Acidogenic fermentation of proteinaceous sewage sludge: Effect of pH

Proteinaceous sewage sludge represents the sludge in which the protein content exceeds the carbohydrate content, accounts for a considerable part of sewage sludge. In this study, volatile fatty acids (VFA) production and synthesis pathway as well as bacterial community in acidogenic fermentation of proteinaceous sewage sludge under different pH conditions were investigated. The results indicated that the alkaline pH improved the solubilization and biodegradation of proteins in the sludge. The soluble protein concentration at pH 11.0 was 67.88% higher than that at pH 3.0. Its biodegradation efficiencies were reduced with a decrease in pH. The pH influenced not only the total VFA production yield, but also the percentage of individual VFA. The total VFA yield at pH 9.0 was 10.70 times of that at pH 3.0. The metabolic conversion pathway of proteins to VFA in acidogenic fermentation systems was investigated using a stoichiometry approach. The Stickland reaction was found to be the main pathway for the VFA accumulation from the decomposition of proteins. pH also significantly influenced the biodiversity and bacterial community in the system. The abundance of microorganisms under alkaline or acidic pH conditions was less than that under neutral pH condition, suggesting that most anaerobic fermentative microorganisms could not tolerate the hostile environment. The terminal restriction fragment length polymorphism analysis showed that, when the pH was reduced from 12.0 to 7.0 and finally to 3.0, the dominant bacterial genuses and percentage of the total microorganisms were Granulicatella genus (65%), Clostridium genus (28%) and Bacillus genus (71%), respectively.