Fouling models rely on knowledge of foulant accumulation and removal mechanisms. In this study, a foulingvisualization apparatus is developed to elucidate centimeter-scalemechanisms of organic fouling and cleaningin reverse osmosis (RO) and forward osmosis (FO). Alginate is used as a model organic foulant and dyedwith methylene blue, which is shown not to affect fouling or cleaning, and to sufficiently highlight the gelfor visualization at low salinity (up to 1% NaCl). When cleaning by increasing the cross-flow velocity, with orwithout reverse permeation, foulant peels off the membrane in discrete pieces in both RO and FO. Videos ofcleaning show that foulant cake swelling and wrinkling can facilitate gel detachment and removal. Despitetheir effectiveness in slowing fouling, spacers can hinder removal of detached foulant pieces by obstructingtheir 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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