Enhanced anaerobic digestion of waste activated sludge by alkaline protease-catalyzing hydrolysis: Role and significance of initial pH adjustment

Abstract

Anaerobic digestion has been regarded as a preferred technology for carbon/energy recovery from waste activated sludge via volatile fatty acids (VFAs) and methane production. Although the enzyme pretreatment has been investigated, specific roles of initial pH adjustment on alkaline protease (AP) catalysis and its influence on anaerobic digestion have been rarely reported. This work demonstrated that the increasing initial pH displayed positive mutual promotion of AP-catalyzing hydrolysis for facilitating sludge solid reduction and biodegradable organic matter solubilization. The release of carbon source and phosphorus were also promoted, while the soluble NH4+-N content was decreased at increasing alkaline conditions, which was beneficial for hydrolysate bio-utilization. Nevertheless, initial pH = 10 was the optimum condition for maximizing VFAs and methane recovery after AP-catalyzing sludge hydrolysis. Under such condition, the sludge solid reduction and sludge hydrolysis rate constants were 0.7981 h−1 and 1.1288 h−1, respectively, which were 6.40 and 2.45 times higher than those in the control. Meanwhile, the biochemical acidogenic potential of 447.0 mg COD VFAs/g VSS and the estimated biochemical methane potential of 437.6 mL CH4/g VSS were achievable. The identification of the mutual promotion distribution revealed that initial pH adjustment presented more close association with anaerobic digestion than AP catalysis and its contribution was amplified with increasing pH. Apparently, such AP catalysis with initial pH adjustment is a promising technology for sustainable sludge management with economic benefits and carbon-emission reduction.