| The biological treatment of municipal sewage produces abundant excess sludge.After mechanical dewatering,the water content of sludge is still as high as 80%,so deep dewatering is needed to reduce the cost of subsequent treatment and disposal.Through electro-dewatering(EDW)under direct-current electric field,the water content of sludge can quickly be reduced to less than 60%,which has been widely concerned by the industry.The dewaterability limit and energy consumption of EDW can be further optimized through sludge conditioning.However,the current physicochemical conditioning technologies are difficult to realize in the industrial application of EDW due to its high cost and secondary pollution risk.In this paper,based on experimental phenomenon of improved EDW efficiency of sludge after anaerobic biochemical reaction process,and focusing on the changes of sludge characteristics,a kind of deep dewatering process of EDW with biochemical conditioning(BC)was explored by using sludge decomposition capacity to change sludge structure.And,it was evaluated from the aspects of dewaterability limit and comprehensive performance of EDW.In addition,the factors and mechanism of improving EDW performance were discussed in depth through the correlation analysis of sludge characteristics and EDW characteristics,the systematic analysis of electrokinetic process,and the exploration of sludge conduction mode.Experimental study on the EDW characteristics of biochemically conditioned sludge showed that BC can effectively improve the dewaterability limit,increase the dewatering rate,and optimize the comprehensive performance of EDW.After 3 days of BC at 35 ~oC,the dry solids content of sludge in dewaterability limit of EDW increased from 35.9%to 49.2%,breaking through the problem that the dry solids content of dewaterability limit can hardly increased to more than 40%in EDW process.Compared to raw sludge,when the sludge was dewatered to the dry solids content of35.9%,the dewatering efficiency for sludge with 35 ~oC-3 days increased significantly,and its water removal per unit time was 2.8 times that of the raw sludge.However,there was no significant increase in electrical energy required for unit water removal,showing good EDW performance.It was found that the destruction of extracellular polymeric substance(EPS),the release of bound water,and the increase of the absolute value of zeta potential during BC were the main reasons for improving EDW effect.The basic research of EPS,an important substance which determines the characteristics of sludge dewatering,was studied by the EDW characteristics of its layered structure.The results showed that the existence of the bound-EPS has strong non-polarity,which reduced the absolute value of zeta potential and the exchange current density of sludge,and it was not beneficial to EDW.On the other hand,the polarity of soluble-EPS was stronger,which increased the electrical conduction ability of sludge and promoted the EDW.These results indicated that the enhancement of electrical conduction ability caused by the changes of sludge EPS during BC process were the key factors affecting EDW performance.According to the above experiments,it was found that the electrical conduction ability of sludge was very important to predict the evolution of EDW.By studying the influence of the amount and type of ions in sludge on the electrokinetic process during EDW,the conductive mechanism of sludge in the EDW process was further analyzed.The results showed that the increase of ions amount changed the electric double layer structure of sludge,increased the voltage drop used for ion migration,and decreased the efficiency of electro-osmosis dewatering.The increase of cation valence states hindered ion conduction and was not conducive to EDW.Through the analysis of the conductive mode of sludge and based on the regional characteristics of sludge cake during EDW process,the conductive mechanism of unsaturated dispersible medium of sludge with gas-liquid-solid three-phase system in the process of EDW was proposed.It was revealed that the fluidity and ionization of dissolved components in sludge were dominant in the initial stage of EDW,and EPS or cell dissolution were dominant in the reconstruction stage.And the weight of surface conductivity gradually increased during EDW process.Based on the experimental phenomenon that BC can improve sludge EDW efficiency,the deep dewatering method and technology of BC to improve EDW efficiency was explored in this paper.And,the conditioning mechanism of the change of electrical conduction ability was clarified to be the main reason of improving EDW.Besides,the electrical conductive mechanism based on the conductivity model of unsaturated porous media was proposed to describe and predict the characteristics of sludge EDW process.All the results provide reference and support for the low cost improvement of sludge EDW efficiency and its industrial application. |
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