Evaluating properties of protein in waste activated sludge for volatile fatty acid production: effect of pH

Protein is the primary organic constituent of waste activated sludge (WAS) and its properties are related to pH. In order to investigate the effect of the sludge protein on the production of volatile fatty acid (VFA), the hydrolysate from the pretreated WAS with thermo-alkaline method was used as the substrate to conduct batch anaerobic acidification over a wide pH range 3–12. Results showed that the percentage of consumed soluble protein in consumed volatile organic substance was more than 75 % at various pHs. The soluble protein was responsible for VFA production. When the pH value decreased from 12.0 to 3.0, some soluble proteins were precipitated and a decrease of 42.2 % of soluble protein was observed. There was no significant decrease in the amount of precipitated protein after anaerobic acidification. Differential scanning calorimeter analysis showed that the enthalpy change (∆H over 1.470 J g−1) of precipitated protein was higher than that of soluble protein except at pH 12. A high level of aggregation between/among precipitated protein particles might be the main reason that precipitated protein displayed a poor bio-degradability and disfavored the accumulation of VFAs during 240 h of anaerobic acidification. WAS acidification for VFA production could be an attractive waste treatment practice in which both pollution control and VFA recovery can be achieved.