| Anaerobic digestion is a common process for waste activated sludge stabilization. However, in anaerobic digestion, the biological hydrolysis is so difficult, which results in the slow degradation, long residence time and large volume. In order to reduce the impact of rate-limiting step, several pretreatments for sludge hydrolysis are presented. In this thesis, the effects of ultrasonic pretreatment, alkaline pretreatment and combination of these two methods are compared. The efficiency and mechanism of anaerobic digestion of sludge by alkaline and ultrasonic pretreatment are studied. In addition, basing on the Monod equation, the kinetics equation of anaerobic digestion of pretreated sludge is deduced. The main conclusions of this study are as follows:It has been found that the combined ultrasonic and alkaline treatment is more effective than alkaline or ultrasonic treatment alone in releasing SCOD and VSS solubilization, which can not only improve the hydrolysis rate, but also reduce the pretreatment time in ultrasonic pretreatment and alkaline dose in alkaline treatment. There are two distinct phases in sludge hydrolysis with pretreatment. The first phase is a very rapid increase in solubilization, followed by a much slower second phase. According to kinetic analysis for first rapid phase, the initial hydrolysis rate of the simultaneous alkaline and ultrasonic treatment is the highest among these methods being 130.3mgSCOD / (L·min).Comparing with the non-pretreated sludge, the reduction of organic matters and biogas production in the anaerobic digestion with simultaneous alkaline and ultrasonic disintegrated sludge can be improved significantly. After pretreatment, the sludge retention time (SRT) of anaerobic digestion can be reduced from 20d to 8d, and the digestion system performs well. When the retention time is 8d, the anaerobic digestion of pretreated sludge shows 2.6 times higher gas production than that of untreated sludge (SRT=20d), and removal efficiency of volatile solid (VS) and total chemical oxygen demand (TCOD) increased by 22.87% and 14.38%, respectively.Comparing with the non-pretreated sludge, the percentage hydrolysis, acidification and methanogenesis of TCOD in the anaerobic digestion with pretreated sludge can be improved significantly. When the sludge retention time is 8d, the hydrolysis, acidification and methanogenesis of TCOD with pretreated sludge increased by 82.38%, 76.47% and 86.54%, respectively, compared to the control group. For the non-pretreated sludge, the hydrolysis is found to be the rate-limiting step of the whole digestion process. For the pretreated sludge, when the SRT≥8d, hydrolysis was the rate-limiting step. However, when the SRT<5d, the hydrolysis and acidification of TCOD are higher than the methanogenesis, and the methanogenesis become the rate-limiting step in the anaerobic digestion.By analyzing the component of released liquid in the anaerobic digestion, the mechanism of enhancing the anaerobic digestion of sludge by alkaline and ultrasonic pretreatment is explored. The results show that when the sludge is pretreated by the simultaneous alkaline and ultrasound, the substances inside and outside the cell can be released into the liquid rapidly, and the utilization ratio of organic matter is increased. The solubilization rate of sludge in pretreatment process is higher than the decay rate of the sludge. Corresponding to concentration of the soluble substance in the sludge, the higher quantity of VFA is production in the anaerobic digestion with pretreated sludge, and when the SRT≥8d, the produced VFA in the anaerobic digestion can be converted to the biogas quickly. It also found that the activity of methane forming bacteria in the anaerobic digestion fed with pretreated sludge is higher than the control group and at the reduced retention time it can be stable. In order to determine the improvement of the process kinetics a first-order degradation model of organic matter is used over the anaerobic digestion system. Comparing with the control, the K of digestion process with pretreated sludge is increased by 89.58%.Basing on the Monod equation, the kinetic equation of anaerobic digestion of pretreated sludge is deduced. Four basic parameters for reactor are YG=0.0726, b=0.0413d-1,K=-9.65 andk0=0.025, respectively.
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