Most of the wastewater treatment plants are being upgraded to serve as water resource recovery facilities, reducing and in some cases eliminating, the dependence on the electricity grid for energy requirements remains one of the preeminent targets. Methane yields from conventional anaerobic digestion systems are not often sufficient to fulfil the requirements of these modern-day resource recovery facilities. Although there are numerous articles on co-digestion of municipal sludge with fats, oils and grease (FOG) waste reporting noticeable increase in biogas production, there are enough reports of inhibition and digester upset to warrant further study. Our present study is focused on characterizing formation and consumption of intermediates at various stages occurring in biochemical methane potential tests. This information is crucial in understanding the nature and the mechanism of inhibition. Preliminary studies were conducted to find a sludge to FOG ratio which enabled us to produce maximum possible biogas and use that ratio for setting up bench-scale digesters for further investigations. Among all the compositions tested, FOG 25 was the only reactor to overcome inhibition after lag phase of 45 days. We also found that FOG-acclimated culture improved the performance of the digester by increasing cumulative methane production by 29.9%, during batch operation. When operated in semi-continuous mode, methane production rate per g-VS added in FOG 25 was 7.5% greater than control and %COD conversion to methane increased by 30.34%. For samples with FOG content higher than 25%, methanogenesis was inhibited, with very little methane produced. These results suggest possible recovery of digesters inhibited after a certain lag phase and contradicts earlier hypothesis of LCFA inhibition to be irreversible. Results obtained from the VFA and LCFA analysis suggest that accumulation of palmitate and stearate at high concentrations can be inhibitory to the methanogens as well as the microbes degrading VFAs and LCFAs. Additionally, absence of fatty acids with carbon chains of 14\u3en(C)\u3e6 could be indicative of degradation mechanisms other than β-oxidation
