Disinfection Byproducts (THMs)

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To protect drinking water from disease-causing organisms, or pathogens, a disinfectant is added to drinking water.  However, disinfection byproducts can form when organic-rich water is disinfected. A major challenge is how to control and limit risks from pathogens and minimize disinfection byproduct formation at the same time.  Disinfection byproducts tend to be highest during the warmest time of year or during periods of long detention times in the distribution system or storage tanks.  For neighborhoods farthest from treatment and storage facilities, the detention times in the distribution system tend to be longest, allowing more time for these byproducts to form.  The particular byproducts at issue are Trihalomethanes (THMs).  As a result of regulatory disinfection requirements, the City occasionally exceeds the total Trihalomethane MCL, which requires that a notice be sent to customers in the area where the limit was exceeded.

This issue has led to inquiries to the Public Works Department for more information. Public Works staff have spoken at length with many customers about the complex nature of this new water quality regulation (called DBPR-II, or Stage II of the Disinfection Byproduct Rule), the presence of THMs, the causes of THMs, and steps the City has been taking to prevent their formation. No specific corrective actions are needed by customers who receive a DBPR-II notice (e.g., no need to boil water). If you have additional questions, please call the Public Works Department at (707) 647-4558.

Frequently Asked Questions:

Expand/Contract Questions and Answers

  • Are personal home filters effective?

  • I’ve never heard of disinfection byproducts before, so why am I hearing about this now?

  • Should I drink bottled water instead?

  • What about other water systems?

  • What is a THM, and how is it formed?

  • Why do we need to add chlorine to the water?