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.


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

For the Stage 2 DBPR, each public water supply system must develop a monitoring plan to be kept on file for State and public review (40 CFR §141.622(a)(1)). If you are a subpart H system (system using a surface water source or a ground water source which is under the direct influence of surface water) that serves greater than 3,300 people, you must submit a copy of the Stage 2 monitoring plan to DENR prior to the date you conduct your Stage 2 monitoring. We recommend that all systems serving over 500 people submit a monitoring plan.

*Sample taken in month with the warmest water temperature.

Governor's Capacity Development Report

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.

Disinfection Residual Report - (Microsoft Excel)

Monthly Disinfection Residual Worksheet - (Microsoft Excel)

Quarterly DBP Report - (Microsoft Excel)

TOC/TTHM/HAA5 Reporting Form - (Microsoft Excel)

DBP Quarterly Reporting Forms

Standard Monitoring Plan

4030 Certification Letter

IDSE Standard Monitoring Report Form

DBP Quick Reference Guide

Comprehensive disinfectants and disinfection byproducts rules (stage 1 and stage 2) quick reference guide (PDF)