Effect of feed-to-inoculum ratio on anaerobic digestibility of high-fat content animal rendering wastewater

Anaerobic treatment of high-fat content wastewater could produce significant methane yields, making anaerobic treatment a promising alternative to energy-consuming aerobic treatment. However, accumulated intermediate LCFA may inhibit the conversions of both SCFA to acetate and of acetate further to methane. The effect of feed-to-inoculum ratio was evaluated in biochemical methane potential (BMP) tests for anaerobic digestibility of animal rendering wastewater (RW). Results showed that more than 90% of RW COD was converted during BMP tests with volumetric feed-to-inoculum ratio of 1. Higher feed-to-inoculum ratios led to RW COD conversion efficiencies below 64% and caused lag periods of >6 days in methane production. Degradation of acetate at different time points in relation to conversion of LCFA suggested that LCFA accumulation partially hindered acetate utilization. The methanogenic community shifted from hydrogenotrophic to acetoclastic methanogenesis in moderately-loaded BMPs. At highest feed-to-inoculum ratio of 9, acetoclastic methanogens were severely inhibited. The bacterial genus Syntrophomonas was present in increasing relative abundance with increasing feed-to-inoculum ratio. Increased relative abundance of the genus Desulfovibrio with increasing feed-to-inoculum ratio was likely a response to the high sulfate concentration in RW. •Anaerobic digestibility of rendering wastewater was evaluated in BMP tests.•Higher feed-to-inoculum ratios reduced methane production rate.•LCFA accumulated in highest feed-to-inoculum ratio tested.•Syntrophomonas relative abundance increased with increasing feed-to-inoculum ratios.•Acetoclastic methanogens were inhibited at higher feed-to-inoculum ratios.