CT is the product of disinfectant concentration (C, in mg/L) multiplied by contact time (T, in minutes). It is the single number that tells regulators whether your disinfection process is actually killing pathogens. The EPA Surface Water Treatment Rule requires specific CT values for inactivation of Giardia lamblia and viruses, and the numbers are not negotiable. If your calculated CT falls below the required value, you are out of compliance, regardless of how good your water looks.
The concept sounds simple: multiply your chlorine residual by the time it stays in contact with the water. But every piece of that calculation has a trap. The residual you measure depends on where you sample. The contact time depends on the hydraulics of your clearwell or pipe, not the theoretical volume. Temperature and pH both shift the required CT target. This guide breaks down each piece so you can calculate CT correctly and stay in compliance year-round.
What CT Means and Why It Matters
CT stands for concentration times time. C is the disinfectant residual in mg/L measured at the outlet of the contact basin (or the point of first customer use, depending on your state). T is the contact time in minutes, which is not the same as how long you think the water sits in the tank. The product of C × T gives you a value in mg·min/L, and that value must meet or exceed the target set by the EPA for the pathogen you are required to inactivate.
The Surface Water Treatment Rule (SWTR) requires 3-log (99.9%) inactivation of Giardia and 4-log (99.99%) inactivation of viruses for surface water systems. The required CT values are published in EPA guidance tables and vary by disinfectant type, temperature, and pH. For free chlorine at pH 7.0 and 20°C, the CT required for 3-log Giardia inactivation is about 104 mg·min/L. For 4-log virus inactivation under the same conditions, it is about 6 mg·min/L. Giardia is the harder target and almost always the controlling requirement.
You can achieve the required CT by having a high residual and short contact time, or a low residual and long contact time. A plant with a large clearwell and low demand might run 0.5 mg/L residual with 200 minutes of contact time (CT = 100). A plant with a small pipeline contact and high demand might need 2.0 mg/L residual with 50 minutes of contact time (CT = 100). Both can be compliant, but the second plant is spending more on chemical and operating closer to the margin.
The critical point: CT is not about average conditions. Compliance is based on worst-case conditions, meaning the lowest residual and shortest contact time that occur during normal operation. If your residual drops from 1.0 to 0.5 during peak demand, and your contact time drops because flow increased, your CT might fall below the target even though it was fine an hour ago.
CT = C × T10
C = disinfectant residual (mg/L) at the outlet
T10 = contact time (minutes) at which 10% of the water has passed through the basin
Units: mg·min/L
Must meet or exceed EPA-required CT for target pathogen at actual temperature and pH.
Disinfection CT Value Calculator
Calculate CT values for chlorine disinfection and verify EPA Surface Water Treatment Rule compliance. Check Giardia and virus log inactivation credits based on residual, contact time, temperature, and pH.
T10 vs Theoretical Detention Time
Theoretical detention time is the volume of the contact basin divided by the flow rate. A 100,000-gallon clearwell at 500 GPM has a theoretical detention time of 200 minutes. But water does not flow through a clearwell in an orderly plug. It short-circuits. Some water zips through in a fraction of the theoretical time while other water sits in dead zones for hours. For disinfection compliance, you need to know how long the fastest 10% of the water takes to get through. That time is called T10.
T10 is estimated by multiplying the theoretical detention time by a baffling factor. The baffling factor ranges from 0.1 for an unbaffled basin (essentially an open tank with inlet and outlet on the same end) to 0.7 for a well-designed serpentine basin with over-under baffling. A typical cylindrical clearwell without internal baffling gets a factor of 0.1 to 0.3. Pipeline contact (water flowing through a long pipe) gets 1.0 because there is no short-circuiting in a pipe.
Using the example above: a 100,000-gallon clearwell at 500 GPM has a theoretical detention time of 200 minutes. If the baffling factor is 0.3 (poor baffling), T10 = 200 × 0.3 = 60 minutes. At a residual of 1.0 mg/L, your CT is only 60 mg·min/L. If you need 104 for 3-log Giardia at your temperature and pH, you are not in compliance. The clearwell is big enough in theory, but the hydraulics cut your effective contact time by 70%.
Improving baffling is one of the most cost-effective upgrades a small system can make. Adding curtain walls or serpentine baffles to a clearwell can raise the baffling factor from 0.3 to 0.5 or 0.6, effectively doubling the T10 without changing the tank size. Some plants have gained enough CT to eliminate the need for a larger clearwell or a higher chlorine dose, saving both capital and chemical costs.
Unbaffled (inlet/outlet same end): 0.1
Poor (single baffle): 0.3
Average (several baffles): 0.5
Superior (serpentine, over-under): 0.7
Pipeline (plug flow): 1.0
T10 = Theoretical Detention Time × Baffling Factor
Detention Time Calculator
Calculate hydraulic detention time for any basin, tank, or lagoon and check against regulatory minimums. Supports rectangular and circular tanks with dead zone correction for actual vs theoretical retention time.
