for the occasional missed result for some parameters. Weekly E. coli data gathered during the bathing season each year was also complete. 5.4 Trade Waste Data Occasional gaps are apparent in the daily trade waste flow data where no daily total flow rate records were available. The clear regular cycle of increased flow rates during weekdays (and Saturdays during some months) enabled the identification of missed days. In total, 25 days of data were blank or unrealistically low, and were therefore filled or excluded from the dataset used for this project. These were as follows: 9 December June and 23 September January May June September 2013 The weekly trade waste composition dataset was complete for ph, BOD 5 and suspended solids from 7 February 2012 to 14 April 2015, and temperature, chemical oxygen demand (COD), and total Kjeldahl nitrogen (TKN) were included in monitoring data from 21 May Temperature was measured sporadically during March to October 2014, and TKN was not recorded for 9 July Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 10

15 6 DATA INTERPRETATION 6.1 WWTP Physical Description The rising main from the Stewart Street pump station follows Stewart Street past its terminus, then turns towards the west to cross Foxton Loop on a dedicated pipe bridge, and then directly across Matakarapa to the north-eastern corner of FWWTP s primary pond. Figure shows the route of the rising main and the location and layout of FWWTP. The rising main is a 300 mm diameter reinforced concrete pipe installed in 1975 with a remaining residual life of 22 years from March 2015 (Gallo Saidy pers. comm.). No inlet grating or screen is provided prior to the FWWTP inlet. Foxton Loop Discharge Point Pump Station Final Maturation First Maturation Primary Pond Figure 6.1.1: Rising Main Reticulation Route and FWWTP Layout The FWWTP consists of a three stage oxidation pond system. The primary pond was constructed in 1970 and the two maturation ponds were constructed in The ponds have a basic earthen liner and a normal operating depth of 1.5 m. Table presents FWWTP s pond dimensions. Table 6.1.1: Foxton WWTP Pond Dimensions Pond Name Length (m) Width (m) Area (ha) Depth (m) Volume (m 3 ) Primary ,000 First Maturation ,000 Final Maturation ,000 Total N/A N/A 6.4 N/A 93,000 A sludge survey of the ponds by Chapman White in August 2013 indicated that the primary pond held about 11,909 m 3 of sludge, the first maturation pond held about 2,057 m 3 of sludge, and the final maturation pond held about 378 m 3 of sludge. The sludge in all three ponds was mostly accumulated along their western and southern portions. In the primary pond, the sludge towards its western side was often more than 0.8 m deep, and in some places the sludge is likely to be within 0.5 m of the pond surface. HDC intend de-sludging the ponds during 2017/18. The ponds are all naturally aerated; no mechanical assistance has been installed. This is partly because electricity is currently not reticulated to the FWWTP site. A small solar panel powers the effluent flow meter telemetry system. Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 11

16 Downer staff manually clear all floating debris from the ponds on a weekly basis. During LEI visits to the FWWTP in January and July 2015, all of the concrete lined wave bands appeared to be in good condition around the upper sides of all three ponds. A section of the northern bund of the main pond seems to be slightly lower than surrounding areas. The reason for this is unclear. The bunds around the outside edges of the ponds are regularly mown. The boundary fencing is being maintained in good condition as appropriate for keeping the neighbouring farmer s stock out of the FWWTP site. The effluent from the outlet of the third pond flows over a V-notch weir into a concrete pipe beneath the pond bund. This discharges into an open drain which is then captured in a corrugated iron half-round flume that traverses the undulating terrace and down the steep slope to the edge of a wetland channel of Foxton Loop where the effluent ultimately discharges (see Figure 6.1.1). 6.2 Population Population data for Foxton that was sourced from Statistics New Zealand s census database indicated the following changes in population: 2001 population = 2, population = 2, population = 2,643 Despite the census data showing a declining population trend for Foxton, for long term planning purposes HDC have adopted an assumed projection of slow population growth rates of 0.4% pa over the next 50 years; this was based on the conclusions of a population forecast report that was prepared for HDC by Infometrics. 6.3 Climate Data Total annual rainfall for Levin during 1980 to 1989 and 1993 to 2014 varied between 811 mm and 1,395 mm, and averaged 1,040 mm. The daily rainfall totals for Levin during 1993 to 2014 are presented in Figure below, while the daily Priestley-Taylor evapotranspiration totals for the same period are presented in Figure below. Figure 6.3.1: NIWA Daily Total Rainfall Data for Levin During Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 12

