Cleaning Reverse Osmosis Membranes
By William F. Harfst
In the 1960s, the first reverse osmosis (RO) membrane was developed for the removal of dissolved salts from water. It was made from cellulose acetate (CA). Later, in the 1970s, thin-film composite (tf) membranes were introduced. These were made from polyamide (PA) and offered higher water permeability (lower operating pressure) and lower salt permeability (higher salt rejection) as compared to CA membranes. Over time, thin-film polyamide membranes have become the predominate RO module, although many CA membranes are still in service. Whether CA or tf type, eventually membrane performance declines due to fouling of the flow channels and membrane surfaces with organics, colloids, suspended solids, and biofilms. This results in an increase in pressure differential, decrease in normalized permeate flow, and an increase in percent salt passage. At this point, it becomes necessary to clean the RO membranes to restore operating performance and extend their useful life.
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