Codigestion Study Helps Turn Industrial Pre-Treatment Problem to Renewable Energy Benefit
- Scott Hardy, Brandt Miller - Hazen and Sawyer
- David Ornelas - El Paso Water Utility
Introduction and Background
El Paso Water (EPW) noticed a spike in influent biochemical oxygen demand (BOD) at the influent of one of their treatment plants that was causing process issues in the Spring of 2015. EPW, which treats on average 60 mgd of wastewater at four treatment facilities, deciphered that the spike in BOD coincided with the start of a new biofuels facility discharging high strength waste (HSW) into their collection system. EPW contracted with Hazen and Sawyer (Hazen) to investigate and quantify process impacts of this new waste stream on their treatment plant. Treatability studies performed by Hazen identified that the HSW was non-toxic to the activated sludge process but required EPW to run an additional blower at the treatment plant which increased their operational cost. In excess of 100,000 mg/ L COD, the HSW seemed to be an excellent candidate for co-digestion which if viable could save EPW aeration costs at the treatment plant and generate electricity with their existing combined heat and power (CHP) system. With excess anaerobic digester capacity and needing additional digester gas to maximize the beneficial use of their CHP system, EPW initiated a co-digestion study with Hazen to explore co-digesting the HSW at their 27.7 mgd Haskell Street Wastewater Treatment Plant (HSWWTP). In addition to the HSW, EPW wanted Hazen to investigate the co-digestability of a sugar waste (SugW).
The scope of the study consisted of periodic sampling and characterization of the HSW and sugar waste to determine any variability on the HSW characteristics, bench-scale ultimate digestibility testing, digester capacity analysis and projection of digester gas production. The bench-scale ultimate digestibility testing was performed by Georgia Institute of Technology (GA Tech).
The HSW was sampled 8 times in a period of two months and characterized by EPW. The HSW is high in soluble COD, which should not result in increased solids production. The pH is around 8, which should help maintain stable digester pH. The ammonia is highly variable and ranges from 2 to 2,700 mg/L NH4-N; however, the average ammonia loading based on available volume of the HSW is minimal compared to the influent ammonia loading of the plant. Issues with measuring volatile solids of the HSW, which resulted in high total COD:VS ratios, will also be discussed.
Bench-Scale Ultimate Digestibility Testing
The primary sludge (PS), waste activated sludge (WAS) , HSW and SugW were tested individually and together to determine the ultimate digestibility.
When each waste stream is digested separately with a seed from the HSWWTP digesters, the SugW has the highest degradability > 95% while the HSW was only 42% degradable. The combined primary and waste activated sludge showed a degradability 50%. However, when all of the wastes are combined, the HSW digestibility increased to 90% assuming the same degradability of the sugar, primary sludge, and WAS. Previous studies have shown that additional degradation of the PS and WAS is achieved during codigestion, which would decrease the degradability of the HSW to about 84%.
Using the ultimate digestibility data, the additional digester gas production from the co-digestion of the HSW alone was estimated using the availability of HSW and impacts to digester solids residence time. The HSW is projected to triple the current digester gas production. The increase in biogas production would allow the existing 1 MW cogeneration system to be utilized on a fulltime basis and has the potential to save approximately $600,000 per year in power costs for EPW.
Following the bench scale results, a full scale pilot in one of HSWWTP anaerobic digesters is planned to confirm digester stability and gas production while the design of the HSW receiving system is being completed. Preliminary results from the full scale will be presented.
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