Optimization of synergistic microwave and zero–valent iron co–pretreatment for anaerobic digestion of waste activated sludge

This study optimized co-pretreatment of microwave temperature (T_MW) and zero-valent iron dosage ([ZVI]) to enhance anaerobic digestion (AD) of waste activated sludge (WAS). WAS was pretreated at T_MW = 100, 150, or 200 °C and [ZVI] = 1, 3, or 5 g/L using a central composite design. Optimal co-pretreatment (T_MW = 168 °C and [ZVI] = 5 g/L) reduced the ratio of volatile solids (VS) to total solids by 21.5 %, increased the solubilization ratio seven-fold, removed 53.5 % of phosphate compared to WAS partly because of lignin fragmentation. Biochemical methane potential identified optimal conditions (T_MW = 164 °C and [ZVI] = 4.8 g/L), enhancing VS removal by 70.9 %, methane yield by 60 %, and reducing hydrogen sulfide by 82.4 % compared to Control. Kinetic analysis indicated 61 - 108 % increase in maximum methane production rate. Microbial analysis revealed increased acetoclastic methanogens and decreased hydrogenotrophic methanogens. Thus, microwave-ZVI co-pretreatment enhanced WAS biodegradability and AD efficiency.