|Operation and Maintenance of Water and Sewerage Systems (Ministry of Water - Tanzania - Rwegarulila Water Resources Institute, 1999, 90 p.)|
|C. General Guide Lines in Construction of Water Structure|
After having treated the raw water in water treatment plants, to the required standards, the last and final stage of water supply schemes comprises distribution of water to the consumers. The main purpose of the distribution systems is to develop adequate water pressure at various points i.e. depends upon the topography of the area of distribution and its elevation with respect to the location of the water treatment plants. The distribution system may be classified into three categories.
(i) Gravity system
(ii) Pumping system without storage
(iii) Dual system with storage
Water from storage tanks can be distributed to consumers by arranging pipelines in two ways.
(i) Dead-end System (Branches system)
In this system water is fed into secondary pipes from on side
(a) It is possible to calculate accurately the discharge and pressure at any point in the distribution system. Calculations are simple and easy to do.
(b) This method requires comparatively less number of cut off valves, hence cheap.
(c) Pipe lines can be laid in the street of any pattern which may not be standardised.
(d) The diameter of mains are to be designed for the population they have to serve. This fact may make the system cheap and economical.
(a) During break downs and repairs large areas which are served by this pipe, go without water and thus cause great inconvenience to the public.
(b) In this system there are large number of dead ends, where water does not circulate but remains static, which may get contaminated due to stagnation.
(c) Water available for fire fighting will be limited is being supplied by only one water main.
(ii) Grid-iron System (looped system)
In this system, water is fed into secondary pipes from two sides.
(a) At the time of repair or breakdown only small portion of the distribution layout is affected.
(b) As there are no dead ends and water circulation is free throughout, it is not liable to contamination.
(c) Water reaches all the points with minimum loss of head.
(d) At the time of fires, by manipulating the cut off valve, plenty of water supply may be diverted and concentrated for fire fighting.
(a) Cost of pipe laying is more because relatively more length of pipe is required.
(b) It is difficult to calculate pressures and discharges at various points of distribution systems
(c) More number of valves are required.