Optimization Of Protein Extraction From Excess Sludge And The Damage Mechanism Of Sludge Flocs And Cell Sructure

Excess sludge is the main by-product of sewage treatment.In recent years,its output is increasing with the increase of sewage treatment volume year by year.The recycling of excess sludge is a hot issue at home and abroad.Excess sludge contains a large amount of usable organic substances and sludge pretreatment is an important way to release the organic substances in sludge.The damage degree of sludge will also vary due to the different mechanism of pretreatment methods.Therefore,thermal-alkali,ultrasonic-alkali and lysozyme method were selected to optimize the parameters first by charactering SCOD,polysaccharide,protein,ammonia nitrogen and TS/VS,and potein was the final judgment index.Meantime,the crude protein in the solution was recovered and purified.Based on the above experiment conditions,thermal,alkali,ultrasonic,thermal-alkali,ultrasonic-alkali and lysozyme methods were selected to treat the sludge.The mechanism of sludge damage was explored by charactering the particle size,specific surface area,extracellular polymeric substances,the amount of organic matter released in supernatant,the damage degree of cell wall and cell membrane,DNA and other indexes.The results show that the cellulolytic effect:thermal-alkali pretreatment(p H 13.0,temperature 140~oC,time 1.5 h,2062.98 mg/L)>ultrasonic-alkali pretreatment(p H12.0,power 5 W/m L,time 30 min,497.76 mg/L)>lysozyme pretreatment(p H 7.0,enzyme concentration 200 mg/g,time 6 h,269.95 mg/L),and the protein purification recovery rate of thermal-alkali pretreatment at p H 3 can reach 62.42%.By comparing the six pretreatment methods to the damaged of sludge floc and cell structure,it can be seen that the damage effect of combined pretreatment is better than the single pretreatment.When the thermal-alkali pretreatment was carried out,the high temperature can effectively make the matter flow out of the cell,and the dissolution effect was greatly enhanced with the participation of alkali(the maximum value of SCOD in the solution was 7895 mg/L).Regarding the ultrasonic-alkali pretreatment,the sludge floc structure was dispersed by ultrasonic,and the intracellular matter was released after the alkali damaged the cell structure.Ultraasonic damage to the floc structure was the strongest(the sludge particle size decreased by 48.15%and the specific surface area increased by 99.13%),but the damage to cell wall was far less than thermal.The thermal energy directly acts on the cell wall and cell membrane to make the intracellular matter release(the content of N-acetylglucosamine increased by 4260 mg/L,and the fluorescence intensity of damaged cell membrane increased by15.312%compared with the original sludge by thermal pretreatment).The damage of floc structure by alkali was better than that of ultrasonic.The damage of sludge floc and cell structure by enzyme was negligible.After each pretreatment method,there is no significant difference in the types of organic matter in the sludge supernatant.In addition,the experiment results exhibit that the highest amount of EPS was extracted from the residual sludge after ultrasonic pretreatment.It can be speculated that the synergistic effect of ultrasonic and thermal on the excess sludge under this condition will produce a better effect,which is worthy of further investigation.