Temperature: Why Winter Is the Hardest Season
Chlorine kills pathogens through a chemical reaction, and chemical reactions slow down in cold water. The EPA CT tables reflect this directly. For free chlorine at pH 7.0 targeting 3-log Giardia inactivation, the required CT at 20°C is about 104 mg·min/L. At 10°C it jumps to about 149 mg·min/L. At 5°C it climbs to about 179 mg·min/L. At 0.5°C (near-freezing source water), the required CT is approximately 278 mg·min/L. That is nearly three times the warm-water requirement.
This hits small northern systems hardest. A plant that comfortably meets CT in summer with a residual of 0.8 mg/L and T10 of 150 minutes (CT = 120) suddenly falls short in January when the required CT doubles. The operator has two choices: increase the chlorine dose or increase the contact time. Increasing the dose is the fast fix, but it raises DBP (disinfection byproduct) formation potential and can cause taste and odor complaints. Increasing contact time usually means reducing flow, which may not be possible during high-demand periods.
The best long-term solution for cold-water CT problems is improving clearwell baffling to maximize T10, or adding pipeline contact by routing the discharge through a longer pipe before the first customer. Every additional minute of contact time at the same residual raises CT without adding more chlorine. Some plants have installed a few hundred feet of looped pipe after the clearwell specifically to gain winter CT.
If you operate in a cold climate, build your CT calculations around your worst-case winter temperature, not your annual average. Your state primacy agency will review your CT reports and they know what your source water temperature is in February. Design your disinfection system to meet compliance at the coldest temperature you expect, and everything else takes care of itself.
pH: The Chlorine Species Problem
When you add chlorine to water, it forms two species: hypochlorous acid (HOCl) and hypochlorite ion (OCl−). HOCl is roughly 80 times more effective as a disinfectant than OCl−. The ratio between the two depends entirely on pH. At pH 7.0, about 75% of the free chlorine is in the HOCl form. At pH 7.5, it drops to about 50%. At pH 8.0, only about 25% is HOCl. At pH 8.5, you are down to about 10% HOCl.
The EPA CT tables account for this. Required CT values increase at higher pH because the chlorine is less effective. At pH 7.0 and 20°C, the CT required for 3-log Giardia is about 104. At pH 8.0 and the same temperature, it jumps to about 167. At pH 9.0, it is approximately 278. The residual on your test kit reads the same, but the killing power of that residual is dramatically lower at high pH.
This creates a conflict for plants that add lime or soda ash for corrosion control. Raising pH to reduce lead and copper leaching pushes the chlorine into the less effective OCl− form, which means you need more CT to meet disinfection requirements. Some plants solve this by adding chlorine before pH adjustment, giving the HOCl form time to work before the pH goes up. Others pre-acidify with CO₂ or acid to hold pH near 7.0 through the contact basin, then raise pH afterward.
The operational lesson: never calculate CT without checking your pH. A residual of 1.0 mg/L at pH 7.0 is not the same disinfection performance as 1.0 mg/L at pH 8.5. The test kit reads the same number, but you may need 2-3 times the CT to achieve the same pathogen inactivation at the higher pH. Check the EPA CT tables for your actual pH and temperature, not just the numbers you memorized for the exam.
pH 6.5: ~90% HOCl (strong disinfectant)
pH 7.0: ~75% HOCl
pH 7.5: ~50% HOCl / 50% OCl−
pH 8.0: ~25% HOCl
pH 8.5: ~10% HOCl (weak disinfectant)
Higher pH = more OCl− = higher required CT for the same kill.
Practical CT Compliance
Calculating CT for your monthly report requires four pieces of data: the disinfectant residual at the outlet of your contact basin, the flow rate through the basin, the basin volume (to calculate theoretical detention time), and the baffling factor. From those, you get C × T10. Compare that to the required CT from the EPA tables at your measured pH and temperature. If your calculated CT exceeds the required CT, you are compliant. The ratio of calculated to required CT gives you your log inactivation credit.
Measure the residual at the right point. The residual used for CT must be measured at the exit of the contact basin, not at the point of application. If you dose chlorine at the clearwell inlet and measure residual at the clearwell outlet, the residual will be lower because of chlorine demand from organic matter and ammonia in the water. That lower number is the one you use. Some operators make the mistake of using the higher residual at the dosing point, which overstates their actual CT.
During high-demand periods, flow increases and T10 decreases. If your plant flow doubles from 250 GPM to 500 GPM, your detention time is cut in half and your CT drops proportionally. This is when compliance failures happen. Monitor flow continuously and have a plan for increasing dose during peak demand. Many plants set a low-CT alarm on their SCADA system to trigger an automatic dose increase when flow rises.
If you cannot meet CT with your existing infrastructure, the options are: increase the chlorine dose (watch DBPs), add contact volume (pipe loops, larger clearwell), improve baffling (curtain walls, serpentine flow), or switch disinfectants (chlorine dioxide and ozone require lower CT for Giardia). Each option has cost and operational tradeoffs. Talk to your state drinking water program before making major changes, because they will need to approve your revised disinfection profile.
1. Measure residual at the basin outlet, not the dosing point.
2. Use actual flow, not design flow, for detention time.
3. Apply the correct baffling factor for your basin configuration.
4. Look up required CT at your actual pH and temperature.
5. Calculate the ratio: achieved CT ÷ required CT = log credit.