150 ILLINOIS STATE ACADEMY OP SCIENCE 



of fine screen. Screens do not happen to be used in the 

 two new installations referred to. The sewage, still 

 flowing at a fairly high velocity and carrying consider- 

 able matter in suspension, flows now into tanks where the 

 velocity is reduced and sedimentation takes place. The 

 tanks are of the so-called Emscher or Imhoff design, in- 

 vented by Dr. Karl Imhoff and first used in the Emscher 

 district of Germany. 



The tank is a sort of two story affair in which the sedi- 

 mentation takes place in the upper chamber. The sludge 

 slips through the slot into the lower compartment where 

 it is subjected to septic digestion. One of the features of 

 this tank is the separation of a biolytic or digestion cham- 

 ber. The vents at the sides allow the escape of gaseous 

 products of digestion without stirring up the sedimenta- 

 tion chamber. This separate digestion of the sludge pre- 

 vents the fouling of the liquor in the sedimentation cham- 

 ber so that the effluent has a mild musty odor rather than 

 the septic odor of the older septic tanks. 



The effluent, now very materially improved and having 

 no large floating particles but still with a distinct milki- 

 ness, may in some cases be discharged into the stream if 

 the stream has sufficient volume. In most cases, how- 

 ever, it is passed through sprinkler nozzles on to a so- 

 called filter in which the action is not one of filtering or 

 straining at all, since the medium is broken rock of 2 to 

 3 inches in size. A slimy film soon develops on the sur- 

 face of the stone, which acts in some way or other to take 

 up all of the milky colloidal organic matter and oxidize 

 much of the nitrogen to nitrate.' The effluent of a trick- 

 ling filter is perfectly clear, and it not only does not con- 

 tain any putrescible organic matter but the nitrate con- 

 stitutes what might be called an excess stability. The 

 effluent is really of better quality than the water in many 

 of our muddy prairie streams. 



From time to time these filters " unload" a sort of 

 black humus which usually is caught in secondary tanks 

 of the Imhoff type. This process has for its net effect 

 the digestion, liquifaction and oxidation of the organic 

 matter, the end products being (NH 4 ) 2 CO.-, , NH 4 N0 3 and 

 a black humus containing 95% or more of water, and 



