| As the most widely applied biological treatment process in the wastewater treatment field, the conventional activated sludge process is technically mature and efficient, but at the same time, has its drawback of high sludge production. The are many harmful components in sludge including pathogenic matter, heavy metals and residual organic matter, which could pollute the environment without proper treatment. At present the main disposal technics include sanitary landfill, agricultural applications and burning. As the environmental and legislative requirements on the discharge of sludge have been strengthened, the disposal of excess sludge is strongly limited. To solve the sludge problem, there are many sorts of study and achievement on it. Among them, sludge anaerobic fermentation is one of the most age-old, usual and economical biological treatment, which needn’t oxygen supply in digesting process, consumes less energy compared with aerobic digestion, and could produce biogas, making sense in the background of energy shortage. However, the disadvantages of the treatment technique are that the waste activated sludge is difficult to degrade, resulting in the slow degradation and long residence time. It is reported that the restricted step of anaerobic digestion is hydrolyzation, so pretreatment before digestion are taken to promote it by disintegrating the sludge flocs and disrupting bacterial cells’wall. After pretreatment the intracellular inclusions are released into the aqueous phase, and rapidly hydrolyzed to small molecular compounds by extracellular enzyme, the hydrolysis can be improved a lot, so that the anaerobic digestion can be enhanced and methane production be significantly increased. However, further research is needed to make sure whether the pretreatment is accommodated to the existing treatment process in wastewater plant, and how its practical economical and energy benefit is.The aim of this experiment is to study and compare the upgrading of anaerobic digestion of ultrasonic, high temperature and peracetic acid pretreatment. It has been found that the SCOD dissolving rate, VFA concentration and TSS, VSS reduction are increased linearly with the argument of 0.4W/ml ultrasonic function time, but in sum the disintegration rate is under 5% level. Setting the sludge temperature at 70℃for 2 days shows a notable disintegration effect, which promotes the SCOD dissolving rate to 22.8%, TSS and VSS reduction to 17.5% and 31.5% separately, but the settlement capability becomes worse slightly. As for the peracetic acid, the higher is the concentration, the better the disintegration effect, but the lower the SCOD increasing rate caused by per unit peracetic acid. Besides, there are peracetic acid and Hydrogen Peroxide left even after the reaction for 12h, if the peracetic acid original concentration is bigger than 0.012%, which may prejudice the following anaerobic digestion. So the 0.01% is the proper peracetic acid concentration to pretreat sludge, it reduces the TSS for 21% after 12h, and improves the settlement, but low the pH to 4.66.In the anaerobic digestion step, the biogas production of sludge pretreated by 0.4W/ml ultrasonic for 10 minutes is 14% more than the original sludge, and the reduction rate of organic matter is 29%. The biogas production of sludge pretreated at 70℃for 2 days is 95% more than the original sludge, and the reduction rate of organic matter is 34%. Because the sludge is acidic after peracetic acid pretreatment, proper pH should be tested to choose. Alkalescent pH is good for increase the VFA, SCOD, biogas production and the methane proportion in biogas, while the acidic pH is only good for SCOD, but bad for others. Considering these factors, the pH of sludge is adjusted to 9.50, and pretreated by 0.011% peracetic acid. Its biogas production increases by 77%, even taking out the theoretic production created by acetic acid which is introduced by peracetic acid, the value is 17%, and the reduction of organic matter is 42%.Though the ultrasonic and heating pretreatment could promote the production of biogas to reclaim energy, they are also energy costing. Taking out the produced energy, in theory,236 kJ/L sludge more energy are needed by the ultrasonic pretreatment, and only 3.2 kJ/L sludge more energy are produced by the heating pretreatment, than the traditional anaerobic digestion system. further more both pretreatments need an extra reaction tank. On the contrary the peracetic acid pretreatment needn’t extra energy or extra reaction tank, which could be taken in the concentrating tank, and increases energy production 23kJ/L sludge. |
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