2 THE MANURE HEAP AND SEWAGE 
septic tank, so that simple gases, like CO, 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 chemical and bacterial agents, so that it flows out below 
quite changed in its nature. Sewage treated in this way does not 
look clear after treatment, but the organic products in it have 
undergone a change that makes them non-putrescible, and they 
will not undergo any further putrefactive changes. This method of 
treating sewage is recent and as yet not fully understood. 
Effect on Organic Products.—As a result of these two types of 
decomposition the various organic bodies in the sewage are very 
largely destroyed by processes similar to those that occur in the 
manure heap. Various gases are liberated (HN, N, COs, CHa, 
HS, etc.), and the total amount of solid matter is thus greatly re- 
duced. Later in the process, especially in the contact beds where 
oxygen is abundant, a vigorous oxidation of the nitrogen compounds 
begins (nitrification) which results in the formation of nitrates. 
These nitrates are, however, thoroughly soluble and become at 
once dissolved in the water of the sewage, which consequently 
clears up. In this way nearly all of the nitrogen which was held 
in high compounds in the original sewage, has either become dis- 
sipated into the air as ammonia or free nitrogen, or has become 
converted into nitrates and has dissolved in the water to form a 
clear solution which is not objectionable when discharged into 
streams. 
