BACTERIOLOGY OF SEWAGE 165 



mixed and very varied material as it enters the main sewers. 



(2) Secondly, stale sewage the ordinary contents of the main 

 sewers. Here there is abundant oxygen, and as the sewage flows 

 along there occurs by bacterial action a certain formation of 

 carbon dioxide and ammonia, which combine to form ammonium 

 carbonate. This is the sewage as it reaches the purification works. 

 Here a preliminary mechanical screening may be adopted, after 

 which it is run into an airtight tank the septic tank. 



(3) It remains there for from twenty-four to thirty-six hours, and 

 becomes a foul-smelling fluid the septic sewage. The chemical 

 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 last 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 particles 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. 



In the earlier stages of any sewage purification, there is little 

 doubt that the albuminous material present is being split up by 

 ordinary putrefactive bacteria. In the mains and where open 

 systems of purification are at work, aerobic forms play the chief 

 part, while in the closed methods anaerobic organisms are those 

 chiefly concerned. In contact and percolating systems there is 

 evidence that at first the purifying action of bacteria is materially 

 furthered by physical processes. Thus Dunbar has shown that 

 when such a substance as coke is used in a sewage filter-bed a 

 considerable amount of the albuminous material is removed in 

 a very few minutes by adsorption, for albumin, being of a 

 colloidal nature, is readily deposited under such circumstances 

 in the pores of the coke in the form of films. After a time such 

 a filter-bed becomes clogged, but on access of oxygen being 



