TOC, an acronym for Total Organic Carbon or Total Oxidizable Carbon, is a non-specific test that measures the amount of carbon bound in organic material and is reported in units of "ppm as carbon". Since the TOC only measures the amount of carbon in organic matter, the actual weight of the organic mass can be up to 3 times higher in natural surface waters. Organics are compounds that contain carbon (with the exception of carbon dioxide, bicarbonate and carbonate). In water treatment, organics can be classified as naturally occurring or man-made.
Naturally occurring organic matter are typically negatively-charged colloids or suspended solids, comprised of tannins, lignin’s, water soluble humic acid compounds resulting from the decay of certain vegetative matter, or fulvic acid compounds resulting from the decay of certain vegetative matter. Naturally occurring organic material can be a foulant to RO membranes, particularly the negatively charged composite polyamides. Neutrally charged RO membranes (e.g. neutrally charged composite polyamides and cellulose acetate) are more resistant to organic fouling. A RO will reject organic compounds. Generally, organic compounds with a molecular weight greater than 200 are rejected at levels greater than 99%. The rejection of compounds with molecular weights less than 200 will vary based on molecular weight, shape and ionic charge. As a rough rule of thumb, alert levels for potential organic fouling in natural water sources are TOC at 3 ppm, BOD at 5 ppm, and COD at 8 ppm.
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