40 WATER SUPPLY FOR DOMESTIC USE. 



Efficiency of filtration will depend very much on the rate 

 the water passes through the filter. The pressure of water 

 should not be great, the depth being not more than 2 feet. 

 From 70 to 75 gallons is the usual quantity which should pass 

 through in 24 hours for each square foot. 



By means of subsidence tanks the water may be freed to a 

 large extent of suspended matters and bacterial organisms, the 

 latter becoming entangled in the suspended matters sink to the 

 bottom with them, and probably undergo changes there some- 

 what similar to those undergone in a filter-bed — 75 per cent, of 

 bacteria may be got rid of in this way. 



Various other means are adopted for the filtration of water 

 besides sand and gravel, such as fine animal charcoal, carferal, 

 which is a black granular matter having some resemblance to 

 granular animal charcoal. It consists of charcoal and iron in 

 small quantities with a basis of clay — but these are only em- 

 ployed on a small scale or for domestic filters. The following 

 are the essentials of a good filter, given by Dr. Parkes : — 



"1. That every part of the filter shall be easily got at for 

 the purpose of cleansing or renewing the medium. 



"2. That the medium shall have a sufficiently purifying 

 power, and shall be present in sufficient quantity. 



" 3. That the medium yield nothing to the water that may 

 favour the growth of bacterial organisms. 



" 4. That the purifying power be reasonably lasting. 



" 5. That there shall be nothing in the construction of the 

 filter itself that shall be capable of undergoing putrefaction, or 

 of yielding metallic or other impurities to the water. 



" 6. That the filtering material shall not be able to clog, 

 and the delivery of the water shall be reasonably rapid." 



The Chamberland-Pasteur Filter is one of the most recent, 

 and owes its filtering power to finely divided porcelain. It is 

 attached to a stand-pipe or tap, and efficiently filters the water, 

 provided that the current be not too strong. It is easily cleaned, 

 and has proved its efficacy by reducing disease among the 

 French soldiers in Cochin China by 50 per cent. At all events, 

 after the introduction of the Chamberlain filter, the health of 

 the army very notably improved. 



Difierent means may be resorted to in order to purify water 

 without filtration. 



