Chapter 15.5 is where California implements the federal disinfection byproduct rules. If you have read the Stage 1 DBPR and Stage 2 DBPR study guides on this site, the MCLs and MRDLs will be familiar. This guide focuses on how California structures those requirements within Title 22, including the specific sections the exam references and the monitoring and compliance framework as it applies in this state.
I deal with these numbers daily at my plant. Bromate is a constant concern with our ozone system, and we monitor TTHM and HAA5 quarterly. This chapter governs the specific monitoring frequencies, reporting timelines, and compliance calculations that determine whether my plant is in compliance.
What Does Chapter 15.5 Cover?
Chapter 15.5 implements both the Stage 1 and Stage 2 Disinfection Byproduct Rules in California. It applies to all community water systems and nontransient noncommunity systems that add a disinfectant or deliver water that has been disinfected. The chapter spans sections 64530 through 64537.6.
The chapter is organized into six articles covering definitions, MCLs and MRDLs, monitoring requirements, and compliance and precursor control. For the T-5 exam, the MCL tables, the OEL calculation, and the difference between RAA and LRAA compliance are the most heavily tested areas.
What Are the DBP MCLs and Disinfectant MRDLs?
These are the same values established by the federal Stage 1 DBPR, implemented in California through Chapter 15.5. They are the numbers you need to know.
DBP MCLs:
| Byproduct | MCL |
|---|---|
| TTHM | 0.080 mg/L |
| HAA5 | 0.060 mg/L |
| Bromate | 0.010 mg/L |
| Chlorite | 1.0 mg/L |
Disinfectant MRDLs:
| Disinfectant | MRDL |
|---|---|
| Chlorine | 4.0 mg/L (as Cl2) |
| Chloramines | 4.0 mg/L (as Cl2) |
| Chlorine dioxide | 0.8 mg/L (as ClO2) |
Bromate applies only to systems using ozone. Chlorite applies only to systems using chlorine dioxide. TTHM and HAA5 apply to any system that disinfects.
The exam tests these numbers frequently. TTHM 0.080, HAA5 0.060, bromate 0.010, chlorite 1.0. Chlorine and chloramine MRDL 4.0, chlorine dioxide MRDL 0.8. Know them all.
What Is the OEL and How Does It Work?
The Operational Evaluation Level is an early warning calculation defined in section 64531. It alerts you when a monitoring site is trending toward a violation so you can act before the fourth quarter pushes the LRAA over the MCL.
The formula is: (Q1 + Q2 + 2 times Q3) divided by 4.
Q3 is double-weighted because the calculation projects what would happen if Q4 is similar to Q3. If the OEL exceeds the MCL after three quarters of data, the system must conduct an operational evaluation to identify what is driving the elevated DBPs and implement corrective measures.
The evaluation must identify the likely cause, describe what the system is doing about it, and be reported to the State Board. The purpose is prevention: fix the trend before it becomes a violation.
The OEL formula is tested directly on the T-5 exam. You may be given three quarterly results and asked to calculate the OEL and determine whether an evaluation is required. Practice the math: add Q1 plus Q2 plus two times Q3, then divide by 4. Compare to the MCL.
What Are the Monitoring Requirements?
Chapter 15.5 specifies monitoring frequencies for each parameter. Knowing where and how often each is sampled is testable.
TTHM and HAA5 are monitored quarterly at each monitoring location in the distribution system. The number of sites depends on population served and source type. Systems must maintain monitoring plans that identify sites representing the highest DBP formation areas. This is the Stage 2 requirement: monitor where the problem is worst, not where it is most convenient.
Bromate is monitored monthly at the entry point to the distribution system, for systems using ozone only.
Chlorite requires daily monitoring at the entry point plus monthly monitoring in the distribution system, for systems using chlorine dioxide only.
Chlorine and chloramines are monitored at the same sites and frequency as total coliform sampling.
Chlorine dioxide is monitored daily at the entry point. If the entry point exceeds the MRDL, additional distribution system monitoring is required.
TOC and alkalinity require monthly paired samples (one source water, one treated water) for systems required to perform enhanced coagulation.
Reporting is quarterly. Results must be submitted to the State Board within 10 days after the quarter ends.
How Does Compliance Work Under Stage 1 and Stage 2?
Chapter 15.5 implements both compliance frameworks.
Stage 1 compliance uses the running annual average (RAA) calculated system-wide across all monitoring locations over four consecutive quarters. The system-wide average must not exceed the MCL.
Stage 2 compliance uses the locational running annual average (LRAA) at each individual monitoring site. Every site must independently stay below the MCL. This is the current compliance standard and the one the exam focuses on.
The practical difference is significant. Under Stage 1 (RAA), a high-DBP site could be masked by low-DBP sites elsewhere in the system. Under Stage 2 (LRAA), every site stands on its own. If one site exceeds the MCL based on its own four-quarter average, the system is in violation regardless of how other sites perform.
Dual sample sets, meaning paired TTHM and HAA5 samples from the same location at the same time, may be required at certain monitoring locations.
What Is Enhanced Coagulation Under California's Framework?
Enhanced coagulation is required for conventional treatment and softening plants using surface water. The requirement is to remove a specified percentage of total organic carbon based on source water TOC and alkalinity levels.
The relationship follows a matrix: higher source water TOC combined with lower alkalinity requires a higher percentage of TOC removal. This is the Step 1 requirement.
If a system cannot meet Step 1, it can qualify for alternative compliance (Step 2) by demonstrating 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.
The best available technologies for precursor removal are enhanced coagulation, enhanced softening, and granular activated carbon (GAC10 or GAC20).
Chapter 15.5 is the California framework for everything related to disinfection byproducts and disinfectant residuals. The MCL tables, the OEL calculation, and the LRAA compliance structure are the most frequently tested areas. If you studied the Stage 1 and Stage 2 DBPR guides on this site, the numbers in this chapter will be familiar. What this chapter adds is the California-specific implementation detail: the Title 22 section numbers, the State Board reporting timelines, and the monitoring plan requirements.
I will continue building T-5 study guides on H2oCareerPro.com. If this guide was useful, share it with a colleague. We are all in this together.
This guide is for educational purposes and reflects California regulations as of April 2026. Always verify current requirements with your state regulatory agency.
Frequently Asked Questions
How does California implement the Stage 2 DBPR?
California implements Stage 2 through Title 22 CCR sections 64530 through 64537.6. Compliance is based on the locational running annual average at each individual monitoring site. Every site must independently stay below the TTHM MCL of 0.080 mg/L and HAA5 MCL of 0.060 mg/L. Systems must maintain monitoring plans identifying sites representing the highest DBP formation areas.
What is the OEL and how is it calculated?
The Operational Evaluation Level is an early warning calculation: (Q1 + Q2 + 2 times Q3) divided by 4. If the OEL exceeds the MCL after three quarters of data, the system must conduct an operational evaluation to identify factors causing elevated DBPs and implement corrective action before the fourth quarter potentially pushes the LRAA into violation.