Removal of organic contaminants

While RO membranes have a wide spectrum of removal of organic contaminants, the nature and extent of rejection will depend upon the nature of the organic solute. RO is effective in rejecting organic solutes with molecular weights greater than 200-300, such as fulvic acids, lignins, humic acids, and detergents. Low-molecular-weight, nonpolar, water-soluble solutes (for example, methanol, ethanol, and ethylene glycol) are poorly rejected.

Undissociated organic acids and amines are poorly rejected while their salts are readily rejected. For example, phenol is poorly rejected by RO membranes but when converted to its salt, rejections as high as 95-99% are observed. Also, rejection of acetic acid is only of the order of 50% but that of sodium acetate is as high as 90-95%.

The variable (and in some cases poor) removal characteristics of RO membranes dictate the use of auxiliary carbon filtration components either before or after (or both) the membrane. As in steam distillation, which has similar problems with organic materials, both RO and distillation require some type of organic removal mechanism such as replaceable carbon filters. The placement of carbon filters in RO systems depends on the type of membrane in use: for cellulose acetate or cellulose triacetate membranes, the carbon element is usually placed after the membrane and captive air tank, and just before the dispensing faucet. For thin-film membranes, a carbon filter is usually placed before and after the membrane. The carbon filter placed in front of the membrane is necessary since various types of organic materials and chlorine are detrimental to the structure of the thin-film membrane. Extra caution must be taken to regularly replace the carbon pre filter so as to ensure reasonable performance and lifetime for the TFC membrane.

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