Water Supply

The amount of water required as a special reserve for fire extinction is usually small as compared with that necessary for other purposes, and is sometimes neglected, but an approximate rule applicable to industrial towns in Britain is that 8o,000Vx, when x is the population in thousands, will give the extra storage in gallons. This storage should be available in the minor service reservoirs, and should be appropriate to the particular district and population supplied by each such reservoir.

Whilst the distribution mains should be capable of meeting the maximum hourly demand, the criterion which really decides their capacity, especially in sub-districts, is the demand for fire extinction, and for this purpose the distribution mains should be capable of carrying of the average hourly rate of ordinary consumption. Thus in a sub-district having a population of 2,25o the average day's supply might be 72,00o gallons, the average hour therefore being 3,00o gallons and the maximum hour of the maximum day 3,000X1•75)0.25=6,60o gallons, whereas the fire demand would be =500% of 3,000 or 1/2•25 15,00o gallons an hour, and the main should therefore be capable of carrying 21,60o gallons an hour.

Source of

Supply.—Schemes of water supply may be divided broadly into two groups according to the general source, and the method of collection. In the first or surface water group we have (i) rain water collected from roofs, etc. ; (2) water from (a) upland rivers, (b) lowland rivers; and (3) water from lakes. In the second or ground water group we have water derived from (I) springs; (2) shallow wells; (3) deep and artesian wells; and (4) horizontal galleries. Whilst rain water from roofs or specially constructed tanks and ground water from shallow wells may, in general, be sufficient for individual supplies or for small groups of consumers, such supplies are not suitable for public supplies and are theref ore not further considered here.

Public water supplies may, from the nature of the works required, be divided into two broad groups--gravitational and pumping schemes. Gravity supplies may be obtained from upland rivers and sometimes from elevated lakes and springs, whilst supplies obtained from lowland rivers, deep wells, horizontal galleries, and sometimes from low level lakes and artesian wells, involve pumping.

Gravitational Schemes.

In the case of upland supplies the amount of storage provided must be such as will afford con tinuity of supply and the elevation should be such that when the reservoir is drawn down to the lowest draw-off level, it not only commands the greater part, if not all, of the area to be supplied, but also allows for the necessary fall to overcome the friction in the aqueduct and for the loss of head in filtration.

As most impounded waters are soft, it is now usual to install filters at the head of the aqueduct, and, during or after the filtration process, the water is so treated as to have a small residual alkalinity which inhibits any deleterious action on the materials, especially pipes, of which the aqueduct may be con structed. Whereas a large main conveying soft moorland water may have its capacity reduced by as much as 4o% by tubercula tion or nodular encrustation within twenty years, the same main conveying filtered and hardened water will not suffer any appreciable reduction in carrying capacity. In the older aqueducts, the only way of maintaining a reasonable capacity is by mechanical scraping at intervals, but once this process is adopted, it is found that the necessity of scraping increases in frequency as time goes on.

The necessity for filtration and hardening or softening of spring waters depends upon the possibility of pollution and the geological formation from which the springs derive their water.

Pumping Schemes.

Where the source is a lowland river or a low level lake the water may be abstracted by means of a simple intake or intakes protected by duplicate screens leading to a sump from which the water is pumped into sedimentation basins, and after a period of storage depending upon the amount of matter in suspension and other factors it is filtered, hardened or softened, and, if necessary, sterilized, and finally pumped into the service reservoir. Where the water is derived from deep wells or from infiltration galleries in the pervious bed or banks of a river, sedimentation is not required and, generally, filtration is unnecessary, though such waters may require softening and sterilization.