1900.] on Bacteria and Setoage. 319 



fresh quantities of sewage in tlie same coke-bed may apparently 

 be continued indefinitely. 



The effluent from one coke-bed undergoes a considerable further 

 purification if it is made to undergo similar treatment in a second 

 coke-bed ; and if this second contact with the coke surfaces is followed 

 by ordinary sand filtration, such as is usually applied to river-watsr 

 which is to be used for drinking purposes, an effluent of extraordinary 

 purity is obtained. 



The original method introduced by the Massachusetts experi- 

 ments, and known as the intermittent aerobic treatment, is sometimes 

 preceded by a preliminary anaerobic treatment. This consists in 

 allowing the sewage to remain quiescent in, or to flow very slowly 

 through, a large tank or channel. A thick, tough scum soon forms 

 upon its surface, and protects the liquid from the air. Under these 

 conditions many of the solid suspended particles of an organic nature 

 pass into solution, and are thus rendered rapidly resolvable by 

 subsequent aerobic intermittent treatment. 



The above general description of the bacterial treatment of sewage 

 has been subjected to modification as to details to suit the conditions 

 of particular localities. Thus the sewage is in some places sub- 

 divided by suitable mechanical arrangements into drops, and allowed 

 to fall continuously like rain upon the surface of the coke-bed. The 

 bed never becomes full of liquid, since when the sewage has trickled 

 through the coke, and has been exposed to the coke surfaces and to 

 the interstitial air, it is at once allowed to flow away from the bottom 

 of the bed. 



That these methods of purifying sewage are correctly described 

 as bacterial has been placed beyond doubt. Any conditions which 

 are unfavourable to bacterial life at once retard the purification, 

 while any treatment of the sewage which sterilises it arrests the 

 purification entirely. 



The bacteria in the sewage are considered to be the active agents, 

 and to produce the changes either directly, or indirectly through 

 their products or enzymes. Bacteria and their spores are found to be 

 present in great numbers in sewage. London sewage has been shown 

 by Dr. Houston and others to contain very large numbers of bacteria, 

 varying from about three to six million per cubic centimetre. It 

 seems probable that many of these bacteria form films, or " swarming 

 islands," on the coke surfaces, similar to those which are produced 

 by their growth upon the surface of a gelatine film (Fig. 1) ; the 

 period of formation of these films may be assumed to be the period of 

 " priming " already referred to. Probably the coke-bed aids bacterial 

 action largely by furnishing surfaces of attachment to the bacteria, 

 upon which they may alternately be exposed to air and to the 

 sewage. The useful effect of solid surfaces in promoting bacterial 

 action in the case of other similar changes is well known, and it may 

 be connected with the effect which the surfaces exert in preventing the 

 settling of the bacteria to the bottom of the liquid. . 