17 Figure 6.3.2: Daily Total Evapotranspiration at Levin During Figure above clearly shows the seasonal variations in the daily total Priestley-Taylor evapotranspiration measured for Levin. Significant rainfall during summer months reduces the evapotranspiration for short periods of time. The trend is quite consistent across the 20 years of data, so the gaps in evapotranspiration data could conceivably be filled by referring to similar months and rain events in other years where the dataset is complete. Table summarises the meteorological statistics for Levin during 1 January 1993 to 30 June 2015; the averages and ranges are given for each parameter and month. Table 6.3.1: Monthly Climate Data for Levin During Priestley-Taylor Evapotranspiration Open Water Total Rainfall Evaporation (mm) (mm) (mm) Average Air Temperature ( o C) Month January 61 (10 130) 130 ( ) 115 (79 145) 17 (16 20) February 71 (14 270) 101 (81 121) 95 (77 126) 18 (16 20) March 62 ( (63 92) 84 (64 110) 16 (14 19) April 82 (21 168) 36 (29 42) 49 (29 68) 14 (12 16) May 90 (18 159) 13 (10 15) 35 (22 47) 12 (9 14) June 108 (34 172) 2 (0 3) 30 (22 42) 9 (7 12) July 90 (27 219) 5 (3 7) 32 (24 47) 9 (7 11) August 81 (19 153) 21 (17 25) 37 (27 46) 10 (8 11) September 96 (9 189) 44 (35 50) 49 (38 68) 11 (10 13) October 103 (28 260) 73 (62 86) 68 (55 93) 13 (11 14) November 93 (35 192) 98 (77 118) 88 (65 112) 14 (12 16) December 101 (40 149) 120 ( ) 103 (82 117) 16 (14 18) Annual 1,032 (811 1,395) 720 ( ) 791 ( ) 13 (12 14) Figure presents a wind rose for Levin (NIWA s station number E05620) for the period of October 1991 to June This wind rose shows that the predominant wind direction in Levin is west-north-westerly. East-north-easterly winds are the next most common direction but they are not as strong as the prevailing WNW winds. These most common wind directions will not carry any odours from the FWWTP towards any sensitive residential areas such as Foxton or Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 13

18 Foxton Beach. South-westerly or south-easterly winds respectively would be required to carry odours towards each of these nearby residential communities, and these only occur about 2.5 % and 2.0 % of the time respectively. Typically wind speeds are less than 20 km/hr, but wind speeds reach km/hr for around 1 % of the time (primarily from the west to north-west quadrant). The overall mean wind speed is 9.2 km/h, and calm conditions occur 4 % of the time. Figure 6.3.3: Wind Rose for Levin During Anecdotal local knowledge indicates that Foxton s rainfall is less than Levin s, so rainfall data for the nearby Waitarere Forest meteorological station was obtained for comparative analysis. Total annual rainfall for Waitarere Forest during 1980 to 1989 and 2010 to 2014 varied between 694 mm and 1,073 mm, and averaged 820 mm. Overall, Waitarere Forest receives about 79% of the rainfall that Levin receives each year, which validates the anecdotal local knowledge. The monthly averages for Levin and Waitarere Forest during identical years of available data are presented in Table below. Table 6.3.2: Monthly Average Rainfall Data During and Average Rainfall Total (mm) Month Waitarere Forest Levin January February March April May Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 14

