Promoting Thermal Hydrolysis Of Excess Sludge With External Enzymes

Activated sludge is the most widely used wastewater treatment technology, andthe excess sludge it produced is about0.3%to0.5%of the total amount of sewage(calculated by97%of water content). Sludge may increase by0.5to1.0times if thesewage was treated in-depth. The sewage sludge treatment is an important componentof the system, in which excess sludge treatment accounts for about20~50%or even70%ofthe total operation costs. However, the traditional aerobic sludge digestion was longerand power consumption is high; hydrolysis is the rate limiting step of anaerobic sludgedigestion, the poor biodegradable of the sludge became an bottleneck which restrictedthe anaerobic digestion.Compared with other sludge hydrolysis technology, such as physical (heat treatment,ultrasonic, etc.), chemical (ozone oxidation, alkali treatment, etc.) and biologicaltreatment, enhanced hydrolysis by additional enzyme can not only cut down digesting time,improve sludge digestibility and dewatering, be conducive to further sludge treatment, andits products are harmless to environment. Combined with the following process, it can realizesludge "zero discharge", and simultaneously realize high effective wastewater biodegradationor enhancement of biogas production. Relative to the anaerobic hydrolysis, operation ofaerobic condition is simple, and there is no odor after the reaction. In addition, due tothe presence of oxygen, sludge reduction can also achieved through the cryptic growthof thermophilic bacteria besides the catalyzing of enzymes.The effects of α-amylase to promote excess sludge thermal hydrolysis underaerobic were investigated. It was found that sludge hydrolysis efficiency wasenhanced by amylase. When treated at the dosage of0.5g/L, SCOD/TCOD in sludgereached30.98%under the optimal temperature50℃, which was7.68%higher thancontrol experiment. As a result of the interaction of amylase and thermal hydrolysis,sludge solids dissolved, and then macromolecular carbohydrate was hydrolyzed intosmall-molecular sugar, solid-phase protein released and further hydrolysis. Sugar andprotein in supernatant both showed an initial increase and then decrease. After theaddition of amylase, the concentration of sugar in supernatant reached271.43mg/L at4h, while protein concentration reached1437.37mg/L at6h under50℃,separately.Furthermore, VSS dissolution rate and SCOD/TCOD were consistent with first-orderreaction dynamic within prior4h at50℃.And then the hydrolysis of excess sludge with amylase tended to balance. The VSS dissolution rate reached22.01%at4h.In this investigation, effects of mixed-enzymes (enzyme preparation containing alphaamylase and neutral protease) on hydrolysis of excess sludge under aerobic condition wereevaluated. It was found that mixed-enzymes had great impact on sludge hydrolysis than singleenzyme. Sludge hydrolysis was better with the increase of temperature within the selectedtemperature. After the reaction of4h, the mixture of the two enzymes (amylase: protease=2:1)resulted in optimum hydrolysis efficiency, the SCOD/TCOD and VSS solubilization reached33.5%and25.4%respectively at the total dosage of0.5g/L under50℃. Sugar and proteinconcentration in sludge achieved397.3mg/L and1479.5mg/L separa4ely. Protease was infavor of low tempdrature, and ammonIa obtained effective accumulation. There was nosignificant difference between single enzyme and mixed-enzyme on ammonia concentrationat45℃, which was at the range of84~88mg/L. The largest ammonia concentration of86.4mg/L was obtained in sludge supernatant when the ratio of amylase and protease was2:1at40℃.Temperature, the dosage of total enzyme, mixed-enzymes ratio and reaction timewere selected to establish an orthogonal test with four factors and there levels. Theeffect of each factor on sludge hydrolysis was assessed through range analysis andvariance analysis of test results. Temperature and reaction time were the main factorsinfluence the dissolution of SCOD and VSS dissolution rate. Visual analysis showed thatthe order of factors influenced the sugar cumulated was temperature> Total dosage ofenzymes> mixed-enzymes ratio> time, it was temperature> Time> Total dosage ofenzymes> mixed-enzymes ratio for protein accumulated and time> mixed-enzymesratio> temperature> total dosage of enzymes for NH4+. Considering of the economyand efficiency, optimum conditions for additional-enzyme hydrolysis of the sludgewere50℃, the total enzyme of0.5g/L, ratio of amylase and protease2:1, and reactiontime of4h.