UPGRADES TO POND-BASED WASTEWATER TREATMENT PLANTS TO IMPROVE PUBLIC HEALTH PROTECTION

Transcription

1 UPGRADES TO POND-BASED WASTEWATER TREATMENT PLANTS TO IMPROVE PUBLIC HEALTH PROTECTION Bridget O Brien and Humphrey Archer, CH2M Beca Ltd Abstract There are about 200 pond-based wastewater treatment plants in New Zealand. As the discharge consents for these come up for renewal, there is often pressure from key stakeholders and regulators to improve effluent quality. There are several options to improve the disinfection performance of pond-based wastewater treatment plants, and therefore improve public health protection. These include a series of maturation ponds, membrane filtration, wetlands and UV disinfection. This paper outlines recent s to three pond-based wastewater treatment plants (Patea, Dunedin Airport and Invercargill), and reviews their effectiveness at improving disinfection and the per capita cost of the s. Membrane filtration is a relatively new technology for s to pond-based wastewater treatment plants. It produces an excellent effluent quality and has been proven to be simple to operate at the Dunedin Airport wastewater treatment plant. Key Words Wastewater treatment, waste stabilisation ponds, disinfection, wastewater treatment plant s, public health Introduction There are about 200 pond-based wastewater treatment plants in New Zealand. As the discharge consents for these come up for renewal, there is often pressure from key stakeholders and regulators to improve effluent quality. The main function of a wastewater treatment plant is to provide public health protection and this paper focuses on various types of s to pond-based wastewater treatment plants (WWTPs) to improve disinfection and hence public health outcomes. membrane filtration, wetlands and UV disinfection. This paper outlines recent s to three pond-based wastewater treatment plants (Patea, Dunedin Airport and Invercargill), and reviews their effectiveness at improving disinfection and the per capita cost of the s. Case Studies Case studies for three different pond-based WWTP s are described below. There are several options to improve the disinfection performance of pond-based wastewater treatment plants, and therefore improve public health protection. These include a series of maturation ponds,

2 Patea Wastewater Treatment Plant Patea is a small town with a population of 1,143 (2006 census). The population declined significantly after the closure of the meat works in 1982, when the population of the town was around 2,000. The was completed in June 2008 and the total capital cost for the was $630,000, which equates to $551 per population equivalent (O Dempsey & Kuyl, 2010). In 1973, an oxidation pond was constructed on a terrace 20m above the Patea River, to treat wastewater before discharging into the tidal reach of the river. A pump station near the comminutor was used to lift the wastewater up to the WWTP. The pond had an area of 2.33ha and a design population equivalent capacity of 2,900. The river and river mouth are used by the local community for swimming, fishing and boating. Shellfish gathering and surfing are popular activities at Patea Beach. A key driver for the to the Patea WWTP was to improve public health outcomes, given the popularity of the receiving environment for recreational activities. The involved adding a mechanical screen, dividing the pond into three ponds in series, partially desludging the pond, installing a geotextile liner in Pond 1 and replacing the river outfall pipe with an in-bank rock diffuser. As the current population was significantly lower than the original design population for the pond, and is not expected to increase, it was possible to create three ponds in series within the existing footprint, to prevent shortcircuiting from the inlet to the outlet (see Figures 1 and 2). The bunds were constructed by end dumping fill material, allowing the WWTP to remain operational throughout the. Figure 1: Patea WWTP showing internal dividing bunds. The combined pond area is 2.2ha. At the average design flow rate of 455m³/day, the retention time is 55 days. Figure 2: Patea WWTP plan layout of.

3 Dunedin Airport Wastewater Treatment Plant Dunedin Airport is situated on the Taieri Plains approximately 30km south of Dunedin s CBD. It is remote from reticulated systems serving Dunedin City. Wastewater from the Airport Terminal and the nearby village of Momona, is conveyed to a standalone STP operated by Dunedin International Airport Ltd. The d STP retained the Imhoff Tank and ponds, but these were augmented with a trickling filter for ammonia nitrogen reduction, and a membrane filtration unit for faecal indicator organism reduction (see Figures 3 and 4). The design population equivalent for the was 633 and the peak flow rate for the membrane filtration unit was 130L/s. The cost $304,000, which equates to $480 per population equivalent (Archer, Roberts & Simpson, 2011). Prior to upgrading in 2007, the WWTP comprised an Imhoff Tank and four waste stabilisation ponds in series. The STP discharged to a land drainage system (Main Drain) which flows into Lake Waihora that is surrounded by nationally significant wetlands. The key driver for the to the WWTP was to significantly improve the effluent quality so that the discharge achieved recreational quality guidelines for indicator bacteria. Other improvements in effluent were sought in terms of ammonia nitrogen, BOD and suspended solids, to reduce the effects on the sensitive receiving environment. Raw Wastewater Raw Wastewater Imhoff Tank Imhoff Tank Waste Stabilisation Ponds in Series Prior to Upgrading Trickling Filter 1 After Upgrading Waste Stabilisation Ponds in Series Discharge to Main Drain Membrane Filtration Figure 4: Dunedin Airport WWTP simplified process flow diagrams, before and after. Invercargill (Clifton) Wastewater Treatment Plant Discharge to Main Drain The Invercargill WWTP at Clifton treats wastewater from Invercargill and has a design population of 75,000, including 25,000 population equivalent for industrial wastewater. Before the in 2003, the WWTP consisted of a mechanical screen, a preaeration tank for grit removal, three primary sedimentation tanks, two trickling filters and a secondary clarifier. The wastewater discharged to the New River Estuary. Figure 3: Dunedin Airport WWTP Membrane Filtration module. A key driver for the to the Clifton WWTP was to improve public health outcomes, as the estuary is used for recreational activities.

