Acidogenesis and Two-phase Codigestion of Fats, Oils, and Greases and Municipal Biosolids

Acidogenic codigestion of fats, oils, and greases (FOG) was studied using suspended sludge digesters operated as batch fed reactors that were fed twice daily. The digesters were maintained at a 2-day retention time and at 37°C. Volatile fatty acid (VFA) production was found to increase with larger FOG loading rates. This increase was insignificant compared to the theoretical VFA production from FOG addition due to inhibition. Long chain fatty acids (LCFAs) were found to have accumulated in the reactor vessel in semi-solid balls that were primarily composed of saturated LCFAs. Two-phase codigestion of FOG was studied at 37°C using SBRs as acid-phase (APD) followed by gas-phase (GPD) digesters operated with 2-day and 15-day retention times, respectively. The two-phase systems were compared by FOG addition to the APD versus GPD. FOG addition to the APD resulted in 88% destruction of LCFAs, whereas FOG addition to the GPD resulted in 95% destruction of LCFAs. Accumulated LCFAs were again found in the APD receiving FOG and were composed mostly of stearic acid (18:0).