Disinfection CT Value Calculator - EPA Surface Water Treatment Rule Compliance

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

Assumptions

• CT tables are based on EPA Surface Water Treatment Rule data for free chlorine as the primary disinfectant
• Contact time (T) uses T10 value — the time for the fastest 10% of water to pass through the contact basin, not theoretical detention time
• Chlorine residual (C) is measured at the outlet of the contact basin as free chlorine in mg/L
• Temperature and pH interpolation between EPA table values uses linear interpolation for intermediate conditions
• Giardia and virus inactivation requirements assume surface water or groundwater under the direct influence of surface water
• The baffling factor used to estimate T10 from theoretical detention time follows EPA guidance categories (poor=0.1 to superior=0.7)

Limitations

• Only covers free chlorine disinfection — chloramines, ozone, chlorine dioxide, and UV disinfection have separate CT/dose requirements
• Does not calculate Cryptosporidium inactivation, which requires the Long Term 2 Enhanced SWTR bin classification and additional treatment
• CT tables assume steady-state conditions — transient events (filter breakthroughs, sudden flow increases) may temporarily violate CT requirements
• Does not account for combined chlorine residual, which provides less disinfection credit per mg/L than free chlorine
• Baffling factor estimates are approximate — actual T10 should be determined by tracer study for compliance purposes
• pH above 9.0 and temperatures below 32 degrees F are outside the published EPA CT table range

References

• EPA — Guidance Manual for Compliance with the Filtration and Disinfection Requirements (SWTR CT tables)
• EPA — LT1ESWTR Disinfection Profiling and Benchmarking Technical Guidance Manual
• AWWA Manual M20 — Water Chlorination/Chloramination Practices and Principles (CT concepts and application)
• Ten States Standards — Recommended Standards for Water Works (contact time and disinfection requirements)
• EPA 815-R-99-013 — Disinfection Profiling and Benchmarking Guidance Manual
• White's Handbook of Chlorination and Alternative Disinfectants, 5th Edition (CT theory and disinfection kinetics)

Frequently Asked Questions