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Concentration-Time Study For Water Treatment Plants

A Concentration-Time (CT) study is a formal determination of the disinfectant concentrations and the effective contact time provided by a water treatment plant.

The disinfection process of a water treatment plant is evaluated in a CT study based on analyses of the concentration (C) of the disinfectant and the theoretical contact time (T) of the disinfecting chemical in each stage of treatment. The purpose of the CT study is to:

  • Identify the number of disinfection zones at a water plant;
  • Determine the effective contact time, or T10, that is provided in each zone;
  • Determine if disinfection requirements are being met; and
  • Provide parameters used to determine daily compliance with disinfection requirements.

The information obtained in the CT study is used to complete surface water monthly operating reports (SWMORs) for surface water systems and groundwater systems under the influence of surface water (GUI), and 4-log groundwater monthly operating reports Microsoft Excel (4-log GWMORs) for groundwater systems.

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)Exit the TCEQ] 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, 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) Spreadsheet Microsoft Excel.

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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:

  • A cover page with the reason the CT is needed and the submitter’s contact information;
  • The Groundwater CT Study Template Microsoft Excel; 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
Technical Review and Oversight Team, MC-159
PO Box 13087
Austin TX 78711-3087

By fax:


Customer Service: (512) 239-4691

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