Design and modelling of electroosmotic dewatering

Abstract

Many previous studies have demonstrated that electroosmotic dewatering is an effective technique to dewater sewage sludge. However, the technology has not yet been successfully applied in industry. This is because there are several technological barriers to the commercial exploitation of the technology that are yet to be resolved. One of these barriers is a scientifically robust design methodology. This aim of this study is to establisha qualitative and quantitative understanding model of electroosmotic dewatering of sewage sludge and from this produce a robust design methodology. This study is divided into three parts: The first part evaluates the feasibility of electroosmotic dewatering under constant voltage for range of sludge types. A number of experiments with four different sewage sludges were undertaken to examine the dewatering efficiency in terms of rate of dewatering, final solid concentration and energy consumption. The results showed that electroosmotic dewatering of sludge is both feasible and potentially economic. It is also shown that the use of the Helmholtz-Smolucowski flow eqPation, together with the assumptions of no electrochemical reaction and a constant electroosmotic permeability, to predict the dewatering process is not sound. The second part of this thesis presents an integrating framework for electroosmotic dewatering under constant voltage and constant current, founded on the mathematics of simple electricalc ircuits and demonstrated by laboratory experimentation. The derived equations and experimental results showed that electroosmotic flow rate decreases with time when dewatering with constant voltage and is constant when dewatering with constant current. Having a linear relationship between flow and time, electroosmotic dewatering with constant current not only enhances the sludge dewatering efficiency, but also has the advantage of simplifying design procedures. The third part explores in further detail each of the design parameters of electroosmotic dewatering under constant current, including sample thickness, current, time over which constant current could be maintained, power supply, pressure and type of electrodes. A detailed design methodology, including design equations to predict final solid content and treatment time, design requirements for each parameter and design procedures, is presented.