19 Average Rainfall Total (mm) Month Waitarere Forest Levin June July August September October November December Annual 820 1,033 Levin s rainfall and evaporation data has been adopted for flow normalisation and design purposes due to the length, reliability, and completeness of this NIWA dataset. Levin s higher overall rainfall rates also result in conservative estimates of annual total wastewater inflows to the FWWTP. 6.4 Trade Waste Flow Rates A consistent and obvious weekly total wastewater inflow cycle also occurs throughout the data set which generally reflects the week-day trade waste flows generated by the trade waste customers. During weekdays, daily total wastewater inflows consistently increase by an average of 419 m 3 /d over intervening weekend daily total wastewater inflows. The daily trade waste flow statistics for are summarised in Table and graphed in Figure below. Table 6.4.1: Summary of Trade Waste Flow Statistics for Average Working Day Flow (m 3 /d) Average Daily Flow (averaged across full weeks) (m 3 /d) Percentage of Total Daily FWWTP Flow (averaged across full weeks) Month January % February % March % April % May % June % July % August % September % October % November % December % Annual % Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 15

20 Figure 6.4.1: Mean Daily Trade Waste Flows for Averaged Across Full Weeks The averages across full weeks reflect the number of public holidays in each month, which changes the ranking of the working day averages. For example, April has the highest working day volume, but the high number of holidays during this month reduces its position to the fifth lowest when averaged across full weeks. Saturdays are often working days during November and December, which also helps to elevate their rankings in the full weekly average dataset. There has also been some variation of total annual flows over The lowest annual flows of 97-98,000 m 3 were recorded during 2010 and 2014, while the highest annual flow of 126,000 m 3 was recorded during 2012; this is nearly 30% higher than 2010 and The 2011 and 2013 flows were both 112,000 m 3 which was close to the average for the entire flow dataset. 6.5 Wastewater Flow Rates The mean daily wastewater flow rate into the FWWTP is approximately 1,300 m 3 /d. Mean daily inflow wastewater rates during the hottest and driest months of January to May are approximately 1,100 m 3 /d, while mean daily inflow wastewater rates during winter months are approximately 1,550 m 3 /d. The volume of stormwater infiltration and inflow (I & I), particularly during winter months, elevates the daily total wastewater inflow rates; the month with the highest average daily total wastewater inflows is August (1,638 m 3 /d during ). The minimum daily total wastewater inflow rates during summer months are m 3 /d, but they rise to 900-1,000 m 3 /d during the wetter months of July to October as a result of increases in groundwater infiltration from seasonally elevated groundwater levels. Mean daily total wastewater inflow rates for each month (based on flow data for ) are presented in Table and graphed in Figure below. Table 6.5.1: Mean FWWTP Wastewater Inflows ( flow data) Month Mean Daily Inflow (m 3 /d) Mean Monthly Inflow Totals (m 3 /month) January 1,095 33,940 February 1,128 31,875 March 1,071 33,214 April 1,042 31,253 Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 16

21 Month Mean Daily Inflow (m 3 /d) Mean Monthly Inflow Totals (m 3 /month) May 1,146 35,511 June 1,342 40,249 July 1,543 47,825 August 1,638 50,767 September 1,522 45,657 October 1,517 47,015 November 1,259 37,772 December 1,217 37,738 Annual Average 1,293 39,401 m 3 /month (472,800 m 3 /y) Figure 6.5.1: Mean FWWTP Wastewater Inflows ( flow data) The actual daily flow data is recommended for use for design review purposes. Due to the time duration and incomplete continuous record limitations of the actual flow data, a synthetic flow data set was generated from meteorological data for use as the basis for calculating discharge scenarios; the process of generating this data set is called flow normalisation, and the report that describes the flow normalisation process and outcomes is LEI, 2015:B5. The calculated mean flow statistics for the normalised flow data set during are presented in Table below. Table 6.5.2: Mean Normalised FWWTP Wastewater Inflows ( flow data) Month Mean Daily Inflow (m 3 /d) Mean Monthly Inflow Totals (m 3 /month) January 1,114 34,791 February 1,105 31,265 March 1,102 34,242 April 1,101 33,031 May 1,254 38,880 June 1,435 43,064 July 1,539 47,724 August 1,516 46,990 September 1,458 43,729 Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 17

