Factors governing combined fouling by organic and colloidal foulants in cross-flow nanofiltration

• Synergistic effects are controlled by organic–colloid concentration ratio.
• Combined fouling is controlled by colloid type.
• At different concentrations, calcium can have different effects on fouling.
• Flux decline in fouling is partly due to organics penetrating the colloidal layer.
• Organic–colloid affinity interactions do not influence combined fouling.

There is some controversy in the literature on the effects of combined fouling of organics and colloids on water flux. There are reports of more flux decline for combined fouling than that of fouling by individual foulants, which has been attributed to synergistic effects. However, some authors observed lower flux decline in combined fouling. In addition, some authors have reported on increased flux declines by calcium addition, whereas others have shown improved fluxes after calcium addition. The discrepancies observed in the literature are systematically investigated in more detail in this manuscript. The organic–colloid concentration ratio was varied in order to explain discrepancies in previously observed effects in combined fouling. Further, the probable contributing factor of colloid type and calcium concentration in combined fouling was also examined. Organic, colloidal and combined fouling was found to be aggravated in the presence of Ca2+ due to organic–Ca2+ complexation and the reduction in foulant–membrane repulsive interactions, but this only occurred up to certain Ca2+-concentrations. Above these concentrations, the opposite effect was observed. In addition, membrane–foulant as well as foulant–foulant affinity interactions were determined and related to initial and later fouling rates. A novel type of “sequential” fouling experiments were carried out for the first time, and yielded information on how colloids and organics interact when both are present in fouling solutions. It was concluded from these experiments that hindered back diffusion of organics by a layer of colloids was the main factor responsible for flux decline in combined fouling.