DISTRIBUTION AND - WHO

1 Chapter 7 DISTRIBUTION AND USE In many rural areas, wells equipped with hand pumps and conveniently located are used as sources of domestic water-supply for individual houses and villages. Since the water obtained in this way must be carried in cans or buckets from the wells to the houses, the amount of water is usually limited and too small for the effective promotion of health and personal hygiene. Although the use of wells in villages is inevitable and often dictated by economic and engineering considerations, 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, because of its obvious advantages. The most important benefits are convenience and an increased supply which may be made, at the same time, wholesome and safe for the protection of health.

2 Water- DISTRIBUTION systems for rural areas are essentially similar to those of urban communities. Often it is possible for the sake of economy to group a number of villages relatively close to each other and to supply them from a central water source. The difference between rural and urban water- DISTRIBUTION systems is to be found in the standards and assumptions upon which engineering designs are based. While not compromising on essential water quality standards, such differences include the amounts of water for domestic consumption, the standards of protection against fire, and the degree of water treatment. The design of a rural water- DISTRIBUTION system involves : (I) the deter- mination of storage; (2) the location and size of feeders; (3) the location and sizes of DISTRIBUTION pipes, valves, and hydrants; and (4) the determi- nation of the pressure required in the system .

3 Before the design is under- taken, it is necessary to make a reconnaissance survey of the village and of the area leading to the source of supply. Often such a survey, carried out by a trained engineer, will be sufficient to enable him to decide on the route to be followed by the feeder pipe and on the location of distributing reservoirs and main pipes. This reconnaissance should be followed by a topographic survey in order to determine the locations and elevations of street intersections; of all low and high points within the village; of streams, gullies, depressions or similar topographical features which may bear upon the design; and, finally, of the feeder line and the distributing reservoir. INSTALLATION OF WATER-SUPPLY SYSTEMS 195 When the construction of the DISTRIBUTION system is completed, similar maps should be prepared for future reference showing the exact location of all pipes, reservoirs, valves, hydrants, and other appurtenances.

4 Small-Community DISTRIBUTION Systems Distributing reservoir 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) the need to satisfy hourly variations in the rate of consumption (in small systems, such variations may be three times the average hourly consumption and sometimes more); (2) the desirability of maintaining adequate pressure throughout the DISTRIBUTION system ; (3) the possibility of repairing adduction pipes between the source of supply and the reservoir, without interruption of the village water service. (4) The need to provide for fire protection. Other advantages which, under certain circumstances, may assume considerable economic importance include the following : (1) Where the water is pumped to the reservoir, pumps can be operated uniformly throughout the day.

5 Such pumps may be much smaller than would be required otherwise. (2) With such a reservoir, 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. In most small rural schemes no special arrangements are made for fire protection. Notes on fire services are included on pages 203-205, 206, 239 but the paragraphs preceding these are based on the assumption that no provision for fire hydrants is being made. 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 main- tenance interruptions.

6 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 and 8 ; 30% between 5 and ; 35 %during the other hours of daylight; and 5% between sunset and sunrise. Local customs will produce local variations; for instance, in Moslem countries the demand during Ramadan will be high at about 3 , and in other parts of the world where Monday is the traditional " wash-day " the Monday morning 196 WATER SUPPLY FOR RURAL AREAS draw-off may be equivalent to the total supply of another day. These and similar considerations must be taken into account when assessing the extent and duration of peak draw-offs : 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 capital cost, as well as most satisfactory from an operational aspect, if a constant flow is maintained throughout the twenty-four hours.

7 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 petrol-driven pumps, the cost of attendance (necessarily continuous with such engines, but normally unnecessary with electric motors) becomes an important factor, and one shift of eight hours, or two totalling 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 Labour 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.

8 Once the probable DISTRIBUTION of draw-off during the 24 hours and the proposed hours of delivery is decided, two graphs can be drawn, similar to diagrams A and B in Fig. 72. In this case a probable demand of 136000 litres per day is envisaged, to be provided by pumping for eight hours per day between 8 and 4 These graphs are then superimposed, as at diagram C, in such a way that the lowest point on line B touches, but does not come below, line A. This gives a theoretical picture of how much Town requiring 136000 litres per day, with low draw-off at night and peak periods 7 to 8 and 5 to Graph A - shows consumption throughout the day; steep slopes show peak hours. Graph B - shows pumping between 8 and 4 136 001 litres in 8 hours equals 17 000 litres per hour.

9 Graph C - shows graphs A and B combined. The vertical distance between the two lines represents the reservoir capacity required at any time, the maximum distance occurring at 4 and representing 102000 litres required. Reservoir assumed to be empty at 8 Graph D - shows the effect on reservoir capacity required if pumping hours are changed to from to Maximum distance now occurs at and represents 70000 litres required. Reservoir empty at and again at Graph E -shows the same consumption pattern as Graph A but with continuous supply throughout 24 hours by gravity main at 1250 gals. per hour. Maximum reservoir capacity now required at 7 equals 57 000 litres. Reservoir empties completely at and starts refilling immediately as supply exceeds draw-off from that time on.

10 INSTALLATION OF WATER-SUPPLY SYSTEMS 197 Fig. 72. DlSTRlBUTlON RESERVOIRS: THEORETICAL RESERVOIR CAPACITY REQUIRED 50000 I. Hour 8 Noon 16 Noon I Hour 5:30 13:30 150000 1. 100000 I. 50000 150000 I. - 100000 I. - E z ,I_-U,i,''' Hour 0 1 2 3 4 5 6 7 8 7 11) 11 12 13 14 15 16 17 18 17 20 21 22 23 24 - - - Hour 8 Noon 16 198 WATER SUPPLY FOR RURAL AREAS storage will be required, the vertical distance between the two lines represent- ing the amount in the reservoir at any hour of the day. From this example the reservoir would be empty at 8 , filling to a maximum of 102 000 litres at 4 , and thence emptying at varying rates until 8 the next day when pumping starts again. It can be seen that the hours of pumping proposed are uneconomical in this instance, and diagram D shows how the storage required is decreased by arranging the pumping hours between and , the max- imum difference between supply and demand now being 70 000 litres at Diagram E assumes that instead of pumping, the water is supplied by a gravity main from source, running through the day and night at 5665 litres per hour.