1.3 Water Storage & Reservoirs

Types of Storage Facilities

Water storage facilities are vital components of a distribution system. They ensure a reliable supply of water during peak demand periods, provide necessary pressure, and store water for emergencies like firefighting or power outages.

Elevated Tanks are structures supported by a tower or pedestal. Their primary function is to maintain adequate and stable pressure in the distribution system using the force of gravity. Because the elevation of the water dictates the pressure, these tanks are typically built on the highest ground available. Elevated tanks are particularly useful for equalizing pumping rates; pumps can run at a steady, efficient rate to fill the tank, while the tank handles the fluctuating demands of the community.

Standpipes are cylindrical tanks whose height is greater than their diameter. They rest directly on the ground. While they hold a large volume of water, only the water in the upper portion of the standpipe (above the elevation required to provide minimum system pressure) is actually considered "usable" storage for maintaining system pressure. The lower water simply acts to support the water above it. Standpipes are generally less expensive to construct than elevated tanks but are less efficient at providing pressure.

Ground-Level Reservoirs are large storage tanks that rest on the ground and typically have a diameter greater than their height. They are used for massive volume storage. Because they are at ground level, they cannot rely on gravity to pressurize the system (unless built on a high hill), so water must typically be pumped from the reservoir into the distribution system.

Clearwells are specific types of ground-level reservoirs located at the water treatment plant. They store treated water immediately after filtration and disinfection, providing the necessary contact time for chlorine to effectively inactivate pathogens before the water is pumped into the distribution system.

Tank Components and Appurtenances

Properly functioning components are essential for the safe operation and maintenance of storage tanks.

• Altitude Valves: These specialized automatic valves control the water level within an elevated tank or standpipe. They sense the pressure (which corresponds to the water level) and automatically close when the tank reaches its maximum level to prevent overflowing. They reopen when system pressure drops, allowing water to flow out of the tank.
• Vents: As water levels rise and fall, air must be allowed to escape or enter the tank. Vents facilitate this breathing process, preventing dangerous pressure buildup or vacuum conditions that could collapse the tank. Vents must be screened (typically with a #24 mesh) to prevent insects, birds, and debris from entering.
• Overflows: If an altitude valve fails, the overflow pipe prevents water from spilling out the top or damaging the roof. The overflow must terminate above the ground with an air gap to prevent cross-connections and must also be screened or flapped to keep animals out.
• Baffles: Internal walls or curtains inside a reservoir that force water to follow a longer, serpentine path from the inlet to the outlet. Baffles prevent short-circuiting, ensuring all water receives adequate contact time for disinfection and preventing stagnant dead zones.

Water Quality and Turnover

A critical operational challenge in managing storage tanks is maintaining water quality. Water age—the amount of time water spends in the distribution system and storage facilities before being consumed—must be minimized.

When water sits in a tank for too long (high water age), several adverse water quality issues can occur:

1. Loss of Disinfectant Residual: Chlorine degrades over time. If the residual drops too low, pathogens can proliferate.
2. Bacterial Regrowth and Nitrification: Without adequate chlorine, harmless background bacteria can multiply. In chloraminated systems, nitrification can occur, severely degrading water quality.
3. Disinfection Byproducts (DBPs): The longer water containing chlorine sits in contact with naturally occurring organic matter, the more DBPs (like Trihalomethanes) form, potentially violating environmental regulations.

To combat high water age, operators must ensure proper turnover (the regular replacement of old water with fresh water). Ideally, a tank should turn over significantly every 1 to 3 days. Turnover is improved by designing tanks with separate inlets and outlets (to force mixing), installing mechanical mixers, or adjusting altitude valve settings to force the tank to drain deeper before refilling (deep cycling).

Maintenance and Disinfection (AWWA C652)

Storage tanks require regular inspection, cleaning, and maintenance. Sediment can accumulate on the floor, and biofilms can grow on the walls. When a tank is taken out of service for cleaning, painting, or repairs, it must be thoroughly disinfected before being returned to service to ensure public health.

The industry standard for this process is AWWA Standard C652 (Disinfection of Water-Storage Facilities). This standard outlines three primary methods for disinfection:

1. Chlorination of the full tank: Filling the tank to the overflow level with water containing a chlorine residual of at least 10 mg/L and holding it for 24 hours.
2. Spraying or Painting: Spraying the interior walls with a highly concentrated chlorine solution (200 mg/L) and allowing at least 30 minutes of contact time. This method saves massive amounts of water compared to Method 1.
3. Chlorination of a small volume: Adding a high concentration of chlorine to a small volume of water in the bottom of the tank, holding for a period, and then filling the tank to overflow to achieve a final specified residual.

After any of these methods, the water must be tested for coliform bacteria. The tank can only be returned to service if the bacteriological test results are negative (absent for coliforms).