Extraction of single-cell protein from activated sewage sludge: thermal solubilization of protein

The single-cell protein in activated sludge from biological wastewater treatment represents the only source of essentially unutilized protein large enough to alleviate the worldwide shortfall. However, economical processes for extracting it on an adequate scale need to be developed. We have examined partial thermal digestion. Thermal treatment has inherent advantages of low treatment costs and production of no additional waste streams. Digestion in sealed tubes at 150-155°C and ambient pressure for 20 min resulted in optimal extraction of high molecular weight protein. Lower temperatures solubilized less material, whereas higher temperatures resulted in degradation of solubilized protein to low molecular weight fragments. Oxidation did not play an important role in degrading the protein under these conditions. A similar temperature dependence was observed for extraction of RNA and DNA. Elevated pH increased yields of high molecular weight protein at lower temperatures, but required substantial increased reagent input.