22 AMERICAN MUSEUM GUIDE LEAFLETS 



of which in practise docs not become seriously exhausted. The condition 

 is analogous to that which obtains in the process of vinegar manufacture 

 when alcoholic liquor is allowed to run over shavings covered with growths 

 of acetic acid bacteria. Under the name of the trickling bed (called also 

 sprinkling bed or percolating bed I this has come to be considered one of the 

 most promising and effective of all devices for sewage purification. 



As in the case of contact beds, almost any hard non-friable material may 

 be used for the construction of trickling filters. In America, the size of the 

 filling material is generally between one and four inches and the depth of 

 the beds between five and eight feet. Mr. Rudolph Heringina very enlight- 

 ening review of the underlying principles of sewage treatment has recently 

 pointed out that there are three fundamental variables in this process of 

 purification, air supply, total area of bacterial films and time of exposure. 

 The area of bacterial films is conditioned by the size of the filling material 

 and the depth of the bed (the smallest material, of course, giving the great- 

 est surface) and the time of exposure is controlled by the rate at which sew age 

 is applied. Reduced to its lowest terms a trickling filter is simply a heap 

 of stone or other material of such size, depth and texture as to support a 

 bacterial growth sufficient for the work in hand. 



The distribution of the sewage over the surface constitutes the most 

 serious difficulty in the construction and operation of the trickling bed. 

 In England, many of the disposal areas are equipped with mechanical 

 distributors of great complexity. Some are designed on the principle of 

 the lawn sprinkler and are revolved by the propulsive force of the sewage 

 as it is discharged. Others are in the form of great movable weirs which 

 pass back and forth on rectangular beds, dripping sewage as they go. At 

 Hanley a mechanical distributor was installed for a quarter acre bed which 

 weighed twelve tons and wore out a forty-five pound bridge rail in two and a 

 half years. 



At other English plants, like the- most famous of all at Birmingham, 

 and at most American disposal areas, the sewage is distributed by spraying 

 it upward from fixed sprinkler nozzles. This method effects a less perfect 

 distribution than that attained by the English mechanical apparatus hut 

 the cost of construction and renewal i> much less. 



The trickling filter can be operated at a rate of 2,000,000 gallons per 

 acre per day, or four times as fast as the contact bed. Half an acre of 

 trickling beds would care for the sewage from 10,000 persons while a similar 

 area would only do for 2500 persons with the contact bed and for 500 with 

 the intermittent filter. Furthermore trickling beds are practically free 

 from the clogging which menaces the permanency of the contact process. 



