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Concentration-Time Study For Water Treatment Plants

A Concentration-Time (CT) study of a water treatment plant is a technical determination of disinfection performance based on the disinfectant concentrations and the effective contact time of each disinfectant.

The performance of a water treatment plant's disinfection process is evaluated in a CT study based on analyses of the concentration (C) of a disinfectant and the theoretical contact time (T) of a disinfectant used in each stage of treatment. The purposes of the CT study are to:

  • Identify the number of disinfection zones at a water treatment plant;
  • Determine the effective contact time, or T10, that is provided in each disinfection zone;
  • Calculate crucial CT parameters to track the water treatment plant's daily disinfection performance; and
  • Ascertain the total effect of all disinfection zones to verify the water treatment plant complies with disinfection requirements.

The information obtained in the CT study is used to complete surface water monthly operating reports (SWMORs) for surface water systems, including groundwater systems with sources of groundwater under the influence of surface water (GUI). The SWMOR form used for surface water systems using alternate technologies (SWMOR-Alt) can be completed with the CT Calculator Microsoft Excel Document to help report operational data. A groundwater CT study is a treatment tool groundwater systems can use to record a groundwater treatment plant's disinfection performance in a 4-log groundwater monthly operating report Microsoft Excel Document (4-log GWMOR).

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Who must keep a CT study on file?

Every treatment plant that uses surface water or GUI must have a current, TCEQ-approved CT study on file.

If your groundwater system is achieving 4-log inactivation to comply with the groundwater rule, then you must have an approval letter on file and meet all the requirements stated in that letter.

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When should I complete (or update) the CT study?

You must submit a CT study to us for review and approval:

  • When we require a CT study as a condition to the granting of an exception request;
  • When a new plant that has been approved for construction by our Plan Review Team is approximately 90 percent complete or three months from startup; and
  • When a treatment plant undergoes renovation, a revised CT study is required if significant changes to the plant's disinfection processes occurred. You must not make significant changes to your treatment plant's disinfection processes without first getting our written authorization. [See 30 TAC 290.39(j) ] Significant changes to disinfection processes include any of the following:
    • changing the type of disinfectant;
    • moving any point of disinfectant addition or monitoring;
    • changing physical facilities which changes the contact time to be more or less effective;
    • changing the approved capacity of the plant;
    • the addition of a new source; or
    • any other change that we consider significant.

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Who may submit the CT study?

As the licensed operator, you may complete the CT study yourself or hire a Texas-licensed professional engineer to do it for you. Tracer studies must be prepared under the supervision of a Texas-licensed professional engineer.

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How do I prepare a CT study?

To prepare a surface water CT study, use the Surface Water CT template Microsoft Excel Document, completed according to the CT Template Instruction Manual, or calculate the effective T10 time. To calculate the effective T10 time, use either of these methods:

To prepare a groundwater CT study, use the Groundwater Minimum Specified Residual (GWMSR) Template Microsoft Excel Document.

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What information is required in the CT study?

For a surface water or GUI system, include this information:

For a groundwater system, include this information:

  • Items listed in the CT Study Checklist;
  • The GWMSR Template; and
  • A schematic and description of the plant with specific details of each unit including, but not limited to:
    • the dimensions of the unit,
    • the minimum and maximum water levels of the ground storage tanks,
    • the disinfectant injection points, and
    • the location of the monitoring points.


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How do I submit the completed CT study?

Submit your completed CT study one of these ways:

By mail:

Texas Commission on Environmental Quality
Corrosion Control and Disinfection Protocol Team, MC-159
PO Box 13087
Austin TX 78711-3087

By email:

Customer Service: (512) 239-4691

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