Modeling cake filtration under coupled hydraulic, electric and osmotic effects

Pore blocking is considered to dominate the dewatering efficiency in filtration processes. Various engineering measures were adopted to promote the permeate flux of membrane filtration, e.g. electro-osmosis, flocculation and aggregation, membrane flush and pressure adjustment. A new model was proposed to address the coupling effect of hydraulic, electric and osmotic fields on cake filtration on the basis of thorough review on relevant researches. A moving boundary was configured to evaluate the time-dependent cake growth behavior. An instant mass balance was assumed valid at the cake–slurry interface, which was shown to reflect the experimental facts. The coupled governing equations were solved numerically by finite element method (FEM), and were validated by comparing with experimental results. The nonlinear constitutive behaviors of the filter cake under varied pressure head were further included in the models. FEM analysis indicates that the hydraulic conductivity and electro-osmotic coefficient of the formed cake are critical for the permeation flux. Correspondingly the pressure head and electrical potential gradient applied to the filtration system largely determine the separation efficiency. The filter cake (FC) formed in front of membrane filter contributes a lot to the reduction in solvent flux under certain dewatering conditions. Measures to improve the flocculation behavior of dispersible sludge were explained by the new model. Modifying the surface charge of sludge with polyelectrolyte is found to benefit the dewatering only within a limited time/leachate range, which was reported for the first time. Suggestions on cake filtration were provided according to our numerical simulations.