Experimental And Numerical Research Of Electro-osmotic Dewatering Of Municipal Sludge At Constant Voltage

In recent years,with the rapid increase of sewage discharge and treatment,a large amount of residual sludge has been produced.According to latest statistics,the annual production of sludge,whose moisture content is about 80%,is expected to reach 3.5 million tons,which brings enormous pressure on the environment.The follow-up disposal of sludge in china is the usage for agriculture and land landfill mainly.But,the acceleration of the urbanization process has eliminated the farmland,which is used for receiving sludge before,and the follow-up treatment of sludge is limited.At the same time,the related state standards stipulate that the moisture content for land usage and landfill sludge must be less than 60%,so far,the moisture content of the sludge by the conventional mechanical dewatering is far from the requirement,which asks for the new treatment technique for the residual sludge.Furthermore,the reducing of the moisture content of sludge can significantly decrease the volume and save the costs of subsequent off-site transportation and disposal.Sludge electro-dewatering technology is the method that can remove moisture from sludge quickly and efficiently by using electro-osmosis principle.It can reduce the moisture content of sludge to the extent less than 60%.In the process of electro-osmotic dewatering of sludge,the flow field,concentration field and electric field are changed and interact with each other during the process of electro-osmotic dewatering.Understanding the mechanism of water migration,the interaction of multiphysic fields and grasping the key factors,which influence the dewatering efficiency,can optimize the operation conditions of electro-osmotic dewatering to improve the technique.In this paper,we first took the sample sludge,which is produced by the mechanical dewatering in the fourth sewage treatment plant of Xi'an City.Through the model experiment and the numerical simulation,we studied the electro-osmotic dewatering mechanism of municipal sludge and the multifield coupling theory.The main research contents and achievements include:(1)The experimental platform of sludge electro-dewatering was constructed to study the changing of electro-osmotic flux and current at different applied voltage gradient and the processing duration.The experimental results show that the sludge moisture content near the anode decreases rapidly and the resistance increases sharply,causing the electro-osmotic flux gradually damping and moisture content increasing gradually from anode to cathode in the process of dewatering.It is concluded that the main reason for the decrease of electro-osmotic dewatering effect is that the low water content sludge near the anode side share the higher voltage.(2)Based on the results of experiments and the combined change of electric field,flow field,concentration field and interaction laws,the partial differential equations of multiphysic fields was deduced.Finite element software,COMSOL Multiphysics 4.3a,is used to simulate the electro-osmotic dewatering process.The model parameters are obtained by experimental measurement and mathematical derivation.The results show that the model can reflect the electric field distribution and flow field changes in the dewatering unit at different loading voltages and processing time very well.(3)Both the model and the experiment are basically coincident with the trend of the electro-osmotic flux and the sludge moisture content,which means that the model parameters are reasonable.The operation conditions can be optimized through the electro-osmotic dewatering model,such as the effect of the electrode aperture ratio on the dewatering effect,and the optimum value is obtained.The model analysis shows that when the electrode aperture ratio L0/L is 3/10,the dewatering speed is the fastest and the effect is optimal.