Disinfection and Disinfection By-Products

Disinfectants (such as chlorine) are the primary defense against diseases caused by microbiological contaminants in public water systems. Although disinfection is the single most important treatment technique in use in public water supplies, the disinfectants themselves can react with organic materials in water supplies to form disinfection by-products (DBPs) which may prove to contaminate the water with compounds that increase cancer risk.

Disinfectants

Many water supplies add a disinfectant to drinking water to kill germs such as Giardia and E. coli. Especially after heavy rainstorms, your water system may add more disinfectants to guarantee that these germs are killed. Common disinfectants are:

Chlorine
Chloramine
Chlorine dioxide

Disinfection by-products

Disinfection by-products form when disinfectants are added to drinking water to kill germs that react with naturally-occurring organic matter in water.

Total trihalomethanes (THM)
Haloacetic acid (HAA5)
Bromate
Chlorite

Do the disinfection and disinfection by-products regulations apply to my water system?

Yes, present regulations only apply to community and non-transient non-community systems adding a disinfectant to their water supplies. However, future regulations will require all systems that purchase disinfected water to meet regulations as well.

Actions your water system should be taking

If your water system is adding a disinfectant, they must start regular disinfectant residual tests (weekly or monthly) at some consumer taps to determine how much disinfection is available at the “end-of-the-line” in your system. A free chlorine residual of at least 0.2 mg/L should be maintained. This will help them plan for modifications in your disinfection to meet any new standards that are required.

Frequency of Sampling

GUDISW: Groundwater under direct influence of surface water

*Sample taken in month with the warmest water temperature.

System type	Population	Monitoring frequency
Surface/GUDISW	>10,000	4/plant/quarter
Surface/GUDISW	500-9,999	1/plant/quarter
Surface/GUDISW	<500	1/plant/year*
Groundwater	>10,000	1/plant/quarter
Groundwater	<10,000	1/plant/year*

DBP Monitoring Plan Documents

A disinfection by-product monitoring plan consists of three separate sections. Click on the links below to download a copy of the monitoring plan documents.

DBP Quarterly Reporting Forms

Disinfection residuals and DBP monitoring sampling results are to be reported quarterly. Click on the links below to download a copy of the quarterly reporting forms.