HomeMy WebLinkAbout11 Water disinfection Practices Agenda Item # 11
Utdit�y DI.-strict
WORKSHOP
To: Board of Directors
From: Ed Taylor
Date: November 19, 2008
Subject: Discussion About Delivered Water Quality and Water System
Disinfection Practices
1. WHY THIS MATTER IS BEFORE THE BOARD
The Board requested information on water system disinfection practices.
2. HISTORY
The District has been using liquid chlorine to disinfect the water supply since the 1980s.
3. NEW INFORMATION
The purpose of disinfection is to ensure public health by killing and/or inactivating any
bacteria, viruses or other micro-organisms that may be present in the untreated water. A
minimum chlorine level is maintained throughout the distribution system to prevent the re-
growth of any micro-organisms that may be present.
As water moves through the distribution system, the amount of chlorine is reduced as the
chlorine reacts with material that is present inside the pipes. State regulations require a
detectable chlorine residual (0.1 mg/L) at the farthest ends of the water distribution system. In
order to maintain that level at the outer reaches of the system, water is dosed to a level of
about 0.5 mg/L at the well. The majority of the system sees chlorine levels of about 0.3 mg/L.
Both the chlorine dosage at the wells and the chlorine residual levels in the distribution system
are well below the State of California Department of Public Health maximum of 4.0 mg/L.
As of November, the District has taken over 460 bacteriological and chlorine residual samples
throughout the distribution system this calendar year. These samples are taken weekly based
upon sampling requirements set by the State. There have not been any positive
bacteriological samples, demonstrating that the current disinfection procedures are working
effectively.
As noted above, chlorine will react with materials inside the distribution system, in some cases
forming compounds known as disinfection byproducts. State regulations normally require
sampling for disinfection byproducts four times per year and set limits on certain compounds
such as trihalomethanes and haloacetic acids. Since the District maintains chlorine residuals
at a minimal, but effective level, the formation of disinfection byproducts is quite low. Recent
sampling has identified total trihalomethane levels of 4.9 ug/L as compared to a maximum
limit of 80 ug/L. Similarly, haloacetic acids were not detected in recent samples as compared
to a maximum limit of 60 ug/L.
4. FISCAL IMPACT
There is no fiscal impact associated with this item.
5. RECOMMENDATION
iI t i re rt and provide comment.
d T ylor Michael . Holley
Water Utility Manager General Manager