TAHUNA WASTEWATER TREATMENT PLANT STAGE 2 UPGRADE PROJECT

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1 TAHUNA WASTEWATER TREATMENT PLANT STAGE 2 UPGRADE PROJECT Rob Crosbie 1, Humphrey Archer 1 and Brian Turner 2 1. CH2M Beca Ltd (rob.crosbie@beca.com, humphrey.archer@beca.com) 2. Dunedin City Council Abstract CH2M Beca Ltd was engaged by Dunedin City Council (DCC) to undertake detailed design of Stage 2 of the Tahuna Wastewater Treatment Plant (WWTP) Upgrade. The upgrade was driven by new effluent quality standards for the discharge from the 1100m long ocean outfall completed as part of the Stage 1 upgrade in The former WWTP consisted of primary sedimentation and chlorine disinfection. The Stage 2 upgrade to secondary treatment was required to improve the effluent quality so that UV disinfection could replace chlorination. The population served is 120,000 and the peak wet weather flow is 4m³/s, making it the second largest plant in the South Island. The liquid stream upgrading involved inlet Suboscreens (relocated from Musselburgh Pump Station), Grit Removal, a High Rate Activated Sludge Process (within former primary sedimentation tanks), Biological Trickling Filters, and UV Disinfection. The solids stream involved gravity belt thickeners and centrifuges at Tahuna. Dewatered sludge cake is either combusted in the existing Tahuna incinerator, or transported to DDC s Green Island WWTP which had spare digestion capacity due to industry closures. An interactive tendering process resulted in a clear understanding of project scope. During the construction period, CH2M Beca provided the Site Representative and Engineer to the Contract, as well as managing eight equipment supply contracts for equipment that was free-issued to the Main Contractor. Direct construction costs for Stage 2 are approximately $47 million. Key Words Wastewater treatment, high rate activated sludge process, biological trickling filters, UV disinfection, thickening, dewatering, Tahuna WWTP, Dunedin Treatment Overview The population served is 120,000 and the design peak wet weather flow is 4,000 l/s, making it the second largest plant in the South Island. A number of novel features are incorporated in the upgraded plant and these are described in later sections, viz; Long trough screen at an inlet bypass overflow weir Converting former sludge hoppers to grit hoppers Suction removal of sludge from rectangular tanks

2 Modification of existing travelling bridge scrapers Coarse screw collection of scum Submerged tube launders coupled with a weir at the sedimentation tank outlet Very high speed aeration blowers with direct-coupled motors and integral variable speed drives (no gearboxes) Lightly loaded Biological Trickling Filters which avoid the need for secondary clarifiers UV disinfection of Biological Trickling Filter effluent Primary Treatment Prior to Upgrading from 2010 to 2013 The Musselburgh Pump Station delivered wastewater from most of Dunedin to the Tahuna Wastewater Treatment Plant where wastewater was settled in relatively large Primary Sedimentation Tanks housed in a building refer to Photo 3. Primary effluent was dosed with Chlorine (sodium hypochlorite) at a flow measurement flume. Disinfected flow could travel by gravity to the Ocean Outfall via the Papatuanuku Junction Chamber, or was boost pumped via the Outfall Pumps during wet weather flows or during outfall flushing. The 1100m long Ocean Outfall and Outfall Pump Station were completed in 2009 during Stage 1 of the Tahuna Upgrade. After Upgrading to Secondary Treatment The Stage 2 upgrade to secondary treatment was required to improve the effluent quality so that UV disinfection could replace chlorination. The liquid stream upgrading involved inlet fine screening with Suboscreens (relocated from Musselburgh Pump Station and replaced by course screens), Grit Removal, a High Rate Activated Sludge Process (within former primary sedimentation tanks), Biological Trickling Filters, and UV Disinfection Figure 1. The solids stream upgrading involved gravity belt thickeners and centrifuges at Tahuna. Dewatered sludge cake is either combusted in the existing Tahuna incinerator, or transported to DDC s Green Island WWTP which had spare digestion capacity due to industry closures. Some sludge is taken directly to the Green Island Landfill. Flow Capacities Design average flow is 500l/s and maximum flow to the HRAS Process is 2800l/s (or 5.6 times average flow). Flow greater than 2800l/s bypasses the HRAS process via an actuated gate. Flow up to 2800l/s receives grit removal, HRAS aeration and sedimentation. The Contact Tank is retained as a blind tank to shore volume for Outfall Flushing. The existing chlorination system has been retained as a backup if a discharge to the former Lawyers Head Outfall is ever required. HRAS treated flow is pumped to the BTFs along with any recirculation flow from the BTFs. Flow up to 1400l/s is pumped to the BTFs and UV disinfection stage. Flow greater than 1400l/s is spilled to the Outfall Pump Station. Treated flow from secondary treatment (BTFs and UV) is recombined with the BTF Feed Bypass flow and either passes to the Papatuanuku Junction Chamber by gravity, or is pumped to the Papatuanuku Junction Chamber by the Outfall Pumps. The Treatment Plant operates in one of three modes at any given time. These are: Normal Treatment Mode (minimum night time flow to peak wet weather flow of 4000l/s) BTF Flushing Mode (once daily for each BTF at 375l/s for 40 minutes duration) Outfall Flushing Mode (once daily at 3200l/s for 10 minutes duration)

