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close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 1: Water supply sources
View the document Overview:
View the document Background information
View the document Evaluation of sources
View the document Factors influencing the quality of water
View the document The quantity of water
View the document Types of sources
View the document Development of water sources
View the document The basic requirements of a water supply
View the document Selection of the source of supply
View the document Lesson plans

Selection of the source of supply

The choice of a source of supply for development depends on a number of factors, chief among which are: - the quantity and quality of the water available; the possibilities of sanitary control of the catchment area; whether the water can be supplied to the consumers by gravity or has to be pumped; and the distance from the source to the houses In order to obtain full information on these points it is necessary to carry out a very careful preliminary survey.

SOURCE OF SUPPLY

The first step in starting any water scheme is to determine what source of supply are available. Frequently a good source is not difficult to find but it is usually advisable to check all alternatives as some may be more economical and safer to develop. Sometimes suitable sources are not obvious and a search should then be made in the valleys, along the foot of the hills, where the vegetation is greener, and such places. In this reconnaissance the inhabitants are generally very willing to assist with their local knowledge.

If the search fails to reveal a satisfactory source an investigation of the ground-water becomes necessary, and for this a knowledge of the local geological formations is most helpful. Study of any existing wells will provide some information about the layers it penetrates, and the location, quantity and quality of the water. Unless a good deal Is already known about the aquifer it is expedient to sink test holes at various likely spots. These holes may be made with a pipe, about 2 inches in diameter, tipped with a point, and driven into the ground by a hammer, or a pipe sunk by an earth auger or by boring. This method is generally successful provided the water is not more than 30 feet or so from the surface. If it is necessary to probe any deeper, it is usually wise to obtain the services of an engineer possessing the experience and the equipment for this type of work. Deep well exploration and construction are expensive and are not jobs for amateurs.

The next step is to determine the quantity of water available. The rainfall figures may be obtained and the history of springs and existing wells may often be secured from the local residents. An estimate of the capacity of the aquifer may be made by pumping a well and noting the rate at which the well refills but the approximate yield in the dry season must be determined as that is often a decisive factor.

THE SANITARY SURVEY

The sanitary conditions prevailing in the immediate areas of possible sources should be thoroughly investigated. This is most important because the methods of purification of water, under rural conditions, are limited, and the process is too often neglected. Animal contamination of the water is very undesirable, and in some places may be dangerous, but the greatest hazard lies in pollution from human sources. It may be possible to find a spring, or stream, coming from a safe catchment area situated uphill from human habitation, or it may be practicable to render a source safe by moving potential origins of contamination or to protect the source by suitable intercepting drainage etc. Though the water from a stream may be liable to pollution it is often feasible to obtain wholesome water through wells and infiltration channels sunk in sand and gravel layers near the stream. Wherever possible the water should be examined chemically and bacteriologically and results considered in the light of the sanitary survey.

SOURCE AND TREATMENT

In the final selection of a source the following priorities should be adopted.

First-priority Consideration

Water which requires no treatment to meet bacteriological, physical, and chemical requirements and which can be delivered to the consumer by a gravity system should be given first consideration. This would usually be limited to springs and protected drainage areas. Such a system requires no treatment and no pumping and, therefore, is ideal from the point of view of maintenance, which is thus reduced to an absolute minimum.

Second-priority Consideration

Water which requires no treatment to meet bacteriological, physical, and chemical requirements but which must be pumped to consumers, would be the second choice. Well supplies would fall within this category.

Pumping can be an economical and simple solution, but it can also be an expensive and complicated one, according to local circumstances. It depends on the availability of qualified operators and on the local cost of fuel. Such factors vary widely from country to country and even from one rural area to another of a given country; they vary also with the types and efficiency of operation and maintenance programs developed for providing aid to municipalities from centrally located headquarters.

Third-priority Consideration

Water which requires simple treatment before it can meet bacteriological, physical, or chemical requirements but which can be delivered to the consumer through a gravity system should be given third-priority consideration. Simple treatment is considered to be limited to: (1) storage which would provide plain sedimentation and some reduction in bacteria, (2) chlorination without the use of a mechanically operated chlorinator, (3) slow sand filtration; or a combination of these.

For rural areas this is normally an inferior solution. It is usually more expensive than the above solutions and involves operational procedures which are most difficult to maintain in small rural communities. In such places, when the chlorine stock runs out, chlorination is abandoned in almost every instance; and, when the slow sand filter becomes clogged, a by-pass is often considered an easy arrangement. Such is the history of treatment measures in most rural areas where routine technical assistance is not provided by a responsible agency.

Fourth-priority Consideration

Water which requires simple treatment, as mentioned above, and which must be delivered to the consumers by pumping would obviously be the most expensive choice to make,

DETERMINING THE RELATIVE COST OF A DEVELOPMENT

The easiest method of determining the relative cost of a development is to:

(A) Estimate the cost of the various components for a given design;

(B) Calculate the time required for construction;

(C) Determine the total labor cost from the price per hour;

(D) Sum items A and C.