Chlorine was first introduced into drinking water in the early 1900s for protection against water-borne diseases. Since then, it has been the most widely-used disinfectant in the United States. However, the addition of chlorine to water in the presence of naturally occurring organic matter results in the formation of numerous harmful by-products. As more became known about the potential by-products associated with chlorination, the search began for alternate disinfectants, such as ozone and chlorine dioxide.
These alternatives, unfortunately, form harmful by-products of their own. The U.S. Environmental Protection Agency (EPA) has recently introduced regulations that further lower the disinfection by-products limits as well as set new limits to newly identified by-products. In addition, the EPA set new limits to the amount of disinfectant that can be used in drinking water.
The balance between the risk of microbial contamination and disinfection by-product formation is a challenge. Currently, there are several options for the drinking water disinfection, each with merit as a disinfectant and with by-products that must be minimized.
Disinfection by-products formation and control
Disinfection by-products (DBPs) are potentially toxic and/or carcinogenic substances that form when disinfectants are added to water. Depending on the disinfectant used and the precursor materials present in the water, several classes of DBPs may form including trihalomethanes (THMs), haloacetic acids (HAAs), chlorate, chlorite and bromate.
When DBPs are a concern in a water treatment process, there are two main approaches to solving the problem. One approach is to control the precursors that react with the disinfectant to form the unwanted DBP. The other approach is to allow the DBPs to form and then use a separate removal process for them.
Precursor control and removal strategies are mainly focused on natural organic matter (NOM) present in water. NOM is considered to be the major precursor to DBP formation. NOM is very site specific, with the different components of NOM (e.g. humic and fulvic acids, etc) being removed with varying degrees of effectiveness by different strategies. Research into this area has focused on characterizing the behavior of NOM according to apparent molecular weight (AMW). Precursor management is often grouped into three categories:
- Control at the source by managing inputs into the watershed to lower precursor concentrations;
- Physical/chemical removal that involves the removal of precursors by processes such as coagulation, adsorption and membrane separation; and
- Oxidation/transformation which involves processes that change the form of precursors.
After DBPs have formed it is possible to remove them with a subsequent treatment process. EPA has specified air stripping and GAC adsorption as techniques for the removal of THMs.
As chlorine was the disinfectant of choice for nearly 100 years and is used by the majority of water treatment systems, its DBPs are usually considered to be of the greatest concern. Chlorine DBPs form when free chlorine (HOCl) is added to water and reacts with the natural organic matter (NOM) present. The generalized equation describing the formation of the halogenated DBPs is:
HOCl + Br- + NOM
THMs and other halogenated DBPs. The major halogenated DBPs that result from the addition of chlorine to drinking water are THMs, HAAs, haloacetonitriles (HANs), cyanogen halides, halopicrins, haloketones, haloaldehydes and halophenols. Some of the major types of these DBPs are listed in Table 1.
In the absence of bromide ion (Br-), only the chlorinated by-products are formed. In the presence of bromide, free chlorine (HOCl) rapidly oxidizes bromide to hypobromous acid (HOBr), which then reacts, along with the remaining HOCl, with NOM to produce the mixed chloro-bromo DBPs.
It has been found that THMs and HAAs are the most common DBPs found in the treatment process. EPA has set a maximum contaminant level (MCL) of 0.100 milligrams per liter (mg/L) for total THMs (TTHMs) and has proposed a new MCL of 0.080 mg/L. In addition to these standards, a MCL for HAA5 of 0.060 mg/L. TTHMs is defined as the sum of four individual THMs: chloroform, bromoform, dibromochloromethane and bromodichloromethane. HAA5 is defined as the sum of five HAAs listed in Table 1: MCAA, DCAA, TCAA, MBAA and DBAA.
Natural organic matter (NOM) is the predominant precursor for the formation of chlorinated DBPs and, due to the fact that THMs were the first DBPs to be regulated, most methods of precursor removal deal with lowering the concentration of NOM in water. THM precursors have been found to be removed by several processes.
Source control. It has been found in some watersheds that the adsorption capacity of soils can affect the amount of DOC transport in water. In soils with low exchange capacity, it is possible to improve the adsorption capacity by the addition of adsorbents such as lime, gypsum or alum sludge.
Enhanced coagulation. This is among the simplest strategies for utilities already using conventional coagulation. Enhanced coagulation may involve any of the following: An increase in coagulant dose, pH adjustment and alternate coagulants. There are concerns associated with enhanced coagulation, including turbidity removal, corrosion and increases in contaminant concentrations, such as aluminum, in the finished water.
Adsorption. NOM has been found to be adsorbed by granular activated carbon (GAC), powdered activated carbon (PAC) and other adsorbing materials.
Anion exchange. At neutral to basic pH values, much of the NOM present in water exists as negatively charged ions (i.e. anions). These anionic, organic precursors are amenable to removal by anion exchange.
Slow sand filtration. Slow sand filters amended with granular media such as anionic resins and GAC can achieve significant removal of organic carbon and THM formation potential, frequently exceeding 75 to 90 percent.
Reverse osmosis. The reverse osmosis (RO) process is effective in reducing concentrations of nonvolatile organics. RO treatment of waters with high concentrations of THM precursors prior to chlorination can significantly reduce the level of THMs in the final potable water.
High energy electron beam irradiation Chloroform can be controlled at pilot scale by the use of innovative high energy electron beam irradiation.
Advanced oxidation. The use of ozone, in conjunction with other disinfectants to provide a residual, can significantly reduce the formation of THMs
Natural organic matter (NOM) is the predominant precursor for the formation of chlorinated DBPs and, due to the fact that THMs were the first DBPs to be regulated, most methods of precursor removal deal with lowering the concentration of NOM in water.
