Study On Extraction Of Protein From Sludgeby The Different-intensity Ultrasound Assisted Enzymatic Hydrolysis

Recent years have seen an explosion in development of urbanization,resluting in increment of excess sludge production which is apt to cause second pollution.Commonly,30⁶0%of protein content exists in the excess sludge,which can be used as foaming agent,liquid fertilizer,animal feed and so on.Therefore,the techniques of protein extraction from excess sludge have drawn wide attention.In this paper,the excess sludge produced by a municipal sewage treatment plant in Zhengzhou was studied.On the basis of the research on the protein extraction from excess sludge by enzymatic hydrolysis combined with high-intensity ultrasound?HUS-EH?,the protein extraction effect and the operating conditions of enzymatic hydrolysis combined with low-intensity ultrasound?LUS-EH?from excess sludge was studied.The results are as follows:?1?In the LUS-EH method,a certain amount of alkaline protease was added to the sludge which was previously sonicated by the high-intensity ultrasound to carry out the hydrolysis reaction.The influence of ultrasonic power density?P?,ultrasonic treatment time?t₁?,enzyme dosage?m?,enzymatic hydrolysis time?t₂?,enzymatic hydrolysis pH and enzymatic hydrolysis temperature?T?on protein extraction and sludge dewatering performance were studied,and then the operation conditions were optimized by the response surface method.The optimum process conditions were P=1 W/mL,t₁=23 min,m=3500 U/g,pH=10.6,T=56.4?,t₂=3 h.Under this condition,the protein concentration of the sludge supernatant was about 10500 mg/L,which was about 50%higher than that of the enzymatic hydrolysis alone?EH?,and the extraction ratio of protein was about 55%.Meanwhile,the sluge specific resistance decreased to 5.2×10⁹ s²/g with the sludge dewatering performance increasing by 38%compared with the original sludge.?2?In the LUS-EH,the alkaline protease and the sludge were irradiated together for a period of time in a low-intensity ultrasound field firstly to improve the enzymatic activity,and then hydrolysis reaction was carried out of the ultrasonic field.The results showed that the optimum process conditions were:P=0.125 W/mL,t₁=30 min,m=4000 U/g,pH=10.8,T=59.2?,t₂=3 h.In this condition,the protease activity increased by about 13%.The protein concentration of the sludge supernatant was about 8000 mg/L,which was about 20%higher than that of the EH,and the extraction ratio of protein was about 44%.At the same time,the sludge dehydration performance increased by more than 50%with the sludge specific resistance being 4.0×10⁹s²/g.?3?HUS-EH,LUS-EH and EH all induced dramatic changes in the sludge composition.In the sludge supernatant,protein,polysaccharide,polypeptide,amino acid and volatile fatty acid significantly increased,while the proportion of lipid,humic acid and cellulose in the sludge residual decreased slightly.Compared with the EH,LUS-EH led to 20%and 100%increasing of the protein and the amino acid contents in the supernatant without significant energy comsumption increment.Compared with the HUS-EH,the energy consumption of LUS-EH dropped by about50%although the 20%decrement in protein.However,the similar amino acid content was obtained by the two methods.Therefore,the protein extraction by LUS-EH was more economical and efficient,and its products were suitable for liquid fertilizer and animal feed.As to the dewatering preformance of the hydrolyzed sludge,the 38%,50%and 55%increasetment were obtained by HUS-EH,LUS-EH and EH,respectively.Comprehensivly considering the protein extraction rate,energy consumption,drug consumption and sludge dewatering performance,the LUS-EH has a good application prospect as one of the ecconomic and efficent methods in sludge protein extraction methods.