Most water systems contain some means of storing water prior to customer use. The two main reasons for storing water are to balance the peak daily demand for water and to maintain pressure in the distribution system. A reservoir is a common water storage facility that holds a large volume of water.
Reservoirs are sized to meet three general requirements:
- daily balancing of demand
- emergency storage
- fire protection needs
Water stored in a reservoir ensures a system has enough water to maintain water flow to customers during the periods of high usage on any given day. The demand may be greater than the capacity of the water supply system alone, and a pump may not be able to keep up.
Water may also be stored for an emergency, such as a watermain break or a power failure when the water supply works may not be operational. Further, some water systems are designed to include a volume of available water to enable fire departments to fight fires.
Location of Water Reservoirs
Typically, a water reservoir is located close to the water supply and distribution pipes and elevated above the community in order to provide sufficient water pressure. In hilly or mountainous communities, it may be possible to locate a reservoir on a slope. In areas of flat land, a water tower or standpipe structure can provide enough elevation to create water pressure.
Calculating elevation for water pressure
Water pressure results from the weight of water (calculated from its depth) and its elevation above the user.
Every 2.31 feet of water depth equates to one pound per square inch (psi) of water pressure.
Therefore, to maintain water pressure of 70 psi at a house, the height of water in the reservoir would have to be 162 feet above the building.
2.31' x 70 psi = 161.7'
Normal domestic water pressure ranges from 50 to 80 psi. This requires that a reservoir be elevated somewhere between 100 and 200 feet above the user. Where it is not possible to use one of the previous storage methods, water may be stored in a pressurized tank known as a hydropneumatic pressure tank. Water may also be stored in underground tanks and include a booster pump system for adequate water pressure.
Related Water Storage Equipment
Some related water storage equipment:
- vent pipes
- access hatches
- overflow pipes
- drain pipes
Water reservoirs are made from various materials and come in sizes and shapes to suit specific situations. Reservoirs are typically identified as:
- steel reservoirs
- reinforced concrete reservoirs
- impoundment reservoirs
- synthetic material tanks (polyethylene and fibreglass)
Steel Reservoir Tank
Pressure is exerted from the inside of a reservoir by the weight of the water; therefore, steel reservoirs tend to be cylindrical in shape.
Reinforced Concrete Reservoir
Reinforced concrete reservoirs can be designed for cylindrical, rectangular, or box shapes.
Impoundment Reservoir Lined and Covered in Plastic
Impoundment reservoirs are formed from the earth, which resists the pressure with sloped sides.
Synthetic Water Storage Tank: Plastic
Hydropneumatic pressure tanks contain compressed air to maintain water pressure in the distribution system so that the need for an elevated tank is eliminated. As they offer limited storage capacity, they are most often found in systems serving small communities or resorts.
Hydropneumatic Pressure Tank and Controls
The system generally consists of a tank that can be pressurized, a water pump, and an air pump. Water from the water source is maintained in the lower two-thirds of the tank and air is fed into the upper third of the tank under pressure from the air pump. The system has automated controls to regulate the volumes of air and water in the tank.