i 9 2 SCIENCE PROGRESS 



extent in each case the selected material. The water capacity 

 of all filter-beds diminishes to some extent with use owing to 

 the retention of sediment ; but after a time a condition of 

 equilibrium is reached. The eight hours' cycle of treatment 

 is recommended by the Local Government Board for contact 

 beds — i.e. one hour to fill, two hours to rest full (contact), one 

 hour to empty, and four hours to rest empty. If this cycle 

 is maintained, it is found that such contact beds can be worked 

 at the rate of 1,000,000 gallons per acre per diem. 



Percolating Filters. — We have seen that in the so-called 

 contact filter beds the pores of the beds are filled with sewage 

 and allowed to remain full for a certain period of time. During 

 this time (one to two hours) the biological processes and the 

 exchange of gases with the surrounding atmosphere are stopped, 

 and this must be regarded as a disadvantage inseparable from 

 the method. In the case of land filtration the sewage passes 

 through the soil very rapidly (according to the nature of the 

 soil), and there is very little interference during the whole time 

 with the natural interchange of gases between the soil and the 

 atmosphere. In 1893 Corbett, of Salford, developed the artificial 

 bacterial method in another direction. He attempted to apply 

 the principle laid down by Sir E. Frankland that in intermittent 

 filtration the liquid should always be allowed to flow freely 

 away. The chief difficulty was to obtain a uniform distribution 

 of the sewage over the entire surface of the filter. After many 

 experiments Corbett adopted for this purpose fixed spray jets, 

 from which the sewage was distributed under pressure in the 

 form of a fountain. He also had a layer of half pipes under- 

 neath the filter beds in order to increase aeration and facilitate 

 drainage. The more thorough the contact between the sewage 

 and the filtering material the better the purification. Such 

 conditions are best fulfilled by the adoption of such a method 

 as Corbett's. The important point is that the sewage should 

 fall upon the filter bed in single drops if possible. Then it 

 passes through the filter bed, dropping from one piece of coke 

 to another until it reaches the bottom, where it is collected by 

 channels or drains, and so reaches the exit pipe. Many different 

 mechanical devices have been invented within recent years for 

 the purpose of securing uniform distribution of the sewage 

 over the surface of the filter bed — e.g. Mather & Piatt's sprinkler, 

 the Accrington sprinkler, the Fiddian distributor, etc. For a 



