Title 22 Chapter 15.5: Disinfectant Residuals, DBPs & Precursors — Quick Reference
DBP MCLs (Table 64533-A) — Memorize These
| Disinfection Byproduct | MCL (mg/L) |
|---|---|
| Total Trihalomethanes (TTHM) | 0.080 |
| Haloacetic Acids (HAA5) | 0.060 |
| Bromate | 0.010 |
| Chlorite | 1.0 |
MRDLs (Table 64533.5-A) — Memorize These
| Disinfectant | MRDL (mg/L) |
|---|---|
| Chlorine | 4.0 (as Cl2) |
| Chloramines | 4.0 (as Cl2) |
| Chlorine Dioxide | 0.8 (as ClO2) |
Federal vs. California: The DBP MCLs and MRDLs above are identical at both the federal and California levels. However, the public-notification tier for a chlorite MCL violation differs. Federal rules treat chlorite exceedance as Tier 2 (30 days). California treats it as Tier 1 (24 hours), with a more stringent trigger requiring only 2 consecutive daily samples at the entry to distribution to exceed the MCL. See the Public Notification quick-reference card for details. Source: Title 22 §64463.1(a)(7); 40 CFR 141.202.
Stage 1 vs. Stage 2: The Critical Difference
Same MCLs. Different compliance math.
Stage 1: Running Annual Average (RAA) across all monitoring sites system-wide. A system with one site at 0.100 mg/L TTHM and one at 0.040 mg/L had a system-wide RAA of 0.070, which is compliant.
Stage 2: Locational Running Annual Average (LRAA) at each individual site. That same site at 0.100 would be a violation because its own LRAA exceeds 0.080. Stage 2 eliminated the masking effect of system-wide averaging.
The OEL Formula
OEL = (Q1 + Q2 + 2 × Q3) / 4
The Operational Evaluation Level is an early warning calculation. Q1 through Q3 are the first three quarterly results at a monitoring site. If the OEL exceeds the MCL, the system must conduct an operational evaluation and take corrective action before the fourth quarter potentially pushes the LRAA into violation. Think of it as a “you're heading for trouble” alarm.
Monitoring Quick Reference
| Parameter | Frequency | Where |
|---|---|---|
| TTHM/HAA5 | Quarterly | Each monitoring location |
| Bromate | Monthly | Entry point (ozone systems only) |
| Chlorite | Daily + monthly | Entry point daily, distribution monthly (ClO2 systems) |
| Chlorine/Chloramines | Same as TC sampling | Same sites as total coliform |
| TOC/Alkalinity | Monthly paired samples | Source + treated water |
What to Watch on the Exam
- The OEL formula is a favorite exam question. Know how to calculate it and know that it triggers an operational evaluation, not a violation.
- TTHM components: chloroform, bromodichloromethane, dibromochloromethane, bromoform. HAA5 components: monochloroacetic, dichloroacetic, trichloroacetic, monobromoacetic, dibromoacetic acid.
- Bromate applies only to systems using ozone. Chlorite applies only to systems using chlorine dioxide. The exam tests whether you know which DBP goes with which disinfectant.
- MRDL exception: systems may temporarily exceed chlorine or chloramine MRDLs to protect public health (line breaks, storms, contamination events). Must notify State Board immediately.
- Enhanced coagulation removes TOC (DBP precursors). The required TOC removal percentage depends on source water TOC and alkalinity using the Step 1 table.
- Chlorite is Tier 1 in California, not Tier 2 as in federal rules. If the exam asks about notification for a chlorite MCL violation, the answer is 24 hours.
Federal Rule Implemented
Stage 1 and Stage 2 Disinfection Byproduct Rules (40 CFR 141 Subpart L).