| The essence of most current wastewater treatment processes is to concentrate pollutants from wastewater into sludge.Sewage can be treated but it also produces a lot of sludge.Sludge contains a large amount of water,which makes incineration,pyrolysis,gasification and other treatment methods inefficient.These treatments may also produce toxic and harmful gases such as dioxins and cause secondary pollution.Anaerobic digestion converts the organic components of the sludge into methane,and additional energy can be obtained by disposing of the sludge.However,due to the structure of sludge,the anaerobic digestion efficiency is low.However,pretreatment of sludge can greatly improve the anaerobic digestion efficiency of sludge.Thermal hydrolysis was used as sludge pretreatment method in this paper.The optimum process parameters of thermal hydrolysis were determined by setting the reaction temperatures of 120℃,135℃,150℃,165℃,180℃,195℃ and 210℃ and the reaction time of 30min,45min,60min and 75min.The effect of thermal hydrolysis on sludge was studied by comparing the physical and chemical properties of sludge.The intermediate products in the process of thermal hydrolysis of sludge were analyzed by multi-batch experiments,and the law of material transformation in the process of thermal hydrolysis of sludge was revealed.Finally,BMP test was carried out on thermal hydrolysis sludge to evaluate its methanogenic performance.The results showed that the dehydration performance and biodegradability of sludge were improved after thermal hydrolysis.The presence of a large amount of free water increased the fluidity of the remaining sludge,transforming it from solid state to liquid state.The total amount and types of organic matter in the sludge liquid phase were improved,with COD dissolution rate up to 50.05%,soluble protein and total sugar concentration up to 18.81 g·L-1 and 10.36 g·L-1.In addition,the reaction temperature was the decisive factor of the thermal hydrolysis effect,and the reaction time had little influence on the thermal hydrolysis efficiency of sludge.The results of the study on the material changes during the thermal hydrolysis of sludge show that the dissolution of organic matter in the sludge was mainly concentrated in the middle and high temperature stage.However,when the temperature was higher than 180℃,the Maillard reaction of sludge will reduce the effect of thermal hydrolysis treatment.The results of the non-isothermal kinetic analysis of the thermal hydrolysis process of sludge show that the thermal hydrolysis reaction of sludge in the heating process was a first order reaction.In addition,the effect of thermal hydrolysis on sludge was dominated by solubilities of organic matter,and the degree of hydrolysis of macromolecular organic matter was small.The results of BMP test showed that the sludge treated at 180℃ for 45 min had the best methanogenic performance,with the cumulative methane production reaching 849.2 mL,4.89 times that of the original residual sludge.When the temperature was higher than 180℃,the anaerobic digestion performance of the sludge would also be reduced due to the Maillard reaction.In this experiment,the optimal thermal hydrolysis condition was 180℃for 45min.However,in fact,the reaction temperature was the decisive factoraffecting the effect of thermal hydrolysis.The increase of thermal hydrolysis efficiency was limited by long reaction time.Considering the energy consumption,the maximum benefit can be achieved by appropriately reducing the reaction time or using the waste heat.In addition,a new experimental idea was proposed for the study of thermal hydrolysis of sludge:the intermediate products of thermal hydrolysis sludge can be obtained through multi-batch experiments,and the reaction mechanism of thermal hydrolysis can be further studied. |
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