Kinetic Study of the Anaerobic Digestion of Swine Manure at Mesophilic Temperature: A Lab Scale Batch Operation

Abstract

Purpose: The kinetic evaluation was performed for swine manure (SM) degradation and biogas generation. Methods: The SM was anaerobically digested using batch digesters at feed to inoculum ratio (F/I) of 1.0 under mesophilic conditions (<TEX>$36.5^{\circ}C$</TEX>). The specific gas yield was expressed in terms of gram total chemical oxygen demand (mL/g TCOD added) and gram volatile solids added (mL/g VS added) and their effectiveness was discussed. The biogas and methane production were predicted using first order kinetic model and the modified Gompertz model. The critical hydraulic retention time for biomass washout was determined using Chen and Hashimoto model. Results: The biogas and methane yield from SM was 346 and 274 mL/ TCOD added, respectively after 100 days of digestion. The average methane content in the biogas produced from SM was 79% and <TEX>$H_2S$</TEX> concentration was in the range of 3000-4108 ppm. It took around 32-47 days for 80-90% of biogas recovery and the TCOD removal from SM was calculated to be 85%. When the specific biogas and methane yield from SM (with very high TVFA concentration) was expressed in terms of oven dried volatile solids (VS) basis, the gas yield was found to be over estimated. The difference in the measured and predicted gas yield was in the range of 1.2-1.5% when using first order kinetic model and 0.1% when using modified Gompertz model. The effective time for biogas production (<TEX>$T_{Ef}$</TEX>) from SM was calculated to be in the range of 30-45 days and the critical hydraulic retention time (<TEX>$HRT_{Critical}$</TEX>) for biomass wash out was found to be 9.5 days. Conclusions: The modified Gompertz model could be better in predicting biogas and methane production from SM. The HRT greater than 10 days is recommended for continuous digesters using SM as feedstock.