TECHNICAL AND SANITARY PROBLEMS 1079 



turn being devoured by fish, is a biological phenomenon of great 

 practical importance. In this way natural streams succeed in 

 cleansing themselves so that waters once foul become clear and 

 attractive in appearance. 



An excellent illustration of these biological changes is the Genesee 

 river below the city of Rochester. This river now receives practi- 

 cally all of the sewage of the city at a point about six miles distant 

 from Lake Ontario. Below this point the river receives few acces- 

 sions. Studies made during the summer of 1912 showed that the 

 effect of the discharge of the sewage into the river was to increase 

 the number of bacteria and reduce the number of green algae. Im- 

 mediately below the sewer there was a further increase in bacteria 

 and a reduction of the dissolved oxygen in the water. A mile or 

 two down stream the bacteria began to decrease and protozoa in- 

 creased. At the mouth of the river the rotifers disappeared but 

 Crustacea were found in abundance in the lake water around the 

 river mouth. Beyond one-quarter of a mile from the river mouth, 

 however, the Crustacea also showed a noticeable decrease. The 

 chemical changes that accompanied these biological conditions were 

 equally interesting. Below the entrance of the sewage the dissolved 

 oxygen in the water almost disappeared but later increased. As the 

 dissolved oxygen decreased the carbonic acid in the water increased. 

 At the river mouth there was an under run of the lake water back 

 into the river due to the lower temperature of the water in the 

 lake. 



Algae also assist in self purification of streams and lakes by liber- 

 ating dissolved oxygen. Sometimes the growth of algae is so rapid 

 and the quantity of oxygen produced is so large that supersatura- 

 tion occurs. This commonly takes place in lakes in the region of 

 the thermocline, as Birge and Juday have well shown. How great 

 a factor this oxygen production may be is probably not yet realized 

 by sanitarians to its full extent. 



Microscopic organisms in streams are also useful in removing 

 the effects of pollution by manufacturing wastes. On the other 

 hand some kinds of trade wastes are of such a character that they 

 tend to destroy microscopic life; such are acid or strongly alkaline 

 wastes, and wastes containing arsenic, copper, and other poisonous 



