

SEWAGE DISPOSAL 
The coarse matter may first be re- 
moved by screening, after which the finer 
material may be allowed to settle as the 
sewage flows slowly through a sedimenta- 
tion tank. 
Or sedimentation may be combined 
with digestion of the solids removed by 
the use of either a septic or an Imhoff 
tank. 
The principle of both is the same, 
viz., that sludge, if allowed to accumulate 
under water, will be reduced in amount 
by the bacteria present in it. Imhoff 
tanks provide a separate lower chamber 
for this process. 
The treatment may stop here ora 
final stage may be added, in which the 
organic matter in the sewage is oxidized 
and changed to a harmless mineral form. 
This oxidization, which is brought 
about by bacteria, may be effected in 
either an intermittent sand filter, a con- 
tact bed of broken stone alternately filled 
with sewage and emptied, or a trickling 
filter—a bed of broken stone through 
which the sewage percolates after being 
sprayed over the surface. 
In the so-called activated sludge 
process the oxidizing bacteria are culti- 
vated in the sewage sludge itself and are 
supplied with compressed air from the 
bottom of the tank. 
The country dweller may purify his 
sewage by installing a small septic tank 
and allowing its effluent to percolate 
through the sandy soil. 
<a 
CENTER CASE 7 contains models 
of a coarse bar screen, a revolving 
mechanical screen and a_ rotating 
Riensch-Wurl screen. 
A sedimentation tank is also repre- 
sented in CENTER CASE 7. 
Models of each of these will be found 
in CENTER CASE 8. 
CENTER CASE 9 contains an inter- 
mittent sand filter in miniature. 
Models of two contact beds are shown 
in CENTER CASE to. 
In CENTER CASE 11 a trickling 
filter is shown in action. 
An activated sludge tank will be found 
in CENTER CASE 8. 
One possible arrangement is shown by 
a model in CENTER CASE 8. 
