June 7, 1900] 



NATURE 



129 



tending to subside, and to sow the organisms broadcast 

 as they descend. 



It has been shown by more recent bacteriological in- 

 vestigations that many of these minute organisms are 

 normally present in the living organism, and make their 

 appearance in large numbers in the dejecta. It is there- 

 fore not remarkable that sewage, which contains the 

 dejecta of men and animals, as well as the washings of 

 considerable road and other surfaces, should contain 

 micro-organisms and their spores in large number. 



The fact that animal dejecta and sewage are inoffen- 

 sively and gradually resolved into simple chemical com- 

 pounds by contact with different kinds of soil has long 

 been known, but this resolution has, until recently, been 

 attributed to the purifying action of the earth itself, or of 

 the organisms which it may contain. It is now abun- 

 dantly proved that the resolving or purifymg agents are, 

 in the main, the micro-organisms which were originally 

 present in the dejecta themselves, although undoubtedly 

 organisms derived from the air, and those already present 

 in the soil, contribute to the change when they are 

 present. 



The experimental purification of sewage by letting it 

 stand in tanks filled with flints, gravel, coal, coke or other 

 mineral substances, proves that there is no special virtue 

 in soil. These experiments, originally commenced by 

 the Massachusetts Board of Health, in 1887, have been 

 repeated by many public sanitary authorities, and their 

 results have been abundantly verified ; and in various 

 localities broken stone, broken slate, broken clay vessels, 

 " ballast " or burnt clay have been successfully employed 

 in the tanks in place of the materials which were originally 

 used. 



For the successful and inoffensive treatment of sewage 

 by this means, a preliminary " priming " of the material is 

 necessary. This is effected by allowing it to remain im- 

 mersed in sewage for several hours daily for a few weeks. 

 Sewage, which is then introduced and allowed to remain for 

 a few hours in the tank containing the "primed " coke or 

 other material, has the amount of its putrescible dissolved 

 matters considerably and rapidly reduced, while its solid, 

 finely-divided feecal matter is brought into solution, and 

 caused to undergo, in large measure, inoffensive resolu- 

 tion into simple compounds. 



In order that these changes may be completed in- 

 offensively, it is necessary that the "primed "coke sur- 

 faces shall be frequently placed in contact with air, and 

 the process is therefore an intermittent one. The coke- 

 bed is first filled with sewage, which is then allowed to 

 flow out from the bottom and to draw air into the inter- 

 stices of the coke. After the coke surfaces have been 

 for several hours in contact with the air, the cycle of 

 processes is then repeated. The treatment of fresh 

 quantities of sewage in the same coke-bed may apparently 

 be continued indefinitely. 



The effluent from one coke-bed undergoes a consider- 

 able further purification if it is made to undergo similar 

 treatment in a second coke-bed ; and if this second con- 

 tact with the coke surfaces is followed by ordinary sand 

 filtration, such as is usually applied to river-water which 

 is to be used for drinking purposes, an effluent of extra- 

 ordinary purity is obtained. 



The original methods introduced by the Massachusetts 

 experiments, 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 sus- 

 pended 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 



^'O. 1597, VOL. 62] 



of sewage has been subjected to modification as to details 

 to suit the conditions of particular localities. Thus the 

 sewage is in some places subdivided by suitable me- 

 chanical 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, producing the changes either directly or 

 indirectly through their products or enzymes. Bacteria 

 and their spores are found to be present in great num- 

 bers 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 



Fig. 1.— Proteus vulgaris. Impression preparation from "swarminif 

 islands " on gelatine ; 20 hours' growth at 20° C. X 3000. (Houston.) 



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. i) ; 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 bicterial 

 action in the case of other similar changes is well-known, 

 and it may be connected with the effect which the sur- 

 faces exert in preventing the settling of Xht bacteria to 

 the bottom of the liquid. 



Sewage contains many different species of bacteria, 

 some of which have been described and figured by Dr. 

 Houston.^ As is seen in Figs. 2, 3, 4, some of these 



1 The illustrative figures in this article have been selected from Reports 

 on "The Bacteriology of London Crude Sewage" and on " The Bacterial 

 Treatment of Crude Sewage." by Dr. Clowes and Dr. Houston, issued by 

 the London County Council (F. S. King and Son) ; they were originally 

 produced from micro-photographs taken by Dr. Norman from Dr. Houston's 

 cultivations. 



