Synergistic effects in combined fouling of a loose nanofiltration membrane by colloidal materials and natural organic matter

Fouling of nanofiltration (NF) membranes in water treatment and wastewater reuse applications is often caused by both colloidal materials and dissolved organic matter. However, most past mechanistic studies on NF membrane fouling have focused on either colloidal fouling or organic fouling. In this study, the performance of a ‘loose’, low salt rejection NF membrane during combined fouling by both colloidal materials and dissolved natural organic matter (NOM) was compared with that during colloidal fouling and organic fouling alone. A significant synergistic effect was observed during combined fouling, manifested as considerably higher flux decline rate than what expected based on the sum of colloidal and organic fouling alone. Microscopic analysis of the colloid/organic foulant layer structure confirmed faster foulant accumulation on the membrane surface, indicating a mechanism of hindered back diffusion of colloidal and organic foulants. The membrane flux decline during combined fouling was also found to depend on solution chemistry and colloidal particle size. Notably, the effects of these factors are different from those during fouling by colloidal materials or dissolved organic matter alone.