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Disinfection Byproducts in Public Water Systems

What are disinfection byproducts, how do they form, and how can public water systems control them?


Disinfection Byproducts that the TCEQ Regulates

Disinfection byproducts (DBPs) are chemicals that form, during drinking water treatment and distribution, when naturally occurring organic matter reacts with chlorine or other disinfectants used to kill pathogenic organisms. The EPA has determined that four of these chemicals, or classes of chemicals, pose potential health risks and must be regulated. 

Contaminant MCLG(mg/L) MCL or TT (mg/L) Potential Health Effects from Long-Term Exposure Above the MCL (unless specified as short-term) Sources of Contaminant in Drinking Water
Bromate zero 0.010

Increased risk of cancer

Byproduct of drinking water disinfection

Chlorite 0.8 1.0

Anemia; infants and young children: nervous system effects

Byproduct of drinking water disinfection

Haloacetic acids (HAA5) n/a 0.060

Increased risk of cancer

Byproduct of drinking water disinfection

Total Trihalomethanes (TTHMs) n/a 0.080

Liver, kidney or central nervous system problems; increased risk of cancer

Byproduct of drinking water disinfection


 The TCEQ's rules also apply to systems that use chlorine dioxide.

Disinfectant MRDL (mg/L) Systems that must monitor Potential Health Effects from Long-Term Exposure Above the MCL (unless specified as short-term) Sampling Frequency
Chlorine dioxide 0.8 All public water systems that use chlorine dioxide

Anemia; infants and young children: nervous system effects

Daily, in the event of a daily exceedance

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Factors that Influence DBP Formation

By considering conditions that favor DBP formation, public water systems can work to achieve lower DBP levels. Water systems can effectively balance disinfection and limit disinfection byproduct formation. Factors influencing DBP formation:

  • Concentration of precursors - the precursors for DBPs are carbon molecules, so reducing total organic carbon (TOC) with coagulation or filtration can reduce DBP levels.
  • Disinfectant type or dose - free chlorine produces the most DBPs; chlorine dioxide and chloramines produce less; ozone does not produce any TTHMs or HAA5s.
  • Water temperature - the chemical reactions that form DBPs are sped up by warmer temperatures, therefore DBP levels tend to increase in the warmer spring and summer months.
  • Disinfectant contact time - the longer water sits in the distribution system, the more time there is for DBPs to form.
  • Water chemistry - formation of TTHMs in relation to HAA5s change with pH. HAA5 is more likely to form at low pH, and TTHM is more likely to form at high pH.
  • Bromide - elevated levels can lead to increased formation of brominated DBPs.

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Controlling DBP Levels

There are many options available for treating or preventing DBPs that vary in cost and complexity. Your strategy to solve the issue should identify the cause of the high DBPs and then target those factors.

Treatment Options Precursors Disinfectant Temperature Contact Time Chemistry
Change or Blend Water Source X   X   X
Modify Reservoir Operations X        
Purchase Water X   X X X
Eliminate Raw Water Chlorination   X   X  
Optimize Disinfectant Dose   X      
Utilize or Optimize Powdered Activated Carbon (PAC) X        
Optimize Current Coagulation Method X       X
Change Coagulation Method X       X
Change Disinfectant   X   X  
Implement Post Filtration Granular Activated Carbon (GAC) X        
Implement Membrane Treatment X        
Optimize Distribution System Chlorination   X   X  
Change Secondary Disinfectant   X   X  
Optimize Distribution System/Tank Operation       X  
Flush System       X  

Not every option is feasible for every public water system, but many are relatively simple. Disinfectant levels must meet disinfection standards, but there is often room to optimize the dose. Water systems can also change the type of disinfectant. Chloramines, chlorine dioxide, and ozone will form fewer DBPs, but require other considerations. Precursors can be reduced by coagulation. If coagulation is already completed, its performance may be optimized by adjusting dose, methods, or changing coagulants. Activated carbon can also be used to remove precursors.

Typically higher DBP levels are found in areas of the distribution with the highest water age (the oldest water). Systems should  try to decrease the amount of time finished water remains in storage and in the distribution system. Periodic flushing of sections of the distribution system prone to long retention times can reduce DBP levels.

DBP reduction must be balanced with disinfection, corrosion control, and other requirements. For instance changes in pH could corrode pipes and create lead and copper issues. Insufficient chlorine residuals could leave a system vulnerable to pathogenic organisms. As with any change in water operations, it is essential to evaluate compliance with other measures simultaneously.

