Contamination by Toxic Organics (Various Specific Primary MCLs)
Currently there are 57 organic chemicals out of the 83 contaminants regulated under the U.S. Safe Drinking Water Act, with MCLs ranging from 10 mg/L for xylene to only 0.00000003 mg/l for dioxin. It is beyond the scope of this Short Course to discuss each one, or even to try to classify them all in terms of toxicity or solubility, but it is important to know about the sources of many of them and whether they are adsorbable to activated carbon.
Agricultural Pesticides: Half (27) of them are insecticides and herbicides, and most of those are chlorinated hydrocarbons which are quite insoluble, and therefore highly adsorbable to particles and to activated carbon. Those containing nitrogen (amines) and phosphorus (phosphates) are more soluble and Iess adsorbable, and those containing several oxygen atoms (alcohols [hydroxides] and organic acids with – COOH groups) are not well adsorbed to anything and are very difficult to remove, except by RO.
Industrial Solvenis and Byproducts: There are 22 of these. Most industrial solvents are small, volatile, low-molecular-weight molecules that readily penetrate RO membranes and adsorb only poorly to activated carbon. Nine more byproducts of plastics and rubber manufacturing are variable in carbon adsorption: some are adsorbed extremely well, and others extremely poorly. But most of these are larger than the solvents and can be removed by RO membranes.
Disinfection Byproducts (PBPs): This refers mostly to byproducts of chlorine disinfection, acting on naturally-occurring humus and “color bodies,” but ozone can produce the same chemicals because it is a powerful oxidizer that can change chloride ion to chlorine. At present, only one chemical “family” is regulated-the THMs or trihalomethanes, as exemplified by chloroform, CHCI3. The MCL is 100 ppb total THMs (TIHMs), but that is expected to change significantly soon. In addition, two more families of DBPs, the halogenated acetic acids (HAAs) and acetic acid nitriles (HANs), win be regulated in the coming years. All are presumed to be carcinogens, and THMs within the presently permitted levels have also been implicated in human miscarriages.