Effect of pre-treatment on sequential anaerobic co-digestion of chicken litter with agricultural and food wastes under semi-solid conditions and comparison with wet anaerobic digestion

•Increased biogas yield of substrates using sequential alkaline/acid pre-treatment.•Anaerobic digestion potential of semi-solid wastes at 15% TS loading.•Effect of pre-treatment on semi-solid anaerobic digestion.•Correlation of biogas production with the digestate lignocellulose fraction.•Comparison of wet and semi solids anaerobic digestion biogas production. Sequential co-digestion batch assays were conducted using feedstocks of chicken litter (CL), food waste (FW) and wheat straw (WS) mixed to a C/N ratio of 26.5 and 15% TS. Untreated mixture produced biogas of 321.6 ± 13.4 mLN/g VSadded which improved up to 50% when either CL or WS pre-treated substrates were fed. However, when both pre-treated CL and WS were fed, 80% and 88% increase in total biogas were found with associated VS removal of 49% and 55%, respectively, for alkali and sequential acid pre-treatment. Also, reactors received pre-treated substrates showed reduction in ammonia and digestate cellulose fraction with an increase in water soluble contents. Biogas production using sequential AD at 15% was almost 38% less than BMP biogas at 4%, however this was negated with the pre-treatment indicating that co-digestion at high TS of 15% is achievable. Further testing is required to confirm these results under semi-continuous conditions.