Comparison of Fouling Behavior in Cellulose Triacetate Membranes Applied in Forward and Reverse Osmosis

Membrane fouling is inevitable during the mem-brane separation process. The difference in the driving force of reverse osmosis (RO) and forward osmosis (FO) affects the behavior of foulants. Thus, in this work, we examined the behavior of different foulants during the FO or RO process, including before and after physical cleaning of the membrane. The foulants used were alginate (Alg-Na), humic acid (HA), bovine serum albumin (BSA), and colloidal silica. The commercial cellulose triacetate membrane was used for both FO and RO processes to investigate the behavior of foulants fairly. During the RO process, the formation of the gel network between alginate and calcium ions tends to accumulate on the surface of the membrane, leading to the formation of a dense layer of the foulant, consequently decreasing the flux. Having HA in the feed, RO and FO processes had a similar flux decline, whereas having alginate and BSA, the flux decline during the RO process was higher than the FO process. When colloidal silica was presented in the feed, the membrane in the RO process had constant flux throughout the testing, whereas the membrane in the FO process had a remarkable decrease in flux. Silicas were adhered more on the membrane tested in FO. It was presumed that the reverse salt diffusion facilitates the aggregation of the silica on the membrane surface, leading to a reduction of flux by cake-enhanced concentration polarization in the foulant layer of silica. Therefore, the foulant properties, type of draw solution, the structure of the foulant layer, and the interaction between the foulant and membrane are important to consider in the fouling behavior in RO and FO processes. This understanding of the fouling behavior in the FO process will lead to the development of the optimum FO process.