Protecting Public Health and the Environment.

Compliance with Disinfection Byproducts Rules

The purpose of the disinfection byproducts (DBP) rules is to reduce potential cancer, reproductive, and developmental health risks from disinfection byproducts in drinking water, which form when disinfectants are used to control microbial pathogens.

Stage 1 DBP Rule

This rule applies to all community and nontransient noncommunity public water systems that use a disinfectant for either primary or residual water treatment. Transient water systems that apply chlorine dioxide as a disinfectant must also comply.

Stage 2 DBP Rule

This rule applies to community and nontransient noncommunity water systems that produce and/or deliver water that is treated with a primary or residual disinfectant other than ultraviolet light. The Stage 2 DBP rule also covers consecutive systems. Consecutive systems are public water systems that receive some or all of their finished water from one or more wholesale systems.

Monitoring Requirements under the Stage 2 DBP Rule

Compliance Monitoring Plans

Owners and operators of systems subject to the Stage 2 DBP must develop a Compliance Monitoring Plan (CMP) identifying how they intend to sample for compliance. Systems must prepare a plan before they begin their Stage 2 DBP compliance monitoring and must keep the plan on file. Surface water (SW) systems and ground water under the influence of surface water systems (GWUDI) with populations over 3,300 must submit their CMPs to the state for review (unless a system prepared an Initial Distribution System Evaluation [IDSE] report and the IDSE report already contains the required information).

Example Plans:

Routine Monitoring

Routine monitoring must be conducted according to the information in the following table:

All systems will begin Stage 2 compliance monitoring on a routine monitoring schedule, except for the following:

  • Systems that are on Stage 1 DBP increased monitoring must begin Stage 2 DBP compliance monitoring on increased monitoring.
  • Systems that are on Stage 1 DPB reduced monitoring may begin Stage 2 DBP compliance monitoring on reduced monitoring if all the requirements have been met.

Reduced Monitoring

Reduced monitoring must be conducted according to the information in the following table:

Systems that are on reduced Stage 1 DBP monitoring may remain on reduced monitoring for Stage 2 DBP if all of the following criteria are met:

  1. System qualifies for a 40/30 certification or received a Very Small System (VSS) waiver.
  2. The monitoring locations for Stage 1 DBP and Stage 2 DBP are the same.
  3. The average of all samples taken in the year prior to starting Stage 2 DBP compliance monitoring is no more than 0.040 milligrams per liter (mg/L) for total trihalomethanes (TTHM) and no more than 0.030 mg/L for haloacetic acids (HAA5).

Systems that are on routine Stage 2 DBP monitoring may qualify for reduced monitoring as determined by DEQ if the following criterion is met:

  • The locational running annual average (LRAA) of all samples taken for 1 year after Stage 2 DBP compliance monitoring starts is no more than 0.040 mg/L for TTHM and no more than 0.030 mg/L for HAA5.

Systems that are on reduced Stage 2 DBP monitoring may remain on reduced monitoring if the following criterion is met:

  • For systems that are on yearly or less frequent (3 years) monitoring, the LRAA of all samples is no more than 0.060 mg/L for TTHM and no more than 0.045 mg/L for HAA5.

In addition to the criteria listed above for systems using surface water or ground water under the direct influence, the running annual average (RAA) for total organic carbon (TOC) level must be below <4.0 mg/L at each treatment plant based on monitoring conducted under Stage 1 DBP rule.

Note that an LRAA is calculated using four quarters of data. If a water system exceeds the LRAA criteria at any location, the system does not meet the reduced monitoring requirements. If monitoring results indicate that a system is no longer eligible for reduced monitoring, the system must resume routine monitoring or be placed on increased monitoring.

Increased Monitoring

Systems on an increased Stage 1 DBP monitoring schedule must begin Stage 2 monitoring on the increased schedule until they meet the requirements for returning to the routine schedule. 

Systems on Stage 2 DBP that are on annual or less frequent monitoring must go to increased monitoring if any sample at any location exceeds the maximum contaminant level (MCL) for either TTHM (0.080 mg/L) or HAA5 (0.060 mg/L). The system must increase monitoring frequency to dual sample sets once per quarter (taken every 90 days) at all locations.

Systems on Stage 2 DBP that are on increased monitoring can return to routine monitoring if, after four quarters of consecutive monitoring, each monitoring location’s LRAA for TTHM or HAA5 is less than 0.060 mg/L or 0.045 mg/L, respectively.

Note that increased monitoring is not based on an LRAA but rather on individual sample results. If any sample at any location is above either of the MCLs, then increased monitoring applies system-wide, not just at one location.

Calculation of TTHM and HAA5 MCLs under the Stage 2 DBP Rule

Local Running Annual Average

Compliance with the TTHM and HAA5 MCLs for Stage 2 DBP rule is based on the monitoring results and LRAA calculations at each monitoring location as shown in the following example:

The table below summarizes the TTHM and HAA5 compliance requirements:

TTHM and HAA5 compliance Quarterly monitoring Once per year or less frequent monitoring
Is based on LRAA—calculated quarterly for each sampling location Value of yearly or less frequent samples at each sampling location
If LRAA exceeds the MCL Violation if any sampling point exceeds the LRAA* Not immediately in violation;
start quarterly monitoring to determine compliance

*Compliance is based on the LRAA so if any one sample exceeds an annual average (e.g., over four times the MCL) or any combination of samples in the four quarters exceeds the MCL, the system is in violation of the MCL.

For quarterly monitoring:

  • If the system fails to complete four consecutive quarters of monitoring, compliance with the MCL will be calculated based on the average of the available data from the most recent four quarters.
  • If the system takes more than one sample per quarter at a monitoring location, all samples taken in the quarter at that location will be averaged to determine a quarterly average to be used in the LRAA calculation.

Operation Evaluation Levels

As a part of Stage 2 DBP rule compliance monitoring, owners and operators of systems are required to calculate operation evaluation levels (OELs) by calculating the sum of the two previous quarters' results plus twice the current quarter's results divided by 4 as shown in the following example:

This calculated LRAA result should help water systems identify if they are likely to exceed the MCL in the next quarter, and give them a chance to make operational changes.

If OELs are higher than the MCL for TTHM and HAA5 at any location in the distribution system, the owner or operator of the system needs to conduct an operational evaluation and submit a report to DEQ. If the system is able to readily identify the cause of the OEL exceedance, the operational evaluation report may be limited.


Staff Contacts

Drinking Water Rule Analyst
Maureen Pepper
DEQ State Office
Water Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0174
maureen.pepper@deq.idaho.gov

DEQ Resource

Frequently Asked Questions about the Stage 2 DBP Rule

Related Pages

Public Water System Switchboard