December 20, 1900] 



NATURE 



i9[ 



metropolitan area which have been charged with varied refuse 

 matters of our streets, our houses, and our manufactories. 



The nuisance was removed by taking the sewage fifteen 

 miles below London. Since this was found insufficient, the 

 sewage was subsequently subjected to chemical treatment and 

 sedimentation before it was allowed to flow into the river. The 

 treatment ultimately adopted, and still in vogue, consists in 

 straining or "screening" off the larger solid matters and then 

 mixing the sewage with solutions of lime and sulphate of iron ; 

 the chemical precipitate thus produced is then allowed to settle, 

 together with the finer particles in the sewage, by sending the 

 sewage slowly through parallel channels on its way to the river. 

 The settled matter, or "sludge," is sent in tank-steamers to be 

 discharged out at sea beyond the river's mouth ; and the fairly 

 clear "effluent" passes constantly into the river from the 

 northern outfall (Beckton or Barking) and the southern outfall 

 (Crossness) in two streams, which jointly deliver over 

 200.CKX),ooo gallons every twenty-four hours into the river, and 

 which probably constitute the most important tributaries of -the 

 lower Thames near London. Since these processes of chemical 

 treatment and sedimentation have been adopted, the foreshores of 

 the river have become clean, the outrageous foulness of the 

 stream has ceased, and those who live on and near the Thames 

 unanimously express their approval of the improvement effected. 



It must be remembered, however, that the effluent of the 

 sewage, after it has been freed from visible foul matter, still 

 contains in invisible solution a large amount of putrescible sub- 

 stance, which may, under suitable conditions, lead to serious 

 foulness in the stream. The effluent at present discharged into 

 the river is practically only clarified sewage. As long as putre- 

 factive changes are delayed by low temperature of the river 

 water, and an ample flush of upper river water comes down to 

 dilute this effluent and to carry it rapidly out to sea, no sensible 

 foulness occurs in the main stream. But in summer time, when 

 high temperature hastens putrefactive change and diminishes 

 the amount of oxygen dissolved in the river water, and when 

 the flush of water from the upper river is diminished by drought 

 and by the abstraction of larger volumes of the water by the 

 water companies, the condition of portions of the lower river 

 frequently closely approaches that necessary to cause offence. 

 There can be no doubt that as the volume of sewage effluent 

 increases, and the abstraction of upper river water for water- 

 supply also increases with the increasing population, these por- 

 tions of the lower river must pass more frequently into a 

 condition bordering upon or actually causing foulness. It is, 

 therefore, prudent to be prepared to adopt without delay a 

 method of treatment of the London sewage which shall meet 

 the requirements of an increasing population, and .shall enable 

 the more ample effluent to be discharged into the river in a state 

 of greater purity than is at present secured. 



As far back as 1893, the Main Drainage Committee of the 

 County. Council, on the advice of their chemist, Mr. Dibdin, 

 started a large scale experiment on the bacterial purification of 

 sewage, the purification being applied to the effluent from 

 chemical treatment and sedimentation. This experimental 

 treatment has been continued by the committee, on my own 

 advice, and has been considerably extended in its scope. The 

 committee also consented to the association of the eminent 

 bacteriologist, Dr. Houston, with me in these experiments 

 during the three years of their progress. The results which have 

 been obtained have been published by the London County 

 Council in the form of a series of reports which I have laid 

 before them from time to time. The general conclusion to 

 which they point is that the settled sewage may be purified to a 

 far greater degree than it is by the present treatment, by en- 

 couraging the spontaneous purifying action of the bacteria which 

 are present in the sewage itself. The effluent thus produced, 

 without the intervention of chemicals, remains free from foul 

 putrefaction and is able to support the life of fish ; in these and 

 in all other respects it is greatly superior to the effluent which is 

 at present discharged into the river. The minute vegetable 

 organisms, known as bacteria, exist to the average number of 

 300,000 per drop of sewage. They only require to be placed 

 under suitable conditions in order to effect the rapid and in- 

 offensive resolution of the putrescible matters of the sewage into 

 harmless and inoffensive products. 



