4. Conclusion
Co-digestion of mixed waste sludge with 25% FVW showed enhanced process stability and biomethanation compared to co-digestion with 50% FVW. Increased acidification observed in thermophilic phase I of co-digestion (1:1) led to process instability and consequently, 33% less methane yield in subsequent phase II compared to co-digestion (3:1). Application of hybrid (MW-H2O2) pre-treatment significantly enhanced the methane production and quality in co-digestion (3:1). It increased the initial sludge solubilisation by 33% and facilitated 2.91-fold higher methane production in thermophilic phase-I compared to untreated digestion. Besides, the biogas quality was also improved with an average of 1.55 CH4/CO2 ratio in the treated digester. The process stability was enhanced in treated digester with an average FOS/TAC of 0.26 in phase-II and helped achieve a 2.17-fold higher overall methane yield compared to the untreated digester. The improved process stability in the treated digester is mostly attributed to buffer capacity exhibited by biopolymers released during the treatment. A study on the oxidative status of the current anaerobic digestion revealed that superoxide radicals negatively correlate with sludge bioactivity. Further investigations on free radical production in anaerobic digestion could establish the mechanisms of free radical generation and their significance in anaerobic digestion. Overall these results reveal the high potential of co-digestion of FVW with mixed activated sewage sludge to enhance the performance of anaerobic digestion and overcome some of its fundamental operational limitations.