STORAGE AND DISTRIBUTION. 



141 



The impurities usually exist in the suspended form, 

 although there are many impurities in solution, and 

 these will be noted by the analyst who examines the 

 water, who will state if any and what treatment they 

 may require. Small water supplies will, of course, not 

 warrant the laying down of elaborate plant of this 

 description, but if the water is abnormally soft, small 

 quantities (advised by a chemical expert) of powdered 

 carbonate of lime may be added to remove some of 

 the dissolved CO 2 . 



In this connexion, however, the removal of sus- 

 pended impurities by filtration is the most important 

 question. A certain amount of settlement goes on 

 during storage, and very heavy matter is intercepted 

 by wire-gauze strainers attached to the draw-off 

 valves in the reservoir. The usual filtering medium, 

 however, is sand and gravel. The filter consists es- 

 sentially of a brick or concrete tank, rendered water- 

 tight and containing layers of filtering material graded 

 in size, the water 

 being admitted on 

 top and drawn over 

 from the bottom of 

 the tank. A very 

 simple form of 

 filter is shown in 

 fig. 104, and the 



Wboci 



Water 



9 Corvcrete 



FIG. 104. 



drawing being self-explanatory needs no further de- 

 scription. 1 In the case of storage and use of rain- 

 water a very suitable arrangement is shown in figs. 

 105 and 1 06. Rain-water, which used to be stored 

 in butts, which were generally the receptacle for filth, 



1 The Slate Slabs are perforated freely. 



