Hydrogen Recovery from Waste Activated Sludge: Role of Free Nitrous Acid in a Prefermentation–Microbial Electrolysis Cells System

Due to the limited hydrolysis rate of particulate organics and suitable substrates for hydrogen-producing bacteria in raw waste activated sludge (WAS), traditional fermentative hydrogen production has low hydrogen yield and energy recovery efficiency. The role of free nitrous acid (FNA) pretreatment on WAS and hydrogen recovery was investigated in a prefermentation–microbial electrolysis cells (MECs) system. The results demonstrated that WAS hydrolysis and acidification were enhanced by FNA pretreatment. Notably, the accumulation of acetic acid and propionic acid eventually reached to 55% and 22% during prefermentation. During MECs cascade utilization, volatile fatty acids (VFAs) were exhausted and the utilization efficiencies of soluble carbohydrates and proteins reached 62% and 41.5%, respectively. The hydrogen yield from FNA-pretreated sludge was 1.44 mL/g of volatile suspended solids, which was approximately 3 times than that of the control. High-throughput sequencing and canonical correspondence analysis revealed that FNA pretreatment promoted the hydrolysis and acidification of particulate organics, through accumulating anaerobic fermentation bacteria in prefermentation, and, furthermore, stimulated the increase of electrochemically active bacteria, thereby enhancing the current and hydrogen production. This study may provide a sound basis for the potential implementation of FNA pretreatment to accomplish cascading utilization of organics and the synchronous recovery of energy from WAS