A cistern is a watertight tank in which rainwater collected from roof runoff or other catchment area is stored. When the quantity of groundwater or surface water is inadequate or the quality objectionable and where an adequate municipal water supply is not available, a cistern supply may be acceptable as a limited source of water. On the one hand, because rainwater is soft, little soap is needed when used for laundry purposes. On the other hand, rain will wash air pollutants, dust, dirt, bird and animal droppings, leaves, paint, and other material on the roof or in roofing materials or catchment area into the cistern unless special provision is made to bypass the first rainwater and filter the water. The bypass may consist of a simple manually or float-operated damper or switch placed in the leader drain. When in one position, all water will be diverted to a float control tank or to waste away from the building foundation and cistern; when in the other position, water will be run into the cistern. The filter will not remove chemical pollutants. If the water is to be used for drinking or food preparation, it should also be pointed out that because rainwater is soft and acidic, and therefore corrosive, hazardous concentrations of zinc from galvanized iron sheet roofing, gutters, and pipe and lead and copper from soldered copper pipe may also be released, in addition to cadmium.

The capacity of the cistern is determined by the size of the roof or catchment area, the probable water consumption, the maximum 24-hour rainfall, the average annual rainfall, and maximum length of dry periods. Suggested rainwater cistern sizes are shown in Figure 1.15. The cistern storage capacity given allows for a reserve supply, plus a possible heavy rainfall of 3-1/2 inches in 24 hours. The calculations assume that 25 percent of the precipitation is lost. Weather bureaus, the World Almanac, airports, water departments, and other agencies give rainfall figures for different parts of the country. Adjustment should therefore be made in

FIGURE 1.15 Suggested cistern storage capacity and available supply.

TABLE 1.16 Quantity and Type of Chlorine to Treat 1,000 gal of Clean Water at Rate of 1 mg/l

Chlorine Compound


High test, 70% chlorine Chlorinated lime, 25% chlorine Sodium hypochlorite 14% chlorine 10% chlorine Bleach, 5-1/4 chlorine

1/5 oz or 1/4 heaping tablespoon 1/2 oz or 1 heaping tablespoon

1 oz

2-3/5 oz the required cistern capacity to fit local conditions. The cistern capacity will be determined largely by the volume of water one wishes to have available for some designated period of time, the total volume of which must be within the limits of the volume of water that the roof or catchment area and annual rainfall can safely yield. Monthly average rainfall data can be expected to depart from the true values by 50 percent or more on occasion. The drawing of a mass diagram is a more accurate method of estimating the storage capacity, since it is based on past actual rainfall in a given area.

It is recommended that the cistern water be treated after every rain with a chlorine compound of at least 5 mg/l chlorine. This may be accomplished by adding five times the quantities of chlorine shown in Table 1.16, mixed in 5 gallons of water to each 1,000 gallon of water in the cistern. A stack or tablet chlorinator and carbonate (limestone) contact tank on the inlet to the cistern is advised for disinfection and acidity neutralization. In areas affected by air pollution, fallout on the roof or catchment area will contribute chemical pollutants that may not be neutralized by limestone or chlorine treatment. Soft water flowing over galvanized iron roofs or through galvanized iron pipe or stored in galvanized tanks contains cadmium and zinc.145

Example With a roof area of 1,600 ft2, in a location where the mean annual precipitation is 30 inches and it is desired to have a reserve supply of 3,000 gallon, the cistern storage capacity should be about 5,600 gallons. This should yield an average annual supply of about 62 gallons per day.

In some parts of the world, large natural catch basins are lined to collect rainwater. The water is settled and chlorinated before distribution. The amount of water is of course limited and may supplement groundwater, individual home cisterns, and desalinated water.

Renewable Energy Eco Friendly

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.

Get My Free Ebook

Post a comment