Ozone is formed by the passage of air or oxygen through an electrical discharge. The resultant air-ozone stream can be bubbled through water in a contact chamber. Ozone is considered the most effective oxidant and disinfectant used in the water treatment process, but it is unstable and cannot maintain a residual in the water supply system. As a result, when ozone is used as a primary disinfectant a secondary disinfectant must also be used so that the residual can be maintained. Like chlorine dioxide, ozone is an unstable gas and must be manufactured onsite.
The use of ozone for disinfection will produce no chlorinated THMs, HAAs or other chlorinated by-products. It will, however, form a variety of oxidation products in the presence of NOM, following the reaction:
O3 + NOM
Oxidation by-products. Oxidation by-products include aldehydes, aldo- and ketoacids, acids and hydrogen peroxide. These are listed in detail in Table 2. Although ozone itself does not produce halogenated DBPs, it can produce brominated DBPs if bromide-containing waters are ozonated, following the reaction:
O3+ Br- + NOM
Brominated by-products. Ozone will oxidize the bromide (Br-) to hypobromous acid (HOBr), which will react with NOM to produce the fully brominated analogs of the chlorination by-products shown in Table 1 and again in Table 3. Bromate ion is the by-product of greatest concern. It has been classified by EPA as a B2 carcinogen (a probable human carcinogen.) Bromate ion has been found to be removed by the processes shown in Table 3.
Chlorine dioxide (ClO2) is primarily used as an oxidant, although recently it has seen use as a primary disinfectant as well. It requires lower CT values and inactivates Giardia, a disinfectant-resistant pathogen, five times faster than free chlorine. As an oxidant, it is highly effective for taste and odor control and iron and manganese oxidation. ClO2 is an unstable gas, requiring that it be manufactured onsite.
ClO2 will form very few, if any, halogenated DBPs. What few by-products it does form however, are very undesirable. The DBPs of greatest concern with ClO2 are chlorate and chlorite, both of which are toxic and carcinogenic.
While ClO2 is a very effective disinfectant and forms no THMs, there is a concern that 50 to 70 percent of the applied ClO2 dosage will remain as residual chlorite (ClO2-).
Chlorite has been found to be removed by the processes listed in Table 4.
Chloramine has been used as a primary disinfectant in some treatment plants since the early 1920s. Chloramines are formed by adding chlorine and ammonia to water at certain ratios of chlorine to ammonia. It is not as effective as free chlorine in disinfection or oxidation and may take 100 times longer to achieve the same bacteriological kill. Chloramines require significantly greater CT values than free chlorine, and when chloramines are used, it is often in combination with additional disinfectants.
The use of chloramines can greatly reduce the formation of THMs and HAAs, but it may instead form chloral hydrate. While it is currently not regulated, it is being considered in future legislation for classification as a DBP. In water containing cyanide, chloramines will form cyanogen chloride and cyanogen bromide to a greater degree than free chlorine. If the chloramines used in the disinfection process were formed by chlorination followed by ammonia addition, THMs may form. Chloramination may also result in nitrate and nitrite formation as the chloramines decompose. The major chloramine DBPs are listed in Table 6.
Little research has been done on technologies for the removal of chloramine-specific DBPs. However, since it forms DBPs similar to chlorine DBPs, many of the processes applied to chlorination DBPs will be effective for chloramine DBPs.
Ultraviolet (UV) light is an effective disinfectant for bacteria and virus control, but it does not provide a residual and can only be used as a primary disinfectant in combination with another secondary disinfectant. UV light is not a reliable disinfectant for Giardia and Cryptosporidium cysts.
Since the use of UV radiation does not require the addition of any chemicals to water being treated, no by-products are formed. The main concern over the use of UV for disinfection purposes is that a residual cannot be maintained in the water treatment system.
UV irradiation does not form any DBPs. The main concern in its use is the lack of a disinfectant residual to ensure no microbial recontamination occurs. This increase in microorganisms after treatment may occur due to properties of the pipeline system which transports the water or the presence of substances in the pipe that form part of the food chain for bacteria. Despite this concern relating to the lack of residual effects, many communities do not experience the hygiene problems in their pipeline systems that would lead to recontamination.
Lingering questions and concerns
The proposed regulations are confounded by many concerns that remain unresolved, such as the:
- Risks associated with many animal or epidemiological studies on DBPs remaining uncertain;
- Concerns that alternative disinfectants to chlorine have their own consequent problems as well as possibly additional DBPs that may be harmful to health;
- Concerns with the technological and economical feasibility of processes that can meet very low cancer risk levels as well as be applied in all geographic regions of the United States; and
- Concerns that the balance between the D/DBP rule and the ESWTR regulations will lead to higher risk of either microbial contamination or DBP formation.
National Primary Drinking Water Regulations: Disinfectants and
Disinfection Byproducts; Final Rule www.epa.gov/fedrgstr/EPA-GENERAL/1998/December/Day-16/g32887.htm
IESWTR D/DBPR Microbial/Disinfection Byproducts (M/DBP) Center www.awwa.org/dbp/overview.htm
National Primary Drinking Water Regulations: Monitoring Requirements for Public Drinking Water Supplies; Final Rule www.epa.gov/docs/fedrgstr/EPA-WATER/1996/May/Day-14/pr-20972DIR/pr-20972.txt.html
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This article appeared in Environmental Protection magazine, August 2000, Vol. 11, No. 8, p. 38.
This article originally appeared in the 08/01/2000 issue of Environmental Protection.