Freezing pretreatment assists potassium ferrate to promote hydrogen production from anaerobic fermentation of waste activated sludge

Anaerobic fermentation is an eco-friendly technology for waste activated sludge (WAS) treatment, during which resource recycle can be achieved. However, traditional sludge anaerobic fermentation is limited by the poor efficiency. We herein reported a novel high-efficiency technology by combining freezing with potassium ferrate (PF) for sludge pretreatment to promote hydrogen production from anaerobic fermentation. Experimental results demonstrated that freezing coupled with PF pretreatment exerted positively synergetic effect on hydrogen production. The maximal hydrogen production of 12.50 mL/g VSS (volatile suspended solids) was detected in the fermenter pretreated by freezing (−12 °C for 24 h) coupled with PF at 0.15 g/g TSS (total suspended solids), which was 1.34, 2.33, and 7.91 times of that from the individual PF, individual freezing, and control fermenters, respectively. The simulation results based on the modified Gompertz model indicated that both the hydrogen production potential and rate were promoted by freezing coupled with 0.15 g/g TSS PF pretreatment, from 2.14 to 13.52 mL/g VSS and 0.012 to 0.163 mL/g VSS/h, respectively. Thorough mechanism investigations revealed that the sludge EPS (extracellular polymeric substances) and microbial cells were both effectively damaged by combined freezing and PF pretreatment, resulting in the acceleration of sludge disintegration. Further investigations demonstrated that except for the acidogenesis, the other biochemical processes were all inhibited by freezing coupled with PF pretreatment, but the inhibitory extent for hydrogen consuming processes was more serious than that responsible for its generation. Gene sequencing analysis illuminated that both of the hydrolytic and hydrogen generating bacteria were largely enriched in the combined pretreatment fermenter. Moreover, the dewatering performances of fermented sludge were found to be notably enhanced by freezing coupled with PF pretreatment.