84 THE MANURE HEAP AND SEWAGE. 
evolved gases are only partly oxidized and are sometimes collected 
and burned into the final condition or CO 2 , 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 Filter 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withdrawn to be replaced by more sewage.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 
septic tank, so that simple gases, like CO 2 and N are evolved. 
The gases are no longer oxidizable. The action on the sewage is 
made more complete and efficient if the sewage be first passed through 
the septic tank and then over a contact or filter bed. 
Sprinkling, Trickling, or Percolating Filters. This represents a 
third slightly different method of bacterial purification of sewage. A 
mass of stones or some other favorable material, broken into 
fragments not less than half an inch in diameter, is spread in a 
layer several feet thick. The sewage is then sprayed or sprinkled 
upon this mass, and it slowly trickles through the rock layer. The 
broken rock fragments are so coarse that there is no filtering action, 
but, as the sewage slowly trickles downward, it is acted upon by 
