Membrane fouling is inevitable during the membrane 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.