| Water purification, distribution and sewage disposal for Peace Corps volunteers |
|Section 4: Characteristics of an adequate system|
WATER DISTRIBUTION SYSTEMS
CHARACTERISTICS OF AN ADEQUATE SYSTEM
The source of water for the distribution system must meet the requirements that have been established for quantity and quality. The source will meet the quantity requirement by simply being able to provide enough water to meet the demand. If no single source can satisfy this requirement, the construction of storage tanks and/or reservoirs can be built to compensate for this deficiency. The quality requirements for drinking water are generally established by local or national health departments. (If none exist, you should review the minimum standards for drinking water discussed in Section 2) If no satisfactory water sources exist, you should carefully consider the construction of wells before choosing a source that would require extensive treatment. Although the distribution of water from a central source by means of pipes to each village house is a goal towards which every community should strive, the construction of wells (if necessary) would be an incomplete, but very satisfactory step toward that goal.
Once satisfactory water has been located, it is essential that care be taken not to contaminate the water during its distribution. To insure the preservation of potable water, the following protective measures should be taken:
1. Wells and pump bases should be sealed so that surface water is unable to enter the well;
2. Water used to prime pumps must not be polluted (when possible use pumps that do not need priming).
3. Only trained workers should be responsible for maintaining the system once it has been built. (Initially, care should be taken to select workers who are healthy and free from communicable diseases).
In selecting a water source, the goal is one needing no chemical treatment (other than disinfection) prior to use. As stated earlier. if no such source already exists, explore the possibility of a well construction program.* When an acceptable-but-less-than desirable source is the only alternative, a treatment plant will need to be part of the distribution system. The purpose of the plant will be to upgrade the quality of water that does not meet drinking water standards. The methods for evaluating water and selecting treatment processes is discussed in Section 2.
(*For further information refer to the Peace Corps training manual on the Construction of Water Wells)
In small distribution systems, whether the water is obtained by gravity or by pumping, it is always desirable to provide a distribution reservoir. The main reasons are:
1. Hourly variations in the rate of consumption are more easily satisfied (in small systems, such variations may be three times the average hourly consumption and sometimes more);
2. Adequate pressure can be maintained throughout the distribution system;
3. Adduction pipes between the source of supply and the reservoir may be repaired without interruption of the village water service.
4. Provisions may be made for fire protection.
5. Pumps can be operated uniformly throughout the day. (Such pumps may be much smaller than would be required otherwise).
6. The size of the adduction pipe between the supply source and the reservoir can be made smaller than would be necessary if the village were fed directly from the water source.
7. Fluctuations in peak periods of demand can be more easily observed and compensated for when all water is drawn from a distributions reservoir.
The first consideration when designing storage is the capacity which will be provided. This to a great extent depends on the type of supply, and is influenced by two main factors - the necessity of catering for peak demand periods, and the provision of reserve to cover normal breakdown or maintenance interruptions.
Conditions vary in different parts of the world, but a typical pattern of draw-off in a village is as follows - 30% of the day's supply between 7 a.m. and 8 a.m.; 30% between 5 p.m. and 6:30 p.m.; 35% during the other hours of daylight; end 5% between sunset and sunrise. Local custom will produce local variations; for instance, in Moslem countries the demand during Ramadan will be high at about 3 am., and in other parts of the world where Monday is the traditional "wash-day" the Monday morning draw-off may be equivalent to the total supply of another day. These considerations must be taken into account when assessing the extent and duration of peak draw-off; this must then be balanced against the rate and periods of water delivery.
When water is supplied by gravity from the source it is most economical in cost, as well as most satisfactory from an operational aspect, if a constant flow is maintained throughout the twenty-four hours. Obviously in such a method of working a smaller delivery main is needed than if larger quantities are required in shorter periods. When electricity is used for pumping it is usually most economical to operate for about twenty hours a day, leaving the pumps idle during the peak hours of electricity demand. With diesel- or gasoline-driven pumps, the cost of attendance (generally continuous with such engines, but normally unnecessary with electric motors) becomes an important factor and one shift of eight hours, or two totaling 16 hours, is a frequent method of operation.
It is quite common to find schemes designed to operate with a single shift of 8 hours initially, increasing to 16 hours when the demand rises later. More than 16 hours a day is not desirable with such engines) not only do labor costs increase but the wear on machinery working continuously throughout the day and night becomes excessive and the life of the plant is correspondingly shortened.
To determine the amount of storage that will be required to provide uniform serice throughout the day, you must estimate three factors:
1. the hourly consumption throughout the day (measured in gallons)
2. the proposed hours of pumping (as explained above, this is determined by the type of equipment available)
3. the pumping capacity of the system (gallons per hour).