NA TURE 



[December 12, 1901 



by bacterial action. Accordingly by the double sedi- 

 mentation an average of more than 60 per cent, of 

 the suspended solid matter or " sludge," which was 

 present in the raw screened sewage, has been caused 

 to disappear. This implies a very considerable diminu- 

 tion of cost in sludge removal and disposal ; but it 

 secures the further advantage that the still impure liquid 

 flowing from the settling tanks has become admirably 

 adapted for undergoing adequate purification in the 

 bacterial coke-beds. It is noteworthy that the full 

 power of disposing of sludge is only developed in the 

 sludge after it has remained in contact with the flowing 

 sewage for some considerable length of time ; and it is 

 only stale sludge which is efficient in resolving the solid 

 insoluble matter into soluble and gaseous forms. This 

 delay is due to the necessity of cultivating in the sludge 

 the necessary species of bacteria, which are derived from 

 the sewage itself. 



The most efificient and rapid method of dealing with 

 the impure liquid, which flows from the settling or so- 

 called " septic '■ tanks, has been found to consist in treat- 

 ing it intermittently in coke-beds, which have been primed 

 with bacteria by being placed for some weeks frequently 

 in contact with sewage. The complete cycle of treat- 

 ment in the London beds consists in filling the coke-bed, 

 emptying it after a couple of hours, and then leaving its 

 coke contents in contact with the interstitial air for 

 another period of two hours. It has been found 

 possible to repeat this cycle four times in twenty-four 

 hours, and using beds six feet in depth to purify the 

 settled sewage at the rate of two million gallons per 

 acre per twenty-four hours. By this purification an 

 effluent is obtained which is saturated with dissolved 

 oxygen, which remains entirely inoffensive in smell for 

 an indefinite period in an incubator at summer heat, and 

 which, therefore, when discharged into a water-course 

 would maintain the respiration of fish and would never 

 render the water offensive. 



Chemical examination shows that the treatment in the 

 coke-bed has reduced the readily oxidisable dissolved 

 matter in the settled sewage by from 60 to 70 per cent., 

 and the whole oxidisable matter in the unsettled raw 

 sewage by more than 90 per cent. 



Bacteriological examination indicates that the effluent 

 contains large numbers of bacteria ; but the presence of 

 these bacteria is useful in effecting inoffensively the re- 

 moval of the organic substances, which still remain in the 

 effluent, as soon as the effluent mingles with the well- 

 aerated river water. 



It is noteworthy that the sewage capacity of a newly- 

 made coke-bed progressively decreases for a time, while 

 its purifying power is being developed by contact with 

 settled sewage. But the capacity ultimately becomes 

 equal to about 30 per cent, of the whole cubic space 

 which has been charged with coke ; and, if the treatment 

 is carried out regularly under proper supervision, this 

 capacity fluctuates by only a few units per cent, above 

 and below this final capacity throughout the period of 

 many years during which the bed has as yet been worked. 

 The decrease of capacity to 30 per cent, is the so- 

 called "choking" of the bed. It is due to a bacterial 

 jelly-like growth of bacteria and zoogUua upon the coke- 

 surfaces. If this jelly is removed and exposed to air over 

 NO. 1676, VOL. 65] 



mercury, it will rapidly absorb oxygen from the air, and 

 will therefore produce a partial vacuum. It appears that 

 this growth is actually charged with oxygen during the 

 aeration or resting of the coke-beds between the chargings 

 with sewage liquid. The growth upon the coke-surfaces, 

 which reduces the capacity of the bed, appears, therefore, 

 to be the essential element of successful purification. 



It is noteworthy that the growth may be unduly 

 developed, with corresponding decrease in the sewage 

 capacity of the bed, by over-frequently filling the bed ; 

 and by resting the bed, or reducing the number of fill- 

 ings, the growth may be diminished and the capacity 

 of the bed correspondingly increased. A great increase 

 in the development of the jelly involves increased puri- 

 fication, but reduction in the amount of sewage dealt 

 with, and vice versd. Accordingly a working rate which 

 is most advantageous on all grounds must be arrived at 

 by trial and experience. 



Careful examination of the composition of the inter- 

 stitial air, even at the bottom of a coke-bed thirteen feet 

 in depth, proves that the air is not deficient in oxygen to 

 an extent greater than 25 per cent, of that normally 

 present in fresh air. It appears, therefore, that although 

 oxygen is being rapidly absorbed during the resting or 

 aeration of the bed, the oxygen which is absorbed is 

 rapidly replaced by natural diffusion, and mechanical 

 aeration of the bed is unnecessary. 



It has been proved that the chemical refuse which is 

 found in the sewage of manufacturing towns seldom 

 exerts any prejudicial action on the action of the bacteria 

 or upon the coke-beds. In some towns, however, a pre- 

 liminary treatment of the sewage has been adopted in order 

 to remove special chemical refuse when it is present in 

 very large quantity. This is not the case either in 

 London or in Manchester. 



One hears occasionally of so-called failures in securing 

 bacterial purification of sewage. It is not too much to 

 say that such failures have been due to the improper 

 construction or working of the bacteria beds. Apparently 

 we have still to learn of want of success when an 

 intelligent attempt has been made under competent and 

 experienced direction. 



Although the process of natural purification of sewage 

 must eventually become general, its adoption will un- 

 doubtedly be delayed by the lack of knowledge on the 

 part of the majority of our public bodies and even on the 

 part of some of their advisers. Those who wish to see 

 the satisfactory results of experimental inquiry usefully 

 and advantageously applied on the large scale will 

 accordingly welcome the appearance and success of such 

 treatises as the one which has suggested the present 

 ,p„ipw Frank Clowes. 



FIFTY YEARS OF BIOLOGICAL STUDY IN 

 AUSTRIA. 

 Botanik unci Zooh^ie in Oesterreich in den J all r en 1S50 

 bis 1900. Festschiift v.d.K.K. Zoologisch-Botanischen 

 Gcselhchaft in Wicn. Pp. x-t-620 ; with 38 plates and 

 9 cuts. (Vienna ; .-Mfred Holder, 1901.) 



THIS magnificent work illustrates in every way the 

 jubilee of the K.K. Zoological and Botanical 

 Society of \'ienna. Twenty-two authors have collabor- 



