Chlorine Dioxide and Chlorite in Drinking Water
Public water systems that use chlorine dioxide in the treatment of drinking water must comply with the maximum residual disinfectant level for chlorine dioxide, the maximum contaminant level for chlorite, and monitoring and sampling requirements.
On this page:
- Chlorine Dioxide and the Disinfection Byproduct Chlorite
- Maximum Residual Disinfectant Level - Chlorine Dioxide
- Maximum Contaminant Level - Chlorite
- Monitoring, Sampling, and Analysis
- Health Effects
- Reporting Requirements
- Consumer Notification
- Assistance and Tools for Public Water Systems
Chlorine Dioxide and the Disinfection Byproduct Chlorite
Chlorine dioxide can be used in the treatment of drinking water to improve taste, odor, and color, remove iron and manganese, and inactivate chlorine resistant microorganisms. Public water systems (PWS) may choose to use chlorine dioxide because it is less likely to form disinfection byproducts (DBPs) than free chlorine.
Chlorite is a DBP that can form when chlorine dioxide is used to treat water. All PWS that use chlorine dioxide must monitor for chlorine dioxide and chlorite.
Monitoring, Analyzing, and Reporting Chlorine Dioxide and Chlorite (RG-503) - Additional details about requirements and procedures for monitoring, analysis, reporting, and complying with chlorine dioxide and chlorite rules can be found in the regulatory guidance.
Maximum Residual Disinfectant Level - Chlorine Dioxide
PWSs must adhere to requirements for the maximum residual disinfectant level (MRDL). If your PWS exceeds 0.8 mg/L on a daily chlorine dioxide sample, you must collect three additional chlorine dioxide samples.
If additional samples are not collected:
- this is an acute MRDL.
If additional samples are collected:
- and any sample is over 0.8 mg/L - this is an acute MRDL.
- and none of the samples exceed 0.8 mg/L - this is a non-acute MRDL.
An acute MRDL requires the PWS to notify customers as soon as possible but no later than 24 hours after the violation is identified.
A non-acute MRDL requires the PWS to notify customers as soon as possible but no later than 30 days after the violation is identified.
Maximum Contaminant Level - Chlorite
PWS must adhere to requirements for the maximum contaminant level (MCL). Compliance for chlorite is determined based on the average of a 3-sample set of chlorite samples. If the average exceeds 1.0 mg/L, this is a MCL violation and the PWS must notify customers as soon as possible but no later than 30 days after the violation is identified.
Monitoring, Sampling, and Analysis
PWS are responsible for collecting chlorine dioxide and chlorite samples; collection of samples must be performed by a licensed water operator.
- Make sure your monitoring plan is up to date with the frequency and location of chlorine dioxide and chlorite sample sites.
- PWS Monitoring Plans and Lab Approval - Information about monitoring plan requirements.
- Make sure TCEQ has the correct chlorite distribution sample locations.
- Drinking Water Viewer (DWV) - Use DWV to ensure the locations are correct. If the sample locations need to be updated, email DBP@tceq.texas.gov.
- Determine if your facility is approved to analyze chlorine dioxide entry point, chlorine dioxide distribution, and chlorite entry point samples.
- Approved PWS Drinking Water Lab list - Review this list to make sure your facility is approved for those analyses. If you're not on the list, complete Drinking Water Laboratory Approval Form (TCEQ-10450).
- Environmental Laboratory (NELAP) Accreditation - Find a NELAP accredited laboratory to analyze your distribution chlorite samples.
| Number Samples | Frequency | Sampling Location | Monitor For | Sample Analysis By |
| 1 | Daily | Entry Point | Chlorine Dioxide | Approved Facility |
| 3 (a 3-sample set) | If EP daily sample is more than 0.8 mg/L | Distribution | Chlorine Dioxide | Approved Facility |
| 1 | Daily | Entry Point | Chlorite | Approved Facility |
| 3 (a 3-sample set) | Monthly and if EP daily sample is more than 1.0 mg/L | Distribution | Chlorite | Accredited Lab |
Collect one sample for chlorine dioxide and one sample for chlorite daily at the entry point if chlorine dioxide is being used.
If any daily chlorine dioxide level exceeds 0.8 mg/L, three follow-up chlorine dioxide samples must be collected in the distribution system. The location of these samples depends on the presence of chlorination facilities in the distribution system.
If there is a chlorination facility in the distribution system, sample at the following locations and times:
- At the service connection nearest the treatment plant two hours after the exceedance.
- At the first service connection after the point where water is re-chlorinated six hours after the exceedance.
- Near the end of the distribution system eight hours after the exceedance.
If there is no chlorination facility in the distribution system:
- Samples must be collected at the service connection nearest the treatment plant at two-, six-, and eight-hour intervals.
Collect a 3-sample set for chlorite from the distribution system every month and if any daily chlorite level exceeds 1.0 mg/L. The samples in the set must be collected on the same day and when chlorine dioxide is in use. Every entry point providing water treated with chlorine dioxide must have a corresponding 3-sample set. Collect samples from all three of the following locations:
- A location near the first customer.
- A location representative of the average residence time.
- A location representative of the maximum residence time.
Health Effects
Some infants and young children who drink water containing chlorine dioxide in excess of the MRDL or chlorite in excess of the MCL could experience nervous system effects. Similar effects may occur in fetuses of pregnant women who drink water containing chlorine dioxide in excess of the MRDL or chlorite in excess of the MCL. Some people may experience anemia.
Reporting Requirements
Chlorine Dioxide Monthly Operating Report (CLO2 MOR), TCEQ-00690 – The PWS must submit this report and chlorite distribution laboratory analysis results to TCEQ by the 10th day of the month following the monitoring period by mail.
TCEQMC-155, Attn: ClO2/ChloritePO BOX 13087Austin, TX 78711-3087
CLO2 MOR instructions - Use these instructions to complete the CLO2 MOR. Systems that did not use chlorine dioxide for a month are still required to submit a CLO2 MOR.
Consumer Notification
Public Notice Language for Drinking Water Compliance - If your PWS has an MCL, MRDL, or monitoring and reporting violation, you can find required language and certificate of delivery documents organized by contaminant to use for public notification.
Consumer Confidence Report - Community water systems must provide the minimum, maximum, and average levels of any contaminant detected in their annual Consumer Confidence Report to customers by July 1 of each year. A community water system supplies water to the same population year-round.
Texas Drinking Water Viewer - Find sample results and monitoring schedules.
Assistance and Tools for Public Water Systems
Contact TCEQ at DBP@tceq.texas.gov or 512-239-1062 if you have questions about how to calculate the MCL, MRDL, find sample schedules and sample results on Texas Drinking Water Viewer.
EPA Stage 2 Disinfectants and Disinfection Byproducts Rule Operational Evaluation Guidance Manual - Information about specific factors that influence DBP formation and ways to control formation can be found in this EPA guidance.
Financial, Managerial and Technical Assistance - TCEQ offers FREE financial, managerial, and technical (FMT) assistance to help PWSs comply with regulations. Contact FMT for help with issues such as, but not limited to preventing operational problems, training about how to comply with rule requirements and performing capacity assessments.
Technical Guidance for Public Water System Operators and Engineers - Information about technical questions, such as submitting plans, exploring treatment options, or requesting exceptions to design or capacity rules.