What is CT Disinfection?
Under the EPA's Surface Water Treatment Rule (SWTR), water treatment plants must achieve specific 'log removal' or inactivation rates for pathogens. Since operators cannot instantly test for Giardia cysts on the fly, they use a mathematical surrogate known as CT.
CT stands for Concentration multiplied by Time. It represents the exposure level of pathogens to a disinfectant. A higher concentration of chlorine means less contact time is required, and vice versa.
Understanding Baffle Factors and T10
Water does not flow through a clearwell or storage tank uniformly. Due to short-circuiting, some water exits the tank much faster than the average flow rate. To account for this, the EPA requires operators to use the 'T10' time, which is the time it takes for the first 10% of water to pass through the basin.
T10 is determined by multiplying the Theoretical Detention Time (Volume / Flow) by a Baffle Factor. Unbaffled tanks might have a factor of 0.1, meaning severe short-circuiting, while highly engineered serpentine basins have a factor of 0.7 or higher.
How to Calculate the CT Value
Calculating the actual CT achieved by your treatment plant involves determining the flow rate, tank volume, baffle factor, and the residual chlorine concentration at the exit of the tank.
The Formula: CT = Disinfectant Residual (mg/L) x T10 Contact Time (minutes). T10 = (Tank Volume / Max Flow Rate) x Baffle Factor.
Worked Example: A clearwell has a volume of 100,000 gallons. The peak hourly flow is 2,000 gallons per minute (gpm). The tank has moderate baffling (factor of 0.3). The free chlorine residual is 1.5 mg/L. First, calculate Theoretical Detention Time: 100,000 / 2,000 = 50 minutes. Next, calculate T10: 50 x 0.3 = 15 minutes. Finally, calculate CT: 1.5 mg/L x 15 mins = 22.5 mg-min/L.
Meeting EPA Required CT
Once you calculate your actual CT, you must compare it to the EPA's 'Required CT' tables. The Required CT is the target number needed to achieve the mandated log inactivation (e.g., 3-log for Giardia).
Required CT fluctuates based on water temperature and pH. Colder water and higher pH levels slow down chlorine reactivity, meaning the Required CT target will be much higher during winter months.
Frequently asked questions
What is log inactivation?
Log inactivation is a mathematical term for percentage reduction. A 1-log reduction means 90% of pathogens are killed. A 2-log reduction is 99%, 3-log is 99.9%, and 4-log is 99.99%.
Why does cold water require a higher CT value?
Chemical reactions, including chlorine oxidation, slow down as temperature decreases. Therefore, pathogens must be exposed to the disinfectant for a longer period or at a higher concentration to ensure destruction.
How is a tank's baffle factor determined?
While standard tables exist for estimating baffle factors based on physical tank design, precise baffle factors are determined by conducting a chemical tracer study on the actual operating basin.
What happens if my actual CT is lower than the Required CT?
If your plant's actual CT falls below the Required CT, the water is not considered fully disinfected. Operators must immediately increase the chlorine dose, decrease the flow rate, or increase the tank operating level to restore compliance.