Disinfection is essential for controlling microorganisms in drinking water and other water treatment applications. However, when chlorine-based disinfectants react with naturally occurring substances in source water, they can also produce disinfection by-products (DBPs).
The most commonly regulated DBP groups in U.S. drinking water include trihalomethanes (THMs)and haloacetic acids (HAA5). Other inorganic DBPs, such as chlorate, chlorite, and bromate, are associated with specific disinfectants and treatment conditions. Understanding how these compounds form—and how chlorine source, water chemistry, and operating conditions influence their formation—is important for optimizing water disinfection while maintaining control over DBP formation.
What Causes Disinfection By-Products to Form?
When chlorine is added to water, it establishes an equilibrium between hypochlorous acid (HOCl)and hypochlorite ion (OCl⁻):
HOCL ⇌ OCl– + H⁺
The relative concentration of HOCl and OCl⁻ depends primarily on pH. HOCl is generally the more powerful disinfecting species, while increasing pH shifts the equilibrium toward OCl⁻.
DBP formation is more complex than simply adding chlorine to water. It depends on several water-quality and operational parameters, including:
• Natural organic matter (NOM)
• Dissolved organic carbon (DOC)
• Bromide concentration
• Chlorine dose
• pH
• Contact time
• Temperature
• Disinfectant type and application method
NOM—particularly humic and fulvic substances derived from decaying vegetation, soils, and other organic material—is one of the most important organic precursor sources for THM and HAA formation.
Bromide is also important because chlorine can oxidize bromide to hypobromous acid (HOBr). HOBr can then participate in reactions with organic matter, contributing to the formation of brominated THMs and HAAs.
As a result, DBP formation is highly site-specific. Two water systems using the same chlorine product can produce different DBP profiles because their source-water chemistry and operating conditions are different.
How Different Chlorine Sources Influence DBP Formation
Different chlorine sources can influence DBP formation through differences in pH, chlorine concentration, product stability, storage conditions, and the chemical environment in which the disinfectant is introduced.
All chlorine-based disinfectants ultimately provide active chlorine species that participate in disinfection. However, the way the disinfectant is delivered can influence water chemistry and therefore affect DBP formation.
Chlorine Gas vs. Hypochlorites
Chlorine gas (Cl₂) reacts with water to form HOCl and hydrochloric acid, which can lower pH depending on dose and water chemistry. Sodium hypochlorite (NaOCl) and calcium hypochlorite [Ca(OCl)₂] introduce hypochlorite into the treatment system. Sodium hypochlorite solutions are typically highly alkaline, so their use can increase the pH of the treated water unless the system has sufficient buffering capacity or pH control.
Because pH influences the HOCl/OCl⁻ equilibrium and DBP formation kinetics, these differences can affect THM and HAA formation. However, the impact of chlorine source should not be considered independently from source-water characteristics, chlorine dose, contact time, temperature, and bromide concentration.
Chlorate Formation
Chlorate (ClO₃⁻) is an inorganic oxyhalide that can be associated with hypochlorite use and disinfectant degradation.
One important pathway is the disproportionation of hypochlorite during storage:
3 OCl⁻ → 2 Cl⁻ + ClO₃⁻
This is particularly relevant for concentrated sodium hypochlorite solutions because chlorate can accumulate during storage and subsequently enter the treated water.
Storage conditions—including temperature, concentration, exposure to light, and residence time—can influence hypochlorite degradation. For this reason, chemical storage and inventory management are important considerations when evaluating the overall DBP profile of a chlorination system.
Bromate Formation
Bromate (BrO₃⁻) is primarily associated with the ozonation of bromide-containing water. Chlorine-based treatment can also participate in bromine chemistry, but bromate formation should not be attributed exclusively to conventional chlorination.
Bromide concentration and the overall oxidation conditions of the treatment process are important factors when evaluating bromate risk.
Chlorite Formation
Chlorite (ClO₂⁻) is primarily associated with the use and breakdown of chlorine dioxide (ClO₂) as a disinfectant. Therefore, chlorite should be considered separately from THMs, HAAs, and chlorate when evaluating the DBP profile of a water treatment system.
THMs and HAAs: The Main DBPs Associated With Chlorination
The two major regulated organic DBP groups associated with chlorination are:
Trihalomethanes (THMs) which include compounds such as:
• Chloroform
• Bromodichloromethane
• Dibromochloromethane
• Bromoform
THM formation is strongly influenced by NOM, bromide concentration, chlorine dose, pH, contact time, and temperature.
