Getting CT straight

Dear Editor:
Just thought I’d bring to your attention a significant error in Kelly Reynolds’ most recent On Tap article (September 2003, “Coliform Bacteria: A Failed Indicator of Water Quality?”).

On p. 92, she states that “…measures of disinfection such as CT—contact time—values are being increasingly used as indicators of microbial quality.” The term “CT” refers to the product of disinfectant concentration and time (i.e., CT = concentration × time), NOT contact time alone. Disinfection is my field, so that kind of jumped off the page at me.


Michael Templeton, Ph.D. candidate
Department of Civil Engineering
University of Toronto
Toronto, Ontario, Canada

Author’s response: Sorry not to catch this. Mr. Templeton is correct, the T is “contact time” in CT. Thus CT is concentration × contact time.

Editor’s response: Michael: Thanks for your letter. I’ll run this by Kelly, but I believe that was my error and not hers. She probably had only referred to “CT.” More than likely, in the rush to edit and provide a bit more detail, I added “contact time,” which is used in reference to ozone or UV. CT as “concentration × time” is used more commonly with mainstream disinfectants, chiefly chlorine and chloramine. Still, we should have caught the inaccuracy during proofing. I should point out that in the section this is referenced in the article, though, it only refers to “disinfection”—not chlorine; so, while it was not completely clear, “contact time” is often used with respect to effective UV or ozone disinfection. We provided this information as a clarification to our readers in the last issue. Thanks for your interest and keeping us honest!

Mr. Templeton’s response: Thanks for your reply. I’d just like to add that “CT” almost always refers to concentration × time—i.e., the USEPA has “CT” tables that list the required CT (units of mg-min/L) for certain levels of pathogen inactivation (e.g., Giardia, viruses) for particular chemical disinfectants (e.g., chlorine, ozone, chloramines, chlorine dioxide). I think these USEPA tables are probably what Dr. Reynolds was referring to when she stated that CT can be used as an indicator of microbial quality.

Contact time is never an indicator of microbial quality, even for UV or ozone. For UV, the indicator would be “IT,” or intensity × time. Ozone uses “CT” values like chlorine and chloramines. For example, a UV reactor that supplies a high intensity with a short contact time can be more effective than a UV reactor that applies a very low intensity for a longer contact time…   

I don’t mean to drag this out, but I think it’s important plant operators reading this article not be given the wrong idea about how they should evaluate disinfection in their facilities.

Keep up the good work—I enjoy reading your magazine every month.


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