22 Month Mean Daily Inflow (m 3 /d) Mean Monthly Inflow Totals (m 3 /month) October 1,497 46,397 November 1,321 39,626 December 1,251 38,767 Annual Average 1,307 39,875 m 3 /month (478,500 m 3 /y) Figure below presents wastewater inflow data for 2013 and 2014 where the weekly and seasonal cycles are apparent for both the total daily wastewater inflows to the FWWTP and the trade waste flows. Figure presents domestic inflows to the FWWTP (calculated by subtracting the trade waste inputs from the FWWTP total daily wastewater inflows) and daily rainfall data for Levin for 2013 and Figure 6.5.2: Daily Total Inflows and Daily Total Trade Waste Flows ( ) Figure 6.5.3: Daily Domestic FWWTP Inflows and Levin Rainfall ( ) Rainfall does not appear to account for all peak flows which can be up to 1-2,000 m 3 /d higher than adjacent days; some of these large flow spikes might be caused by unusual commercial inputs from a contributor other than the monitored trade waste customers (assuming that the trade waste flow meter records have accurately measured their highest peak flows). Clearances Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 18

23 of reticulation blockages and erroneous readings may also be causing some of these unusually high peaks. Peak total daily wastewater inflows of 3-4,000 m 3 /d occur occasionally each year; rare total daily wastewater inflow peaks of 5-7,000 m 3 /d can also occur. Figure demonstrates this occurring occasionally during 2010 to 2012 after subtracting trade waste inflows from total daily wastewater inflows. Figure 6.5.4: Daily Domestic FWWTP Inflows and Levin Rainfall ( ) After subtracting trade waste flows from the total daily flows entering the FWWTP, it was possible to calculate the total domestic, dry weather, and wet weather flow statistics for the FWWTP during These statistics are presented in Table below. Table 6.5.3: Domestic Total, Dry Weather, and Wet Weather Flow Statistics Statistical Measure Total DWF WWF Minimum: th Percentile: Median: Mean: ,045 95th Percentile: 1,558 1,171 1,676 Maximum: 5,732 1,435 5,732 Based on these figures and the annual mean trade waste flow of 300 m 3 /d, the mean total DWF is 1,137 m 3 /d and the mean total WWF is 1,345 m 3 /d. These figures can then be used to calculate the hydraulic residence times for the FWWTP s ponds, as presented in Table below. Mean Flow Conditions Dry Weather (1,140 m 3 /d) Wet Weather (1,340 m 3 /d) Annual Mean (1,300 m 3 /d) Table 6.5.4: FWWTP Hydraulic Residence Times (days) Original Design Volumes Current Due to Sludge Volumes Primary Pond (69 ML) Maturation Ponds (24 ML) Total FWWTP (93 ML) Primary Pond (57 ML) Maturation Ponds (21.5 ML) Total FWWTP (78.5 ML) Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 19

