We're having a problem with UF membrane fouling; we were wondering whether a high sulphate concentration in the electrolyte can affect the membrane properties or cause membrane fouling?
Bear in mind that alum and ferric sulfate are often used as coagulants, while their hydrolysis products tend to flocculate, what may contribute for membrane fouling. It may help to adjust the pH to approx. 5.5 prior ultrafiltration, if previously higher (but ≤ 7.8). Previous microfiltration may also help, if not implemented yet.
We are not adding any coagulants to the electrolyte so we have an aqueous solution with some metal ions (Na, Ni, Fe, Cr) and NO3-, SO4(2-), OH- at around pH 8.5. We also have some insoluble metal (hydr)oxides and some insoluble sulfur. We were wondering whether the sulphate could chemically react with the PES membrane itself which in turn affect the filtering ability of the membrane?
At high concentrations of sulfate ion, a highly concentrated polarized layer is formed on the membrane surface. This may result in crystallization of the sulfate salts within the narrow pores which have very high potential for nucleation. The crystallization will destroy the surface pores structure and reduces the salt rejection. Probably the use of some anti-scaling for non-batch systems may be effective.
Thank you for your answer Ali; would a concentration of around 70g/L of Na2SO4 cause this layer and subsequent crystallisation to form?
It depends strictly on the velocity profile on the membrane surface, geometry of the module and surface properties of the membrane such as roughness and also the zeta potential between the solution and the surface. Minor impurities such as calcium or barium ions are problematic.
You could share more details then I can guide you better.
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