450 BACTERIA PATHOGENIC TO MAN 



The collection of water in large reservoirs allows not only the living 

 and dead matter to subside, but allows time also for the pathogenic 

 germs to perish through light and antagonistic bacteria and other 

 deleterious influences, sand or mechanical coagulant. Filtration of 

 water exerts a very marked purification, taking out 99.8 per cent, of the 

 organisms in those best constructed and at least 95 per cent, in those 

 cpmmonly used in cities. The construction of filters is too large a sub- 

 ject to enter on minutely here; sand filters consist, as a rule, of several 

 layers, beginning with fine sand, and then smaller and larger gravel, 

 and finally rough stones. A certain time elapses before the best results 

 are obtained; this seems to wait for the formation of a film of organic 

 material on the sand, which is full of nitrifying bacteria. Even the 

 best filters only greatly diminish the dangers of polluted water. Spring- 

 water and well-water are, in fact, filtered waters. 



Water which is subject to serious pollution must be submitted to a 

 preliminary purification before it can be considered a suitable source 

 for a drinking-water supply. The means employed for its purification 

 depend to a large extent upon the character of the water and the nature 

 of the pollution. Filtration through slow T sand filters, three to five 

 feet in depth, removes 98 to 99.5 per cent, of the bacteria and organic 

 matter; so that effluents from the best constructed sand filtration beds 

 constitute safe and reliable drinking waters. Five hundred thousand 

 to one or two million gallons, depending somewhat upon the extent of 

 pollution and the fineness of the sand, can be filtered daily per acre. 

 Only the surface of the sand filter becomes in any way clogged and 

 as thin a layer as can be scraped off is removed one or more times a 

 month. This surface sand is washed with clean water and several 

 scrapings replaced at one time. Sand filtration beds are very widely 

 used abroad and are coming into extensive use in this country. The 

 filter beds at Lawrence, Mass., have been used over ten years with 

 marked success, rendering the highly polluted Merrimac River a safe 

 drinking-water; the filter beds there are scraped about thirteen times 

 a year. 



Mechanical filtration plants find considerable favor where clarifica- 

 tion as well as bacterial purification is desired. A coagulant such as 

 sulphate of aluminum is employed and forms in the water a flocculent 

 precipitate which carries down with it all suspended matter; 50,000,000 

 or more gallons of water can be filtered on an acre daily, but the filters 

 must be washed once or twice daily by reversing the flow and cleansing 

 the clogged filter with a stream of the purified water. The bad clogging 

 is due to the fact that the process is a purely mechanical one, and not 

 comparable in any way with the vital processes carried on in the sand 

 filter by the nitrifying bacteria. 



Other methods are coming into use, such as the passage of ozone, 

 and have proved successful. Such processes should be under the direct 

 supervision of expert sanitary engineers and bacteriologists. 



Domestic Purification. Water which requires private filtering should 

 not be supplied for drinking purposes. Unhappily, however, it often 



