BACTERIA OF WATER AND SEWAGE 303 



contain, killed. All satisfactory methods are fundamentally 

 bacteriological. Whether buried, treated by the dry-earth 

 system, the cesspool, the stream, or better methods, sewage 

 is purified or rendered harmless by bacteria. The modern 

 methods most frequently used are the intermittent filtration, 

 broad irrigation, and the septic tank and contact bed methods. 

 Their importance is in the inverse order from that in which 

 they are enumerated. 



Intermittent filtration is used successfully in certain locali- 

 ties where the coarse material needed in their construction, 

 such as gravel, broken stones, or cinders, are available. One 

 acre purifies the sewage for one thousand people. The most 

 favorable location is along a river bed. These beds, although 

 they must be much more extensive than those required for 

 water, may cost less. Sewage may be purified by this method 

 so that it is indistinguishable from pure water, except perhaps 

 by the amount of sodium chloride. The aerobic bacteria are 

 especially active in this filter. Broad irrigation or sewage 

 farming is successfully practiced where the proper soil exists 

 (dry and porous). The amount of land required is from ten 

 to twenty times as great as is require4 by the intermittent 

 filter. The sewage is run down one side of the field in a ditch, 

 and in lateral ditches or furrows it flows through growing 

 crops towards a stream. The sewage is applied intermittently 

 as above. This method is used in Berlin and Paris, and also 

 extensively in England and to some extent in North America. 

 It is economical, since several crops can be raised instead of 



