The Influence of Electrochemical Properties of Membranes and Dispersions on Microfiltration

Abstract

Membrane filtration processes are widely used in various water and wastewater treatment applications. Crossflow microfiltration is a pressure driven membrane process, which has been shown effective in a great number of processes, including the removal of colloidal inorganic and organic solids. The main factor limiting the application of crossflow microfiltration and other pressuredriven membrane processes is flux decline due to membrane fouling and concentration polarization. The steady value of permeate flux depends on various process variables such as cross-flow velocity, kind of membrane (pore size and chemical composition), the Reynolds number, shear stress and shear rate at the membrane surface in the membrane system and on physico-chemical particle-particle and particle-membrane interactions (Broussous et al., 2000; Huisman et al., 1999; Narong & James, 2006; Velikovska & Mikulasek, 2007). One way of characterizing these particle-particle and particle-membrane interactions is the knowledge of the -potential of both particle and membrane. This electrostatic characterization of membranes is a useful way to predict and interpret the performance of microfiltration process. The magnitude of the -potential gives the information of the stability of the system. Near the isoelectric point – IEP (the value of pH, where the charge and therefore -potential of particles are equal to zero (Takagi & Nakagaki, 2001)) the system is unstable and the particles tend to flocculate. Therefore, the stability of the particles and the particle-membrane system could affect the separation process. Many studies showed that permeate flux, J, can be easily changed by pH, kind of added salt and salt concentration of the microfiltration dispersion (Baik & Seung, 2010; Gustafsson et al., 2000; Martin et al., 2003; Moritz et al., 2001; Mullet et al., 1997; Nazzal & Wiesner, 1994; Oo & Song, 2009). Baik & Seung (2010) investigated surface charge properties of bentonite colloids to study their colloidal stability in a solution as a function of the pH and ionic strength. The results of -potential measurements for the bentonite colloid showed that the bentonite colloids were stable at lower ionic strengths of 0.01 and 0.001M NaClO4 but instable at a higher ionic strength of 0.1M NaClO4 within the whole studied pH range. Faibish et al. (1998) studied the effect of electrostatic double layer interaction on permeate flux decline and deposit cake formation in crossflow membrane filtration of colloidal