BACTERIOLOGY OF SEWAGE 735 



present in sewage have suggested a means of dealing with sewage 

 so as to make use of these properties, and many bacterial systems 

 of sewage disposal have been devised. The principle most widely 

 adopted is to run the sewage into large covered reservoirs (septic 

 tanks), where it remains at rest for twenty-four to forty-eight 

 hours. Here it is under practically anaerobic conditions, and 

 anaerobic bacteria exert their action on the solids, partly dis- 

 solving them, partly disintegrating them, with the formation of a 

 sludge which has to be cleared out from time to time. From the 

 septic tanks the sewage passes on to beds composed of broken 

 brick, coke, or some similar material, through which it slowly 

 percolates, and here it is subjected to the action of aerobic organ- 

 isms, which complete the decomposition to such an extent that 

 the effluent does not affect fish life nor putrefy, so that it may be 

 run into a stream without causing a nuisance. Four sets of 

 these aerobic bacterial beds are usually provided, each set being 

 worked in turn for six hours and resting for eighteen hours during 

 the twenty-four hours. The effluent from such bacterial beds 

 may contain as many bacteria as, or more than, the sewage itself. 

 Pathogenic organisms may be present in it, for Houston found that 

 the B. pyocyaneus added to the beds soon appeared in the effluent. 

 On the survival of the typhoid and cholera organisms in sewage 

 see pp. 423 and 532 respectively. 



Examination of Sewage and Sewage Effluents 



To ensure a fair average sample, the sewage or effluent should 

 be collected in small portions at intervals. The portions are 

 mixed, strained through muslin, and dilutions of 1 in 10, 1 in 100, 

 1 in 1,000, and 1 in 10,000 made with sterile tap-water. These 

 are then examined according to the following scheme (see table 

 on next page). 



Milk 1 



Milk is an admirable nutrient soil for the development 

 and multiplication of micro-organisms, and, though sterile 



1 See Houston, Rep. to the London County Council, No. 933, 1905 ; 

 MacConkey, Journ. of Hygiene, vol. v, 1905, p. 333 ; Hewlett and Barton, 

 ibid. vol. vii, 1907, p. 22 ; Savage, Rep. Med. Off. Loc. Gov. Boardim 1909-10, 

 p. 474, and Milk and the Public Health (Macmillan, 1912) ; Lane-Claypon, 

 Milk and its Hygienic Relations (Longmans, Green & Co., 1916). 



