264 MICROBIOLOGY OF WATER AND SEWAGE 



out the year a larger number of organisms to the cubic centimeter 

 than the water of the river St. Lawrence, which is much clearer and 

 contains much less organic matter. Sewage water is rich in organic 

 matter, and proportionately rich in bacterial life; and bacterial purifica- 

 tion is synchronous with a diminution of organic matter. 



Jordan remarks in this connection that "in the causes connected 

 with the insufl&ciency or unsuitabiUty of the food supply is to be found 

 the main reason for the bacterial self-purification of streams." 



Oxidation. — On the surface of waters, in rapids, falls, and tidal 

 rivers, much oxygen is absorbed, and much impure matter is oxidized. 

 Such oxidation is one of the minor agencies in the purification of water. 



Vegetation and Protozoa. — Low forms of plant and animal life, 

 like certain species of algae, river plants, and the numerous protozoan 

 forms, bring about a reduction of organic matter in water, and thus 

 reduce the amount of food available, for bacteria. There is also the 

 antagonism between these forms and bacteria. The chemical products 

 of the higher forms are considered by some authorities to be injurious 

 to bacterial life;, and many bacteria are ingested by predatory protozoa. 



Dilution. — Sewage flowing into a river or lake is at once diluted 

 with quantities of pure water, and the amount of available food mate- 

 rial is thus diminished; the space occupied by a definite number of bac- 

 teria is increased; and it is easy to see that the greater the dilution, 

 the fewer sewage bacteria will be found. An example will suffice to 

 illustrate. The sewage of the city of Ottawa amounts to about 

 454 L. (100 gallons) per second; and the gelatin count from it gives 

 an average in round numbers of 3,000,000 bacteria per c.c. The 

 yearly mean discharge of the river is about 1,364,511 L. (300,000 

 gallons) a second; and thus the sewage becomes , diluted 3,000 times. 



Sedimentation. — Impurities, suspended matter, and bacteria 

 having weight, naturally gravitate to the bottom; and the subsidence 

 of these matters is spoken of as sedimentation. 



Lake water being still, sedimentation in it is more marked than in 

 moving water; and such water contains but few bacteria. In slow- 

 moving rivers the influence of this factor is also quite pronounced; 

 and, according to Jordan, "The influences summed up by the term 

 sedimentation are sufficiently powerful to obviate the necessity for 

 summoning another cause to explain the diminution in numbers 

 of bacteria" in sewage polluted rivers. The example already given 



