Sand Filter Beds . 83 



the upper layer of the filter bed have become covered 

 with a gelatinous film, which has been found to be the 

 seat of the bacterial scavengers. 



The nitrifying bacteria are found to be quite promi- 

 nent in the filter beds, and it is their function to change 

 the ammonia formed in the processes of decay, into the 

 soluble nitrates. Since the nitrifying bacteria are dis- 

 tinctly aerobic, requiring a plentiful supply of air for 

 their development, the filter beds must be so constructed 

 as to allow the air ready admission. For this reason, 

 layers of coarse materials are placed at the bottom, and 

 on top of that other layers of successively greater fine- 

 ness until the uppermost layer is reached. This consists 

 of sand whose grains are fairly uniform in size. It is 

 recommended that the thickness of the sand layer be 

 about 24 inches and, in any case, not less than 12 inches. 

 Because of the gradually increasing thickness of the 

 gelatinous film around the grains of sand, the filters 

 are apt to become clogged in the course of time. To 

 obviate this difficulty, the surface portion of the sand 

 is pared off from time to time, and the removed material 

 thoroughly cleaned and purified. 



It has been found in practice that the best results 

 are secured when the filters are worked intermittently, 

 that is, when they are given periods of rest in order to 

 allow a more thorough aeration of the bed. It has been 

 demonstrated, likewise, that the efficiency of the filters 

 depends, to a great extent, upon the rate at which the 

 water is made to pass through them. 



Other methods of water purification. The bacteria 

 present in drinking-water may be removed or destroyed 



