What Is the Stage 2 Disinfection Byproducts Rule?

By SubcontractorHub Editorial Team·Published November 2026

A dealer can see TTHM or HAA5 on a city report and treat the number as a filter quote. The Stage 2 Disinfection Byproducts Rule is the monitoring rule behind those names, not a product page. SubcontractorHub is software for the customer record. This page is the rule. It is not a treatment design, and it is not medical advice.

The Stage 2 Rule Is a Monitoring Rule

EPA's Stage 1 and Stage 2 page, fetched the day this page was written, says the Stage 1 Disinfectants and Disinfection Byproducts Rule reduces drinking water exposure to disinfection byproducts. It applies to community water systems and non-transient non-community systems, including those serving fewer than 10,000 people, that add a disinfectant during any part of the treatment process. The same page says the Stage 2 DBPR strengthens public health protection by tightening compliance monitoring requirements for trihalomethanes and haloacetic acids, and that the rule targets public water systems with the greatest risk.

Taken together, the page says, the two rules improve drinking water quality by providing protection from disinfection byproducts. It says byproducts, if consumed in excess of EPA's standard over many years, may increase health risks. It names lakes, rivers, reservoirs, and ground water aquifers as source-water locations. It names Giardia, Cryptosporidium, and viruses as pathogens that can cause gastrointestinal illness, including diarrhea, vomiting, and cramps. It says disinfectants can react with naturally occurring materials to form trihalomethanes, haloacetic acids, chlorite, and bromate.

Those four byproduct names are not four identical rules. Chlorite and bromate have their own pages, chlorite in drinking water and bromate in drinking water. The Stage 2 sentence on the EPA page is specifically the tightening of TTHM and HAA5 monitoring. The contaminant rows live on total trihalomethanes in drinking water and haloacetic acids in drinking water.

January 4, 2006 Is Not the Stage 1 Date

EPA's plain-English guide, fetched the same day, says the Stage 1 DBPR was published in the Federal Register on December 16, 1998, and the Stage 2 DBPR was published in the Federal Register on January 4, 2006. The guide says the rules set enforceable limits for disinfectants and disinfection byproducts, and created monitoring requirements. A dealer who writes "the DBP rule" without the stage has mixed two dates.

The guide says EPA set maximum residual disinfectant levels for chlorine, chloramines, and chlorine dioxide, and MCLs for chlorite, bromate, TTHM, and HAA5. It says Stage 2 analytical and monitoring requirements for TTHM and HAA5 apply to community and nontransient noncommunity systems that add a primary or residual disinfectant other than ultraviolet light, or that deliver water treated with a primary or residual disinfectant other than ultraviolet light. Consecutive systems that do not add a disinfectant but deliver that water must also comply with the Stage 1 chlorine and chloramine monitoring the guide names.

The guide says that after Stage 1, EPA determined the required monitoring sites might not represent higher byproduct concentrations in the distribution system. Stage 2 was promulgated to require more consistent protection across the distribution system and to reduce peaks. Systems serving 10,000 or more people were required to conduct a one-time Initial Distribution System Evaluation to find high-byproduct areas, and those sites became the basis for TTHM and HAA5 compliance sites. The guide says that evaluation should already have been completed for those systems. This page does not reprint the compliance-date table, because copying a truncated schedule would invent a deadline.

What Is a Locational Running Annual Average?

The guide defines a locational running annual average as the average of analytical results for samples taken at a monitoring location. It says you calculate an LRAA for each TTHM site and each HAA5 site. It says you are in violation if the LRAA for any monitoring location, covering any consecutive four-quarter period, exceeds the TTHM MCL of 0.080 mg/L or the HAA5 MCL of 0.060 mg/L. It also says an LRAA based on fewer than four quarters can be a violation if it already exceeds one of those MCLs.

That is a different reading from one jug on a kitchen counter. A single result is not, by itself, the four-quarter average the guide uses for quarterly sites. The guide does say compliance can also be based on a single sample result if the system monitors once a year or less often, and it says to use the single result when the result is four times the MCL. This page does not decide which schedule a named system is on. The schedule depends on population and source, and the guide's date table was not copied here.

The guide also prints reduced-monitoring lines: every TTHM LRAA at or below 0.060 mg/L and every HAA5 LRAA at or below 0.045 mg/L after the monitoring the guide requires, and a tighter pair of 0.040 mg/L and 0.030 mg/L for systems that want to remain on reduced quarterly monitoring. Those lines are not the MCLs. Writing 0.040 as if it replaced 0.080 would mis-state the violation the guide actually names.

What Should the Stage 2 Note Say?

Six lines keep the Stage 2 Disinfection Byproducts Rule from being rewritten as a private-well fine or as a filter price.

1. The stage. Stage 2 is January 4, 2006. Stage 1 is December 16, 1998. Do not merge the dates.
2. The contaminants the Stage 2 sentence names: TTHM and HAA5. Chlorite and bromate are on the EPA list of byproducts and have their own rows.
3. Whether the sample is one result or a locational running annual average, and which monitoring location it came from.
4. The violation lines the guide prints: 0.080 mg/L for TTHM and 0.060 mg/L for HAA5, as an LRAA, not as a removal percent.
5. Whether the system adds a disinfectant other than ultraviolet light, or only delivers water that was treated that way.
6. What you did not claim: no private-well statute, no installed price, and no statement that a kitchen sample is the system's four-quarter average.

Where the Rule Sits on the Job

A city consumer confidence report can show a TTHM or HAA5 number that a homeowner brings to a water dealer. The number still needs the stage, the location, and whether it is an average. How to read that report is how to read a consumer confidence report. A notice the system must send the public is a different rule, the public notification rule. This page does not assign a tier to a byproduct.

The broader family of byproducts is disinfection byproducts in drinking water. The software that holds the customer file is water treatment contractor software. How a dealer reads a full panel is the well water testing guide. This article does not size a filter and does not tell a household the water is safe to drink.

Ultraviolet light is the exception the guide writes into the Stage 2 sentence. A dealer who sells a UV unit still should not tell a customer the city's TTHM average disappeared. The guide's exclusion is about which systems must monitor, not about a device erasing a number that was already formed upstream. Write the stage, the location, and the average. Leave the price off this note.

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