Drinking Water Distribution
Drinking Water Distribution
This domain tests the storage, pumping, piping, and cross-connection controls that keep treated water safe after it leaves the plant. Questions focus on pressure, disinfectant residual, main disinfection, sampling, backflow, and hydraulic math.
Core rules and concepts
- Maintain at least 20 psi in the distribution system under normal operating conditions; below that, contamination can enter through leaks.
- Keep water moving: excessive age raises TTHM/HAA5 and can cause nitrite problems in chloraminated systems. Use flushing, storage turnover, and valve/pump operation.
- New or repaired mains: chlorinate at 50 mg/L for 24 hours (or equivalent CT), flush, and obtain satisfactory bacteriological samples before placing in service.
- Backflow: air gap is the highest protection, then RPZ, then double check; test annually and maintain records.
- Coliform monitoring: routine samples are based on population and system size; a positive repeat or E. coli triggers Tier 1 public notice and corrective action.
Worked example
Disinfect 1,500 ft of 12-inch main at 50 mg/L. Volume = π × (0.5 ft)² × 1,500 ft = 1,178 ft³; × 7.48 gal/ft³ = 8,810 gal. Chlorine needed = 8,810 × 8.34 × 50 ÷ 1,000,000 = 3.67 lb. Using 12.5% hypochlorite at 1.25 lb available chlorine per gallon: 3.67 ÷ 1.25 = 2.94 gal. Add that volume, circulate, hold 24 hours, then flush and sample.
Traps
Confusing pressure with head (1 psi = 2.31 ft of water); mixing gpm and MGD; ignoring chlorine demand so the residual falls below 50 mg/L; and failing to sample for coliforms after flushing.