Enhanced SCFAs production from waste activated sludge by ultrasonic pretreatment with enzyme-assisted anaerobic fermentation

A novel strategy based on ultrasonic pretreatment and enzyme-assistence was proposed in this study to enhance short-chain fatty acids (SCFAs) production from anaerobic fermentation (AF) of waste activated sludge (WAS). The impacts of ultrasonic frequency (20 - 40 kHz), ultrasonic amplitude (60 - 90 %), and treatment duration (0 - 8 min) on sludge solubilization, as well as different enzymes (lysozyme, alpha-amylase, cellulase, and blended enzyme) on SCFAs production were assessed. The results showed that the optimal condition for ultrasonic pretreatment to enhance sludge solubilization was ultrasonic frequency of 20 kHz, amplitude of 80 %, and treatment duration of 5 min. Scanning electron microscope (SEM) and particle size distribution revealed that ultrasonic pretreatment effectively destructed EPS and sludge floc structure. The addition of enzymes enhanced SCFAs production, with the effects following the order of blended enzyme > lysozyme > cellulase > alpha-amylase > control. The maximum SCFAs accumulation for the enzyme-added systems was obtained on the 4th day, which were 1.81 - 3.41 times of that in the control system (8th day). Microbial community structure analysis disclosed that the functional microorganisms were strengthened by the addition of enzymes. The enrichment of phyla Firmicutes and Bacteroidetes in the enzyme-added systems, with enriched acidogenic genera (e.g. Propionispira) in the blended enzyme system and hydrolytic genera (e.g. Romboutsia) in the lysozyme system, hinted the mechanism of SCFAs production enhancement by enzymes.