Distribution Storage Requirements

Water storage requirements should take into consideration the peak daily water use, the maximum-day demand plus the required fire flow, the capacity of the normal and standby pumping equipment, the availability and capacity of auxiliary power, the probable duration of power failure, and the promptness with which repairs can be made. Additional considerations include land use, topography, pressure needs, distribution system capacity, special demands, and the increased cost of electric power and pumps to meet peak demands.

Water storage is necessary to help meet peak demands, fire requirements, and industrial needs; to maintain relatively uniform water pressures; to eliminate the necessity for continuous pumping; to make pumping possible when the electric rate is low; and to use the most economical pipe sizes. Surges in water pressure due to water hammer are also dissipated. Other things being equal, a large-diameter shallow tank is preferable to a deep tank of the same capacity. It is less expensive to construct, and water pressure fluctuations on the distribution system are less. The cost of storage compared to the decreased cost of pumping, the increased fire protection and possibly lowered fire insurance rate, the greater reliability of water supply, and the decreased probability of negative pressures in the distribution system will be additional factors in making a decision.

In general, it is recommended that water storage equal not less than one-half the total daily consumption, with at least one-half the storage in elevated tanks. A preferred minimum storage capacity would be a two-day average use plus fire flow, or the maximum-day usage plus fire requirements less the daily capacity of the water plant and system for the fire-flow period.

Hudson157 suggests the provision of two tank outlets, one to withdraw the top third of tank water for general purposes and a second outlet at the bottom of the tank to withdraw the remaining two-thirds of tank water if needed to supply building sprinkling systems in developed areas with high-rise apartments, industries, shopping centers, office complexes, and the like. In small communities, real estate subdivisions, institutions, camps, and resorts, elevated storage should be equal to at least a full day's requirements during hot and dry months when lawn sprinkling is heavy. Two or three days storage is preferred. The amount of water required during peak hours of the day may equal 15 to 25 percent of the total maximum daily consumption. This amount in elevated storage will meet peak demands, but not fire requirements. Some engineers provide storage equal to 20 to 40 gal/capita, or 25 to 50 percent of the total average daily water consumption. A more precise method for computing requirements for elevated storage is to construct a mass diagram. Two examples are shown in Figures 2.8 and 2.9. Fire requirements should be taken into consideration.158

It is good practice to locate elevated tanks near the area of greatest demand for water and on the side of town opposite from where the main enters. Thus, peak demands are satisfied with the least pressure loss and smallest main sizes. All distribution reservoirs should be covered; provided with an overflow that will not undermine the footing, foundation, or adjacent structures; and provided with a drain, water-level gauge, access manhole with overlapping cover, ladder, and screened air vent.

Water storage tanks are constructed of concrete, steel, or wood. Tanks may be constructed above or partly below ground, except that under all circumstances the manhole covers, vents, and overflows must be well above the normal ground level and the bottom of the tank must be above groundwater or floodwater. Good drainage should be provided around the tank. Tanks located partly below ground must be at a higher level than any sewers or sewage disposal systems and not closer than 50 feet. Vents and overflows should be screened and the tanks covered to keep out dust, rain, insects, small animals, and birds. A cover will also prevent the entrance of sunlight, which tends to warm the water and encourage the growth of algae. Manhole covers should be locked and overlap at least 2 inch over a 2- to 6-inch lip around the manhole. Partly below-ground storage is usually less costly and aesthetically more acceptable than elevated storage.*

Properly constructed reinforced concrete tanks ordinarily do not require waterproofing. If tanks are built of brick or stone masonry, they should be carefully constructed by experienced craftsmen and only hard, dense material laid with full Portland cement mortar joints should be used. Two 0.5-inch coats of 1:3 Portland cement mortar on the inside, with the second coat carefully troweled, should make such tanks watertight. A newly constructed concrete or masonry tank should be allowed to cure for about one month, during which time it should be wetted down frequently. The free lime in the cement can be neutralized by washing the interior with a weak acid, such as a 10 percent muriatic acid solution, or with a solution made up of 4 pounds of zinc sulfate per gallon of water and then flushed clean.

Wooden elevated storage tanks are constructed of cypress, fir, long-leaf yellow pine, or redwood. They are relatively inexpensive and easily assembled, and need

* For small concrete reservoir construction details, see Manual of Individual Water Supply Systems, U.S. EPA, Washington, DC, 1973, pp. 127-128.

FIGURE 2.8 Mass diagram for determining capacity of tank when pumping 7 hours, from 11 p.m. to 6 a.m. (Source: J. E. Kiker, Jr., "Design Criteria for Water Distribution Storage," Public Works (March 1964): 102-104. This illustration originally appeared in the March 1964 issue of Public Works ®, published by Public Works Journal Corporation, 200 South Broad Street, Ridgewood, NJ 07450. © 2002 Public Works Journal Corporation. All rights reserved.)

FIGURE 2.8 Mass diagram for determining capacity of tank when pumping 7 hours, from 11 p.m. to 6 a.m. (Source: J. E. Kiker, Jr., "Design Criteria for Water Distribution Storage," Public Works (March 1964): 102-104. This illustration originally appeared in the March 1964 issue of Public Works ®, published by Public Works Journal Corporation, 200 South Broad Street, Ridgewood, NJ 07450. © 2002 Public Works Journal Corporation. All rights reserved.)

not be painted or given special treatment; their normal life is 15 to 20 years. Wooden tanks are available with capacities up to 500,000 gallons. The larger steel tanks start at 5,000 to 25,000 gallons; they require maintenance in order to prolong their life. Reinforced prestressed concrete tanks are also constructed. Underground fiberglass reinforced plastic tanks are also available up to a capacity of 25,000 to 50,000 gallons. Tanks having exterior lead-based paint needing repair present special problems regarding removal and prevention of air pollution.

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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