inches in diameter, and then in a fine bed, of perhaps half-inch material. 

 The rate of treatment, even so however, is much higher than that commonly 

 used with sand beds, 500,000 gallons per acre per day against 100,000. Half 

 an acre of contact beds would treat the sewage from a population of 2,500 

 against 500 for the intermittent filter. The effluent even from double 

 contact is less highly purified than an intermittent filter effluent. It is 

 dark and somewhat turbid, but it should be free from the tendency to putre- 

 factive decomposition. 



THE TRICKLING FILTER 



Meanwhile the problem of purifying sewage at high rates was being at- 

 tacked in another and even more promising manner. The fundamental 

 combination of bacterial films, sewage and air can be effected in various 

 ways. The late Colonel George E. Waring attempted it at Newport in 

 1894 by blowing air into a bed of coarse stone below, while sewage ran down 

 through it from above. Theoretically this seems a satisfactory process 

 but it has not yet been demonstrated that a sufficient supply of oxygen 

 can be economically provided in this manner. Success was finally reached 

 along another line by resorting to the device of applying sewage, not in bulk, 

 but in a fine spray distributed as evenly as possible over the surface of the 

 bed. By this means the rapid flow of large streams of sewage is prevented 

 and the liquid trickles in thin films over the surfaces of the filling material 

 while the spaces between are continually filled with air, the oxygen content 



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