The Water Quality Association defines the word “disinfect” as “to free from infection by either a chemical or physical means; causing the absence of pathogenic or indicator Coliform bacteria in drinking water.”
Chlorine has been used for more than 100 years as the main water disinfectant in the U.S. In 1974, it was discovered that the use of chlorine in drinking water produced trihalomethanes , due to chlorine reacting with naturally occurring organic matter. As a result of this discovery, the first regulation to address disinfection byproducts occurred in 1979 when the U.S. EPA established an interim maximum contaminant level (MCL) of 0.10 mg/L as an annual average for total trihalomethanes in public water supplies.
In 1996, the amendment of the Safe Drinking Water Act (SDWA) required the EPA to develop interrelated regulations aimed at reducing disinfection byproducts, as well as developing alternative disinfectants to reduce these byproducts and still control microbial contaminants. These regulations are collectively known as Microbial and Disinfection Byproduct Rules (M-DBP Rules).
In December 1998, The Stage 1 Disinfectants and Disinfection Byproducts Rule (Stage 1 DBP) and the Interim Enhanced Surface Water Treatment Rule (IESWTR) were announced. The Stage 1 DBP established maximum residual disinfectant level goals and maximum residual disinfectant levels for three disinfectants: chlorine, chlorine dioxide and chloramine. The rule also established MCL goals and MCLs for some of the common disinfection by-products including: total trihalomethanes, haloacetic acids, chlorite and bromate.
In addition, systems that use surface water, or groundwater under the direct influence of surface water are required to reduce total organic carbon (TOC) levels by using enhanced coagulation or enhanced softening. Measurement of TOC is used to indicate the amount of disinfection byproducts that may be produced; so, reducing TOC will reduce the amount of byproducts produced.
The IESWTR builds upon the requirements of the Surface Water Treatment Rule, and focuses on the control of Cryptosporidium by establishing a 2-log removal for systems filtering an MCL goal of zero for Cryptosporidium, and inclusion of Cryptosporidium in the watershed control requirements for unfiltered public water systems. The rule also tightens standards for turbidity performance and applies to systems serving 10,000 or more people. The Long Term 1 Enhanced Surface Water Treatment Rule became effective on Feb. 13, 2002 and applies all the regulations under IESWTR to smaller systems, those serving less than 10,000 people.
The most recent regulations include the Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 DBP) and the Long Term 2 Enhanced Surface Water Treatment Rule (LT2), both of which were published in the Federal Registry in January 2006. The Stage 2 DBP rule applies to all public water supplies that treat with a primary or residual disinfectant other than UV.
The rule requires all systems to evaluate their distribution systems, identifying locations with high levels of disinfection byproducts, which will become the sampling sites. The rule also requires systems to meet the MCLs for the TTHM and HAA5 as an average at each of the new monitoring locations instead of as a system-wide, average as was allowed with the Stage 1 DBP. The LT2 rule also builds upon established regulations by requiring systems using surface water, or groundwater under the direct influence of surface water to monitor water for Cryptosporidium monthly for two years.
Based on the results of the monitoring, the systems will be classified into one of four categories. Systems classified in the higher categories will be required to provide additional water treatment to further reduce Cryptosporidium levels. Any system that stores treated water in a reservoir must either cover the reservoir or treat the discharge to inactivate 4-log virus, 3-log Giardia lambia and 2-log Cryptosporidium.
Monitoring will begin in October 2006 and will continue for four years, depending on the schedule of the system. It is apparent with all the regulations that the EPA is concerned about the risks of microbial contamination, as well as the resulting disinfection byproducts.
As a result of these regulations, many water supplies have looked at alternative means of disinfection to reduce the formation of disinfection byproducts while still providing adequate microbial protection. Many systems are switching to chloramines—the combination of chlorine and ammonia—as the primary disinfectant. Because chloramines are not as reactive as chlorine, there is significantly less formation of disinfection byproducts. In addition, chloramines are more stable than chlorine, so they can provide better residual protection throughout the distribution system.
Chloramines, however, are toxic to fish and harmful to patients on dialysis. Additionally, chloramines react with certain types of rubber, specifically those used in hot water tanks, toilets and washing machines. Black particles are an indicator that these rubber materials are breaking down.
Chlorine dioxide is the third disinfectant regulated under the SDWA. Chlorine dioxide has been used for many years in treating municipal water for taste, odor, iron and manganese. Chlorine dioxide is a gas that is explosive under pressure, so it cannot be stored or shipped and must be generated on site. Chlorine dioxide is more effective than chlorine or chloramine for the inactivation of viruses, Cryptosporidium and Giardia. Its biocidal properties are not affected by the pH of the water, and it provides residual protection within the distribution system. Chorine dioxide can be expensive though, due to the cost of personnel training, raw materials to generate on site, and the sampling and testing for the potential byproducts, chlorite and chlorate.
UV and ozone are two other non- chlorine based disinfectants that are being explored as alternatives. In 1982, the FDA declared ozone as “generally recognized as safe” for use in bottled water, and in 1987, Los Angeles brought online an ozonation plant. Ozone is a powerful disinfectant and oxidant that can destroy viruses and other waterborne parasites that can be resistant to chlorine. Ozone is also used to oxidize nuisance inorganics such as iron, manganese and hydrogen sulfide, but it can also oxidize naturally occurring bromide, causing the formation of bromate. Bromate is a carcinogen and is regulated at 0.010 mg/L. Ozone is a gas that must be generated on site and, because of the electricity needed, can be much more expensive than chlorine.
UV is another option for disinfection processes that is gaining popularity in the U.S. UV is a non-chemical disinfectant that penetrates the cell wall of an organism, damaging its genetic material and preventing reproduction. UV has demonstrated its effectiveness against Cryptosporidium and other viruses, bacteria and cysts, and there are no known byproducts. The biggest drawbacks of UV are that it provides no residual protection and requires high-clarity water in order to be effective.
There are currently 11 disinfection byproducts that are regulated under the SDWA. These include four trihalomethanes and five haloacetic acids that are primarily formed in systems using chlorine-based disinfectants. Chlorite is commonly formed for systems using chlorine dioxide as the disinfectant, while bromate is a result of ozone application.
According to the Nationwide Disinfection Byproduct Occurrence Study undertaken by the EPA, there are more than 500 disinfection byproducts that have been identified. Because it is not feasible for the EPA to test more than 500 compounds for toxicity, it has prioritized them and has come up with a list of 50 “high priority” DBPs for further study. These high-priority DBPs include brominated, chlorinated and iodinated species of halomethanes, brominated and chlorinated forms of haloacetonitriles, haloketones, haloacids, halonitromethanes and analogues of MX (See Figure 1). Several new methods were developed in this study to help identify the presence of these new DPBs in drinking water.
Disinfection of our drinking water is necessary to reduce waterborne illness. The discovery of the formation of trihalomethanes from the use of chlorine disinfection led to the use of alternative disinfectants, which in turn has led to the formation of other DBPs. Many public water supplies are still struggling to reduce the formation of DBPs while still providing water that is free of microbial contaminants. Studies continue on the various disinfectants and disinfection byproducts; in the meantime, however, residential water treatment offers homeowners options for reducing the most common disinfection byproducts: trihalomethanes and haloacetic acids in their tap water.