Water Supply

Service Reservoir.

When the main service reservoir is sup plied with filtered water or deep well water it is usually covered in order to prevent contamination and algal growths. Covering also helps to maintain the water at an even temperature and, espe cially in hot climates, prevents loss of water by evaporation. Even if reservoirs are not covered initially, it frequently happens that the building up of the surrounding land has rendered cover ing necessary, and as this can only be done by throwing the res ervoir out of commission, it is really economy to cover all but the largest reservoirs at the outset. The smaller service reservoirs are located in populous districts and are invariably covered.

Both large and small reservoirs are constructed of concrete, re inforced concrete or brickwork and concrete. They may be rec tangular, circular or hexagonal in plan, and their shape depends not only upon the contour of the ground, but also to some extent on the form of roof covering. For capacities up to one million gallons, the economic depth may be from i o to 12 feet, whilst at 3o millions, the depth may be 3o feet. Usually service reservoirs are surrounded by an earthen bank and the roof is soiled over.

The concrete floor is usually flat with sufficient fall for drainage during cleaning, and in all but the smallest reservoirs, a division wall of part or full height may be provided to enable a part of the storage to be available during cleaning. The roof may be of flat reinforced concrete slabs resting upon a system of main and secondary beams of reinforced concrete, or steel beams encased in concrete. It may consist of longitudinal arches in brickwork or concrete, which spring either from girders or from transverse flying arches, these arches being supported on massive piers, or, as is frequently found in American practice, the piers may support a system of concrete groined arches. Still another and economical form of covering consists of a series of relatively thin plain con crete domes springing from concrete groined arches which, in turn, are supported by hexagonal concrete block columns. Ample pro vision for ventilation should be made especially in tropical coun tries, and in these countries mosquito netting is essential.

Apart from the main service reservoir it is usual to provide one or more minor reservoirs upon suitably elevated sites to meet the hourly and possibly daily fluctuations of supply in the more or less immediate vicinity, and to hold an appropriate reserve for fire extinction. Where no site at the required elevation

is available, watertowers supporting elevated tanks of cast iron, steel or reinforced concrete, holding 20,000 to 500,000 gallons, and standing 5o to Ioo feet above the ground, are used. Minor service reservoirs, located at the ends of main distribution pipes remote from the main service reservoir, are advantageous in that they enable smaller and less costly connecting mains to be used, and also give more regular pressures in the districts served.

Distribution.

There are two methods of charging for domes tic supplies : as in Britain, it may be by a water rate at so much per cent. of the annual value, or, as in many American and Con tinental cities, at so much per i,000 gallons by meter. Where the supply is metered, it is to the user's interest to maintain his fittings in proper condition, and in America, the introduction of meters invariably leads to a considerable reduction in consumption.

Quite apart from leaky fittings there may be loss of water in the distribution mains, and in a well-organised Water Depart ment, the detection and prevention of leakage and waste is an important part of the work of maintenance.

The outflow from all service reservoirs should be metered, and a close observation of the rates of flow during the night hours will indicate whether there is any serious loss of water in the area supplied from that reservoir. For more detailed observa tions, the town is divided up into districts, the night supply to each of which can by an appropriate system of valve control be made to pass through a Deacon Differentiating Waste Water Meter. This meter carries a paper diagram mounted on a drum, which is caused by clockwork to revolve uniformly; each house service is controlled by a stop cock accessible to the Waterworks Officials ; these cocks are closed in turn and the time of doing so is noted ; where there is leakage taking place the closing of the cock will cause a step in the diagram, and the time at which the step occurs will, on reference to the night inspector's log, enable that particular house to be located. The fittings in that house are examined and the user is called upon to remedy the defects which are notified.