ANTISEPTICS 141 



changes which take place in the septic tank are of a most complex 

 nature. The sewage entering it contains little free oxygen, and 

 therefore the bacteria in the tank are probably largely anaerobic, 

 and the changes which they originate consist of the formation of 

 comparatively simple compounds of hydrogen with carbon, 

 sulphur, and phosphorus. As a result there is a great reduction 

 in the amount of organic nitrogen, of albuminoid ammonia, and 

 of carbonaceous matter. The latter fact is important, as the 

 clogging of ordinary filter beds is largely due to the accumulation 

 of such material, and of matters generally consisting of cellulose. 

 One further important effect is that the size of the deposited 

 matter is decreased, and therefore it is more easily broken up 

 in the next stage of the process. This consists of running the 

 effluent from the septic tank on to filter beds, preferably of coke, 

 where a further purification process takes place. By this method 

 there is first an anaerobic treatment succeeded by an aerobic ; 

 in the latter the process of nitrification occurs by means of the 

 special bacteria concerned. The results are of a satisfactory nature, 

 there being often a marked diminution in the number of coli 

 organisms present. 



Often the effluent from a sewage purification system contains 

 as many bacteria as the sewage entering, but, especially by means 

 of the septic tank method, there is often a marked diminution. 

 It is said by some that pathogenic bacteria do not live in sewage. 

 The typhoid bacillus has been found to die out when placed in 

 sewage, but it certainly can live in this fluid for a much longer 

 period than that embraced by any purification method. Thus 

 the constant presence of b. coli, b. enteritidis, and streptococci 

 which has been observed in sewage effluents must here still be 

 looked on as indicating a possible infection with the typhoid 

 bacillus, and it is only by great dilution and prolonged exposure 

 to the conditions present in running water that such an effluent 

 can be again a part of a potable water. 



ANTISEPTICS. 



The death of bacteria is judged of by the fact that when 

 they are placed on a suitable food medium no development 

 takes place. Microscopically it would be observed that division 

 no longer occurred, and that in the case of motile species move- 

 ment would have ceased, but such an observation has only 

 scientific interest. From the importance of being able to kill 

 bacteria an enormous amount of work has been done in the way 

 of investigating the means of doing so by chemical means, 



