The effect of high carbon co-substrates on the stability and performance of anaerobic co-digestion in the batch reactor

In this study, the anaerobic co-digestion of a mixture of sawdust, chlorella microalgae and cow manure was examined. Sawdust was added to the mixture to increase the amount of carbon available. This study has been designed to determine the impact of incorporating cellulosic material as a rich source of carbon into main substrates. Using co-digestion to create a C/N gradient in the influent, a relationship between substrate composition and biogas production was investigated. During this study, 19 reactors of 500 ml capacity were filled to a level of 350 ml and operated at a mesophilic temperature of 37.5 Celsius degree. Total solids (TS) in the digester feed were 3.8% and retention time in the reactors was 30 days. Design Expert 13 software and mixture design was used to design the experiments. Adding sawdust to the reactors (7%-10%) improved the carbon/nitrogen (C/N) ratio of the mixture in the reactors to an optimal value (22) and produced 25%-35% more biogas. The maximum average amount of gas produced per day from a mixture of 8.3% sawdust, 33.3% algae and 58.3% cow manure was 231 ml. Adding sawdust when the TS is low in organic load prevents the reactors from acidifying, increases the stability and improves biogas output. Highlights · Adding high carbon co-substrate improved the anaerobic co-digestion process of algae and animal waste. · Mixture design method on RSM was successfully applied to predict the responses for AD process. · Adding sawdust to the reactors improved the C/N ratio, stability of the reactors, and biogas production. · The maximum biogas production was increased by nearly 35%.