CHAPTER XIII 

 BACTERIAL PURIFICATION OF SEWAGE 



The decomposition of manure and humus in the soil, 

 the destruction of organic matter in surface waters, 

 and its gradual disappearance in the cess-pool, filter- 

 beds, or septic tanks, are all brought about by the 

 same forces — the vital activities of bacteria. The uni- 

 versal distribution of the microorganisms in the air, 

 dust, water and soil, assures a speedy inoculation of 

 all materials capable of undergoing decay. Over and 

 above these sources of inoculation, sewage contains enor- 

 mous numbers of bacteria derived from human excreta 

 and kitchen wastes. It is no wonder, therefore, that 

 sewage becomes foul so rapidly. 



Sewage-farms. — The purifying power of soil, due to 

 its bacteria, was recognized in early times. Hence, when 

 the purification of sewage was absolutely necessary for 

 a number of cities, land treatment was one of the first 

 means considered for such purification. Sewage-farms 

 thus came into existence after the middle of the last 

 century. Some of these have been successfully managed 

 to this day. In many towns, however, there were no 

 suitable areas of land available for this purpose and 

 sewage-irrigation was entirely impracticable. 



Sewage tanks and filter beds. — It was discovered that 



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