The San Francisco Public Utilities Commission is interested in evaluating the economic feasibility of co-digesting various high strength waste material, mainly grease trap waste and food waste at their anticipated new anaerobic digestion facility expected to be operational at the Southeast Water Pollution Control Plant in 2022. High strength waste material increases the production of biogas when co-digested with municipal sludge, resulting in an additional renewable energy source that can be beneficially used. However, processing high strength waste at the new Southeast Plant's digestion facility results in increased digestion capacity, increased biogas recovery system capacity, increased dewatering requirements, and increased biosolids for hauling and beneficial use.The quantities and characteristics of yellow grease, grease trap waste and food waste available to the SFPUC were first estimated. Characterization of the waste was based on the commission's own work and available literature. The baseline anaerobic digestion process chosen for this evaluation was a temperature-phased anaerobic digestion (TPAD) process sized to process the projected Southeast Plant's 2045 biosolids loads. It was also assumed that the high strength waste would be pre-processed offsite and arrive at the Southeast Plant in a slurry form ready for anaerobic digestion.Various options for co-digesting the available high strength waste were considered and the impact on anaerobic digestion and biogas generation was quantified. Economic analysis compared the baseline TPAD digestion process with the considered options in terms of increased project cost, increased operation and maintenance costs, and increased electricity generation potential using co-generation technology.The results of the various co-digestion options considered in this study showed no economic benefit from co-digesting the available high strength waste. Strategy of co-digestion should be based on using the extra digestion capacity that will be available at the new facility's startup, and gradually curtailing high strength waste addition as the digestion facility reaches its design capacity in 2045. BACKGROUND

The Las Virgenes Municipal Water District/ Triunfo Sanitation District Joint Powers Authority (JPA) (Calabasas, CA) operates the Tapia Water Reclamation Facility (WRF), which currently treats an average dry weather flow of approximately 34,000 m 3 /day (9 MGD). In May 2010, the facility was required to comply with a new maximum monthly average NOx-N concentration limit of 8 mg-N/L. In anticipation of this lower limit, various minor low-cost improvements were implemented and plant staff modified the secondary process operations in an attempt to demonstrate that the facility could consistently comply with the future lower NOx-N limit. Although these changes resulted in the reduction of the effluent NOx-N to a range of 8-12 mg-N/L from 12-20 mg-N/L, further facility improvements were required.After an extensive analysis of the WRF secondary process and spare tanks available at the WRF and at the Rancho Las Virgenes Composting facility, the most cost-effective options to improve NOx-N removal were: (1) the implementation of RAS denitrification (endogenous) in spare tanks formerly used for aerobic digestion of primary and waste activated sludges at the WRF;(2) retrofitting tanks at the composting facility to equalize and biologically treat the high strength recycle stream generated by the dewatering of the anaerobically-digested sludge ("centrate"); and (3) conversion of the main secondary process to a 4-stage Bardenpho configuration. The combined effect of these three modifications resulted in the reduction in the Tapia WRF effluent NOx-N concentration to concentrations consistently below the new maximum monthly permit limit of 8 mg-N/L. Two existing raw sludge storage tanks at the composting facility were retrofitted for equalization and biological treatment of the centrate nitrogen load, which was contributing approximately 5 to 7 mg/L of ammonium-N to the plant influent or about 15% of the TKN load. The biological reactor is operated as a Sequencing Batch Reactor (SBR) with intermittent aeration. To avoid the purchase, storage and use of an external carbon source to enhance denitrification and generate alkalinity for nitrification in the SBR, a portion of the raw combined sludge was found to be a suitable "in-house" source of carbon for the process. Approximately 6.5% of the raw combined sludge is diverted to the centrate treatment process for this purpose. Ammonium-N and Total Inorganic Nitrogen (ammonium plus NOx-N) removal efficiencies up to 90% and 85%, respectively, were demonstrated.