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Protein can be extracted effectively from excess sludge by thermal alkaline process. During the extraction process, insoluble protein in sludge was turned into soluble protein, and part of the soluble protein was hydrolyzed into peptide. Then part of the soluble protein and peptide were degraded into amino acids, generating by-products: water, carbon dioxide and ammonium. Therefore, to obtain the maximum concentration of soluble protein, it is necessary to establish a kinetic model describing the process of sludge protein extraction. The report indicated that the extraction mechanism of sludge protein was according to first-order continuous reaction kinetic. A kinetic model of the reaction was established to explain the relationship between the rate constant, temperature and hydrolysis time of the first-order reaction. The optimum conditions were as follows: 0.1 M NaOH, reaction temperature 70°C, hydrolysis time 4 h. Protein separated and obtained from sludge hydrolysate by centrifugation and filtration, which could be used as a feed additive or foaming agent.

Protein could be extracted effectively from excess sludge by barium hydroxide (Ba(OH)2) hydrolysis process. During the extraction process, insoluble protein in sludge was turned into soluble protein, and part of the soluble protein were hydrolyzed into peptide, amino acids, water, carbon dioxide and ammonium. To obtain the maximum concentration of soluble protein, the effects of barium hydroxide, temperature and time on the hydrolysis degree of the sludge protein and protein concentration were investigated. Under optimal conditions of 0.2 M Ba(OH)2, 110 ℃ of the reaction temperature, and 4 h of hydrolysis time, the degree of sludge protein hydrolysis and the protein concentration was 79.10% and 1106.11 mg/L, respectively. Through building a SPSS multiple nonlinear regression model, the optimal protein concentration was calculated. Barium ions were converted into barium sulfate precipitates by adding an equal amount of sulfuric acid in the hydrolysate. Then the precipitates were washed with 0.1 M NaOH to obtain pure barium sulfate and protein solution. Protein was separated and obtained from sludge hydrolysate by centrifugation and filtration, which could be used as a wood binder or foaming agent.

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Thermal alkaline method can effectively extract protein from excess sludge. During the extraction process, insoluble proteins in sludge were turned into soluble proteins, and partly soluble proteins were hydrolyzed into peptides, amino acids and by-products: water, carbon dioxide and ammonium. The optimum conditions were as follows: 0.1 M NaOH, the temperature of 70 °C, reaction time of 5 h. Then the soluble proteins were hydrolyzed into amino acids with hydrochloric acid furtherly. The optimal process parameters of hydrochloric acid hydrolysis were as follows: the hydrolysis temperature of 110 °C, heating time of 12 h, and acid to supernatant ratio of 0.8. The results show that proteins of 46.33% were extracted from the excess sludge and the amino acids yield was 75.49%, namely approximately 9 g per 100 g of dry sludge under the optimum hydrolysis conditions. The contents of amino acids in hydrolysate were determined by automatic amino acid analyzer and the functional groups of hydrolysate were characterized by Fourier transform infrared (FTIR) spectrum. There were 17 kinds of amino acids in the hydrolysate，which can be used as deodorants, food additives, preservatives and corrosion inhibitors.