Distribution System Interface and Finished Water Storage
This domain covers the interaction between treatment and distribution, finished water storage, and maintaining water quality throughout the distribution system. The exam tests your ability to manage storage tanks, prevent stagnation, and ensure residual maintenance.
Distribution System Basics
- Distribution systems must maintain adequate pressure (typically 20–100 psi) and disinfectant residual. Cross-connection control and backflow prevention are critical.
- Water age affects quality; excessive age can lead to disinfectant loss, DBP formation, and nitrification. Manage by tank turnover and looping.
Finished Water Storage
- Storage tanks provide fire flow, emergency supply, and pressure stabilization. They must be covered, vented, and secured against contamination.
- Turnover should occur every 1–3 days to maintain quality; poor turnover leads to stagnation, sediment accumulation, and biofilm growth.
- Tank inspections are required regularly; look for structural integrity, coatings, vents, and security.
- Mixing systems (e.g., baffles, aerators, or mechanical mixers) can reduce stratification and stagnation.
Residual Maintenance
- Booster chlorination may be needed in large systems to maintain residual. Monitor at representative points.
- Chloramine systems require careful control to avoid nitrification; monitor ammonia and nitrite.
- Dead-end mains should be flushed regularly to prevent stagnation.
Storage Tank Operation
- Altitude valves control tank filling and prevent overflow. Level controls and telemetry monitor tank levels.
- Emergency storage must be protected from contamination; hatches locked, vents screened.
Worked Example
A 500,000-gallon storage tank serves a community with average demand of 0.5 MGD. What is the approximate turnover time?
- Turnover time = tank volume / average daily demand = 500,000 gal / 500,000 gal/day = 1 day.
- This is within the recommended 1–3 days, so turnover is adequate. If demand were 0.1 MGD, turnover would be 5 days, indicating potential stagnation and need for operational changes (e.g., cycling, mixing, or reducing tank size).
Common Traps
- Neglecting tank turnover, leading to water quality degradation.
- Forgetting that altitude valves can fail, causing overflow or empty tanks.
- Overlooking security of tank hatches and vents.
- Assuming chloramine residual is stable; nitrification can consume it.
- Failing to flush dead-end mains, causing stagnant water and customer complaints.