DBPs are formed upon the reaction of chemical disinfectants with DBPs precursors. Natural organic matter (NOM), commonly measured by total organic carbon (TOC), serves as the organic precursor, whereas bromide ion (Br-) serves as the inorganic precursor. DBP formation is influenced by water quality (e.g., TOC, bromide, pH, temperature, ammonia, carbonate alkalinity) and treatment conditions (e.g., disinfectant dose, contact time, removal of NOM before the point of disinfectant application, prior addition of disinfectant).
Chlorine in the form of hypochlorous acid/hypochlorite ion (HOCl/OCl-) reacts with bromide ion, oxidizing it to hypobromous acid/hypobromite ion (HOBr/OBr-). Hypochlorous acid (a more powerful oxidant) and hypobromous acid (a more effective halogenating agent) react collectively with NOM to form chlorine DBPs, including trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HAAs), haloketones, chloral hydrate and chloropicrin. The dominance of chlorine DBP groups generally decreases in the order of THMs, HAAs and HANs. The relative amounts of TOC, bromide and chlorine will affect the species distribution of THMs (four species: chloroform, bromoform, bromodichloromethane [BDCM] and dibromochloromethane [DBCM]), HAAs (up to nine chlorinated/brominated species) and HANs (several chlorinated/brominated species). Generally, chlorinated THM, HAA and HAN species dominate over brominated species, although the opposite may be true in high-bromide waters. Although many specific chlorine DBPs have been identified, a significant percentage of the total organic halogens still remain unaccounted for. Another reaction that occurs with chlorine is the formation of chlorate (ClO3-) in concentrated hypochlorite solutions.
Ozone can directly or indirectly react with bromide to form brominated ozone DBPs, including bromate ion (BrO3-). In the presence of NOM, non-halogenated organic DBPs, such as aldehydes, ketoacids and carboxylic acids, are formed during ozonation, with aldehydes (e.g., formaldehyde) being dominant. If both NOM and bromide are present, ozonation forms hypobromous acid, which, in turn, leads to the formation of brominated organohalogen compounds (e.g., bromoform).
The major chlorine dioxide DBPs include chlorite (ClO2-) and chlorate ions, with no direct formation of organohalogen DBPs. Unlike the other disinfectants, the major chlorine dioxide DBPs are derived from decomposition of the disinfectant as opposed to reaction with precursors.
Use of chloramine as a secondary disinfectant generally leads to the formation of cyanogen chloride (CNCl), a nitrogenous compound, and significantly reduced levels of chlorine DBPs. A related issue is the presence of nitrite (NO2-) in chloraminated distribution systems.