24 6.6 FWWTP Total Wastewater Inflow and Outflow Correlation Figure presents a comparison of the HDC total wastewater inflow and HRC total wastewater outflow data. This clearly shows an overall reduction in total wastewater outflows within the pond system. It is also apparent that the total wastewater outflows have occasionally dropped to very low rates and then dramatically risen to well above the total wastewater inflows. HDC staff have attributed these events to blockages of the effluent outlet that restrict outflows, followed by surges of outflows when the blockages clear. Figure 6.6.1: Comparison of Daily Total Wastewater Inflow (HDC) and Outflow (HRC) Data for May 2012 to March 2015 HRC s telemetry data for the FWWTP effluent flow data and weekly manual readings of the effluent flow meter indicated an overall loss of 12% (about 200 m 3 /d) from the influent flow rates. This is inferred to reflect losses due to evaporation and leakage through the base of the ponds. Due to the large area of the ponds, this equates to an average loss of about 2-3 mm/d, which is a combination of leakage and evaporative loss during summer months and mostly leakage during winter months. The accuracy of this loss rate is to be confirmed as a part of a separate investigation and report (LEI, 2015:A6). Figure below shows the average monthly variations in rainfall, evaporation, and losses of wastewater volumes during May 2012 to March This clearly shows the seasonal variations in all of these parameters. The October data indicates net zero gains from combined rainfall and evaporation, which suggests that pond leakage is around 150 m 3 /d. The July and August variations appear to partly reflect pond outlet blockages that stored excess rain inputs during July which were released during August when the outlet blockages cleared and outflows surged. Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 20

25 Figure 6.6.2: Comparisons of Rainfall, Evaporation, and Wastewater Losses for May 2012 to March Trade Waste Composition and Loads The trade waste water quality composition and load data for February 2012 to April 2015 is summarised in Tables and below. Table 6.7.1: Statistics for Trade Waste Concentrations for Parameter Units Range Mean Median 95 th Percentile BOD 5 go 2/m , ,823 COD go 2/m ,850 1,557 1,385 2,622 TSS g/m , ph ph TKN g/m Table 6.7.2: Statistics for Trade Waste Loads for Parameter Units Range Mean Median 95 th Percentile BOD 5 kgo 2/d 2.7 2, COD kgo 2/d 24 2, ,045 TSS kg/d 7.9 1, TKN kg/d These tables of data indicate that the trade waste compositions can vary considerably up to high values, but its composition is normally less than 25% of the maximum concentrations. The typical concentration ranges were stable for all parameters during the three years of monitoring data. Table indicates the percentage load contributions of trade wastes to the total wastewater load of FWWTP for some parameters. Most parameters monitored at each location do not match, so direct comparisons are only possible for two parameters. Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 21

26 Table 6.7.3: Trade Waste Contributions to FWWTP Median Loads for Parameter Units Trade Waste FWWTP Influent Trade Waste Contribution 1 ScBOD 5 kgo 2/d Not monitored 393 BOD 5 kgo 2/d 279 Not monitored COD kgo 2/d 494 Not monitored TSS kg/d % Total Kjeldahl kg/d % Nitrogen Note: 1 The trade waste contributions to the FWWTP influent loads were calculated using direct daily comparisons of trade waste and FWWTP influent loads when sampling at both locations coincided. 6.8 FWWTP Raw Wastewater Composition and Loads The raw wastewater composition and load statistics for FWWTP are summarised in Tables and below. Table 6.8.1: Statistics for Wastewater Concentrations Entering FWWTP ( ) Parameter Units Range Mean Median 95 th Percentile ScBOD 5 kgo 2/m , TSS g/m ph ph units Total Kjeldahl g/m Nitrogen Ammoniacal N g/m Nitrate N g/m Nitrite N g/m Total Nitrogen g/m DRP g/m TP g/m E. coli cfu/100 ml x x 10 6 (geomean) 4.6 x x 10 7 Table 6.8.2: Statistics for Wastewater Loads Entering FWWTP ( ) Parameter Units Range Mean Median 95 th Percentile ScBOD 5 kgo 2/d , ,130 TSS kg/d , Total Kjeldahl kg/d Nitrogen Ammoniacal N kg/d Nitrate N kg/d Nitrite N kg/d Total Nitrogen kg/d DRP kg/d TP kg/d Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 22