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Stage 2 Disinfection Byproducts for TTHMs and HAA5s

All community and non-transient, non-community water systems must comply with the Stage 2 Disinfection Byproducts Rule. In addition to setting monitoring and reporting standards, the Stage 2 DBP Rule changes the compliance calculation for TTHM and HAA5 and establishes the Operational Evaluation Level requirement. The Operational Evaluation Level helps public water systems identify high DBP levels and act to decrease levels to avoid a maximum contaminant level (MCL) violation. More information about the OEL can be found at Operational Evaluation Requirements.

What are the MCLs for TTHM and HAA5?

The MCL for TTHM is 0.080 mg/L and 0.060 mg/L for HAA5.

Who conducts TTHM and HAA5 monitoring?

TCEQ uses contract samplers to collect all TTHM and HAA5 samples used for compliance. TCEQ pays for the sample collection and the public water system is responsible for paying the laboratory for the analysis of the sample. TTHM and HAA5 samples are analyzed by the Lower Colorado River Authority (LCRA) or the Department of State Health Services (DSHS).

You can view your sample results on Drinking Water Watch. If you need help navigating the website please contact the Drinking Water Quality Team at (512) 239-4691 or

How do I calculate the MCL under the Stage 2 DBP Rule?

The Stage 2 DBP Rule establishes the compliance calculation for the locational running annual average (LRAA). Calculate the LRAA for each site by adding the four most recent quarters of data and dividing by four. Compliance is no longer calculated by using all results from all sites.

Q1 + Q2 + Q3 + Q4 / 4 = LRAA

Example data set: 

 Example Data Set

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Locating your DBP Sample Results and Sample Schedule

Use Drinking Water Watch to access DBP sample results and sample schedules.

Sample Schedules

1. Go to Drinking Water Watch.

2. Enter your public water system ID or your public water system name and click Search for Water Systems. Do not enter both.

3. Click on the public water system ID number on the left side of the screen. 

4. Click on Sample Schedules/FANLs/Plans located in the yellow text at the top of the page, first column, fourth row. 

5. Scroll down, past the TCR Sample Schedules. 

Sample Results

6. Click on Name in Chemical Results: Sort by: Name Code located in the yellow text at the top of the screen, third column, fourth row.

7. Scroll down to the bottom of the page. Click on the code 2456 for total haloacetic acids or 2950 total trihalomethanes. 

8. The most recent sample results are located at the top of the page. The sample result is in the second to last column named Concentration

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Choosing Sample Sites for TTHM and HAA5

The intent of the Stage 2 DBP Rule is to identify areas that are high in TTHMs or HAA5s or have the potential to have high results. In most cases these sites were designated using the Initial Distribution System Evaluation (IDSE) and should not be changed except in a few distinct circumstances:

  • Water flow or water age has changed within the distribution system,
  • A new plant, source, or entry point has come on-line,
  • A customer refuses to allow water system personnel to collect samples,
  • Sample site is at a dead end main.

Notify TCEQ of the change request at least two weeks before sampling.

Water system personnel need to evaluate the distribution system to identify locations that have the potential to form increased DBP levels.

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TTHM and HAA5 Violations

A maximum contaminant level (MCL) violation is assigned:

  • If the locational running annual average (LRAA) exceeds 0.060 mg/L for HAA5, or 
  • If the LRAA exceeds 0.080 mg/L for TTHM.

A monitoring and reporting violation is assigned if a system:

  • Fails to collect or submit sample results to TCEQ,
  • Fails to perform or submit an operational evaluation after an OEL exceedance.

A public notice violation is assigned if a system:

  • Fails to distribute a public notice to customers,
  • Fails to submit the public notice and certificate of delivery to TCEQ, or
  • Fails to submit the public notice to customers or TCEQ by the required deadline.

Violation Process

When a violation is assigned, TCEQ will mail your public water system a letter notifying you about the violation. The letter is addressed to the administrative contact as listed in Drinking Water Watch and includes an explanation about the violation, sample site the violation occurred at, results used to calculate compliance, public notice template, certificate of delivery, and TCEQ contact information. If you receive a violation letter please read it and follow the instructions. It is your responsibility to ensure the notice you provide to your customers is correct and complete. You can include additional information about the violation or an explanation about how the system is solving the issue. For community water systems all public notices must be mailed or hand delivered to bill paying customers and must be posted in public places. You can find public notice due dates in Drinking Water Watch under Violations

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  • To request free assistance for your DBP issues contact the Financial, Managerial, and Technical Assistance program at (512) 239-4691 or
  • To view your water system's DBP results, visit Drinking Water Watch.
  • For help navigating Drinking Water Watch contact the Drinking Water Quality Team at (512) 239-4691 or