Impact of Free Long Chain Fatty Acids on Completely Mixed Anaerobic Digesters Treating Municipal Sewage Sludge

In this study, different dosages of high purity long chain fatty acids (LCFA) including linoleic acid, oleic acid, and a mixture of oleic acid and stearic acid were added to laboratory completed mixed anaerobic digesters by the percentage of the total solids (TS) in weight. After reaching steady state, the performance of these digesters was compared with those without LCFAs addition. Although higher residual solids and COD were observed in the effluent of the reactors with LCFA addition, higher biogas production was found due to over 90% of degradation of added LCFA. A higher dosage of LCFA resulted in higher biogas production, solids destruction and COD destruction, but also caused increased residual solids and COD. For reactors with the same dosage of LCFA addition, linoleic acid addition led to more solids destruction and less residual solids and COD at a concentration up to 20%. Up to 30% of pure linoleic acid, oleic acid and mixed acid addition are able to enhance the performance of anaerobic digesters. However, it is recommended that the dosage of LCFAs be below 30% to avoid LCFAs accumulation and to increase reactor stability. Palmitic acid as well as stearic acid is the rate limiting intermediates of oleic acid degradation, while oleic acid is an intermediate of linoleic acid degradation. A higher dosage of LCFA addition led to greater accumulation of degradation intermediates.