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 FOS/TAC ratio (volatile fatty acids to alkaline buffer capacity) compared to mixture of 50% MWAS and 50% FVW. Application of hybrid (MW-H2O2) pretreatment in the former mixing ratio increased sludge solubilisation 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 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.