SEWAGE PURIFICATION 287 



of nitrogenous food. Apart from undigested food, it has 

 been shown that the bulk of fseces consists of intestinal 

 secretions, epithelium detritus, etc., and masses of bacteria. 

 Under the conditions maintained within the anaerobic tank 

 these will all gradually break down, and an important con- 

 sideration here arises, viz., as to how far this breaking down 

 should be carried. We have seen that eventually, by the 

 decomposition of albuminous matter, evil-smelling substances 

 such as hydrogen sulphide, indol, skatol, and various amines 

 are produced. A frequent error in the design of septic tanks 

 has been to make these too large, so that the decomposition 

 of nitrogenous matter is carried more or less to its limit, with 

 the production, in many cases, of serious nuisance. The 

 design of anaerobic tanks should be directed to the rapid 

 deposition of solid matter, and its retention for a period 

 sufficiently long to enable it to be broken down as far as is 

 economically possible, while the liquid portion of the sewage 

 should be led away quickly, sufficient time only being allowed 

 for ammoniacal fermentation and incipient proteolysis to 

 take place therein. The hydrolytic tank of Travis, and the 

 Emscher-Brunnen of ImhofE, have this object in view, but 

 any design which distinguishes between the changes taking 

 place in the solid matter and in the supernatant liquid is 

 likely to be more or less successful. It has been found, e.g., 

 that in latrine-tanks, where the greater part of the faecal 

 matter is retained in a compartment at the inlet, separated 

 from the main tank by a pigeon-holed wall, that a very large 

 amount of liquefaction of retained solids takes place in this 

 inlet chamber. 



Decomposition of fats. — There is evidence that besides 

 the decomposition of cellulose and nitrogenous products a 

 considerable change takes place in the fatty constituents of 

 the sewage in the anaerobic tank. Fat is always present in 

 household refuse from the washing of plates and dishes ; all 

 the soap which is used finds its way' into the sewage, and 



