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Water Quality Drinking Water 3.1 Water Treatment Suspended particulates that contribute to total organic carbon are generally removed during the pre-treatment step usually by a combination of coarse and fine screens, micro-strainers and/or simple gravity settling (National Health and Welfare 1993). The
major mechanism for removal of dissolved substances such as
humic and fulvic acids is the coagulation/sedimentation process
(National Health and Welfare 1993). In British Columbia, very
few water purveyors apply treatment beyond disinfection. Organics
removal by coagulation has been found to be optimal at pH 4
to 6. Removal of organic material by aluminum and iron salts
is effective with removal efficiencies varying from 55 to 90%
between source waters (Reckhow and Singer 1990). Oxidation
of organic carbon by chemical oxidants other than chlorine
(e.g., hydrogen peroxide, ozone, UV radiation) are possible
methods for removing organic carbon while also avoiding the
formation of trihalomethanes. Removal of organic carbon compounds
by activated carbon has had limited success because of early
breakthrough problems (Symons et al. 1982).
Few human or animal toxicity studies of dissolved organic carbon compounds such as humic and fulvic acids have been undertaken. The limited studies that have been conducted indicate that these compounds are not toxic at the levels that could occur in drinking water. For example, male rats supplied with soil fulvic acid for up to 90 days at levels of 10, 100 and 1000 mg/L showed no significant changes in body weight, intake rates, organ/body weight ratios or tissue histology (Health Canada 1996). Little information is available on the toxicities of metals and their humate complexes (Health Canada 1996). The primary reasons for reducing organic carbon in drinking water are not related to the toxicity of the organic carbon compounds themselves but rather to the desire to reduce the formation of trihalomethanes (THMs) following chlorination (Young and Singer 1979; Vogt and Regli 1981; Rathbun 1996; and numerous others), and avoid the objectionable colour that arises when humic and fulvic acids are present at high levels (Smith and Davies-Colley 1992; Smith et al. 1991). Potential harm to humans can arise due to the reaction between humic and fulvic acids and chlorine to form THMs, trichloroacetic acid, dichloroacetic acid, haloketones and haloacetonitriles (Reckhow and Singer 1990; Rook 1977). The reaction of hypochlorous acid with the methyl ketone groups (acetyl groups) in humic acids to form trihalomethanes is shown in the equation below (Manahan 1984; Bunce 1990).
R-CH3O + 3 HOCl = CO2 + CHCl3 + 2H2O The haloform reaction shown in equation 1 is base-catalyzed and, as such, the rate is dependent on the pH of the reaction medium. The base-catalyzed reaction removes a proton from the carbon double bond causing the formation of the enolate ion. This is the rate determining step and the enolate ion will subsequently undergo chlorine substitution at the carbon in the alpha position (the carbon next to the carbonyl carbon). The resulting trichloromethyl ketone then hydrolyzes, causing the cleavage of the carbonyl carbon and alpha carbon bond and yielding chloroform and a carboxylic acid. Although humic substances are likely the major source of trihalomethanes, other organic carbon substances can also be trihalomethane precursors. For example, with some alkenes, chlorine adds to an activated double bond, the byproduct of which may be oxidized to methyl ketones, and may then undergo the haloform reaction. Similarly, meta-hydroxy phenolic compounds and cyclohexanes containing a methylene group flanked by two carbonyls can yield trihalomethanes. Many of the compounds produced as a result of chlorination of organic carbon compounds are probable carcinogens to humans or have been shown to be mutagenic (Health Canada 1996; Reckhow and Singer 1990). Reckhow and Singer (1990) observed that the average yield of THMs in drinking waters of seven U.S. cities was 52.2 µg/mg total organic carbon. In cities with high organic carbon concentrations in the raw water (about 15 mg/L), concentrations of THMs were observed in the low mg/L range (Reckhow and Singer 1990).
Health Canada has not established a drinking water quality guideline for either dissolved or total organic carbon (Health Canada 1996). Since 1978, a number of water quality parameters have been identified as not requiring a guideline, of which total organic carbon was one (Health Canada 1996). No rationale was given for total organic carbon specifically. In general, drinking water quality guidelines are not developed for parameters in which: (i) currently available data indicate no health risk or aesthetic problem, or (ii) the parameter is composed of several constituents for which individual guidelines exist or may be required. Guidelines do exist for several parameters related to dissolved and total organic carbon. For example, an aesthetic objective of 15 TCU has been established for true colour (Health Canada 1996), which obviates the need for a dissolved organic carbon guideline. Similarly, an aesthetic objective of 500 mg/L for total dissolved solids and a maximum acceptable concentration of 1 NTU have been established (Health Canada 1996), thus addressing some of this drinking water quality issues that would be associated with high levels of total organic carbon. No criteria for dissolved or organic carbon for protection of drinking water were found in the world literature or on the worldwide web. The US EPA recently issued the Disinfectants and Disinfection By-Products Rule that specifies, amongst other things, maximum TOC levels of 2 mg/L in treated water and 4 mg/L in source water to ensure that disinfection byproducts such as trihalomethanes are present at acceptable levels (Pontius 1993).
The water quality criteria for total organic carbon are 2 mg/L for treated water and 4 mg/L for source water. The criteria should not be exceeded at any time in drinking water systems that use chlorination for disinfection. For systems that do not disinfect or which use other methods for disinfection (e.g., ozonation), the criteria do not apply. The appropriate methodology for determining total organic carbon is discussed in section 1.4 and in BC Environment (1994).
The production of haloforms in drinking water as a result of the haloform reaction between organic carbon compounds and hypochlorous acid is a serious drinking water quality issue. The removal of excess organic carbon prior to chlorination will reduce the production of THMs and other substances that complex to humates at low levels. The interim maximum acceptable concentration for trihalomethanes derived by Health Canada (1996) for drinking water is 0.1 mg/L. A comparison of data from drinking water sources in the United States shows that, if TOC can be maintained below 2 mg/L in the drinking water effluent, there is a high probability that the THMs guideline of 0.1 mg/L will not be exceeded (Martin 1994). Several
studies have shown that dissolved and total organic carbon
levels are strongly correlated with water colour (e.g., Effler
et al. 1985; Gorham et al. 1986; Evans 1988). Concentrations
of approximately 3 to 5 mg/L are associated with waters having
a true colour of approximately 15 to 30 mg/L Pt (Hutchinson
and Sprague 1987; Henriksen et al. 1988; Brakke et al. 1988;
Matuszek and Beggs 1988; Welsh et al. 1993). This suggests
that the drinking water quality criteria for both organic carbon
parameters to address aesthetic concerns would be about 3 mg/L,
thus further supporting the criteria proposed above.
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