SEWAGE PURIFICATION 285 



shown to be due to two organisms, one of which produced 

 mainly hydrogen and the other marsh gas. In both cases 

 carbon dioxide (C0 2 ) and fatty acids are also produced as 

 by-products. The production of gases is a visible indication 

 that fermentation is taking place in the sewage. The evolution 

 of nitrogen has often been regarded as taking place in septic 

 tanks ; the author's experience, however, would tend to show 

 that such nitrogen, if it is produced, arises either from the air 

 dissolved in the incoming sewage, or from the reduction of 

 nitrates present therein, and not from the anaerobic decom- 

 position of nitrogenous matter. It may be taken, therefore, 

 that the gases which are evolved in the septic tank arise 

 chiefly from the decomposition of cellulose. The researches 

 of Omelianski, described in Chapter X, showed that the 

 optimum temperature for this fermentation was above 

 90 F. For this reason the activity of septic tanks in this 

 country, measured solely by the gas evolved, is much greater 

 in summer than in winter, and it never attains the intensity 

 observable in tropical countries. There, where the tempera- 

 ture seldom is less than 70 F. and often of course much 

 higher, a quite extraordinary development of gas may take 

 place. The illustration on Plate IV (i) is from a photograph 

 taken by the author at the installation attached to the leper 

 colony at Matunga near Bombay. Here the tanks are pro- 

 vided with gas-tight iron covers, and the gas is withdrawn from 

 below these into a gas-holder. The carbon dioxide is removed 

 by lime purifiers, and the inflammable marsh gas and hydrogen 

 used for driving the engine which pumps the sewage, and also 

 for lighting and cooking purposes. The gas-holder, lime 

 purifiers, and engine-house are indicated in the photograph. 



Such economic use of the gas from septic tanks has been, 

 to a limited extent, adopted in this country, but, owing to the 

 temperature conditions, it is hardly likely to find wide applica- 

 tion on the large scale, and artificial raising of the temperature 

 of large volumes of sewage is out of the question. It is, 



