Combined organic and colloidal fouling in forward osmosis: Fouling reversibility and the role of applied pressure

In this study, we systematically investigated the propensity and reversibility of combined organic-colloidal fouling in forward osmosis (FO) under various solution chemistries (pH and calcium ion concentrations) and applied hydraulic pressure on the feed side. Alginate, silica colloids, and their mixture (i.e., combined organic-colloidal) were used as model foulants. Our findings demonstrate that combined organic-colloidal foulants caused more rapid flux decline than the individual foulants due to the synergistic effect of alginate and silica colloids. As a result, much lower flux recovery was achieved by physical cleaning induced by increasing the cross-flow rate, in contrast to single foulants of which the fouling layer was easily removed under all solution conditions. Interestingly, less flux decline was observed at neutral pH for combined fouling, while acidic conditions were favorable for alginate fouling and basic solutions caused more silica fouling, thereby providing clear evidence for the combined fouling effect. It was also found that calcium ions enhanced water flux decline and induced the formation of less reversible combined organic-colloidal fouling layers. Lastly, the role of applied hydraulic pressure on the feed side in FO was examined to elucidate the mechanism of fouling layer formation, fouling reversibility, and water flux recovery. Higher fouling propensity and lower fouling reversibility of combined organic-colloidal fouling were observed in the presence of applied hydraulic pressure on the feed side. This observation suggests that the lower fouling propensity and greater fouling reversibility in FO compared to reverse osmosis (RO), are attributable to unpressurized operating conditions in FO.