Water Distribution
This section covers Water Distribution. The exam tests how you keep treated water safe from the plant clearwell to the customer tap: pressure, storage, pumping, cross-connection control, residual maintenance, flushing, and sampling.
Core rules
- Maintain positive pressure throughout the system; 20 psi is the usual regulatory floor at all points under normal conditions. Low pressure creates an intrusion risk.
- Keep a detectable disinfectant residual in the distribution system. Monitor at representative locations, including dead ends and high-residence-time areas.
- Storage tanks need turnover and mixing. Oversized or stratified tanks can develop nitrification in chloraminated systems, lose residual, and accumulate sediment.
- Cross-connection control follows a hierarchy: air gap first, then reduced-pressure zone, double-check, and pressure vacuum breaker where allowed. Test backflow assemblies on schedule.
- Flushing removes sediment and stale water. Unidirectional flushing uses enough velocity to scour mains, often about 3 ft/s; dechlorinate if discharge rules require it.
- Coliform sampling follows the routine, repeat, and triggered sampling rules in the RTCR. Sample at designated sites, not convenient taps.
Worked example
A 500,000-gallon tank serves an average demand of 250 gpm. Turnover time = 500,000 ÷ (250 × 1,440) = 500,000 ÷ 360,000 = 1.39 days. For a 3-day turnover target, required throughput = 500,000 ÷ (3 × 1,440) = about 116 gpm. Reasoning: storage volume must be matched to demand and mixing, not just fire-flow capacity.
Traps
Ignoring dead ends; commissioning oversized tanks without a mixing plan; sampling at a kitchen tap instead of a designated site; skipping annual backflow testing; flushing without checking residual and dechlorination; treating pressure transients as harmless.