The Stage 1 DBPR is where the tension in water treatment becomes a regulation. We disinfect because pathogens will make people sick. We create DBPs because disinfection reacts with organic matter in the water. The rule does not eliminate this tension. It manages it by setting limits on both sides: how much disinfectant you can add (MRDLs) and how much byproduct you are allowed to create (MCLs).
I deal with this balancing act on every shift. My plant uses ozone as the primary disinfectant, which means bromate is a constant concern. We also use chloramines for residual, which means TTHM and HAA5 are in play. The DBP rule is not academic for me, and it should not be academic for you either. If you operate a plant that disinfects, which is every plant, you are managing DBPs whether you think about it that way or not.
Let me walk you through what you need to know for the T-5 exam, and more importantly, for running your plant.
What Is the Stage 1 DBPR and Why Does It Exist?
The Stage 1 DBPR established the first enforceable limits on disinfection byproducts in drinking water. It applies to all community water systems and nontransient noncommunity systems that add a disinfectant or deliver water that has been disinfected. California implements the rule through Title 22 CCR sections 64530 through 64537.6.
Before Stage 1, there was no federal limit on how much TTHM or HAA5 could be in your finished water. The rule recognized that the chemicals we use to kill pathogens create their own health risks, and that those risks need to be managed alongside the microbial risks.
The fundamental challenge is that reducing disinfection to lower DBPs can increase microbial risk, and increasing disinfection to lower microbial risk can increase DBPs. Every operational decision you make about disinfectant dose, contact time, and precursor removal is a negotiation between these two concerns. The Stage 1 DBPR sets the boundaries of that negotiation.
What Are the DBP MCLs?
These are the numbers you need to know cold. The exam will test them directly, and you will reference them throughout your career.
| Disinfection Byproduct | MCL | Compliance Basis |
|---|---|---|
| Total Trihalomethanes (TTHM) | 0.080 mg/L (80 ppb) | Running annual average (RAA), system-wide |
| Haloacetic Acids (HAA5) | 0.060 mg/L (60 ppb) | Running annual average (RAA), system-wide |
| Bromate | 0.010 mg/L (10 ppb) | Running annual average; ozone systems only |
| Chlorite | 1.0 mg/L | Monthly average at entry point; ClO2 systems only |
TTHM is the sum of four compounds: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. HAA5 is the sum of five: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. You do not need to memorize every individual compound for the exam, but you should know how many compounds make up each group.
The bromate MCL is particularly relevant if your plant uses ozone. At 0.010 mg/L, there is very little room between detectable levels and the limit. I have seen our plant's bromate results get uncomfortably close to the MCL, and it focuses your attention in a way that a comfortable margin does not.
The TTHM and HAA5 MCLs are the two most frequently tested DBP numbers on the T-5 exam. Memorize 0.080 for TTHM and 0.060 for HAA5. If you can only remember two numbers from this entire guide, make it those two.
What Are the MRDLs for Disinfectants?
MRDLs limit how much disinfectant residual you can carry. They are the other side of the DBP equation: the MCLs limit the byproducts, the MRDLs limit the disinfectants that create them.
| Disinfectant | MRDL | Compliance Basis |
|---|---|---|
| Chlorine | 4.0 mg/L (as Cl2) | Running annual average in distribution system |
| Chloramines | 4.0 mg/L (as Cl2) | Running annual average in distribution system |
| Chlorine Dioxide | 0.8 mg/L (as ClO2) | Daily measurement at entry point |
Notice the difference in compliance basis. Chlorine and chloramines are averaged annually, which gives you room for temporary spikes. Chlorine dioxide is measured daily, which does not.
There is an important exception. Systems may temporarily exceed the chlorine or chloramine MRDL to protect public health during emergencies such as line breaks, storms, or contamination events. However, the system must notify the State Board immediately. This exception does not apply to chlorine dioxide.
The MRDL for chlorine dioxide (0.8 mg/L) is significantly lower than for chlorine and chloramines (4.0 mg/L), and it is measured daily rather than as an annual average. This reflects the fact that chlorine dioxide byproducts (chlorite) have acute health effects, while chlorine and chloramine effects are chronic. The exam may ask why these are different.
What Monitoring Is Required?
The monitoring requirements under Stage 1 vary by which disinfectant your system uses and what byproducts it produces.
TTHM and HAA5 are monitored quarterly at representative locations in the distribution system. The number of monitoring sites depends on your source type and population served. These samples should be collected from locations where DBP formation is expected to be highest, typically at the far ends of the distribution system where water age is greatest.
Bromate is monitored monthly at the entry point to the distribution system. This applies only to systems using ozone. If you do not use ozone, you do not monitor for bromate.
Chlorite is monitored daily at the entry point plus monthly in the distribution system. This applies only to systems using chlorine dioxide.
Chlorine and chloramines are monitored at the same sites and frequency as total coliform sampling. This makes practical sense because both measurements reflect distribution system water quality.
TOC and alkalinity require monthly paired samples, one from source water and one from treated water, for systems required to perform enhanced coagulation. The paired sampling is necessary because enhanced coagulation compliance is based on the percentage of TOC removed, which requires knowing both the starting and ending concentrations.
