Determining the limits of anaerobic co-digestion of thickened waste activated sludge with grease interceptor waste

Anaerobic co-digestion of thickened waste activated sludge (TWAS) with grease interceptor waste (GIW) from a food service establishment was conducted in lab scale semi-continuous digesters. GIW included the entire contents of the grease interceptor (GI) including fat, oil, and grease (FOG), food residuals, and associated wastewater. GIW was added in step increases to identify the maximum methane production and the corresponding threshold input of GIW that led to inhibition of methanogenesis. The experiment was performed at mesophilic conditions (37 °C) with a solids retention time (SRT) of 20 days. The highest GIW addition rate achieved without digester failure was 20% (v/v), or 65.5% (w/w) of volatile solids (VS) added, enhancing the methane yield from 0.180 to 0.752 m3(CH4)/kg(VS added), biogas production from 2.2 × 10(-3) to 1.4 × 10(-2) m(3)/d, and methane content from 60.2% to 70.1%. The methane yield of 0.752 m3(CH4)/kg(VS added) is the highest value reported to date for co-digestion of GIW. Stepwise increases in co-substrate addition led to better microbial acclimation and reduced the GIW inhibitory effect. The limit for GIW addition leading to an inhibited digestion process was identified to be between 20 and 40% (v/v) or 65.5 and 83.5% (w/w) of VS added. The results show the significant benefits of anaerobic co-digestion of GIW and the positive impacts of gradual addition of GIW.