BACTERIOLOGY OF WATEB AND SEWAGE 441 



encounters obstacles larger or smaller which produce a cer- 

 tain amount of filtering action. The water in a stream 

 which is passing slowly through a wide bed of water plants 

 or algae is quite effectually filtered. Microorganisms tend 

 to cling to the surface of the plant stems, sand, etc., with 

 which they come in contact. 



Sixth, food and temperature conditions are in general 

 unfavorable to the growth of pathogenic bacteria and these 

 die off relatively rapidly in consequence. 



The distance which any water which has become con- 

 taminated must flow in order to become adequately puri- 

 fied differs with the conditions. Rapidity of flow, temper- 

 ature, amount of dilution, and amount of contamination 

 will all have their influence. In some cases evidence of 

 sewage contamination will disappear within a few miles; 

 under other conditions it will exist for some hundreds of 

 miles down stream. 



Coagulation and Sedimentation. When certain chem- 

 icals such as iron salts, alum, etc., are added to waters con- 

 taining sediment or organic material, the hydroxides which 

 are more or less gelatinous in nature are generally formed. 

 These cause coagulation or agglutination of the particles in 

 suspension and lead to relatively rapid sedimentation. Most 

 of the microorganisms are removed with the suspended 

 material in this process. Usually, however, mere coagula- 

 tion and sedimentation is not trusted to remove all harmful 

 bacteria from polluted water, but it is an essential pre- 

 liminary process in the purification of water of most 

 streams used as a source of supply by cities. 



Chemical Treatment. In some cases where it has not 

 been found practicable absolutely to eliminate harmful 

 bacteria by other processes, the water supply of the city is 

 treated with some chemical which will, on the one hand, 

 destroy such harmful microorganisms, and on the other not 

 injure the flavor of the water, prove poisonous, or injurious 



