Anaerobic co-digestion of foodwaste with liquid dairy manure or manure digestate: Co-substrate limitation and inhibition.

Process instability has been a challenge to anaerobic digestion of foodwaste at higher organic loading rates. Co-digestion is one of the measures to improve stability. This study conducted batch experiments to compare liquid dairy manure and dairy manure digestate as a co-substrate for anaerobic digestion of foodwaste. The batch co-digestion experiments showed a two-stage biogas production process, which could be simulated with a modification of the Gompertz model. The specific biogas yields derived with the two-stage biogas production model was further simulated against the co-substrate ratios with substrate limitation - inhibition models for identifying the optimal co-substrate ratio. The Haldane model was the best to simulate co-substrate limitation - inhibition kinetics in anaerobic co-digestion of foodwaste. A higher ratio of dairy manure could result in co-substrate inhibition to biogas production due to recalcitrance of cellulose and toxicity of lignin and lignin derivatives. Kinetic modeling shows that the optimal volatile solids (VS) ratio of liquid dairy manure is 16.6%, at which the maximum specific methane yield is 0.54 L/g VS. Semi-continuous co-digestion of 88% foodwaste and 12% liquid dairy manure at a hydraulic retention time of 14 d attained 94% of the simulated maximum methane yield. Although co-digestion of foodwaste and manure digestate resulted in lower biogas yields than co-digestion with liquid dairy manure, manure digestate is still an attractive co-substrate that has several operational advantages compared with liquid dairy manure.