The RO membrane is a thin film made up of microscopic pores. In normal operation, the membrane in reverse osmosis elements can become fouled (contaminated) by mineral buildup, biological matter, colloidal particles and insoluble organic constituents. Besides fouling, membranes can get clogged to the extent that they cause a drop in the output of purified water; or become ineffective in removing the salts effectively, or both. Because of this fouling problem most RO purifiers used in homes have UV purification post membrane to eliminate the chances of contamination.
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