202 MICROBIOLOGY OF WATER AND SEWAGE. 



considerable organic matter derived from vegetable debris, etc., contain, 

 as a rule, more organisms than rivers in which there is but little of such 

 material. Thus the Ottawa River, which drains a large area of forest 

 lands and is characterized as an upland peaty water carrying a rather 

 high percentage of organic and volatile matter, contains throughout 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 proportion- 

 ately rich in bacterial life; and bacterial purification is synchronous 

 with a diminution of organic matter. 



Jordan remarks in this connection that "in the causes connected with 

 the insufficiency or unsuitability 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 thus 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 material 

 is thus diminished; the space occupied by a definite number of bacteria 

 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 1. (100 gal- 

 lons) 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 1. (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- 



