INTESTINAL OR COLON-TYPHOID GROUP 291 



for purification of city supplies has in many cases resulted in a 

 great diminution of the death-rate from typhoid and other intes- 

 tinal diseases. 



Sewage Disposal. The question of proper disposal of sewage 

 is closely related to the topic of pure water for domestic purposes. 

 Usually sewage is allowed to flow into a suitable stream, and is 

 purified as it passes down-stream. There is no valid objection 

 to this, providing there is a sufficient and constant flow of water 

 in the stream to insure dilution, and the water of this stream is 

 not used as a city supply further down. Unfortunately, too little 

 attention has been paid to this subject, and the high typhoid 

 death-rates in some cities are due directly to the use of such 

 sewage polluted water. Berlin and Paris purify their sewage by 

 using it in the irrigation of large tracts of land, and recollecting 

 the water in underdrains. Such a system is highly efficient, but, 

 as it requires a particular type of soil, large areas, and suitable 

 conditions, it is not often practicable. For sewage disposal in 

 small cities and towns, and even private residences or farms, some 

 of the numerous modifications of the septic tank and filter-bed have 

 been shown to be most efficient. The sewage is first carried to a 

 septic tank, so called a large tank, usually of brick or concrete, 

 and commonly covered. This tank is planned so that the sewage 

 flow of twelve to twenty-four hours will fill it, or, in other words, 

 that a given portion of sewage will require that time to pass 

 through. Here much of the solid material settles out. The dis- 

 solved oxygen, if any be present in the raw sewage, is quickly 

 used up, and anaerobic conditions are established. Under such 

 treatment the decomposition of the organic matter occurs rapidly. 

 Most of the sediment of the septic tank is soon dissolved by 

 bacterial action. Gases, particularly H 2 S, CH 4 , H 2 , and NH 3 , are 

 produced. These rise to the top, and are there intercepted by the 

 heavy scum which forms, and oxidized to H 2 S0 4 , or free S 2 , CO 2 , 

 H 2 O, -and HN0 3 ; hence there is but little disagreeable odor to be 

 noted about such a plant. The organic material is, for the most 

 part, broken down into soluble, easily oxidizable substances. 

 The sewage must not be held under these conditions for too long a 

 period, otherwise the decomposition will go too far. The sewage 

 then usually passes to a dosing chamber. This is simply a chamber 



