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Portland Water Bureau

From forest to faucet, we deliver the best drinking water in the world.


1120 SW Fifth Ave, Suite 600, Portland, OR 97204

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Disinfection Byproducts

Trihalomethane Rule (1990)
Effective in 1981 for large systems, this rule requires quarterly monitoring for trihalomethanes and set a maximum contaminant level for the annual average.  Trihalomethanes are formed as a byproduct of chlorination. The regulation has been strengthened and updated in the Stage 1 Disinfectants and Disinfectant Byproducts rule.

Stage 1 Disinfectants and Disinfection Byproducts (D/DBP) Rule (2002)
Stage 1 Disinfectants and Disinfection Byproducts Rule
The Stage 1 Disinfectants and Disinfection Byproducts (D/DBP) Rule was promulgated in December 1998 and went into effect January 2002. The D/DBP Rule applies to all community water systems that add a disinfectant during any part of the treatment process.

Disinfectants control waterborne disease, but disinfectants can react with naturally occurring compounds in water (precursors) to form disinfection byproducts which pose health risks. These regulations attempt to balance risks to assure control of waterborne disease while limiting exposure to disinfectant residuals and disinfection by-products. The Stage 1 D/DBP Rule requires monitoring and set Maximum Residual Disinfection Levels (MRDLs) for the disinfectants chlorine, chloramines, and chlorine dioxide. It also set Maximum Contaminant Levels (MCLs) for the disinfection byproducts total trihalomethanes, haloacetic acids, chlorite, and bromate.