Protein Extraction From Excess Sludge By Enzymatic Hydrolysis

In the process of sewage biological treatment,a large amount of surplus sludge mainly composed of microorganisms is produced.The relative lag in the development of sludge treatment and disposal technologies inevitably poses risks to the environment and affects the harmonious development of human health and economy.Therefore,reducing the amount of sludge and transforming it into material which is beneficial to social is a green and environmentally friendly path to development.The sewage sludge is mainly composed by bacteria,protozoa,metazoan,algae and suspended matter.the protein content in the sewage sludge is known to accont for 30-60%.The protein can be used to prepare protein feed,organic fertilizer,biodegradable vesicant,foam extinguishing agent and foamed concrete insulation material,etc.In this paper,the sewage sludge was the test subject.The effect of enzymatic hydrolysis was measured by protein rate and sludge dewatering performance.On the basis of single-factor experiments,Response surface method was used to optimize the enzymatic hydrolysis conditions,the composition of sludg were analyzed before and after hydrolysis.The main research conclusions are as follows:(1)The study of enzymatic hydrolysis experiment was carried by four differents proteolytic enzymes(Acid protease,neutral protease,alkaline protease,papain).The rate of protein extraction and SCOD dissolution,the content of amino acids and peptides and sludge specific resistance was used to evaluate the optimal reaction condition for these protease.The results showed the alkaline protease hydrolysis has the highest enzymatic hydrolysis effect.Response surface method was analyzed to obtain the optimal process parameters of alkaline protease hydrolysis sludge.The optimal hydrolysis condition are initial pH 10.1,hydrolysis temperature 57.3 ?,enzyme amount 6500U/g,the sludge solids content 4%,the reaction time 4h,Experimental results have been shown that the protein extraction rate was 34.1%,the sludge dewatering performance was increased by 79%.Under the optimized condition,Humic acid decreased from 11%to 7.7%,hemicellulose decreased from 4.7%to 4.1%,crude fat decreased from 0.55%to 0.35%.The content of the polysaccharide solution was about 1296 mg/L,which increased by 162 times.the content of the polypeptide and amino acid were 693 mg/L and 611 mg/L,respectively.(2)On the basis of protein extraction from excess sludge and sludge specific resistance by single enzymaitia hydrolysis,selected three enzymes(neutral protease,papain and alkaline protease)constitute three groups of compound enzymes.By the three groups of compound enzymes with their suitable condition,the enzymaitic hydrolysis experiment was carried out.The results shown that the optimal compound enzyme was neutral protease and alkaline protease,and its enzyme activity ratio was 1:3.Further optimization the optimal conditions of composite enzyme catalytic hydrolysis.The results showed that the optimal hydrolysis condition are initial pH 9.16,enzyme amount 12200 U/g,temperature 54.75 ?,sludge solids content 4%,reaction time 4h.Under the optimized condition the protein content was 8541 mg/L,which was increased 25.3%more than alkaline enzyme hydrolysis,the sludge specific resistance was 1.75×108 S2/g,and dehydration performance was increased by 98.5%more than alkaline enzyme hydrolysis.Under the optimized condition,Humic acid in sludge decreased to 7.1%,hemicellulose reduced to 3.1%,and crude fat decreased to 0.31%.Compared with the alkaline protease enzyme hydrolysis,these substances degrade significantly.The polysaccharide content in the supernatant was 1456 mg/L,which was increased by 12%than alkaline enzyme.The content of polypeptide was 1130 mg/L and the amino acid was 778 mg/L,which was increased by 63%and 27.3%respectively.Compared with the sludge before hydrolysis,it was concluded that the compound enzyme has stronger hydrolysis ability than alkaline protease.(3)The enzymatic hydrolysis kinetics of alkalin protease and compound enzyme were studied using Michaelis-Menten equation combination with enzymatic hydrolysis experiments.The kinetic rate equations were v?????=95[S]/12+[S]and v???=139[S]/9.9+[S]in the optimal hydrolysis conditions of alkaline protease enzyme and complex enzyme,respectively.The decreasing order of these protease for Km and Vmax was alkaline protease,compound enzyme;and compound enzyme,alkaline protease,respectively.