3 Each BTF is sequentially flushed for a period of 40 minutes once a day to control the biofilm inventory within the BTFs. The flushing of each BTF is undertaken during the late evening or night to avoid conflict with daily Outfall flushing and to coincide with lower influent flow and load. The timing of the flush (afternoon, evening, early morning) can be adjusted to select the desired incoming flow conditions for flushing. The Ocean Outfall is flushed daily to scour settled solids and trapped air. The Ocean Outfall flushing is undertaken mid-morning to coincide with the diurnal peak incoming flow (dry weather conditions Qi of 500 to 800l/s) to assist achieving the desired flush duration (10 minutes) and flow rate (3200l/s). Inlet Screening and Screenings Handling Description The original influent channels were modified to house the three relocated Suboscreens and screw conveyors from Musselburgh Pump Station. The Suboscreens are in-channel, rotating drum, fine screens (2mm slot apertures), which remove solids larger than 2mm. Screenings are washed, dewatered and compacted in three new Kuhn Wash Presses. During normal operation, Tahuna WWTP inflows range from a minimum night time flow of 220L/s, up to the maximum of 4,000L/s during wet weather. Each Suboscreen is capable of screening a flow rate up to 1,800L/s. As the flow increases towards 3,600L/s and the subsequent level in the forebay area rises, the third Suboscreen and wash press are brought into operation. Flows greater than the available capacity of the Suboscreens, overtop the Bypass Weir and are screened by a new Huber Trough Screen with 6mm diameter holes. HRAS Advanced Primary Treatment Process Overview During the Stage 2 upgrading ( ), the former primary sedimentation treatment plant was converted to grit removal and an HRAS (High Rate Activated Sludge) process including sedimentation. Grit Removal The original plant did not have a separate grit removal process. Grit was removed along with sludge and incinerated. In wet weather, the grit quantities increased substantially and due to the greater inorganic content, the sludge could not be incinerated. Because the proposed upgrading required recirculation of sludge and centrifuge dewatering, a grit removal stage was needed, as is normal practice to avoid excessive wear of equipment. The inlet end of the former rectangular PSTs (Primary Sedimentation Tanks) was modified with partition walls and flow distribution alterations, to become grit removal tanks. Grit settlement is achieved by inducing a roll current using coarse bubble aeration, which acts to sweep settled grit into the former sludge hoppers. Grit is pumped from the grit hoppers to the grit washer/classifiers on a timed sequence, where the grit is washed and dewatered. Dewatered grit is discharged to skips for disposal to landfill. HRAS Aeration After the Grit Tank, the wastewater enters the HRAS Aeration Tank which is a further section of the old PSTs modified with partition walls, and fitted with grids of fine bubble air diffusers. The HRAS aeration stage is a short retention, highly loaded aeration process that removes BOD by incorporating colloidal solids in the recirculated biomass. Some soluble BOD reduction occurs with associated biomass growth.