27 6.9 FWWTP Treatment Performance HDC s monthly wastewater monitoring data for the FWWTP influent and effluent quality during September 2010 to April 2015 is summarised as median concentrations and percentage removal rates in Table below. These statistics indicate the FWWTP s median treatment performance. Table 6.9.1: Foxton Wastewater Treatment Performance Median Values Parameter Parameter Units Trade Waste FWWTP Influent FWWTP Pond 3 Parameter Removal Rate Effluent ScBOD 5 go 2/m % BOD 5 go 2/m COD go 2/m 3 1,385 TSS g/m % Total Kjeldahl N g/m % Ammoniacal N g/m % Total Oxidised N g/m Total Nitrogen g/m % DRP g/m % Total P g/m % E. coli cfu/100 ml 4.55 x log Table also includes median concentrations for trade wastes based on February 2012 to April 2015 data to indicate its contributions to the FWWTP influent loads. The residential and other commercial inputs dilute the trade wastewater concentrations to some extent before they enter FWWTP. Since October 2013, HDC have also monitored the wastewater quality at the inlets of Ponds 2 and 3 on a monthly basis to assess the treatment performance of each maturation pond. This data is relevant to the performance assessment of FWWTP (HDC, 2105:C8), but is not considered to be relevant design information that should be included in this report. The treated wastewater effluent composition and load statistics for FWWTP are summarised in Tables and below. Table 6.9.2: Statistics for FWWTP Wastewater Effluent Concentrations Discharged ( ) Parameter Units Range Mean Median 95 th Percentile ScBOD 5 kgo 2/m TSS g/m ph Total Kjeldahl g/m Nitrogen Ammoniacal N g/m Nitrate N g/m Nitrite N g/m Total Nitrogen g/m DRP g/m TP g/m E. coli cfu/100ml 4 370, (geomean) 400 6,100 Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 23

28 Table 6.9.3: Statistics for FWWTP Wastewater Effluent Loads Discharged ( ) Parameter Units Range Mean Median 95 th Percentile ScBOD 5 kgo 2/d TSS kg/d Total Kjeldahl kg/d Nitrogen Ammoniacal N kg/d Nitrate N kg/d Nitrite N kg/d Total Nitrogen kg/d DRP kg/d TP kg/d Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 24

29 7 CONCLUSIONS AND RECOMMENDATIONS 7.1 General The design parameters for FWWTP presented in this report have been developed using historical flow and performance data. These design parameters are recommended to be used for describing the FWWTP design and performance, as baseline data for future design reviews and upgrade decisions, and as baseline data for resource consent applications. The values presented in this report for each of these parameters are valid for minimal changes to the wastewater sources and population base of Foxton, but will require adjustment for forecasting and design review purposes if significant changes to trade wastes or Foxton s population are to be accounted for. 7.2 WWTP Design FWWTP consists of a three stage oxidation pond system. Stage one consists of a 4.6 ha pond with stages two and three being 0.8 ha maturation ponds respectively. The ponds have a basic earthen liner and are approximately 2 m deep with a normal operating depth of 1.5 m. FWWTP has a theoretical working volume of 69,000 m 3 for the primary pond, and 12,000 m 3 for each of the two maturation ponds. No screening is provided and the ponds are aerated naturally without mechanical assistance. 7.3 Sludge A sludge survey in August 2013 estimated that the three ponds contained a total sludge volume of some 14,400 m 3 ; the primary pond contained nearly 12,000 m 3 of this sludge volume. In some areas the sludge level is likely to be within 0.5 m of the water level surface. HDC intend to desludge the existing ponds during 2017/18 in order to return the WWTP ponds to their optimum performance levels. The de-sludging process is likely to cause a short term change to the historic range of wastewater discharge quality due to disturbance of the deposited sludge, but the long term discharge quality should improve somewhat once the full pond volume has been restored and the suspended solids have resettled. The values of the parameters presented in this report should still be used as the basis for completing the FWWTP design review and consent application documents, as they represent the usual ranges and averages of the FWWTP parameters. 7.4 Flows The daily total raw wastewater inflow averages about 1,300 m 3 /d and increases from summer flows of about 1,100 m 3 /d to winter flows of about 1,550 m 3 /d. Foxton s trade waste customers generate average daily flow rates of about 420 m 3 /d during weekdays and some Saturdays. When averaged across full weeks, the trade waste daily flow contributions are about 300 m 3 /d and represent about 23% of the average total daily FWWTP flows. After subtracting trade wastes from the total flows, dry weather domestic flows average 837 m 3 /d and wet weather domestic flows average about 1,045 m 3 /d. The hydraulic residence times for FWWTP are 72 days for the original pond dimensions, and 61 days for their reduced volumes when accounting for accumulated sludge volumes. Comparisons of manually recorded total wastewater inflows and outflows indicated an overall loss of 12% (about 200 m 3 /d) from losses due to evaporation and leakage through the base of the ponds. Due to the large area of the ponds, this equates to an average loss of about 2-3 mm/d, Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 25

