Marsh. — Seivage and Fish Culture 171 



sewage into something valuable enough to pay the cost 

 of the conversion, and on a small yet practical scale 

 they seem to have succeeded. They invoke the aid of 

 fish culture, use the sewage to rear young food fish to 

 marketable size and persuade the market to buy them. 

 This method has been developed by Dr. Bruno Hofer, 

 the director of a government fisheries research station 

 in Munich, Germany. Its practical use is best exempli- 

 fied in Strassburg where a large pond system consumes 

 a portion of the output of the city sewage in growing 

 the German carp and other staple food fishes. Here the 

 writer had the good fortune in May, 1914, to see this 

 combined fish cultural and sewage disposal plant in 

 operation. 



As is well known, streams purify themselves finally 

 of the sewage poured into them. The process is com- 

 plicated and not thoroughly understood, but is known 

 to depend upon a variety of conditions, such as tempera- 

 ture, rate of flow, oxygenation of the water, and to in- 

 volve complicated chemical changes among which oxida- 

 tions are of prime importance. Water bacteria in great 

 numbers are necessary to the process and many higher 

 forms of both animal and vegetable life play an essential 

 part. A rather slow current is favorable to the abun- 

 dance of these organisms and affords the necessary time 

 for their action upon the sewage. In general then the 

 slower the stream, other conditions being equal, the more 

 minute life it is apt to contain and therefore the greater 

 its sewage digesting ability or power to purify itself. 

 Slow streams, however, have little comminuting effect 

 on the gross particles of sewage, which requires there- 

 fore to be mechanically screened of the larger bodies 

 in suspension in order that purification may proceed 

 rapidly. If such sewage is evenly distributed through- 

 out a slowly flowing stream containing a suitable fauna 

 and flora, these latter digest and incorporate the sewage, 

 and increase thereby. If the stream is made a fish pond 

 with a very slow current and conditions are under con- 

 trol and nicely adjusted, a profitable cycle may be estab- 

 lished, consuming sewage on the one hand and produc- 



