H AMERICAN MUSEUM GUIDE LEAFLETS 



the bacteria growing in the liquid consume oxygen much taster than it can 

 be absorbed from the surface, and anaerobic conditions arc easily main- 

 tained. In such a still pool of sewage sludge, the putrefactive bacteria effect 

 a hydrolytic cleavage of the organic compounds and ultimately split them 

 up into such simple forms as nitrogen, hydrogen, carbon dioxide and marsh 

 gas. 



Aside from certain minor details as to si/.e and construction, the ( anieron 

 septic tank is simply a brick or masonry basin, covered perhaps with a 

 wooden roof to protect it from the wind, hut with no special features to 

 distinguish it from any other tank. If in operation the sludge is removed 

 at frequent intervals, the tank is merely a sedimentation basin. If the 

 sludge is not removed putrefaction sets in, the liquid becomes dark colored, 

 hubbies rise from the bottom and hurst at the top and sometimes a thick 

 crust or scum forms over the whole surface. The solids are changed first 

 to liquid and then to gaseous form. The amount of gas evolved is large, 

 four or five gallons from a hundred gallons of sewage, and with closed 

 tanks it is possible to collect this gas and burn it. 



The net practical result of the septic process is an appreciable reduction 

 in the amount of stored suspended solids, due in part to the liquefying action 

 of the bacteria and in part to consolidation of the sludge, which makes it 

 more compact and easier to handle. The action of the tank falls far short, 

 however, of the hopes entertained by its original promoters. Half or two- 

 thirds of the sludge still remains to he handled, and the tank itself frequently 

 becomes a nuisance from the evolution of odors of decomposition. Several 

 improved types of liquefying tanks have been suggested during the last few 

 years, of which the one designed by Iniholf for the Kmschcr Drainage 

 Hoard of North Germany has in particular attracted wide attention. It 

 is a tank with an upper portion through which the fresh sewage Hows and a 

 deep compartment helow in which the sludge accumulates and liquefies, 

 and it is said to effect a remarkable destruction of sludge with no obnoxious 

 odors. 



DISPOSAL OF SEWAGE BY DILUTION 



The processes SO far considered are preliminary processes only, which 

 remove from the sewage a larger or smaller proportion of its burden of sus- 

 pended solids hut which do not attempt ultimate purification of the organic 

 constituents, either in solution or suspension. The final aim of sewage 

 purification is to effect a transformation of these organic compounds into 

 innocuous mineral substances by the action of oxygen, and this action is 

 nitrification, practically brought about by the action of certain bacteria. 



