Anaerobic co-digestion of activated sludge and fruit and vegetable waste: Evaluation of mixing ratio and impact of hybrid (microwave and hydrogen peroxide) sludge pre- treatment on two-stage digester stability and biogas yield

Co-digestion of mixed waste activated sludge (MWAS) and fruit and vegetable waste (FVW) was studied in a two stage (thermophilic followed by mesophilic) semi-continuous anaerobic digestion to evaluate anaerobic digester performance and stability. A mixing ratio of 75 % MWAS and 25 % FVW showed a 1.6-fold increase in overall methane yield and achieved 0.38 volatile fatty acids to alkaline buffer capacity ratio (FOS/TAC) compared to mixture of 50 % MWAS and 50 % FVW. Application of hybrid (MW-H2O2) pretreatment in the former mixing ratio increased sludge solubilization by 33 % and consequently enhanced overall methane yield by 2.17-fold. The treated digester showed increased process stability with a FOS/TAC ratio of 0.26 as a consequence of buffer capacity offered by released biopolymers during pre-treatment. The generation of superoxide radicals during anaerobic digestion was studied and found to negatively correlate with sludge bioactivity. Two-stage digestion also minimizes the issue of high acidification due to co-digestion involving FVW.