The general conclusions derived from the experimental 

 bacterial treatment of raw sewage at the out-falls of the London 

 sewage into the Thames are as follows : — 



(i) The following results were obtained by treating the raw 



NO. 1625. VOL. 63] 



sewage bacterially in coke-beds. In the process adopted, the 

 sewage was allowed to flow into large tanks which contained 

 fragments of coke about the size of walnuts. As soon as the 

 level of the liquid had reached the upper surface of the coke-bed, 

 its further inflow was stopped, and it was allowed to remain in 

 contact with the bacteria coke surface for two or three hours. It 

 was then allowed to flow slowly away from the bottom of the 

 coke-bed. This out-flowing liquid constituted the "sewage 

 effluent. " After an interval of from three to seven hours, the 

 processes of emptying and filling the coke-bed were repeated 

 with a fresh portion of sewage. The coke-bed was at first filled 

 in this way twice in every twenty- four hours, but later on it was 

 filled three and four times in twenty-four hours. 



(2) A considerable purifying action has been effected by the 

 coke-bed. This is produced by the introduction of bacteria from 

 the sewage. The maintenance of the purifying action is due to 

 the presence of bacteria or their enzymes upon the coke sur- 

 faces, and to the adequate aeration of these surfaces by frequently 

 exposing them to the oxygen of the air. 



(3) The oxygen undergoes absorption by these surfaces, and the 

 aeration of even the lowest portions of a deep coke-bed seems to 

 be satisfactory in the above method of working, since the air 

 present in the interstices of the coke, between two fillings with 

 sewage, usually contains as much as 75 per cent, of the Jamount 

 of oxygen present in the air. 



(4) Raw sewage, which had been deprived of its larger 

 particles by screening it through coarse gratings, lost practically 

 the whole of its suspended matter by remaining in such a coke 

 bacteria bed for two or three hours. It appears that the 

 suspended particles of faecal matter underwent liquefaction by 

 the bacteria, since they did not collect upon the surface of th^ 

 coke. 



(5) The sand and grit and finer mud, arising mainly from the 

 wear of road surfaces, however, were deposited upon the 

 coke surfaces, and gradually reduced the capacity of the 

 coke-bed. 



(6) Hair, fibrous matter and woody fibre derived from the 

 wear of wooden street pavements, and particles of chaff and 

 straw mainly derived from the dejecta of horses employed in 

 the street traffic, were also deposited upon the coke surfaces 

 and gradually choked the coke-bed. These substances, which 

 consist mainly of cellulose, are apparently only acted upon by 

 bacteria with extreme slowness under the above conditions. 

 They arrive, however, in a water-logged condition, and rapidly 

 settle down from the sewage if its rate of flow is reduced. 



(7) In dealing with the sewage of the metropolis, it seems 

 best to submit the roughly screened raw sewage to a somewhat 

 rapid process of sedimentation, in order to allow these suspended 

 mineral and cellulose matters (5,6) to subside ; and then to pass 

 the sewage direct into the coke-beds. The dissolved matters 

 and the small amount of suspended matters which are still 

 present in the sewage are then readily dealt with by the bacteria 

 of the coke-bed, and no choking of the beds occurs. 



(8) The sewage effluent which is thus obtained from the coke- 

 beds is entirely free from offensive odour and remains inoffensive 

 and odourless even after it has been kept for a month at summer 

 heat, either in closed or open vessels. It is clear, except when a 

 turbidity is produced by fine mud particles washed down by heavy 

 rain. Many pond and river fish have been kept in the constantly 

 renewed effluent for a month, and have been found to be perfectly 

 healthy at the end of that period. 



(9) The chemical character of this effluent may be briefly indi- 

 cated by stating that on an average 51 "3 per cent, of the dissolved 

 matter of the original sewage, which is oxidisable by permanganate, 

 has been removed by the bacteria, and that the portion which has 

 been removed is evidently the matter which would become 

 rapidly offensive and would rapidly lead to de-aeration of the river 

 water if it were allowed to pass into the river. The above per- 

 centage removal (51 '3) was effected by coke-beds varying from 

 4 to 6 feet in depth. A similar bed, 13 feet in depth, has proved 

 more efficient, and has for some time produced a purification of 

 64 per cent., while an old bed, 6 feet in depth, has given a 

 purification of 86 per cent. A repetition of the treatnhent of the 

 effluent in a second similar coke-bed has produced an additional 

 purification of I9'3 per cent., giving an average total purification 

 of 70 '6 per cent, (.^e^ Table I.). It should be noted that the 

 above purification is reckoned on the dissolved impurity of the! 

 sewage ; the suspended solid matter is not taken into account. 

 No difficulty has been found in maintaining this bacterial- 

 purification. 



