Combined electrical-alkali pretreatment to increase the anaerobic hydrolysis rate of waste activated sludge during anaerobic digestion

[Display omitted] •Combined electrical-alkali pretreatment for improving sludge anaerobic digestion was proposed.•Combined process enhanced the cell lysis, biopolymers releases, and thus sludge disintegration.•Increased solubilization of sludge increased the anaerobic hydrolysis rate.•Increased solubilization does not always induce an improved anaerobic digestion efficiency. Pretreatment can be used prior to anaerobic digestion to improve the efficiency of waste activated sludge (WAS) digestion. In this study, electrolysis and a commonly used pretreatment method of alkaline (NaOH) solubilization were integrated as a pretreatment method for promoting WAS anaerobic digestion. Pretreatment effectiveness of combined process were investigated in terms of disintegration degree (DDSCOD), suspended solids (TSS and VSS) removals, the releases of protein (PN) and polysaccharide (PS), and subsequent anaerobic digestion as well as dewaterability after digestion. Electrolysis was able to crack the microbial cells trapped in sludge gels and release the biopolymers (PN and PS) due to the cooperation of alkaline solubilization, enhancing the sludge floc disintegration/solubilization, which was confirmed by scanning electron microscopy (SEM) analysis. Biochemical methane potential (BMP) assays showed the highest methane yield was achieved with 5V plus pH 9.2 pretreatment with up to 20.3% improvement over the non-pretreated sludge after 42days of mesophilic operation. In contrast, no discernible improvements on anaerobic degradability were observed for the rest of pretreated sludges, probably due to the overmuch leakage of refractory soluble organics, partial chemical mineralization of solubilized compounds and sodium inhibition. The statistical analysis further indicated that increased solubilization induced by electrical-alkali pretreatment increased the first-order anaerobic hydrolysis rate (khyd), but had no, or very slight enhancement on WAS ultimate methane yield (fd), demonstrating that increased solubilization of WAS from a pretreatment does not always induce an improved anaerobic digestion efficiency.