Sewage and Sewage Effluents. 185 



pure effluents are discharged into streams used for sources 

 of water-supply the town taking water may protect itself 

 by filtration. It should so protect itself, at any rate, 

 from the pollution necessarily incident to surface-waters; 

 and, unless the bacterial condition of a stream or lake is 

 made very materially worse by the discharge of sewage 

 effluents, it is fair that the responsibility of purification 

 should rest on the water works, rather than on the sewage 

 purification plant. Shellfish, on the other hand, cannot 

 be purified. Either pollution must be prevented, or the 

 industry abandoned. Under such circumstances sanitary 

 authorities may rightly demand, as they have demanded 

 at Baltimore, that bacteria, as well as putrescible organic 

 matter, shall be removed in sewage treatment. Under 

 such circumstances the bacterial control of purification 

 plants is as essential as in the case of water filters. 



In England, considerable attention has been devoted to 

 this subject and numerous methods have been recom- 

 mended as furnishing valuable criteria of the bacterial 

 quality of sewage effluents. Houston (1902''), for 

 example, suggests various tests involving the use of litmus 

 milk, pepton solution, gelatin tubes, and neutral-red 

 broth, as well as the inoculation of animals. He con- 

 siders the determination of the numbers of B. coli and 

 B. sporogenes as of greatest moment, while the identifi- 

 cation of streptococci is of value in certain cases, and the 

 "enumeration of liquefying bacteria, spore-forming aerobes, 

 thermophilic bacteria, and hydrogen sulphide producing 



