At the end of 2006 the City of Millbrae, California completed a $5.5 million wastewater treatment plant improvement project at no additional cost to the City's ratepayers. The project improved cogeneration facilities, provided an automated grease trap waste receiving station, improved digester heating, replaced the aging main electrical switchgear, and enhanced digester mixing. Costs for the project were not passed on to rate payers because the overall project is self-funding, which means that the savings and revenue generated by the project will be used to pay the debt service for the improvements. For the term of the loan, the City will spend the same amount of money it does each year for operations, except now a portion of that money will pay for plant improvements rather than paying for utility-purchased power. The project would not have been financially viable if grease trap waste receiving were not included. Grease trap waste receiving resulted in an increase in digester gas production and added revenues from tipping fees.

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This manuscript reports the first 30 months performance of POTW anaerobic digesters when feeding the pre-processed slurry produced from combining inedible kitchen grease (IKG) composed of fats, oils, and greases (FOG) with anaerobically digesting sewage solids to produce biogas for a combined heat and power (CHP) system. In 1993, the Millbrae facility began recording electronically solids processing data, such as the number of hauled loads, the wet ton weight of biosolids, polymer use relative to filter cake density/weight, electricity produced and bought, and digester stability. Comparison of these historical performance data with those of the first thirty months shows surprising results that yield benefits far greater than anticipated. These data indicate feeding pre-processed IKG into the Millbrae wastewater anaerobic digesters steadily and substantially increases the amounts of biogas produced, decreases the volume of biosolids disposed of by a sustainable 32%, improves the stability of digester operations, and improves biosolids dewaterability.