Synergism and effect of high initial volatile fatty acid concentrations during food waste and pig manure anaerobic co-digestion

• Co-digestion of food waste with pig manure had synergistic effects on methane yields and kinetics.
• Butyrate degradation was inhibited.
• High initial VFA concentrations affected kinetic modelling.
• Dual-pooled first order model provided the best fit to the data.

Anaerobic co-digestion of food waste (FW) and pig manure (PM) was undertaken in batch mode at 37 °C in order to identify and quantify the synergistic effects of co-digestion on the specific methane yield (SMY) and reaction kinetics. The effects of the high initial volatile fatty acid (VFA) concentrations in PM on synergy observed during co-digestion, and on kinetic modelling were investigated. PM to FW mixing ratios of 1/0, 4/1, 3/2, 2/3, 1/4 and 0/1 (VS basis) were examined. No VFA or ammonia inhibition was observed. The highest SMY of 521 ± 29 ml CH4/gVS was achieved at a PM/FW mixing ratio of 1/4. Synergy in terms of both reaction kinetics and SMY occurred at PM/FW mixing ratios of 3/2, 2/3 and 1/4. Initial VFA concentrations did not explain the synergy observed. Throughout the study the conversion of butyric acid was inhibited. Due to the high initial VFA content of PM, conventional first order and Gompertz models were inappropriate for determining reaction kinetics. A dual pooled first order model was found to provide the best fit for the data generated in this study. The optimal mixing ratio in terms of both reaction kinetics and SMY was found at a PM/FW mixing ratio of 1/4.