The Impact of Chlorination Disinfection Byproducts on Water Quality

Chlorination disinfection is a disinfection technology that has been used in my country for many years and is still commonly used in water treatment. However, in the past two decades, it has been gradually discovered that chlorination reacts with certain organic and inorganic components in water during disinfection, generating a series of halogenated organic byproducts, most of which pose a potential threat to human health. In particular, in traditional pre-chlorination processes, high concentrations of chlorine react directly with high concentrations of organic pollutants in the raw water, resulting in even higher concentrations of chlorinated byproducts. Therefore, chlorination disinfection byproducts are a significant factor affecting drinking water quality.

Volatile trihalomethanes (THMs) and non-volatile haloacetic acids (HAAs) are considered the two major categories of chlorination disinfection byproducts. Their formation in water depends on the type and concentration of organic precursors, the amount of chlorine added, the chlorination time, the pH of the water, temperature, and the concentrations of ammonia nitrogen and bromides. The precursors of trihalomethanes and haloacetic acids are mainly humic acid, fulvic acid, algae, and some small organic molecules with active carbon atoms. As pH increases, the formation of trihalomethanes increases, but the formation of haloacetic acids decreases. In the presence of ammonia nitrogen, the yield of trihalomethanes is lowest before the inflection point of the chlorination curve. The formation of trihalomethanes increases significantly when free residual chlorine is present in the water. In recent years, it has been found that brominated trihalomethanes pose a greater potential hazard to human health. When bromides are present in water, bromide ions (Br-) are oxidized by hypochlorous acid (HOCl) to hypobromoic acid (HOBr), which reacts more readily with precursors than hypochlorous acid to form brominated trihalomethanes and brominated haloacetic acids. The formation of brominated trihalomethanes and brominated haloacetic acids in water generally increases with the initial [Br-]/[HOCl] molar ratio and [Br-]/[DOC] ratio. Because the concentration of dissolved organic carbon in the filtered water is lower than that in the original water, while the concentration of bromide ions remains essentially unchanged, the [Br-]/[DOC] ratio of the filtered water is higher than that of the original water, leading to an increase in the content of brominated trihalomethanes and brominated haloacetic acids after chlorination.

Trihalomethanes and haloacetic acids are potential carcinogens. Current water quality standards for total THMs (THMs) in water generally limit their total concentration or the concentration of chloroform. The drinking water standards in the United States and the United Kingdom stipulate a maximum allowable concentration of THMs (TTHMs) in tap water of 100 μg/L. my country's current drinking water standards use chloroform concentration as the limiting indicator, setting its maximum allowable concentration in tap water at 60 μg/L, but do not limit the concentration of brominated trihalomethanes (BTHAs). In 1994, the World Health Organization proposed reference concentrations of chloroform and bromodichloromethane in tap water of 200 μg/L and 60 μg/L, respectively, at a carcinogenic risk level of 10⁻⁵; while the reference concentrations for dichloroacetic acid and trichloroacetic acid were 50 μg/L and 100 μg/L, respectively. In its newly drafted "Restriction of Disinfection Byproducts," the United States sets the allowable concentration of THMs in tap water at 80 μg/L; and the maximum allowable concentration of haloacetic acids (THAAs) at 60 μg/L. Currently, my country's drinking water standards only regulate chloroform among total hydrogen monoxide (THMs), neglecting to regulate brominated trihalomethanes (BHMs) and haloacetic acids. Given the greater health risks of BHMs and haloacetic acids, their restrictions in drinking water should be given sufficient attention.

Halogenated phenols are also non-volatile chlorination disinfection byproducts. The following chlorophenols are primarily detected in water after chlorination disinfection: 2-chlorophenol, 3-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol, and 2,4,6-trichlorophenol. Chlorophenols are products of the reaction between chlorine and phenolic compounds, and are also degradation products of some pesticides. During the reaction of chlorine and phenol, when the concentration ratio of chlorine to phenol (Cl2/phenol, in mg/mg) is between 1 and 4, chlorophenols are the main chlorination disinfection byproduct. The maximum formation of several chlorophenols occurs at a chlorination disinfection byproduct ratio of approximately 3. When the chlorine-to-phenol ratio exceeds 4, the formation of trihalomethanes increases significantly. The main problem caused by chlorophenols is odor. Chlorophenols are compounds with a strong, pungent odor that significantly affects the sensory properties of water. However, some chlorophenols, such as 2,3,4-trichlorophenol and 2,4,6-trichlorophenol, have high Ames mutagenic activity. Since the latter is generated in large quantities during chlorination disinfection, its impact on drinking water quality cannot be ignored.

In addition, various other chlorination disinfection byproducts have been detected in tap water, such as MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone] and its isomer E-MX [E-2-chloro-3-(dichloromethyl)-4-oxobutadiene acid] and its methyl ester form Me-MX [3-chloro-4-(dichloromethyl)-5-methoxy-2(5H)-furanone], as well as haloacetonitrs, haloketones, haloacetaldehydes, and halonitromethanes. Ames test results showed that MX, E-MX, and Me-MX are strong mutagens. Some haloacetonitrs (such as dichloroacetonitrile and bromochloroacetonitrile) were positive, with dichloroacetonitrile causing skin cancer; trichloroacetonitrile and bromochloroacetonitrile caused lung cancer. Some haloketones (such as 1,1,1-trichloroacetone, 1,1,3-tetrachloroacetone, pentachloroacetone, hexachloroacetone, etc.) were all positive in the Ames test. All chloroacetaldehydes were positive, with monochloroacetaldehyde and trichloroacetaldehyde decreasing liver enzyme activity. Halogenated nitromethanes are indirect mutagens.

The World Health Organization (WHO) set the reference concentrations for dichloroacetonitrile, dibromoacetonitrile, and trichloroacetonitrile in tap water at 90 μg/L, 100 μg/L, and 1 μg/L, respectively, in 1994.

my country's drinking water sources are not only severely polluted, but also, due to factors such as soil erosion, surface humus enters water bodies with surface runoff, resulting in a generally higher concentration of organic matter in surface water compared to developed countries. This higher concentration of organic matter in filtered water inevitably increases chlorine consumption during the chlorination disinfection process, affecting disinfection effectiveness. Furthermore, the increased chlorine consumption leads to the formation of higher levels of halogenated organic compounds, impacting drinking water quality.