| Activated sludge method is used widely in biological wastewater treatment, but it will produce large amounts of excess sludge. It almost takes up 0.3%-0.5% of the amount of wastewater treatment. Annual sludge production is about 25 million tons in China presently (calculated by 80% of water content), which will pose a significant threat to the ecology system if it is not properly disposed. Costs for traditional treatment and disposal of excess sludge are quite expensive and it accounts for up to 60% of the total operation costs of a wastewater treatment plant. Sludge hydrolysis is a hot topic in the area of sludge reduction, and hydrolysis is the rate limiting step of sludge digestion. Therefore, enhancing sludge hydrolysis is important premise and base to reduce sludge. Compared with other sludge hydrolysis technology, such as physical, chemical and biological treatment, enhanced hydrolysis by additional enzyme can not only cut down digesting time, improve sludge digestibility, and reduce disposal costs, but also can be easily controlled, and its products are harmless to environment. Combined with the following process,it can realize sludge "zero discharge", and simultaneously realize high effective wastewater biodegradation or enhancement of biogas production.The technology of sludge reduction is the minimum of sludge biomass through physical, chemical, biological and chemical methods to meet the government effluent standard. The possible approaches for sludge reduction based on following aspects:uncoupling metabolism, cryptic growth enhanced through cell lysis and microfauna pry, sludge hydrolylsis by enzymes. Characteristics of each technology were compared, and the sludge reduction based on additional enzyme was discussed particularly in relevant research.In this investigation, the effects of commercial enzyme preparation containing alpha amylase and neutral protease on hydrolysis of excess sludge were evaluated. Protease and a-amylase were added singly, the dosage of enzyme and the hydrolysis time are 0,30,60,120, 180 mg enzyme/gTS and 4,8,12 h, respectively. The results indicated that additonal enzymes can enhance sludge hydrolysis. The hydrolysis basically completed within 4 h under anaerobic digestion, and the optimal dosage of enzyme is 60 mg enzyme/gTS. a-amylase had higher efficiency than protease, SCOD/TCOD rose to 22.3% and 18.5%, respectively.VSS reduction achieved respectively 54.24% and 39.70% at the optimal dosage of 60 mg enzyme/g TS. The hydrolysis rate of sludge improved with temperature increasing from 40 to 50℃, which could be well described by the amended Arrhenius equation. Mixed-enzymes (enzyme preparation containing alpha amylase and neutral protease) had great impact on sludge hydrolysis than single enzyme. The mixture of the two enzymes (protease: amylase=1:3) resulted in optimum hydrolysis efficiency, the efficiency of solids hydrolysis increased from 10%(control test) to 68.43% at the temperature of 50℃. Correspondingly, the concentration of reducing sugar and NH4+-N improved about 377% and 201%, respectively.According to the mechanism and kinetic analysis of enzymatic hydrolysis process, VSS solubilisation process within prior 4 h followed first-order kinetics, dring which the rate of the reaction is fast firstly, and then it slows down.The hydrolysis rate constants Kh for protease, amylase, mixed-enzyme treatment were 0.12,0.20 and 0.24 h-1, which were 5,8,10 times that of the blank, respectively. Obviously, the first-order hydrolysis rate constants obtained in this study followed an Arrhenius type of behavior R=0.964).The relationship between reaction rate constant and temperature was in accordance with Arrhenius equation, with activation energy 20.19 kJ/mol and preexponential factor 263 L/(mg.min).The variation of enzyme activity during sludge hydrolysis (protease and a-amylase) was also investigated. It was found that enzyme activity were both rose firstly, and reduced thereafter. The maximum enzyme activity were obtained at around 4h after hydrolysis,2.57 U/mL and 4.64 U/mL respectively for protease and a-amylase, which could well explain the curve of enzymztic reaction dynamics.
|
PDF Full Text Request