Effect of operating conditions on CaSO4 scale formation mechanism in nanofiltration for water softening

The effect of the operating condition, such as transmembrane pressure and crossflow velocity on the scale formation pathways, was investigated in nanofiltration (NF) of a CaSO4 solution. The examination of scanning electron microscopy of a fouled membrane surface provided strong evidence that there are two mechanisms in NF fouling (e.g., scale formation on the membrane surface) in nanofiltration; surface and bulk crystallization. The hydrodynamic operating conditions determined which fouling mechanism was dominant. The fouling due to surface crystallization augmented with an increase in operating pressure, but reduced with an increase in flow velocity. Concentration polarization was found to be responsible for this type of fouling. On the other hand, the extent of fouling by bulk crystallization appeared to be the most important at intermediate crossflow velocity and higher operating pressure (e.g., higher flux) because both secondary nucleation effect and deposition probability depends on crossflow velocity and permeation rate. An experimental and theoretical method for differentiating one mechanism from another was explored in order to elucidate the dominant fouling mechanism according to the hydrodynamic conditions.