7G8 REPORT — 1903. 



Simpson, Past-President of the Institution cf Civil Engineers. The rate of filtra- 

 tion, to be thoroughly eSective, must depend on the condition of the water to be 

 filtered, but a rate of from 450 to .550 gallons per square yard of surface of sand 

 per day {i.e., twenty-four hours) is usually found to be efficient. Filter-heds are 

 generally open to the sky, but occasionally, when situate at considerable elevations, 

 they are covered by roofs to prevent interruption by the formation of ice in times 

 of severe frost. In certain exceptional cases in which the water is difficult to treat 

 it is twice filtered with excellent results. The water after filtration should be 

 discharged into a pure-water tank or service reservoir of sufficient capacity to 

 enable the process of filtration to proceed at a uniform rate by night as well as by 

 dav, without regard to irregularities in the rate of demand in the district of supply. 

 " The particles in suspension in the water, which are intercepted by the process 

 of filtration, gradually form a film over the surface of the sand, and thus improves 

 the filtration ; but this film at last becomes so thick as to unduly reduce the rate 

 at which the water passes through the sand. The filter-bed is then laid off and, 

 the water having been withdrawn, the surface of the sand is scraped otf to a depth 

 of about a quarter of an inch ; the sand thus removed is waslied in suitable machines 

 to free it from the matter intercepted daring the process of filtration, audi s after- 

 wards replaced in the filter-bed either immediately or after several similar ^ciap- 

 ings have taken place, care being taken that the thickness of the sand left in the 

 bed shall not at any time be reduced below that required to ensure efticient filtra- 

 tion. From time to time the sand is removed to a depth of several inches and 

 washed, and occasionally it is taken out and washed to its full depth. From the 

 foregoing description it will be understood that the filtration of water, although 

 a simple process, is one which necessitates constant watclifulness on the part of 

 those responsible for the management of those waterworks undertakings in which 

 the water undergoes filtration. 



As near to the termination of the aqueduct conveying the water from the 



impounding reservoir to the point of distribution as the levels of the ground will 



permit, a service reservoir should be constructed for the purpose of equalising the 



flow of water along the aqueduct, and for maintaining the supply to the district 



during any temporary interruption on the line of aqueduct due to a burst pipe or 



otherwise. The service reservoir should contain not less than one day's supply, 



two or three days, and, in exceptional cases, even more being sometimes desirable. 



Service reservoirs should bj' preference be covered so as to exclude light, and 



thus prevent the growth of vegetation which would otherwise take place. The 



covering, when consisting of brick arches, has also the advantage of keeping the 



water cool in summer, and preventing the temperature from becoming too much 



reduced in winter. The rate of draught on the service reservoir is continually 



varying throughout the day and night according to the hourly requirements of 



the population which it serves. This variation is very considerable, amounting 



during certain hours of the day to at least twice the average rate of consumption 



during the twenty-four hours. It will therefore be apparent that were it not for 



the equalising efi'ect of the service reservoir the aqueduct must have a capacity at 



least double that which is needful where a service reservoir is available. At 



Sonthport, for example, although the water is distributed from a service reservoir, 



that reservoir is situate at a distance of about seven miles from the town, because, 



owing to the great extent of comparatively flat land in the neighbourhood of 



Southport, it was impossible to obtain a suitable elevation nearer to the town 



than Gorse Hill, on the summit of which the reservoir stands. Consequently the 



main pipes thence to the town have to be of sufficient capacity to convey the water 



at a rate corresponding with the demand at the time of viaxinmm consumption, 



or, in other words, of about twice the capacity which would have been needed if 



the service reservoir could have been placed close to the town, when these pipes 



would, for the greater part of their length, have been situate on the inlet instead 



of on the outlet side of the reservoir. 



Having now followed the water in the case of a gravitation supply from its 

 source to the service reservoir from which it is to be distributed to the consumers, 

 it will be eonve?]ient to follow in a similar manner water obtained by means oi" 



