Experimental Study On Electrokinetic Dewatering Mechanism Of Sewage Sludge And Theoretical Analyses Of Multi-field Coupled Phenomenon

There is a tremendous output of sewage sludge in China which seriously threatens the environment, accompanied by the rise in the output of domestic sewage as well as the disposal rate of sewage. Dewatering is key points of treatment and disposition of sludge, which can significantly reduce the difficulty in techniques of sludge disposition and treatment fee. Developping an efficient dewatering technology is the key to the problems about the treatment and disposal of sludge. Electrokinetic dewatering is a method utilizing electroosmosis to dewater the sludge, which has been proven to be fast and efficient. Seepage field, chemical concentration field and electric field interactions during the process of electrokinetic dewatering, causing attenuation in dewatering efficiency. Based on laboratory experiments and numerical simulations, the study on electrokinetic dewatering mechanism of sewage sludge and theoretical analyses of multi-field coupled phenomenon were carried out. The main research works and conclusions are as follows:(1) The mechanism and process of dewatering were obtained based on experimental results of electrokinetic dewatering of sludge under different applied voltage gradients, electrode distances and treating times. It is found that, as the dewatering extended from the anode to cathode, the electrical resistance of the dewatered section and the corresponding voltage consumption gradually increased, and hence the voltage left for the undewatered section decreased. The phenomenon resulted in a significant attenuation in electroosmotic flow, dewatering effect and treating efficiency over time. Non-uniform distribution of residual water content was observed along the dewatering direction. It turns out that the increasing energy consumption by the dewatered section is the fundamental reason for the attenuation in the treating efficiency of EKD with fixed-space electrodes.(2) Based on analyses of testing results, the change regularity of seepage field was simplified and a multi-field coupled model of electrokinetic dewatering of sludge was proposed. This model is able to analyze the change of electroosmotic flow rate and voltage distribution, as well as ion transport during the electrokinetic treatment under different applied voltage gradients, electrode distances and treating times. The analyzed results indicate that the ions in sludge migrated under the action of electric field and seepage field, causing reduction in concentration of ions and rise of the electrical resistance as well as the voltage in dewatered section of the sludge. The increasing voltage further accelerated the migration of ions and intensified the reduction of ions concentration in dewatered sludge. Consequently the applied voltage was concentrating in dewatered sludge and unequally distributed between the electrodes.(3) Based on simulation and testing results, it is found that shorter electrode distance was more suitable for energy conservation. Both the energy consumption and the dewatering effect increased with the rise of the voltage under the condition of certain electrode distance. Optimization selection of the voltage could be made through comparing the treatment fee and the cost saved through dewatering. The analyzed results indicate that 750 RMB Yuan/hundred tons could be saved through electrokinetic dewatering with voltage ranging from 2～3V and 1cm electrode distance for sludge using landfill disposition method,2300 RMB Yuan/hundred tons could be saved through electrokinetic dewatering with voltage ranging from 4.5～5.5V and 1cm electrode distance for sludge using incineration disposition method.(4) A new electrokinetic dewatering method using movable electrode was invented, through using of which the voltage could be concentrated on undewatered sludge and the dewatering efficiency could be maintained. The comparison tests and analyzed results show that the movable electrode method achieved better dewatering effect with lower energy consumption comparing with traditional electrokinetic dewatering method using fixed-spacing electrodes.(5) Tests of electrokinetic dewatering of silt and red mud were carried out to analyze the applicability of electrokinetic treating method in different subjects. Results show that electrokinetic dewatering was efficient for silt. However, it was invalid for red mud.