Historical perspective - Crystal Clear
Of all disinfectants currently in the market, free chlorine (hypochlorous acid) has been used for potable water disinfection since the early 1900s. It is the most widely used of all the oxidative disinfectants due to its low cost and proven effectiveness. It is an excellent bactericide, viricide and cysticide. In 1974, the Dutch scientist Johnnes Rookidentified chloro and bromo trihalomethanes (THMs), the first class of halogenated disinfection by-products (DBPs) in chlorinated drinking water.
In 1975, the U.S. Environmental Protection Agency (EPA) conducted a survey that identified chloroform as being dominant in most chlorinated drinking water. In cases where bromide was present in the source water, the addition of chlorine formed brominated THMs. The concentrations of THMs in the finished water were correlated to the total organic carbon (TOC) concentrations in the raw water. Natural organic matter (NOM), which is the major constituent of TOC, was found to be a major component of the precursors that react with chlorine to form THMs.
Chloroform was identified in 1976 by the National Cancer Institute as a suspected human carcinogen, which led EPA to set a maximum contaminant level (MCL) for total THMs (TTHMs) of 0.100 milligrams per liter (mg/L). This standard applied to systems serving more than 10,000 people. The decision to regulate TTHMs was made because the health effects of individual THMs were not well known.
EPA published a guidance manual in 1981 for controlling THMs in drinking water. Aeration and activated carbon adsorption were considered to be effective removal technologies and several options were considered effective for DBP precursor removal, including:
- Activated carbon adsorption;
- Oxidation by ozone or chlorine dioxide;
- Clarification by coagulation, precipitate softening or direct filtration;
- Oxidation with potassium permanganate;
- Lowering pH; or
- Moving the point of chlorination downstream in the treatment process to allow for the removal of precursors to DBPs prior to disinfection.
Many water utilities adopted several strategies in response to the TTHMs MCL. A common modification was to move the point of chlorination downstream from raw water application to sedimentation basin effluent, decrease chlorine dosages and use chloramine as an alternative primary or secondary disinfectant in place of free chlorine. Many utilities were able to meet the TTHMs MCL with such modifications, but questions arose regarding the integrity of the finished water in terms of microbial inactivation.
In the 1980s, dichloroacetic acid, trichloroacetic acid, haloacetonitriles, haloketones, chloropicrin, cyanogen chloride and chloral hydrate, to name a few, were found in chlorinated drinking water. Several of these halogenated DBPs were found to have adverse health effects by the National Academy of Sciences in 1987. More concerns about waterborne viral diseases escalated, leading EPA to promulgate the Surface Water Treatment Rule (SWTR) and the Total Coliform Rule (TCR) in 1989. The SWTR required three logs, or 99.9 percent inactivation, of Giardia cysts and four logs, or 99.99 percent inactivation, of viruses (Giardia cysts cause giardiasis disease). Log credits were given to utilities depending on the level of treatment.
The CT concept was later introduced, requiring that water be in contact with sufficient concentration of disinfectant (C) for a sufficient time (T) in order to provide adequate disinfection under different water quality conditions. CT values for different disinfectants were developed. The SWTR and TCR also required a residual disinfectant level and disinfection practices in the distribution system for continued microbial protection.
Some utilities found that they could meet either the TTHMs MCL or the SWTR/TCR regulations, but not both. Further complicating the issues were the findings that, in addition to chlorine DBPs having adverse health effects, alternative disinfectants also formed by-products with adverse health effects. In 1983 Haag and Hoigne found that bromate ion and brominated organics were formed in bromide containing ozonated water. Bromate was later identified as a suspected human carcinogen. Chlorite, which causes hepatotoxicity in animals, was found in chlorine dioxide disinfected water.
In addition to the findings of other disinfection by-products, the emergence of cryptosporidium a waterborne parasite responsible for major disease outbreaks in three U.S. cities, the most recent of which infected 370,000 people in Milwaukee, WI in April of 1993. The EPA proposed mandatory testing for cryptosporidium as a result of these outbreaks in water supplies further complicated the issue. The main challenge was and still is to effectively disinfect water while minimizing the formation of harmful by-products.
EPA has attempted to confront this challenge and balance the risk associated with DBPs against the risk associated with microbial disease, but several factors have made this difficult, including the lack of occurrence data for the viruses Giardia and Cryptosporidium and halogenated DBPs, uncertainty about the health effects of various DBPs and the lack of data on DBPs of alternative disinfectants such as ozone and chlorine dioxide which became widely adopted. Another vital piece of data is the public identification of more halogenated DBPs, given that the hundreds of halogenated DBPs found to date account for only about 50 percent of total organic halide (TOX).
Because of the complex issues that EPA faced, the agency had to draw on the expertise of others to prepare a rule on its own. In 1992, EPA initiated a negotiated rulemaking process, termed RegNeg, which consisted of an advisory committee of individuals who had interest in the regulation, including utilities, state drinking water agencies, environmental groups, consumer advocates and public health officials.
Due to the complexity of the issues, the regulations were proposed in two stages. Stage one of the D/DBPs rule was proposed in 1994 and became effective in 1998. It lowered the TTHM MCLs to 0.80 mg/L and provided MCLs for three other classes of DBPs. The rule also provided maximum residual disinfectant level goals (MRDG) for three disinfectants:
- An MRDG for chlorine was set at 4 mg/L;
- An MRDG for chloramines was set at 4 mg/L; and
- An MRDG for chlorine dioxide was set at 0.8 mg/L.
Enhanced coagulation and granular activated carbon (GAC) were considered as best available technologies (BAT) for precursor control. The information collection rule (ICR) was proposed to provide necessary data for stage two of the D/DBP regulation, which is anticipated to be proposed this year. The ICR requires utilities serving more than 10,000 to begin monitoring for microbial contaminants and DBPs. Also, GAC and membrane pilot testing is required for surface water utilities with raw water TOC greater than 4 mg/L and serving more than 100,000 people as well as for groundwater with finished water TOC greater than 2 mg/L and serving more than 50,000 people.
Click here to post comments about this topic, and read what others have to say.
This article appeared in Environmental Protection magazine, August 2000, Vol. 11, No. 8, p. 41.
This article originally appeared in the 08/01/2000 issue of Environmental Protection.