For the exam, know that TTHM/HAA5 monitoring is quarterly in the distribution system, bromate monitoring is monthly at the entry point, and chlorine/chloramine monitoring follows the total coliform schedule. The exam tests whether you know where and how often each parameter is sampled.
What Is Enhanced Coagulation and When Is It Required?
Enhanced coagulation is required for conventional treatment and softening plants that use surface water. Its purpose is to remove total organic carbon (TOC) before disinfection, reducing the precursor material available to form DBPs.
The concept is straightforward. TOC is what reacts with your disinfectant to form trihalomethanes and haloacetic acids. If you remove more TOC before you disinfect, you create fewer byproducts. Enhanced coagulation is the regulatory mechanism for ensuring that plants take this step.
The required TOC removal percentage is based on a matrix of source water TOC and source water alkalinity. The relationship is straightforward: higher source water TOC and lower alkalinity require higher removal percentages. At the extreme end, source water with TOC above 8.0 mg/L and alkalinity between 0 and 60 mg/L requires 50% TOC removal.
The designated best available technologies for TOC removal are enhanced coagulation, enhanced softening, and granular activated carbon (GAC10 or GAC20, where the number refers to the empty bed contact time in minutes).
If a system cannot meet the Step 1 TOC removal requirements, it can qualify for alternative compliance under Step 2 by demonstrating one of several conditions: source water TOC below 2.0 mg/L, TTHM below 0.040 mg/L and HAA5 below 0.030 mg/L, or source water SUVA of 2.0 L/mg per meter or less. These alternatives recognize that some source waters are naturally low in DBP precursors and do not need aggressive TOC removal.
How Does Stage 1 Compliance Work?
Compliance for TTHM and HAA5 under Stage 1 is based on the running annual average (RAA). You take all samples from all monitoring locations over four consecutive quarters, average them system-wide, and compare that average to the MCL.
That is the strength and the weakness of Stage 1 compliance. The strength is that it smooths out seasonal variation and gives systems credit for managing DBPs over time rather than punishing a single high result. The weakness is that system-wide averaging can mask individual locations where customers are consistently receiving water with DBP levels above the MCL.
Consider this example. A system has five monitoring sites. Four consistently produce TTHM results around 0.040 mg/L. One site at the end of a dead-end main consistently produces results around 0.120 mg/L. The system-wide average is 0.056 mg/L, well below the 0.080 MCL. The system is in compliance under Stage 1 even though customers at the fifth site are drinking water with TTHM levels 50% above the MCL.
That problem is exactly what Stage 2 DBPR was designed to fix. Stage 2 requires compliance at each individual monitoring location using the locational running annual average (LRAA), eliminating the ability to average away localized problems.
The difference between Stage 1 (system-wide RAA) and Stage 2 (individual-location LRAA) compliance is one of the most commonly tested DBP concepts on the T-5 exam. If a question asks about the limitation of Stage 1, the answer is that system-wide averaging can mask high DBP levels at individual locations.
The Stage 1 DBPR is the foundation for understanding how drinking water regulations manage the tension between disinfection and its byproducts. Every operational decision about dose, contact time, and precursor removal connects back to this rule. The numbers in this guide, particularly the TTHM and HAA5 MCLs, will show up on the exam and in your daily work.
I will continue building T-5 study guides on H2oCareerPro.com. Next up is the Stage 2 DBPR, which directly addresses the compliance limitation we discussed here. If this guide was useful, I would appreciate you sharing it with a colleague. We are all in this together.
This guide is for educational purposes and reflects federal and California regulations as of April 2026. Always verify current requirements with your state regulatory agency.
Frequently Asked Questions
What are the MCLs for TTHM and HAA5 under Stage 1 DBPR?
The MCL for total trihalomethanes is 0.080 mg/L (80 ppb) and the MCL for haloacetic acids is 0.060 mg/L (60 ppb). Both are calculated as running annual averages of all monitoring locations system-wide over four consecutive quarters. TTHM is the sum of four compounds and HAA5 is the sum of five.
What is the MRDL for chlorine in drinking water?
The MRDL for chlorine is 4.0 mg/L as Cl2, measured as a running annual average in the distribution system. The same value applies to chloramines. Chlorine dioxide has a stricter MRDL of 0.8 mg/L as ClO2, measured daily at the entry point. Systems may temporarily exceed the chlorine or chloramine MRDL during emergencies with immediate State Board notification.
What is enhanced coagulation and when is it required?
Enhanced coagulation is required for conventional treatment and softening plants using surface water. It requires removing a specified percentage of TOC, the primary precursor for DBP formation. The required removal is determined by source water TOC and alkalinity levels, with higher TOC and lower alkalinity requiring more aggressive removal. If Step 1 requirements cannot be met, alternative compliance under Step 2 is available for systems with naturally low precursor levels.
How did Stage 1 DBPR compliance differ from Stage 2?
Stage 1 compliance for TTHM and HAA5 was based on a running annual average of all monitoring locations system-wide. A system with one high-DBP location and several low-DBP locations could average to compliance even though some customers received water above the MCL. Stage 2 requires compliance at each individual monitoring location using the locational running annual average, eliminating the ability to average away localized problems.