Optimize Sustainability – Case Study on How to Transform into a Water Resource Recovery Facility
- Scott A. Hardy - Hazen and Sawyer
Sustainability and Resource Recovery is the new paradigm for wastewater treatment plants. Two tangible means of increasing sustainability are to produce renewable energy using anaerobic digestion and recover phosphorus through struvite precipitation reactors. This paper will present a case study of how to optimize current plant processes with the addition of some new processes to transform your wastewater treatment plant into a Water Resource Recovery Facility.
The F. Wayne Hill Water Resources Center is Gwinnett County, Georgia’s advanced wastewater treatment facility that has a design capacity of 60 million gallons per day (mgd) and is currently operating at an average flow of 30 mgd. The facility has primary clarification followed by biological nutrient removal (BNR) activated sludge process. Primary sludge and waste activated sludge are digested in five 1-million gallon egg-shaped anaerobic digesters.
To produce renewable energy, the county installed a 2.1 MW combined heat and power (CHP) engine generator in 2011 using the American Recovery and Reinvestment Act funding. The engine generator uses digester gas as a fuel to produce power to offset the plant’s purchased power and heat for digester heating.
The full load digester gas requirement to run the engine generator is about 523 standard cubic feet per minute (SCFM); however, the current gas production at the plant was around 250 SCFM. To increase digester gas production, the F. Wayne Hill WRC implemented the following optimization strategies:
1. Primary Clarifier Removal Performance Optimization – Approximately 60% of the volatile solids (VS) in primary sludge can be converted to digester gas, whereas, typically only 30% of VS is waste activated sludge (WAS) is converted to digester gas. Maximizing primary sludge loading to the digesters has a major impact on digester gas production. H&S performed an optimization study that included computation fluid dynamic (CFD) modeling that resulted in increase total solids removal through the existing primary clarifiers from negative removals to 50% total solids removal.
2. Fat Oil and Grease (FOG) and High Strength Waste (HSW) Co-Digestion – GCDWR installed a FOG and HSW receiving facility to add highly digestible, high concentrated organic waste streams to increase digester gas production and provide an additional revenue stream from tipping fees. Bench-scale digestion studies were performed to evaluate which FOG and HSW produced the most digester gas per gallon.
3. Increasing Digester Solids Residence Time (SRT) – Increasing the SRT in the anaerobic digesters increases the volatile solids destruction and digester gas production. Also, with the increase in primary sludge production and FOG co-digestion, the hydraulic loading to the existing digesters was increasing and lowering the SRT. To de-bottleneck the anaerobic digester, co-thickening of WAS and primary sludge is being implemented to increase solids concentration from an average of 2.5% to 5.5% TS, thus increase the solids retention time by over 50%.
4. Accept Other WWTP Solids – Gwinnett recently completed expansion of their Yellow River WWTP and decided to send 14 mgd worth of primary sludge and WAS to the F. Wayne Hill WRC through the collection system, which increases the influent loading to the plant. This emphasized the importance of properly working primary clarifiers and the additional loading to the anaerobic digesters for gas production.
Another means of increase sustainability of the facility is to recover the phosphorus removed in the BNR process to make an inorganic fertilizer product. This system not only decreases phosphorus loading in the recycle stream, which helps optimization the BNR process, but decreases maintenance by preventing struvite formation. The process removes phosphorus from the waste activated sludge to produce an inorganic, fertilizer grade, end product. Pilot results of two phosphorus recovery technologies will be presented with a current update on design of a new Nutrient Recovery Facility.
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