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Executive Summary DISINFECTION Chlorination is the most widely used method for disinfecting water supplies in the United States. It is convenient to use, elective against most waterborne pathogens, and continues disinfectant activity within the distribution system. Chlorination is the standard disinfectant against which others are compared. However, chlorination can result in the formation of trihalomethanes (THM's) and other halogenated hydrocarbons. The discovery that some of these products are carcinogenic for experimental animals has prompted a reexamination of alternate disinfectants and procedures. The comparative electiveness of 12 disinfectants or processes tor inactivating microorganisms (bacteria, viruses, protozoa) were evaluated. Chlorination, ozonization, and the use of chlorine dioxide come closest to meeting the criteria established for a drinking water disinfectant. The ultimate choice among methods will require weighing the disinfectant efficacy, detailed in this evaluation, against the toxicity of the products produced by the use of a particular method of disinfection. CHEMISTRY The major objective of the review of disinfectant chemistry is the identification of products that are likely to be formed by the use of 1
2 DRINKING WATER AND H"LTH . . specific disinfectants. The identification of known and theoretical products of disinfection, which is attempted herein, is intended to be a guide to those contaminants that might require removal or toxicological evaluation. There is a large and rapidly growing body of scientific literature on the products of chlorination in drinking water. Comparable information for other disinfectants is scarce. This lack of data on alternative disinfectants should not lead to the conclusion that they are free of the difficulties encountered with chlorine. Quite apart from the question of their efficacy as alternative disinfectants, there remains the question, "Will the substitution of a disinfectant for chlorine in water treatment merely produce a different set of by-products whose ejects on human health may be as significant, or more so, than those products known to be produced from chlorine?" Clearly, each disinfectant chemical that was examined in this survey produces by-products that may occur in actual water treatment applica- tions. Of particular concern are the following substances that are either known to or could result from the use of the various disinfectants. From chlorine: the trihalomethanes (THM's), trichloroacetone (CC13COCH3), and other largely uncharacterized chlorinated and oxidized intermediates that are formed from the complex set of precursors in natural waters; chloramines; chlorophenols; and the largely unknown products of dechlorination. From ozone: epoxides, which theoretically result from unsaturated substrates such as oleic acid, although none have yet been found in drinking water; peroxides; and other highly oxidized intermediates such as glyoxal (OHCCHO) and methylglyoxal (CH3COCHO) from aromatic precursors. From bromine and iodine: THM's and other bromine and iodine analogs of chlorinated species; bromophenols; bromoindoles; bromoani- soles; plus the halogens themselves, which may remain in drinking water as residual. From chlorine dioxide: chlorinated aromatic compounds; chlorate (C1O3-) and chlorite (C1O2-), which are often present as by-products or unreacted starting material from production of chloride dioxide; and chlorine dioxide itself. This list, incomplete as it is, is compelling in that it shows that the use of each disinfectant could result in products that should be examined in more detail.
Executive Summary 3 GRANULAR ACTIVATED CARBON Raw water and disinfected water supplies may contain organic com- pounds that have been demonstrated to be carcinogenic or otherwise toxic in experimental animals or in epidemiological studies. Properly operated granular activated carbon (GAC) systems can remove or effectively reduce the concentration of many of these harmful compounds. Less is known about synthetic resins than about GAC, but it is known that they can be applied to remove certain types of organic contaminants. This study of GAC provides data on adsorption iso- therms, percent removal, and competitive equilibria of a wide variety of organic compounds. The information available as of this date on the treatment of water with GAC provides no evidence that harmful health effects are produced by the process under proper operating conditions. However, there are incomplete studies on the possible production of such effects with virgin or regenerated carbon through or · reactions that may be catalyzed by the GAC surface; · reactions of disinfectants with GAC or compounds adsorbed on it: · reactions mediated by microorganisms that are part of the process; · by the growth of undesirable microorganisms on GAC. Studies are also needed on the properties of regenerated activated carbons and on the adsorption of additional contaminants with potential health ejects. The data regarding microorganisms on GAC beds and their metabolic products, including endotoxin production, are quite limited. The efficacy of biodegradation is believed to be less than adsorption, although microbial activity has been shown to remove organic compounds as measured by group parameters such as total organic carbon, potassium permanganate demand, chemical oxygen demand, and W absorbance. No evidence was found for removal of specific organic compounds of potential harm to health by microbial activity. The evidence for and against prechlorination and preozonization enhancing biodegradation of organic compounds is presented. GAC serves as a catalyst for some reactions in water systems, e.g., oxidation, reduction, and polymerization. These reactions can produce organic and inorganic species that were not present originally. However, little can be said concerning the degree of their occurrence during water treatment and their possible impact on public health.
4 DRINKING WATER AND H"LTH Regeneration procedures influence the chemical properties of GAC. This, in turn, influences adsorption, catalytic properties, and leachable chemicals. The frequency of GAC regeneration is determined by the organic compounds in the water and their competitive interactions. The types and concentrations of organic compounds may vary widely in different locations and seasons of the year. Competitive interactions are complex and presently cannot be predicted without data from laboratory and/or pilot scale tests on the water to be treated. While there is ample evidence for the effectiveness of GAC in removing many organics of health concern, more data are needed in the quantification of any harmful health effects related to the use of GAC. This need, however, should not prevent the present use of GAC at locations where analysis of the water supply clearly indicates the existence of a potential health hazard greater than that which would result from the use of GAC. Clarification processes (coagulation, sedimentation, filtrations remove significant amounts of some organics, especially some types of THM precursors and relatively insoluble compounds that may be associated with Articulates. In some cases, the removal of THM precursors by clarification may be sufficient to eliminate the need for an adsorption process.