| Rapid urbanization in many areas of the world has resulted in an increasing of waste activated sludge (WAS) from wastewater treatment plants (WWTPs), which has become a serious environmental issue. Costs for traditional treatment and disposal of WAS are quite expensive and would account for up to50%or60%of the total operating cost of WWTPs. Sludge is a kind of substance that are rich in organic matter, and it can be turn into wealth if rational use of the waste. The aim of hydrolysis and acidfication of WAS is to improve its biological degradability in the terms of higher soluble chemical oxygen demand (SCOD) and short chain fatty acids (SCFAs) productions. SCOD and SCFAs, especially acetate and propionate, are recognized as the most suitable carbon substrates for enhanced biological phosphorus removal (EBPR). Therefore, the hydrolysis and acidification of WAS has caused wide public concern, and it has important practical significance for sludge treatment. In order to reach the goal, the additional enzymes (protease and a-amylase) were selected to enhance the hydrolysis and acidification of WAS, thus provided a new technique for the treatment of WAS.Hydrolyis of particulate organic matter to soluble substance is believed to be the rate-limiting step of anaerobic digestion. The hydrolysis of complex organic molecules in the degradation of biodegradable particulate organic matter depends heavily on hydrolytic enzymes. However, the dosages of enzyme in the sludge are low, and they are usually entrapped by, adsorbed by, or bound to the sludge, thus leading to the low hydrolysis efficiency of the conventional sludge treatment. Recently, the enzymatic hydrolysis of sewage sludge has not been conclusively determined, thus the effect of enzyme dosage, operating temperature and the mixture ratio on the solubilisation of WAS were evaluated with additional enzymes used alone or in combination in this investigatin. It was found that during sludge anaerobic fermentation, the solubilization of sludge particulate organic-carbon and the hydrolysis of solubilized substrate (protein and carbohydrate) were all increased in the presence of enzyme. VSS solubilisation process could be almost completed within4h. Single enzyme had less impact on sludge solubilisation, of which amylase treatment displayed higher hydrolysis efficiency than that of protease. The mixture of two enzymes (protease:amylase=1:3) resulted in optimum hydrolysis result, the efficiency of solids hydrolysis could be increased to68.43%at the temperature of 50℃.In order to clearly understand the substantive characteristics of the hydrolysis process enhanced by enzyme, the kinetic analysis and the material transformation rule during this process is very important. The hydrolysis kinetics of WAS hydrolysis process enhanced by additional a-amylase was evaluated. The results showed that VSS solubilisation process could be almost completed within4h, and the rate constants obtained in this study followed the Arrhenius type of behavior. The Arrhenius equation of a-amylase hydrolysis (lnk=2429/T+5.57, R2=0.964), the reaction activation energy for VSS hydrolysis reduced from62.72kJ/mol (control test) to20.19kJ/mol (a-amylase treatment). Kinetics analysis indicated that the conversion coefficients (a) of VSS to soluble chemical oxygen demand (SCOD), carbohydrates and NH4+-N was found to be0.266±0.012,0.043±0.001and0.038±0.002, respectively.Based on the WAS hydrolysis enhanced by enzyme, the effect of enzyme on WAS acidification and the destruction mechanisms of WAS enhanced by additional enzymes was further analyzed and explored. It was found that enzyme could promote the hydrolysis and acidification of WAS. After different enzymes treated, acetic acid was the most prevalent product, which almost accounted for50%of the total SCFAs. The propionic and iso-valeric acids were the second, and the n-valeric acid was the lowest. During the entire fermentation, the pH values kept increasing no matter in the presence or absence of enzyme, and the same pH variations were observed.The study on the destruction mechanisms of WAS during anaerobic digestion is helpful for the understanding of substantive characteristics of the hydrolysis enhanced by enzyme. The Excitation-emission matrix (EEM) fluorescence spectroscopy has been applied in the distinguishing of the characteristics of extracellular polymeric substances (EPS) and dissolved organic matter (DOM), thus the objectives of this research were to obtain the EEM fluorescence characteristic of the EPS and DOM during WAS anaerobic digestion enhanced by enzymes and attempt to well understand this process from a novel insight. It was found that the protein-like (aromatic and tryptophan protein-like substances) and humic acid-like substances were the mainly fluorophores in the sludge, of which the release and transformation of tryptophan protein-like substances was more rapidly than that of aromatic protein-like substances. The EPS could be seriously disrupted by the action of hydrolytic enzymes, and the complex DOM was also broken down into low molecular-weight intermediates, thus the hydrolytic enzyme could enhance WAS solubilization and destruction. The comparisons of SCFAs production and hydrolytic enzymes activities between autoclaved and unautoclaved sludge showed that the production of SCFAs was the result of organics degraded by microbial. The diversity of microbial community structure (mainly bacterium) was studied through the application of16S rDNA-PCR amplification, DGGE fingerprinting and phylogenetic analysis. The results showed that the microbial existed in the enzymatic treatment and the raw sludge was partially similar, and each stage also had their special microbial. The amount of Clostridiaceae bacterium for the enzyme systems were much more than the blank during WAS hydrolysis and acidificationThe enzyme added or originally exist in the sludge are usually entrapped by, adsorbed by, or bound to the sludge, thus leading to the low hydrolysis efficiency of the conventional sludge treatment. Surfactant can cause an apparent increase in the aqueous solubility, thus accelerate the rate of nonaqueous phase substance dissolution into the aqueous phase, which may be able to compromise the floc integrity, liberate the trapped enzyme (within the floc matrix and on the cell-surface) and also expose more substrate. In addition, the hydrolysis of complex organic molecules in the degradation of biodegradable particulate organic matter depends heavily on hydrolytic enzymes. In consideration of the special characteristics of both enzyme and surfactant, the combination of surfactant and enzyme might exhibit a positive synergetic effect on waste solids hydrolysis, which could be used in the treatment of WAS to enhance its reduction. Thus, the combined effect of SDS and enzyme on WAS hydrolysis and acidification was investigated. It was found that the combined system was more effective in the promotion of sludge hydrolysis than sole SDS and sole enzyme. The composition analysis of SCFAs produced in SDS+mixed-enzymes (ME) system indicated that acetic acid was the most prevalent product and propionic acid was the second one. |
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