12.3 Distribution Storage: Types, Sizing & Turnover

What Storage Buys

$\text{Total Storage} = \text{Operating} + \text{Fire} + \text{Emergency}$

Worked example. MDD is 3.0 MGD; fire flow is 3,000 gpm for 3 hours; emergency policy is one average day at 1.6 MGD. Operating at 20 percent of MDD = 0.20 x 3,000,000 = 600,000 gal Fire = 3,000 x 180 = 540,000 gal Emergency = 1,600,000 gal Total = 2,740,000 gallons

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Storage Types and Hydraulic Position

Level and Pressure

$\text{Pressure at the base (psi)} = \text{Water depth (ft)} \times 0.433$

$\text{Pressure at a service (psi)} = (\text{Tank water surface elevation} - \text{Service elevation}) \times 0.433$

Example: a hilltop tank with an overflow elevation of 640 ft, currently 8 ft below overflow, serving a home at elevation 465 ft. Water surface = 640 - 8 = 632 ft. Difference = 632 - 465 = 167 ft. Static pressure = 167 x 0.433 = 72.3 psi

A tank that floats on the system is a pressure-setting device. Every foot the level drops removes 0.433 psi from every customer in the zone. That is why level, not volume, is the operational control variable.

Control Valves

• Altitude valve - closes to prevent overflow when the tank reaches a set level; a two-way altitude valve also allows the tank to feed back into the system on demand.
• Level transmitters - pressure transducer, ultrasonic, radar, or float; feed SCADA and the pump control logic.
• Overflow - must terminate with a screened, downturned outlet and a proper air gap, and may never be directly connected to a sewer or storm drain.
• Drain and washout - sized to empty the tank for inspection, also air-gapped.

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Turnover: The Water Quality Problem

Storage is the place in a distribution system with the longest detention time, and long detention causes four predictable problems:

1. Loss of disinfectant residual - decay is continuous and accelerates with temperature
2. Disinfection byproduct formation - free chlorine plus organic precursors plus time equals TTHM and HAA5, and Stage 2 evaluates each monitoring site on its own LRAA
3. Nitrification in chloraminated systems - the warm, low-residual, low-velocity interior of a tank is the ideal nitrifier habitat
4. Thermal stratification - a tall tank in summer develops a warm surface layer that never mixes with the cold bottom, so a nominally "turned over" tank may be exchanging only the top few feet

Managing Turnover

• Cycle through a wider level band. A tank operated between 90 and 95 percent full exchanges 5 percent of its volume per cycle. Widening the band to 70-95 percent exchanges five times as much.
• Separate inlet and outlet, arranged to minimize short-circuiting - a design requirement for new reservoirs under 22 CCR 64585. Where a common inlet/outlet pipe exists, the incoming water tends to leave immediately and a large volume never exchanges.
• Active mixing systems - submersible mixers or nozzle systems that break stratification.
• Seasonal deep cycling - deliberately drawing a problem tank far down during low-demand periods, with residual and turbidity monitoring during recovery.
• Booster disinfection at the tank outlet where residual cannot be maintained by turnover alone.
• Right-size the storage. Storage sized for a growth projection that never happened is a permanent water quality liability.

[!TIP] Water age is not the same as detention time. A tank with a 3-day theoretical detention time and a common inlet/outlet may have a well-mixed 20 percent and an 80 percent that is weeks old. Tracer studies, temperature profiling at multiple depths, and residual profiles at multiple depths are how operators find out which one they have.

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Inspection and Maintenance

Any reservoir taken out of service for repair or inspection must be disinfected and sampled under AWWA C652-02 before return to service, and if coliform is detected the reservoir must be resampled with results submitted to the State Board for review and approval.