370 
material, the sewage being automatically discharged on to the bed, re- 
tained a fixed period, and then discharged from the bed. In such a bed 
absorption and oxidation of the organic matter is accomplished by aerobic 
bacteria, viz., those which thrive in the presence of air. (See figure 5.) 
(3) Sand filters: As the sewage from tanks can be discharged on 
sand filters—automatically dosed as in contact beds. Such sewage covers 
the surface of the bed and gradually works through to the underdrains 
below, the action being that of filtration and nitrification. If a very pure 
effluent is desired the sewage can be discharged from the tank to the con- 
Fie. 5. 
Contact Beds, Julietta, Ind. 
tact bed and then be treated through the sand filters. In a properly de- 
signed plant this will give a very clear effluent. (See figure 6.) 
Sprinkling Filters: In the larger plants sprinkling filters are largely 
used. These consist of beds of broken stone, usually of a depth of six feet 
or more and are arranged for good underdrainage. The sewage is auto- 
matically discharged over the top of the bed by sprinkling nozzles; trickles 
down through the stone and out through the underdrains. Such beds can 
be worked at a higher rate than any of the preceding methods, hence a 
smaller area is required, which makes this method more adaptable for 
large installations. 
