Restaurant Trap Waste Characterization and Full Scale FOG Co-Digestion at the San Francisco Oceanside Plant

In recent years there has been growing interest in co-digestion of grease trap wastes (GTW) from food service establishments for enhanced biogas production. Even though numerous full-scale studies have been reported, there is a paucity of reliable characterization data for GTW or fats, oils, and grease (FOG) available in the literature. This paper presents findings from a full-scale co-digestion at the Oceanside Wastewater Treatment Plant in San Francisco, CA. During this study, best practices for GTW sampling and GTW characterization were identified and hundreds of samples were analyzed. The GTW received at OSP had an average total volatile solids (TVS) concentration of 5.6% and an average COD concentration of 14,837 mg/L. Over a 7-month period, there was an average increase of 27% in methane gas production when FOG injection represented an average of only 6% of the total volatile solids loading to the digesters. This paper also documents that there was no negative impacts on the overall digester performance despite a FOG loading representing up to 47% of the total volatile solids fed to the digesters.