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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 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. EPA has determined that four of these chemicals, or classes of chemicals, pose potential health risks and must be regulated. The table provides the maximum contaminant level goal (MCLG), maximum contaminant level (MCL) and potential health effects for these disinfection byproducts. 

Contaminant MCLG(mg/L) MCL (mg/L) Potential Health Effects from Long-Term Exposure Above the MCL
Bromate zero 0.010 Increased risk of cancer
Chlorite 0.8 1.0 Anemia; infants and young children: nervous system effects
Haloacetic acids (HAA5) N/A* 0.060 Increased risk of cancer
Total Trihalomethanes (TTHMs) N/A* 0.080 Liver, kidney or central nervous system problems; increased risk of cancer

 *Although there is no collective MCLG for this contaminant group, there are individual MCLGs for some of the individual contaminants:

  • Haloacetic acids: dichloroacetic acid (zero); trichloroacetic acid (0.3 mg/L)
  • Trihalomethanes: bromodichloromethane (zero); bromoform (zero); dibromochloromethane (0.06 mg/L)

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 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

Other Disinfection Byproducts Rule

For those systems using Chlorine Dioxide or Ozone, they must comply with the requirements listed under the Other Disinfection Byproducts Rule. Information on these requirements can be found on the webpage Public Water Systems using Chlorine Dioxide or Ozone. If you have questions relating to this rule or about these disinfectants, please email

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 Total Trihalomethane (TTHM) and the Total Haloacetic Acids (HAA5) and establishes the Operational Evaluation Level requirement.

Operational Evaluation Level Requirements:

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.

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).

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

The Stage 2 DBP Rule establishes the compliance calculation uses the locational running annual average (LRAA) and is compared against the MCL. Calculate the LRAA for each site by adding the four most recent quarters of data and dividing by four.

LRAA formula: (Q1 + Q2 + Q3 + Q4) / 4 = LRAA

  • If the LRAA for TTHM is more than 80 µg/L then you have an MCL Violation.
  • If the LRAA for HAA5 is more than 60 µg/L then you have an MCL Violation.

Locating Your DBP Sample Results and Sample Schedule

Use Drinking Water Watch to access DBP sample results and sample schedules. General instructions on where to locate sample results and the sample schedules can be found on the TCEQ webpage Instructions for DWW.

DBP Sample Schedules

DBP sample schedules could be listed under two headers on the Sample Schedule/FANLs/Plans page:

  • Other DBP Schedules can be found:
    • Under Individual Non-TCR Sample Schedules as multiple schedules under the different sample points with the individual analytes of 1009 – Chlorite and/or 1011 – Bromate
  • Stage 2 DBPR Schedules can be found:
    • Under Group Non-TCR Sample Schedule with the analyte group code of DBP2
    • or under Individual Non-TCR Sample Schedules as two separate schedules with the individual analyte codes of 2456 – Total Haloacetic Acids (HAA5) and 2950 – TTHM.

DBP Sample Results

DBP samples can be found under the following analyte codes when looking under chemical results sorted by name.

  • Chlorite sample results can be found under the code 1009
  • Bromate sample results can be found under the code 1011
  • TTHM sample results can be found under the code 2950 total trihalomethanes. 
  • HAA5 sample results can be found under the code 2456 for total haloacetic acids.

The most recent results will be found at the top of the Results List table.

If you are a system trying to interpret the results received from the lab for TTHM and HAA5, please review the YouTube video for DBP sample results.

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. If the system does not notify TCEQ, it puts the system at risk of receiving a monitoring and reporting violation.

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

If you are looking for the requirements for the other Disinfection Byproducts Rule, please visit the webpage Public Water Systems using Chlorine Dioxide or Ozone.

Violation Process

When a violation is issued, TCEQ will mail your public water system a letter notifying the public water system 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 the system receives a notice of violation letter, they must read and follow the instructions. It is the water systems responsibility to ensure the notice is provide to the customers and is correct and complete. A system can choose to include additional information about the violation or an explanation about how the system is solving the issue, but it cannot refute or contradict the mandatory language. For community water systems all public notices must be either 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. If there are any questions relating to the public notice process, please visit the the Drinking Water Public Notice Page or email

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.

Unregulated Contaminant Monitoring Rule

There are several unregulated DBPs that are under consideration for regulation under the Unregulated Contaminant Monitoring Rule (UCMR). The EPA uses the UCMR to collect data for contaminants that are suspected to be present in drinking water and do not have health-based standards set under the Safe Drinking Water Act (SDWA). If you are looking for information on this rule, the contaminants currently under consideration, or need assistance with your UCMR sampling, please visit the EPA UCMR Webpage .

Contacts and Assistance

  • 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