610 A MANUAL OF BACTERIOLOGY 



are the B. fluorescens liquefaciens and varieties, several varieties of 

 Proteus, the B. filamentosus, varieties of the B. mesentericus, B. 

 mycoides, B. subtilis, B. cloacce, and the colon bacillus. The latter 

 numbers from 20,000 to 2,000,000 per c.c., and the other bacilli 

 mentioned number 200,000 to 2,500,000 per c.c. Many anaerobic 

 sporing bacilli are also found, especially the B. Welchii, the spores 

 of which number from 30 to 2000 per c.c., averaging 500-600. 

 Foreign bacteria introduced into sewage are probably soon sup- 

 pressed by the predominant species of the sewage. 



The air of well- ventilated sewers differs but little from that of 

 the external air, and the organisms in it contrast with those of 

 sewage by the abundance of moulds. Specific organisms may, 

 however, gain access to it (p. 365). 



The powerful liquefying and solvent actions of the bacteria 

 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 dissolving 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 organisms, which com- 

 plete 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. 363 and 437 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, 



