Water Distribution
Delaware's Water Distribution domain tests the part of the system the customer actually touches: pressure, storage, mains, hydrants, cross-connection control, disinfectant residual, and coliform monitoring. Delaware systems range from the large New Castle County network to small community and mobile-home systems, so the exam favors practical hydraulics and public-health protection over design theory.
Core rules
- Pressure conversion: 1 psi = 2.31 ft of water; 1 ft of water = 0.433 psi.
- Maintain at least 35 psi normal working pressure and at least 20 psi under fire-flow or emergency conditions.
- Storage provides equalization, fire volume, and emergency supply. Operate tanks to turn over water and prevent stagnation, which drives down residual and raises DBPs.
- Maintain a detectable disinfectant residual throughout the distribution system. Chloraminated systems must also monitor ammonia and nitrification indicators.
- Backflow protection: an air gap is the strongest method; a reduced-pressure-zone assembly is used for high-hazard connections; a double-check valve is limited to low hazard. Test devices annually.
- Revised Total Coliform Rule: follow the sample siting plan and population-based sample count, collect repeat samples within 24 hours after a positive, and complete an assessment when triggered.
- New and repaired mains: flush, disinfect, and collect acceptable bacteriological samples before placing the main in service, following AWWA C651 practice.
Worked example
A storage tank water level is 150 ft above a hydrant. Static pressure = 150 × 0.433 = 65 psi. If friction loss at the design fire flow is 8 psi and the hydrant is at the same elevation as the tank base reference, residual pressure = 65 − 8 = 57 psi. That passes the 20 psi emergency minimum, but a smaller main or a higher flow could fail it.
Traps
Confusing static pressure with residual pressure; multiplying by 2.31 when converting feet to psi; omitting friction loss; selecting a double-check valve for a high-hazard chemical connection; sampling at a non-approved site; flushing without checking the receiving water for chlorine and permit requirements.