Study On Anaerobic Digestion Efficiency Of Sludge Enhanced By Biological Pretreatment Of Endogenous Hydrolase

Anaerobic digestion has been widely adopted for the treatment of municipal sludge before final diposal; it is considered a major and essential part of a modern WWTP. In terms of the composition of Extracellular Polymeric Substance, adding hydrolase may be another pretreatment way to enhance sludge biogegradability and reduce hydraulic retention time.In this work, hydrolyse producing bacteria were isolated from waste activated sludge; effects of cultivation condition and nutritional factors on enzyme producting capability, the basic property of the model of batch liquid fermentation kinetics and enzymatic characteristics of stains were investigated; the fementation liquid which containing hydrolase was adding to waste activated sludge as endogenouse hydrolase to strengthen the sludge hydrolysis efficiency, and effect of different temperatures, time, enzyme on hydrolytic and acidification. Biochemical Methane Potential (BMP)tests were carried out to assess sludge biodegradability, in the same time, mechanism of endogenouse hydrolase strengthen anaerobic digestion of sludge was discussed.Four strains with high and steady amylase or protease activity were isolated from sludge; furthermore, the strains can be cultured in sterilized sludge, retaining partial enzymatic activity. Physiological and biochemical analysis showed that the strain was Bacillus subtilis, cillus licheniformis, Aeromonas hydrophilaand and Aeromonas hydrophila respectively, and we named them WYDF、WYZZ、WYC and WYE. WYDF and WYZZ produce amylase, mainly; WYC and WYE produce protease. Bacteria growth mass and enzyme producing activity were effected by carbon source, nitrogen source and initial pH; the growth and enzyme producing is not simultaneous, and both of them kinetics were established. The crude enzyme fementation of the four strains were separation and purification, the results show that each of each crude enzyme is different and the mixture of several kinds enzyme.The effects of temperatures, time and mixed proportion of two kinds of endogenous enzyme on sludge pretreatment efficiency were investigated. The results showed that temperature of pretreatment play a critical role in the enzyme hydrolysis sludge progress; at the middle temperature,37℃is suitable for endogenous enzyme pretreatment，and7h is enough for a efficient sludge solubilization. Endogenous which procuded by strain WYDF presents the strongest ability to hydrolyze and acidize the sludge. The concentration of SCOD and VFAs increased by as much as142%and219%when the sludge was treated for28h, at37℃compared to a control (without adding of endogenous hydrolysis); SCOD/TCOD reach up to11.1%; solid particle ranged from200to1000m was reduced, the number of3070m increased. All samples, the acetic acid content increased with respect to pretreatment duration.Effect of proportion between amylase and protease on pretreatment efficiency was studied. The result showed that the ratio was1:3which was beneficial to produce VFAs and organic dissolution.Biochemical Methane Potential tests were carried out to evaluate the effect of different enzyme pretreatment condition on sludge subsequent anaerobic digestion. It shows that pretreated sludge by endogenous no matter which strain the anaerobic digestion efficiency was impoved. For the sludge pretreated by WYDF, volatile solids removal rate, VS removal rate and methane productivity were improved from633to702mL,36.02%to39.14,137to161mL CH4/g VSadded, respectively. It was compared to untreated sludge, at the end of the test (25d). Mixed enzyme improved sludge anaerobic degradation compared with hydrolase used alone; optimal proportion exists in different endogenous hydrolase for sludge pretreatment.The change of sludge surface configurationin were observed before and after enzymatic treat by scanning electron microscope, and the change of particle size was messured, in order to provide some insights into sludge disintegration process. Results indicate that endogenous enzyme disintegrate sludge, improve the bioavailbility, reduce the number of larger size solid by hydrolysis EPS and destroy the structure of filamentous bacteria. In addition, enzyme will not only act on sludge in the process of pretreatment, but it will also continue to play a role in the subsequent anaerobic digestion.Microbial community was detected by DGGE technology; the figure demonstrated that pretreated sludge results in an increase in microorganism species in the earlier stage of anaerobic digestion, compared to control; it observed no change with further digestion.