Effect of high-carbon co-substrates on anaerobic co-digestion stability and performance

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 available carbon. 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 carbon/nitrogen (C/N) gradient in the influent, a relationship between substrate composition and biogas production was investigated. In 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°C. 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 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.