Enhanced volatile fatty acid production by a modified biological pretreatment in anaerobic fermentation of waste activated sludge

• Enhanced VFAs production was obtained by a novel PEH pretreatment in WAS anaerobic fermentation.
• PEH obtained higher carbon release and utilization rate simultaneously.
• EDTA-2Na solubilized the EPS and then protease lysed sludge cell in PEH process.
• Quality of carbon released by biological pretreatment greatly exceeded physicochemical pretreatment.

A novel protease/EDTA-2Na hydrolysis (PEH) pretreatment method was developed with the combination of EDTA-2Na hydrolysis (EH) and protease hydrolysis (PH) to improve volatile fatty acids (VFAs) production by anaerobic fermentation of waste activated sludge (WAS). Efficiencies of PEH, PH, EH and thermal alkaline hydrolysis (TAH) were compared in releasing SCOD from WAS. Results indicated that physicochemical pretreatment, TAH, had higher SCOD release but less utilization rate of the released SCOD in anaerobic fermentation, biological pretreatments, PH, showed less SCOD release but higher SCOD utilization rate, and PEH could simultaneously obtain higher SCOD release and utilization rate. By TAH, released SCOD could reach 19,273.21 mg/l, but VFAs was only 11,820.36 mg COD/L. By PEH, SCOD and VFAs were 13,628.98 and 12,704.44 mg COD/L, respectively. SCOD released by PEH showed good biodegradability with BOD5/COD of 0.64%, 25.49% higher than that of TAH. By anaerobic fermentation, volatile suspended solids (VSS) of WAS pretreated by PEH could be reduced by 18.50%, but only 6.16% for TAH treated WAS. In PEH process, synergistic effects between protease and EDTA-2Na were observed, in which EDTA-2Na prevented protease from being trapped on the surfaces of sludge floccules and finally greatly improved protease efficiency. Therefore, it is feasible to apply PEH on enhancing VFAs production in anaerobic fermentation of WAS due to its higher and better bio-available carbon release than other pretreatments.