4 Very high speed K-Turbo aeration blowers supply air for the grit tanks and HRAS aeration tanks. The blowers do not have gearboxes. The direct-coupled motors have integral variable speed drives, the speed of which is controlled to provide total air required for the Grit and HRAS Aeration Tanks. The required air flow to each Grit Tank is fixed. The required air flow to each HRAS Aeration Tank varies according to the DO concentration. If a low DO concentration is detected, a greater air flow is required, and vice versa. Air is delivered from the blowers via a common discharge pipeline, and then dedicated lines with modulating control valves supply air to each tank. The modulating control valves are automatically controlled between 30 and 70% open, to maintain the DO setpoint to its associated tank. Air discharge from the Grit and HRAS Aeration Tanks is malodorous. Thus the tanks are fitted with low level covers and the air is continuously extracted to the odour control system. Refer to Photo 2. HRAS Sedimentation The remaining area of each former PST, and the existing travelling bridge, are used in the sedimentation section of the HRAS process. Travelling bridge scrapers are rare in NZ, but are common in Europe. Because the original units had required minimal maintenance (in comparison with chain and flight scrapers) and were in reasonable condition, it was decided to adapt the travelling bridge scraper to remove sludge from close to where it settles, rather than moving the sludge to one end of the tank. This required unique features to be designed. Sludge is pushed towards central walls by an angled scraper hung from the travelling bridge, where it is removed by multiple wall pipes that withdraw sludge as the angled scraper passes. The scraper is angled in both directions so that sludge is pushed to the central wall in both bridge travel directions. The bridge travels at the same speed in both directions. Two range finders on each bridge are set up to detect excessive bridge skew. The return activated sludge (RAS) suction manifolds in the middle of each tank have 12 intakes with the intake spacing varied to reflect the fact that the majority of the solids settle out in the inlet portion of the tank. Each intake has an air actuated pinch, which fails open on loss of air. One valve on a suction manifold is open at all times. Each valve opens when the range finders sense that the travelling bridge is in the opening range for that valve. Once the next valve has opened, the previously opened valve closes. Refer to Photo 1. The RAS pumps have variable speed drives, the speed of which is set so that the RAS flow rate is a percentage (in the range of 20 to 75%) of the influent wastewater flow. The RAS pumps have a combined discharge into the PST influent channel. A Tschuda floating screw scum collector is located at the inlet end of the HRAS sedimentation tank. These were supplied from Austria and this is thought to be the first use of these in Australasia. Scum that floats to the surface in the sedimentation tank, is moved to the new scum screw by the travelling bridge. The coarse screw can remove scum that comes from both directions; the sedimentation tank, and the aeration tank. The screws concentrate the scum around small scum troughs at one side of the tanks. The screw and trough floats so that the trough is always at a fixed distance below water level. A submerged scum pump, adjacent to the trough, discharges to the Gravity Tank Thickener (GTT). Refer to Photo 2. The original PSTs had a simple weir along the end wall of the tank. The overflow per metre length of weir was much higher than recommended to minimise solids carryover caused by upwelling currents below the weir. Submerged tube launder pipes were retrofitted to reduce the upflow velocity.

5 Co-Settlement of Flushed Solids from the BTFs The two BTFs (Biological Trickling Filters) are flushed for a period of 40 minutes daily to control the biofilm thickness. The flushed BTF effluent is returned back to the HRAS influent for removal of solids. The BTF flush is undertaken at night to coincide with low influent flows in order to minimise the effects of BTF flush solids on the HRAS process. Note that the mass of BTF solids flushed does not exceed the typical raw wastewater solids in the HRAS process during the morning peak flow. Biological Trickling Filter (BTF) Secondary Treatment Process The ocean outfall consent requires the following median BOD and TSS to be < 50mg/l and faecal coliform < 1,500cfu/100ml. The UV disinfected BTF effluent is predicted to comply with the above parameters. Wastewater from the HRAS process flows by gravity to the BTF Pump Station, for flow up to 1400l/s (the BTF/UV treatment design flow). A BTF bypass weir, with hypochlorite disinfection, spills flow in excess of 1400l/s directly to the Outfall Pump Station. The BTFs are loaded to maximum of 0.4kgBOD/cu.m/day, which is a relatively light loading rate. This allows for significant treatment of the wastewater to be achieved, as well as stabilisation of the biofilm solids. Effluent from the BTFs is fed to the UV disinfection channels without conventional secondary clarification. A key driver fot the process selection, was to avoid production of secondary sludge which is more difficult to dewater. UV Disinfection Because some biofilm solids are present in the BTF effluent, the UV disinfection dose is relatively high. Solids Processing The duty solids handling configuration comprises Gravity Belt Thickeners (GBT), flow balancing, GBT feed pumping, GBT polymer batching and dosing, thickened waste activated sludge pumping, thickened waste activated sludge storage, centrifuge feed pumping, centrifuge polymer batching and dosing, centrifuge dewatering and cake storage in an elevated hopper and load out to trucks. The standby solids handling configuration comprises the existing Gravity Tank Thickener (GTT), thickened waste activated sludge pumping, thickened waste activated sludge storage, centrifuge feed pumping, centrifuge polymer batching and dosing, centrifuge dewatering and cake storage in an elevated hopper and load out to trucks. The duty solids handling configuration is shown in the block flow diagram, Figure 2. Thickened sludge ranging from 2.5 to 3.0%DS (target concentration of 2.7%DS) is pumped from the TWAS storage tank to the centrifuges. The centrifuges are configured for duty/standby operation only. Thickened sludge from the TWAS tank can also be pumped to the existing DEWA dewatering belt press. The DEWA can be operated in parallel with the duty centrifuge if additional dewatering capacity is required. Target concentration for dewatered sludge from the centrifuges is 25% DS. Odour Control Because the Tahuna WWTP is located across the road from a large residential area, and is adjacent to major sporting facilities, a high standard of odour control is required refer to aerial Photo 3. Consequently, the aeration tanks, BTFs, sludge storage tanks and other equipment, were covered and ventilated to large bark biofilters for odour treatment. The two new biofilters supplement the two existing bark biofilters which have performed effectively