In general, higher pH and longer reaction times can favor THM formation, although the actual result depends on the characteristics of the source water and treatment process.
Haloacetic Acids (HAA5) which refers to five regulated haloacetic acids:
• Monochloroacetic acid
• Dichloroacetic acid
• Trichloroacetic acid
• Monobromoacetic acid
• Dibromoacetic acid
Like THMs, HAAs are formed when disinfectants react with organic precursors in water.
The conditions that favor THM and HAA formation are not necessarily identical. Consequently, optimizing a treatment process requires evaluating the complete DBP profile rather than focusing on a single compound.
What About Organic or Stabilized Chlorine?
Chlorinated isocyanurates, such as dichloroisocyanurate and trichloroisocyanurate, are chlorine-based products that release active chlorine while also interacting with cyanuric acid in solution. Research has investigated whether these products can influence DBP formation and chlorine residual stability compared with conventional free chlorine. Some studies have reported differences in THM and HAA formation under specific test conditions, but these results should not be interpreted as a universal reduction in DBPs for every water source or application.
The presence of cyanuric acid can reduce the concentration of immediately available HOCl, which can change both disinfectant kinetics and DBP formation pathways. Therefore, the use of stabilized chlorine requires consideration of the intended application, required disinfection performance, water chemistry, and applicable regulatory requirements.
For drinking water applications, disinfectant selection should always be based on the specific treatment objective and applicable regulatory approvals.
Comparing Common DBPs by Disinfectant
A practical way to understand DBP formation is to associate each DBP with the treatment chemistry most strongly connected to its formation:
| DBP | Primary formation pathway / association |
| THMs | Chlorine reactions with natural organic matter; influenced by bromide, pH, dose, contact time, and temperature |
| HAA5 | Chlorine reactions with organic precursors; formation depends on source-water chemistry and treatment conditions |
| Chlorate | Particularly associated with hypochlorite degradation and storage |
| Chlorite | Primarily associated with chlorine dioxide use |
| Bromate | Primarily associated with ozone treatment of bromide-containing water |
This distinction is important because DBP control is not simply a matter of choosing one chlorine source over another. Effective control requires understanding the complete water chemistry and treatment process.
How Can Water Treatment Operators Control DBP Formation?
The most effective DBP management strategies generally focus on controlling the conditions that promote DBP formation.
1. Reduce DBP Precursors
Removing or reducing natural organic matter before chlorination can significantly reduce the amount of material available to form THMs and HAAs.
2. Optimize Chlorine Dose
Applying only the amount of disinfectant required to achieve the desired microbial control can help reduce unnecessary chlorine demand and DBP formation.
3. Control pH
Because pH influences chlorine speciation and DBP formation kinetics, maintaining appropriate pH conditions is an important part of process optimization.
4. Manage Contact Time
Longer chlorine contact times can increase DBP formation. Treatment systems should therefore balance the required disinfection contact time with DBP control objectives.
5. Control Chemical Storage Conditions
For hypochlorite products, proper storage and inventory rotation can help minimize product degradation and the accumulation of chlorate.
6. Monitor Source-Water Chemistry
Regular monitoring of parameters such as DOC, TOC, bromide, pH, temperature, and chlorine demand provides valuable information for predicting and controlling DBP formation.
PROVICHLOR TABLET™ and Water Disinfection
PROVICHLOR TABLET™ is an organic chlorine-based disinfection product designed to provide a controlled source of active chlorine for water treatment applications.
Like other chlorine-based disinfectants, its interaction with source-water chemistry can result in the formation of disinfection by-products under certain conditions. DBP formation should therefore always be evaluated as part of the complete treatment process rather than attributed solely to the disinfectant itself.
The use of a stabilized chlorine tablet system can provide operational advantages by delivering chlorine in a controlled, consistent format while simplifying chemical handling and dosing.
When used with PROVITAB 3™ chlorinators, PROVICHLOR TABLET™ provides a practical approach to continuous chlorine dosing for applications where consistent disinfectant delivery and simplified chemical handling are important operational priorities. The most effective DBP management strategy is not simply selecting a different chlorine product. It is optimizing the entire disinfection process—including source-water quality, chlorine dose, pH, contact time, residual control, and chemical storage.
By understanding these variables, water treatment professionals can improve microbial control while making informed decisions to minimize unwanted disinfection by-products.
For more information about PROVICHLOR TABLET™ and PROVITAB 3™ chlorination systems, contact our technical team to discuss your water treatment requirements.