THE BACTERIA Or SOIL, AIR, WATER, ETC. 1 27 



Storage of Water. — ^When water is kept in large reservoirs, 

 the solid particles in it, including bacteria, tend to fall to the 

 bottom. The number of bacteria in a water-supply may be 

 considerably reduced in this way. 



Filtration. — Filtration on a large scale has been more 

 commonly in use in the cities of Europe than elsewhere, until 

 lately. But filtration-plants now exist in several cities of the 

 United States. By this method 98 per cent, to 99 per cent, 

 of the bacteria in water may be removed. 



Slow Sand Filtration.* — The filter consists of successive 

 layers of stones, coarse and fine gravel. The uppermost layers 

 are of fine sand. The whole filter is from i to 2 meters thick. 

 The sand should be 60 cm. to 1.2 meters in thickness. The 

 upper layers may be removed from time to time, the remainder 

 not becoming less than 30 cm. in thickness. The first water 

 coming from the filter is discarded. The actual filtration is 

 done largely by the slimy sediment which collects on the surface 

 of the layer of fine sand. The filter-beds may be several acres 

 in extent, and in cold climates should be protected by arches of 

 brick or stone. They require renewal occasionally. This 

 kind of filtration has come largely into use since the cholera 

 epidemic of 1892-93, and it appears to be very effective. It is 

 often advisable to use storage basins in connection with sand 

 filtration, to permit of settling of part of the solid matter before 

 filtration. 



Mechanical Filtration. — This method of filtration is also 

 called the American system. It is more rapid than the preced- 

 ing method and does not require a large area for filter beds. 

 Although sand is required also, filtration is accomplished by a 

 jelly-like layer of aluminum hydroxide. This product is formed 

 by adding to the water a small quantity of aluminum sulphate 



* For a full discussion see Journal American Medical Association. Oct. 3, 

 to 31, 1903. 



