Fouling control mechanisms of demineralized water backwash: Reduction of charge screening and calcium bridging effects

This paper investigates the impact of the ionic environment on the charge of colloidal natural organic matter (NOM) and ultrafiltration (UF) membranes (charge screening effect) and the calcium adsorption/bridging on new and fouled membranes (calcium bridging effect) by measuring the zeta potentials of membranes and colloidal NOM. Fouling experiments were conducted with natural water to determine whether the reduction of the charge screening effect and/or calcium bridging effect by backwashing with demineralized water can explain the observed reduction in fouling. Results show that the charge of both membranes and NOM, as measured by the zeta potential, became more negative at a lower pH and a lower concentration of electrolytes, in particular, divalent electrolytes. In addition, calcium also adsorbed onto the membranes, and consequently bridged colloidal NOM and membranes via binding with functional groups. The charge screening effect could be eliminated by flushing NOM and membranes with demineralized water, since a cation-free environment was established. However, only a limited amount of the calcium bridging connection was removed with demineralized water backwashes, so the calcium bridging effect mostly could not be eliminated. As demineralized water backwash was found to be effective in fouling control, it can be concluded that the reduction of the charge screening is the dominant mechanism for this.

► Demineralized water backwashing significantly improves ultrafiltration fouling control.
► The charge screening effect and the calcium bridging effect are two of the mechanisms responsible for the NOM fouling on UF membranes. On the other hand, the hypotheses for the demineralized water backwashing were the reduction of the charge screening effect and the calcium bridging effect.
► Results show that the reduction of the charge screening effects is the dominant mechanism of the demineralized water backwashing. UF membranes and NOM used in this study became less negatively charged when the concentration and valence of cations in electrolytes increased (e.g., cations in feed during filtration, enhancing fouling). By backwashing with demineralized water, the reduced negative charge of membranes and NOM during filtration was restored by the demineralized water, enhancing the repulsion force between membranes and NOM and consequently fouling removal.
► Although a small amount of calcium bridging connections could be reduced in the process of demineralized water backwashing, this reduction is limited.