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Disinfection CT Value Calculator - EPA Surface Water Treatment Rule Compliance

Verify Giardia and virus log inactivation from chlorine residual, contact time, temperature & pH

Free CT value calculator for drinking water treatment operators. Enter chlorine residual, contact time (T10), water temperature, and pH to calculate your CT value and check compliance with EPA Surface Water Treatment Rule requirements. Includes required CT lookup tables for Giardia and virus inactivation at 0.5 to 3.0 log removal credits. Automatically interpolates between temperature and pH data points for accurate results.

Pro Tip: Winter is when CT compliance gets tight. At 35°F and pH 8.0, you need roughly 5 times the CT value compared to summer conditions at 70°F and pH 7.0. If your contact basin barely passes in August, it will fail in January unless you increase chlorine residual or reduce flow rate.

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Disinfection CT Value Calculator
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How It Works

  1. Enter Chlorine Residual

    Input the free chlorine residual in mg/L measured at the end of your contact basin. Typical values: 0.5-2.0 mg/L for surface water treatment.

  2. Enter Contact Time (T10)

    This is the T10 value - the time for 10% of the water to travel through your contact basin at peak flow. T10 is NOT the same as theoretical detention time. Use tracer study results or multiply theoretical detention time by a baffling factor (0.3-0.7 depending on basin design).

  3. Enter Temperature and pH

    Use your coldest water temperature (worst case) and highest pH. Both increase the required CT value significantly.

  4. Check Compliance

    The calculator shows your CT ratio (actual/required). A ratio of 1.0 or higher means you meet the requirement. Below 1.0 means you need more residual, more contact time, or both.

Built For

  • Daily compliance verification for surface water treatment plants
  • Monthly reporting of CT values for state regulatory agencies
  • Evaluating impact of seasonal temperature changes on disinfection adequacy
  • Planning for pH or flow changes that affect CT compliance
  • Training new operators on the relationship between CT components

Frequently Asked Questions

CT stands for Concentration times Time. It is the product of disinfectant residual (C, in mg/L) multiplied by contact time (T, in minutes). CT is measured in mg·min/L and represents the total disinfection exposure. Higher CT means more pathogen kill. The EPA requires minimum CT values based on target pathogen, temperature, and pH.
T10 is the time for the fastest 10% of water to pass through the contact basin. It accounts for short-circuiting - the reality that some water moves through faster than average. Using theoretical detention time would overstate your actual disinfection contact time and give a false sense of compliance.
Cold water requires much higher CT values because chlorine reacts more slowly at lower temperatures. At pH 7.0, the required CT for 3-log Giardia inactivation roughly doubles when water temperature drops from 50°F to 35°F. This is why winter is the critical period for CT compliance.
Your CT ratio (actual CT divided by required CT) must be 1.0 or higher to meet EPA requirements. Many states recommend maintaining a ratio of 1.5 or higher to provide a safety margin. A ratio below 0.8 is a significant compliance risk that needs immediate corrective action.
Disclaimer: This calculator uses EPA CT tables for free chlorine disinfection. Actual compliance determination must follow your state primacy agency requirements and approved monitoring plan. CT values for chloramines, ozone, and chlorine dioxide use different tables not included here. Always consult your state drinking water program for compliance questions.

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