Stage 2 Disinfectant/Disinfection ByProducts Rule - Total Trihalomethanes (TTHMs) / Haloacetic Acids (HAA5s)

The Stage 2 Disinfection Byproducts Rule (DBPR) was promulgated in January 2006 and applies to all community water systems and non-transient non-community water systems that add a primary or residual disinfectant other than ultraviolet light (UV) or deliver water that has been treated with a primary or residual disinfectant other than UV. Systems that purchase disinfected water are included in this rule.

Nym Veach

Rule Manager

P: 701-328-6624

Fact Sheet


  • How do I determine the maximum residence site in the distribution system?
    • The maximum residence site is typically the point in the distribution system furthest from the point of disinfection ("oldest water"). A reasonable approach would be to measure chlorine residuals at various points in the distribution system and pick the point at which the chlorine residual is the lowest.
  • How is the term disinfection by-product defined?
    • A disinfection by-product is a compound formed by the reaction of a disinfectant such as chlorine with organic matter in the water supply.
  • What are the maximum contaminate levels (MCLs) for Total Trihalomethanes (TTHMs) and haloacetic acids five (HAA5)?
    • Maximum contaminants levels (MCLs) for TTHM and HAA5 are 0.080 mg/L and 0.060 mg/L respectively.
  • What is a combined distribution system?
    • A combined distribution system consists of all systems that provide water (wholesaler/seller) and the systems that receive finished water (consecutive/buyer).
  • What is a “dual” TTHM/HAA5 disinfection byproduct sample set?
    • A set of two samples collected at the same time and same location, with one sample analyzed for TTHM and the other analyzed for HAA5.
  • What is an “individual” TTHM/HAA5 disinfection byproduct sample set?
    • A set of two samples with one sample collected at the high TTHM location and one sample collected at the high HAA5 location.
  • What is the Stage 2 DBPR operational evaluation level (OEL)?
    • A system may still be in full compliance with the Stage 2 DBPR even when individual DBP sample results are above 0.080 mg/L for TTHM and 0.060 mg/L for HAA5 since compliance is based on a locational running annual average. Because of concerns that high levels of DBPs can occur even when the system is in compliance, there is an additional requirement in the Stage 2 DBPR for operational evaluations.
      As a part of Stage 2 DBPR compliance monitoring, water systems are required to monitor operational evaluation levels (OELs). The OELs are determined by calculating the sum of the two previous quarter's results plus twice the current quarter's results divided by 4. This locational running annual average (LRAA) threshold should help water systems identify if they are likely to exceed the MCL in the next quarter, and gives them a chance to make operational changes.
  • What is the criterion to qualify for reduced TTHM/HAA5 monitoring?
    • TTHM Locational Running Annual Averages (LRAAs) ≤ 0.040 mg/L at all locations
    • HAA5 Locational Running Annual Averages (LRAAs) ≤ 0.030 mg/L at all locations
    • Annual average source water TOCs at each treatment plant ≤ 4.0 mg/L for surface water systems