Effect of pH on the Accumulation of Volatile Fatty Acids from Proteinaceous Excess Sludge

Abstract

Proteins were the primary organics of excess sludge. Their properties were related to pH, which in turn affected the production of volatile fatty acids (VFAs). Excess sludge was firstly pretreated using the thermo-alkaline method, and the centrifuged supernatant was then taken as the substrate to investigate the effect of pH on the accumulation of VFAs from proteinaceous waste. The heating method was used to treat the inoculums in order to inhibit the generation of bio-methane during the whole anaerobic fermentation. The results showed that pH had an obvious influence on the bioconversion of proteins. Although the conversion ratios were higher than 50% when pH was controlled at 7.0-11.0, the protein biodegradation was sensitive to too low or too high pH values. Soluble proteins were precipitated to become insoluble proteins with the adjustment of pH from alkaline to acid except at pH 3.0. Acetic acid was the main accumulated end-product at most of pH settings, and at pH 10.0, its concentration reached the maximum of 5.7 g·L-1 which was around 57.8% of total VFAs. At various pHs, it was soluble proteins that were responsible for VFAs production, while the precipitated proteins displayed a poor bio-degradability and disfavored the accumulation of VFAs during a 240 h of anaerobic fermentation.