Drinking Water Treatment and Distribution
The Drinking Water Treatment and Distribution domain tests the operator's ability to run a conventional surface-water plant and protect quality after the water leaves it. Expect questions on coagulation and flocculation (jar tests, alum dose, pH adjustment), sedimentation, granular media filtration, free chlorine or chloramine disinfection, CT, turbidity, distribution hydraulics, cross-connection control, and monitoring under the Surface Water Treatment Rule and Stage 2 DBPR.
Core targets
- Combined filter effluent turbidity: ≤0.3 NTU in at least 95% of monthly samples and never >1 NTU.
- Free chlorine residual in distribution: detectable at ≥0.2 mg/L, maintained throughout the system.
- Minimum distribution pressure: 20 psi under normal conditions.
- CT = disinfectant residual (mg/L) × contact time (min) × baffling factor. Compare with the required CT for the target log inactivation at the measured temperature and pH.
Worked example: CT compliance
A clearwell has 1.4 mg/L free chlorine, 25 min theoretical contact time, and a baffling factor of 0.6; water is 10°C at pH 7.4. CT achieved = 1.4 × 25 × 0.6 = 21 mg·min/L. The table requirement for 3-log Giardia at 10°C and pH 7.4 is about 120 mg·min/L. Because 21 < 120, the plant does not meet 3-log Giardia credit; increase residual, contact time, or baffling, or adjust pH.
Traps
- Using theoretical detention time without the baffling factor.
- Comparing CT at the wrong temperature or pH, or claiming the wrong log credit.
- Treating 0.3 NTU as an absolute daily ceiling instead of a 95th-percentile limit; 1 NTU is the absolute limit.
- Forgetting that chloramines need much higher CT than free chlorine for virus inactivation.
- Ignoring cross-connections: an unprotected hose bibb or irrigation line can back-siphon contaminants into the main.