Proteins recovery from waste activated sludge by thermal alkaline treatment

Abstract

Extracellular polymeric substances(EPS) are the major organic component of waste activated sludge (WAS), and proteins are the dominant organic component of EPS. To recover the protein effectively, hydrolysis treatment should be carried out so that the protein in the sludge flocs and microbial cells is ultimately released into the aqueous phase. In this study, a chemical method(thermal alkaline treatment(TAH)) was used to extract proteins from WAS. The factors affecting the extraction of proteins from sludge by TAH method were optimized with the extraction rate of protein as the index. The peak concentration of 549.86 mg/L and extraction rate of 60.72% of sludge protein for TAH, was achieved at pH of 13, temperature of 70 °C, reaction time of 9 h, and sludge concentration of 20 g/L of experimental conditions. Partial macromolecular protein was broken down into low molecular protein and less melanoid was produced. Further analysis of sludge residue by X-ray diffraction(XRD) and X-ray photoelectron spectroscopy(XPS) indicated it might be related to the formation of calcium carbonate(CaCO3) at pH of 13 and temperature of 70 °C. Relative molecular weight of sludge proteins was tested by sodium dodecyl sulfate polyacrylamide gel electrophoresis(SDS-PAGE) with coomassie blue staining. The relative molecular weight of extracted sludge protein was approximately between 6.5 kDa and 16 kDa. All these indicated that macromolecular protein was changed into low molecular protein during the thermal alkaline hydrolysis process and that released carbon dioxide combined with calcium ions to form insoluble calcium carbonate. However, complete purification of the sludge protein still needs further study.