6 in handling foul air from existing covered processes and the sludge incinerator. Conclusion Standard ATV-DVWK-A 131E Dimensioning of Single-Stage Activated Sludge Plants, German Association for Water, Wastewater and Waste, May 2000 The new 1100m long ocean outfall constructed in Stage 1 of the Tahuna WWTP upgrading, resulted in a marked improvement in shoreline water quality. The Stage 2 upgrading from primary to secondary treatment with UV disinfection, provides further benefits by avoiding the use of chlorine disinfection and reducing the discharge of solids and oxygen demand load to the ocean. Acknowledgements The assistance of others in completing this major project successfully, is gratefully acknowledged: Brian Turner, DCC Project Manager William Cockerill, OCTA Programme Manager Chris Henderson, DCC Treatment Plant Manager Terry Dodd, DCC Process Engineer Richard Bennett and the team at MWH References Tahuna Stage 2 Project Design and Development Process Phase 2 Final Report, Final Options Assessment by MWH and CH2M Beca, May Sludge Removal Systems for Secondary Sedimentation Process Working Report of the ATV Specialist Committee 2.5 Sedimentation Processes, Korrespondenz Abwasser (KA), Issue 13, 1988, pg

7 1.2 m³/s HRAS By- Pass 2.6 m³/s Trickling Filter and UV By-Pass Trough Screen Relocated 6mm Hole Suboscreens 2mm Slot Gap (1) Existing Primary Sedimentation Tanks Grit Removal HRAS Aeration HRAS Sedimentation TF Pump Station Biotransformation Trickling Filters UV Lift PS UV Disinfection (4) Outfall PS 4.0 m³/s 2.8 m³/s (3) (3) (3) 1.4 m³/s (3) (3) (2) (3) (2) Screenings to Landfill HRAS Return Sludge BTF Flush PS Papatuanuku Junction Chamber Filtrate From Sludge Processing (2) Grit Washer Grit to Landfill To Sludge Processing HRAS Waste Sludge (2) (1) 4.0 m³/s To Ocean Outfall Wastewater Sludge / Biosolids / Ash Filtrate Flow values indicate the distribution of flow with an inlet flow of 4.0 m³/s ( ) indicates the number of process units in parallel Figure 1 - Liquid Stream - High Rate Activated Sludge with BTF and UV (no secondary clarifiers)

8 From HRAS GBT Balance Tank Gravity Belt Thickener TWAS Storage Tank Dewatering Centrifuges Dewatered Sludge Storage (1) (2) (1) (3) Conveyor (1) Dewatered Sludge to Green Island WWTP or Landfill Wastewater Sludge / Biosolids / Ash Filtrate ( ) indicates the number of process units in parallel Figure 2 - Solids Stream GBTs, centrifuges, sludge cake load out

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10 Photo 1 - Looking to the outlet end of a HRAS tank showing the travelling bridge scraper and sludge removal ports

11 Photo 2 - Looking to the inlet end of the HRAS tank showing the scum removal screw, closer spaced sludge removal ports and covered aeration tank in the background

12 Photo 3 - Aerial Photo of Tahuna WWTP

13 Author Biography and Photograph Humphrey Archer Humphrey has 40 years experience with specialist knowledge of wastewater treatment plants and a range of biosolids management methods. In addition, Humphrey has successfully facilitated consultation for resource consent procurement over a wide range of environmental improvement projects. He was involved in the preparation of the Dunedin Wastewater Strategy in 1992, and was the process design leader for upgrading both the major WWTPs in Dunedin -- Green Island ( ) and Tahuna ( ). Rob Crosbie Rob has 34 years experience covering a wide range of civil engineering projects. He was the Engineer to the Contract for the ocean outfall constructed in Stage 1 of the Tahuna WWTP upgrading and continued in that role for the Stage 2 upgrading of the treatment processes. In addition, he was Design Manager during the detailed design phase.