30 which is a combination of leakage and surface evaporative loss. The flow data indicates that pond leakage is around 150 m 3 /d during October when net zero gains from combined rainfall and evaporation occur. The accuracy of this loss rate is to be confirmed as a part of a separate investigation and report (LEI, 2015:A6). 7.5 Loads The raw wastewater composition and load statistics for FWWTP are summarised in Table below. Table 7.5.1: Statistics for Wastewater Loads Entering FWWTP ( ) Parameter Units Range Mean Median 95 th Percentile ScBOD 5 kgo 2/d , ,130 TSS kg/d , Total Kjeldahl kg/d Nitrogen Ammoniacal N kg/d Nitrate N kg/d Nitrite N kg/d Total Nitrogen kg/d DRP kg/d TP kg/d Table indicates the percentage load contributions of trade wastes to the total wastewater load of FWWTP for some parameters. Most parameters monitored at each location do not match, so direct comparisons are only possible for two parameters. Table 7.5.2: Trade Waste Contributions to FWWTP Median Loads for Parameter Units Trade Waste FWWTP Influent Trade Waste Contribution 1 ScBOD 5 kgo 2/d Not monitored 393 BOD 5 kgo 2/d 279 Not monitored COD kgo 2/d 494 Not monitored TSS kg/d % Total Kjeldahl kg/d % Nitrogen Note: 1 The trade waste contributions to the FWWTP influent loads were calculated using direct daily comparisons of trade waste and FWWTP influent loads when sampling at both locations coincided. The FWWTP s median influent and effluent quality and its median treatment performance during are summarised in Table below. Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 26

31 Table 7.5.3: Foxton Wastewater Treatment Performance Median Values Parameter Parameter Units Trade Waste FWWTP Influent FWWTP Pond 3 Parameter Removal Rate Effluent ScBOD 5 go 2/m % BOD 5 go 2/m COD go 2/m 3 1,385 TSS g/m % Total Kjeldahl N g/m % Ammoniacal N g/m % Total Oxidised N g/m Total Nitrogen g/m % DRP g/m % Total P g/m % E. coli cfu/100 ml 4.55 x log Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 27

32 8 REFERENCES Beca and Lowe Environmental Impact (2014:E0) Resource Consent Application and Assessment of Environmental Effects Foxton Wastewater Discharges Infometrics (2014) Review of Projections for the Horowhenua District Lowe Environmental Impact (2014:B1) Summary of Existing Environmental Data for Foxton WWTP Lowe Environmental Impact (2015:A6) Foxton WWTP Pond Leakage Assessment Lowe Environmental Impact (2015:B2) Foxton WWTP Consent Compliance Summary Lowe Environmental Impact (2015:B5) Foxton WWTP Flow Normalisation Modelling Horowhenua District Council (2015:C8) Foxton WWTP Design Review and Upgrade Options Horowhenua District Council Foxton WWTP B6 Design Parameter Summary P a g e 28

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