PAPERS ON CHEMISTRY AND PHYSICS 151 



known as the sludge. This sludge is siphoned off from 

 the digestion chamber of the tanks from time to time, 

 drained on gravel or sand beds and hauled away. It is 

 comparatively poor in nitrogen, most of that element 

 having been converted into soluble salts. In some cases 

 it is sold for as high as $2.00 per load. 



The activated sludge process which is rapidly coming 

 into favor substitutes an aeration tank and settling tank 

 for Imhoff tanks and niters and is very much more com- 

 pact. 



The sewage, together with about 25$ by volume of 

 returned sludge, enters one end of the tank and the two 

 are thoroughly mixed and aerated by means of air 

 blown in through porous plates at the center and bottom 

 of the tank. In from three to six hours the sewage is 

 completely clarified, considerable nitrates are produced 

 and the mixture passes on to settling tanks. The set- 

 tled liquor is clear and stable and is discharged into the 

 stream. The sludge, which contains 97-99% moisture, is 

 partly returned to maintain the process while the re- 

 mainder is dried and sold for fertilizer. 



The sludge differs radically from Imhoff sludge. 

 Activated sludge has practically the same chemical com- 

 position as microbial protein, containing from 7 to 10% 

 of nitrogen. It is a very valuable fertilizer, but un- 

 fortunately no cheap method of drying it has been 

 worked out. Much progress is being made by the Sani- 

 tary District of Chicago and elsewhere at the present 

 time and the problem appears far from insoluble. 



I have purposely avoided discussing the question of 

 sludge drying from lack of time. 



CHEMISTRY OF SEWAGE PURIFICATION 



Two rather opposing theories have been proposed for 

 the explanation of the reactions in sewage treatment, 

 more especially the clarification and nitrification which 

 takes place in the activated sludge tank or on the trick- 

 ling filter. 



The one is the Hampton Doctrine of Travis which 

 Ardern summarizes as follows: " According to this 

 theory the purification process is primarily and essen- 



