Self- Purification 79 



River-water ,and surface drainage. — The differe":ces 

 in the compositon of river-water, as well as their in- 

 fluence on the water bacteria, may be further modified 

 by the quantity and quality of the surface drainage. 

 When the surface drainage includes notable additions 

 of sewage, the influences on the bacterial flora of the 

 stream may be quite far-reaching. The modifying effect 

 of sewage may be due to its quality as well as to its 

 quantity. For example, the sewage of European cities 

 is more concentrated than that of American cities be- 

 cause of the larger volume of water used by the latter. 

 It is to be expected that equivalent quantities of the 

 two will not have the same effect on any given stream. 

 Moreover, the ratio between the volume of sewage 

 added to a stream, and the volume and velocity of the 

 latter, is of direct significance. A slight quantity of 

 sewage added to a large river would not contaminate 

 the water to such an extent as the addition of a large 

 volume of sewage to a small stream. 



Self-purification of rivers. — The activities of the water 

 and sewage bacteria rapidly exhaust the store of readily 

 decomposable organic material. The less readily avail- 

 able residues do not furnish the large quantities of food 

 required by vast numbers of bacteria, and the less 

 resistant species are rapidly eliminated in the struggle 

 for existence. The gradual disappearance from the water 

 of the organic substances, as well as of the bacteria 

 introduced with the sewage into flowing streams is 

 designated as self-purification. 



Differences of opinion still exist as to the extent 

 to which self-purification may be depended upon for 



