Fouling models rely on knowledge of foulant accumulation and removal mechanisms. In this study, a fouling visualization apparatus is developed to elucidate centimeter-scale mechanisms of organic fouling and cleaning in reverse osmosis (RO) and forward osmosis (FO). Alginate is used as a model organic foulant and dyed with methylene blue, which is shown not to affect fouling or cleaning, and to sufficiently highlight the gel for visualization at low salinity (up to 1% NaCl). When cleaning by increasing the cross-flow velocity, with or without reverse permeation, foulant peels off the membrane in discrete pieces in both RO and FO. Videos of cleaning show that foulant cake swelling and wrinkling can facilitate gel detachment and removal. Despite their effectiveness in slowing fouling, spacers can hinder removal of detached foulant pieces by obstructing their path. Finally, photographs point to a new mechanism of internal fouling in FO: vapor formation due to sub-atmospheric pressure within the membrane. Awareness of these mechanisms allows for better modeling of fouling and motivates optimization of swelling-inducing cleaning procedures.

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