TREATMENT OF CITY SEWAGE QI 
tion proceeds rapidly, the organic bodies becoming partly broken 
down. Gas is evolved in quantity and bubbles up through the 
liquid to find exit from the tank by special vents. The gases 
represent the partial destruction of the organic matters in the 
sewage, and as fast as they are evolved the organic ingredients in 
the sewage disappear and the sewage becomes clearer. When it 
leaves the outlet, after flowing through the tank, it is much purer 
than when it entered, and may then be discharged into streams 
without greatly contaminating them, if the process has been 
efficient. The evolved gases are only partly oxidized and are 
sometimes collected and burned into the final condition or COkx, 
etc. The fermentation, being of the nature of putrefaction, gives 
rise to unpleasant odors, since the gases contain various compounds 
of sulphur and phosphorus that are only partly oxidized. 
The Filier Bed and Contact Bed.—These two methods, though 
differing in detail, are identical in principle, and both are designed 
to stimulate the activities of the aerobic bacteria. In the filter 
beds the sewage is received upon great open beds, the bottoms of 
which are made of masses of coke, broken stone, clinkers, sand, 
etc., arranged in layers of different degrees of fineness, the finest at 
the top. Through these the sewage filters and appears below, 
greatly purified. It was at first supposed that the process was a 
mechanical filtering through the sand, but it is now known that the 
mechanical filtering has little to do with it. The contact beds are 
similar, large, open beds, filled with coarse coke, clinkers, or other 
material, but not arranged for filtering. The sewage is conducted 
upon these beds, allowed to remain there for a few hours, and then 
withdrawn to be replaced by more sewage. Although no filtering 
takes place, this sewage is purified by its sojourn in the contact 
bed. 
In both cases the primary action is that of aerobic bacteria, 
aided, doubtless, by direct chemical activities of the oxygen of the 
air. The bacteria rapidly cause the decomposition of the organic 
products, and the decomposition is more complete than in the 
