WATER SUPPLY. A sufficient supply of potable water is essential to man. The word potable means that the water shall be bright, clear and sparkling, free from suspended matters, reasonably soft, free from chemical poisons and in such condition that it cannot cause typhoid fever, cholera, diarrhoea or other water-borne disease, nor have any injurious action on metals. The water should be available at such pressures as will enable a supply by gravity to reach the upper floors or cisterns.
Water is needed for domestic purposes, including sanitation, also for industries, where a high degree of purity is often needed. The availability of suitable supplies determines the location of such industries. Water must also be supplied for fire extinction, street watering, supplies to public buildings and institutions and the flushing of sewers.
Estimation of Requirements.—A local estimate should be made of the average daily quantity which will be required in from 25 to 5o years' time. This estimate involves the prediction of the probable increase of population and of the quantity to be used for all purposes expressed in gallons per head per day. Factors involved are the character of the town, the habits of the people and the various industries which are established or likely to be established. The rate per head varies widely not only throughout any particular country but also as between countries. General experience indicates that almost invariably the rate per head shows an upward trend.
For example a small country town in England where there is but little industrial usage may require 18 or 20 gallons : a large commercial city may use for all purposes 35 or 4o gallons, and in America supplies up to goo gallons are fairly common and in several instances in the United States much larger quantities are used. Liverpool may be cited as a typical large city and of the total supply of 34.7 gallons per head in 1927 domestic purposes account for 25.7 gallons, trade and shipping (supplied by meter) 9.1 gallons, public purposes 1.6 gallons and sundry other purposes including fire extinction 3.3 gallons.
In America the higher consumption is partly due to a higher standard of living and a more lavish use of water and partly to waste due to faulty house fittings and to leaky distribution mains; an analysis of statistics covering 136 American cities having a population exceeding 25,00o shows that, on the average, where less than io% of the services are metered the consumption per head is i 28 gallons, whilst with 5o% or more metered services the consumption is only 52 gallons.
The maximum hourly demand for a town with a population of several hundred thousand will be 50% to i00% in excess of the average hour taken over the day: the demand of the maximum day will be 20-4o% in excess of the average day's demand : the maximum week will exceed the average week by 15-30% and the maximum four weeks will exceed the average four weeks by 1 o-2o%.
The hourly fluctuations and to some extent the daily fluctua tions are usually catered for in the minor service reservoirs which will also be first drawn upon in the event of fire, but the storage to meet the fluctuations over longer periods and the reserve to cover periods when the aqueduct may be out of commission is provided in the main service reservoir, and this in practice would have a capacity equal to 2-4 days of the average supply in the case of a gravitation supply; this storage would be increased if the aqueduct traversed ground liable to subsidence and reduced if the ground were good and the aque duct, where of pipes, was in duplicate with fairly frequent cross connections. Where a supply is pumped the aqueduct, or rising main, is usually fairly accessible, and pumping machines in duplicate, so less storage would be sufficient. Should the pumps work only part of the day the storage must also be capable of meeting the demand when the pumps are idle.