280 PUKIFIOATIOE" OF PUBLIC WATER SUPPLIES. 



sand can be quickly and thoroughly cleaned. This is 

 sometimes accomplished by means of reversed currents 

 of filtered water forced upward through the bed of sand 

 and run to waste until it runs clear. Mechanical de- 

 vices are sometimes added for the purpose of agitating 

 the sand while the reverse current is passing. In others 

 the entire body of sand is forced upward with the wash- 

 ing current into a chamber and then allowed to flow 

 back into its proper position. This process of cleaning 

 the sand is one of the essential features of so-called me- 

 chanical filtration. It is usually done V)nce in twenty- 

 four hours and is ordinarily accomplished in about half 

 an hour. 



The various mechanical devices adopted for this pur- 

 pose, as well as the appliances before mentioned for re- 

 taining the sand while permitting the water to pass, are 

 the subjects of many patents held by different inventors' 

 and covering, apparently, every possible variation of ar- 

 rangement. This method of filtration is, therefore, at 

 the present, entirely controlled by patents. 



With the rapid rate of passage of the water through 

 the sand, this substance alone is able to accomplish 

 very little in the way of purification, only the very 

 coarsest matters in suspension being retained by the 

 sand, the effluent otherwise differing in no degree from 

 the applied water. The use of coagulants is, therefore, 

 a necessary feature in this method of filtration. Any 

 substance that vv^ill group together the minute particles 

 of clay or other impurities will serve the purpose. 



Sulphate of alumnia has, however, been found to be 

 not only cheaper but equally efficient and the most 

 easily applied of such substances. It is, therefore, 

 generally adopted for this purpose. A solution of lump 

 alum is prepared of known strength and this solution is 

 applied to the water entering the filter in the precise 

 proportion requisite to accomplish the necessary coagu- 



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