The Effluent 119 



still capable of comtrning with oxygen. In other words, 

 they are combustible. Hence, the gases generated in 

 the septic tank and consisting of nitrogen, carbon dioxid, 

 marsh gas and hydrogen, are, in a number of places, used 

 for illumination purposes after the previous removal 

 of the inert carbon dioxid. 



Treatment of the effluent. When the sewage is very 

 strong, the effluent from the septic tank may contain a 

 high proportion of ammonia and other decomposition 

 products, and the concentration may be great enough to 

 retard, for a time, the development of aerobic bacteria. 

 With proper aeration, however, and with dilution in 

 extreme cases, the effluent may be subjected to further 

 rapid change in the contact beds, or filters. Contact 

 beds, when worked on the intermittent plan, do not 

 exclude the activities of aerobic bacteria. The alternate 

 filling, emptying and resting interfere, however, with 

 the best development of either aerobic or anaerobic 

 organisms. In the filters proper, on the contrary, ade- 

 quate underdraining and intermittent working permit 

 a better aeration and higher aerobic efficiency. 



The latter is further enhanced by sprinkling the 

 effluent over the entire surface of the filter. By reducing 

 the amount of liquid passed through the filter, so as to 

 admit a large volume of air, or by forcing air through 

 the filter by some artificial means, the oxidation pro- 

 cesses may be made quite intense. The nitrifying bac- 

 teria become more aggressive under such conditions 

 and change the ammonia in the septic-tank effluent into 

 nitrites and nitrates in an incredibly short space of 

 time. The destruction of the organic matter in the 



