| In recent years, the municipal sludge production has maintained a high growth in China. The amount of dewatered sludge has reached 35 million tons in the year of 2015, but only less than 20 per cent of these sludge has been treated by effective method. From the view of energy saving, sewage treatment facilities only completed half or two-thirds of the task. Therefore sludge treatment problems must be solved immediately. Anaerobic digestion is the best way to achieve the goal of reduction, stabilization, resource and harmless of sludge. However, it has shortcomings in traditional anaerobic digestion, such as the slow rate of hydrolysis, the huge volume of digester, and the low gas production rate. Considering the various features of existing pretreatment technology and the nature of municipal sludge, we chosen ultrasonic and amylase-protease to dissolve the sludge and we aimed to improve the effect of anaerobic digestion and to increase the production rate of biogas.Before anaerobic fermentation, optimal parameters such as The length of time of ultrasonic pretreatment, the total use of compound enzymes, the enzymes radio and the reaction temperature. Pretreated sludge was used in single batch and continuous batch fermentation. During each time of fermentation the value of SCOD, pH, VFAs, etc. are studied. The main findings are as follows:(1) The optimal length of time of ultrasonic was 16 minutes and frequency of 40 KHz, power of 600 W and dissolved sludge was 18.4% at this time. Compound enzymes was better than single amylase or protease enzyme in dissolving sludge and the optimal ratio of 2:1 (starch enzymes:protease), temperature of 35℃, and the SCOD reached 20250 mg/L in 6 hours under this condition.(2) The pretreated group have greater cumulative gas production than the control group. Ultrasonic pretreatment and enzymatic pretreatment group were 8.76% and 11.14% higher than the control group and they ended prematurely than the control group in single batch anaerobic digestion. When the control group was 45 days; enzyme pretreatment group with the shortest, only 31 days; ultrasonic pretreatment group was 37 days. Pretreatment greatly reducing the required anaerobic digestion time, while increasing the rate of degradation of the sludge VS.(3) For enzymatic pretreatment and ultrasonic pretreatment, the gas production averages were 24.71 L·d-1 and 22.87 L·d-1, higher than the control group (15.28 L·d-1). The pretreated sludge showed an increase in continued batch anaerobic digestion process, and the organic matter removal significantly increased. The value of VSr in the order:ultrasonic pretreatment group (44.42%)>enzyme pretreatment group (42.65%)>control group (28.98%).(4) Pretreatment technology provides enough nutrients for anaerobic bacteria breeding, and pH value of the sludge is more conducive to the survival of methanogens. VFAs, FAN showed great accumulation in the process of anaerobic digestion, but methane bacteria has not been inhibited and their ability to tolerate high concentrations of ammonia and VFAs enhanced. Sludge bacteria was 19.7% cracked under ultrasonic pretreatment, and 3.61% under enzyme pretreatment. During the anaerobic digestion process enzymes continued to play a role.(5) It took 10 days and 13 days for ultrasonic and enzymes group to start the reactor, while the control group required more than 20 days, and the pretreatment improves the rate of VS reduction in the Initial start-up phase. Pretreated sludge anaerobic digestion system can withstand minor SRT and high sludge loading, and get higher gas production. The best SRT of pretreated sludge in batch anaerobic digestion were 12 days for ultrasonic group and 16 days for enzymes group, but non-pretreated sludge lasted more than 20 days. The VSr has improved markedly in anaerobic digestion of pretreated sludge. |
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