Determination of the inhibitory concentration level of fat, oil, and grease (FOG) towards bacterial and archaeal communities in anaerobic digestion

Full-scale application of anaerobic digestion (AD) of any substrate is made possible through identifying its impact in terms of biomethane inhibition and optimum levels along with degradation and microbial community composition. This study focused on utilisation of FOG in AD by investigating varying FOG amounts to determine optimum and inhibitory concentration levels; in addition, we also studied its effect on microbial populations and biomethane production. Increased FOG levels extended the lag phase and eventually displayed complete inhibition. Operating AD with FOG levels in the concentration range of 0.1–1.5% (v/v) enhanced biomethane production, resulting in a 2 to 19-fold increase. The highest biomethane production was observed at 1% FOG (19-fold increase), with a lag phase of 25 days. LCFAs were degraded by 80–90% during AD, following a 0.5–1.5% FOG loading. An addition of either 2 or 3% FOG permanently inhibited biomethane production due to high VFA accumulation (17–19 g/L) and low LCFA reduction (29 and 18%), respectively. Under optimum biomethane production (1–1.5% FOG), Syntrophomonas and Fermentimonas were abundant, indicating their role in LCFA degradation and acetogenesis. Methanosaeta archaea were initially dominant, owing to the increased presence of LCFAs, but were replaced by Methanosarcina after the conversion of LCFAs to acetate and other acids. The current study proposes to run AD at around 1% (v/v) FOG loading to achieve maximum yield. To avoid inhibition during high FOG loadings (>1.5%), this would require microbial acclimatisation or prior substrate treatment.