Treatment Processes · Disinfection Practice
What Is CT in Drinking Water Disinfection?
Killing germs takes both enough disinfectant and enough time. CT is the disinfectant concentration multiplied by the contact time. It is how plants prove they gave the germs enough exposure. The math box shows the arithmetic.
Contact time is not simply basin volume divided by flow, because some water short-cuts through the basin faster than the rest. Federal rules use T10 as the contact time. T10 is the time it takes the first 10 percent of the water to pass through, found by a tracer study or another method your regulator approves. T10 is figured at peak hourly flow, when contact time is shortest.
Flow, water level, routing, and residual change the CT you actually get. Temperature and pH change the CT you need: colder water and, for chlorine, higher pH need more. Higher flow or a lower basin level means less contact time, so you need more residual for the same credit. Know which measurement points and basin setup the calculation assumes.
Record the residual, flow, level, and time used, so the calculation can be checked later.
Key point: CT uses the approved T10 contact time at peak hourly flow, not the full basin detention time.
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Water Treatment Part 2: Treatment Processes in Practice
This page comes from the course. It covers coagulation and jar tests, flocculation and sedimentation, filters and backwash, membranes, disinfection, water chemistry and plant residuals, one step at a time, with short knowledge checks and a certificate of completion.
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