4 The involved constructing a series of four maturation ponds in series with a combined area of 13.2ha, followed by two wetlands in parallel with a combined area of 7.6ha (see Figures 5 and 6). The retention time in the maturation ponds and wetlands at the average design flow rate of 25,000m³/day is 12 days, which is relatively short. The was completed in August The cost for the was $2.9M, which equates to $38 per population equivalent. Results Patea Wastewater Treatment Plant As required by the resource consent for the discharge from the Patea WWTP, the effluent quality and receiving environment water quality is measured three times per year by the Taranaki Regional Council (TRC, 2012). The faecal indicator results before and after the are summarised in Table 1. Figure 5: Clifton WWTP plan layout of. It can be seen that there has been a dramatic improvement in the effluent quality following the. In their annual consent monitoring report, TRC (2012) state The effect of the oxidation pond s effluent discharge generally has been limited to occasional small rises in bacteria numbers near the right bank Patea River site immediately upstream or downstream of the discharge (dependant on tide conditions) with bacterial water quality measured a further 600m downstream usually similar to that measured upstream of the discharge at SH3 bridge. Figure 6: Clifton WWTP wetlands after.

5 Table 1: Patea WWTP Effluent Quality Before and After Upgrade Table 2: Dunedin Airport WWTP Effluent Quality Before and After Upgrade Faecal coliforms Enterococci Faecal coliforms before Faecal coliforms after Before After Before After Number of samples Number of samples Minimum 20 1 Minimum Median 26, , Median 9, th percentile 74, th percentile 146, ,500 1,040 Maximum 3,700, Maximum 190, ,000 1,200 Invercargill (Clifton) Wastewater Treatment Plant Dunedin Airport Wastewater Treatment Plant As required by the resource consent for the discharge from the Dunedin Airport WWTP, the effluent quality is measured monthly. The faecal indicator results before and after the are summarised in Table 2. It can be seen that the WWTP has produced a very good effluent quality since the, and the concentration of faecal coliforms has always been below the resource consent limit of 260cfu/100mL. As required by the resource consent for the discharge from the Clifton WWTP, the effluent quality is measured weekly. The faecal indicator results before and after the are summarised in Table 3. It can be seen that the WWTP has produced a reasonable effluent quality since the, although the concentrations of faecal coliforms and enterococci are still quite variable. While the WWTP generally complies with its effluent faecal coliforms limit of a rolling 12 month geometric mean of 6,000cfu/100mL, this was exceeded on occasions when the Clifton Wool Scour discharged to the WWTP. Consent limits were complied with after the wool scour closed in 2009, with a significant improvement in final effluent quality since the closure of the wool scour (see Table 4). Oxidised wool grease compounds are known inhibitors of UV

6 light, which affects the performance of both UV lamp and sunlight disinfection. Table 4: Invercargill WWTP Effluent Quality After Upgrade and Closure of Wool Scour Table 3: Invercargill WWTP Effluent Quality Before and After Upgrade Faecal coliforms Enterococci Faecal coliforms Enterococci Number of samples Minimum 10 5 Before After Before After Median Number of samples th percentile 4, Maximum 24,196 4,700 Minimum 30, ,000 3 Median 1.3x10 6 1, x Conclusion 90 th percentile 3.17x , x10 5 3,600 Maximum 25x x x x10 5 The effluent quality for three case studies for different types of pond-based WWTP s was compared in terms of disinfection of pathogenic indicators. The best effluent quality was achieved by the addition of membrane filtration after ponds (Dunedin Airport WWTP), as this provides a physical barrier which largely prevents the discharge of pathogen indicators. The next most effective was dividing an existing pond into multiple ponds in series (Patea WWTP). It is likely that this was effective due to the serpentine flow path preventing short circuiting, combined with a very long retention time of 55 days at average flows. The least effective was constructing multiple ponds in series followed by wetlands (Invercargill WWTP). While the pond and wetland layout prevents short circuiting, the effectiveness of disinfection is limited by the relatively short retention time of 12 days at average flows. The effluent quality improved significantly following the closure of the wool scour in 2009, as oxidised wool grease

7 compounds are known inhibitors of UV light and so were affecting disinfection performance. References Archer, HE, Roberts, R and Simpson, S, Membrane Filtration Retrofit to Dunedin Airport Waste Stabilisation Pond. IWA 9th International Waste Stabilisation Pond Conference, Adelaide, Australia, O Dempsey, BM and Kuyl, VL, Time is of the Essence to a Coliform Reduced Presence. Proceedings of Water NZ Conference, Taranaki Regional Council, South Taranaki District Council Waverley, Kaponga, Manaia and Patea Municipal Wastewater Treatment Plant Systems Monitoring Programmes Annual Report , Technical Report

8 Author Biography Bridget O Brien Bridget is an Associate Environmental Engineering at Beca and has 12 years experience in environmental engineering projects, focussing particularly on assessments, consenting, designing and upgrading pond-based wastewater treatment plants. She has also been project manager for a wide range of environmental engineering projects. Contact details: CH2M Beca, PO Box , Christchurch 8141, bridget.obrien@beca.com Humphrey Archer Humphrey is a Technical Director Environmental Engineering at Beca has 40 years experience with specialist knowledge of wastewater treatment plants, including waste stabilisation ponds. In addition, Humphrey has successfully facilitated consultation for resource consent procurement over a wide range of environmental improvement projects. Contact details: CH2M Beca, PO Box , Christchurch 8141, humphrey.archer@beca.com