13* 



NATURE 



[Dec. ii, 1879 



annum, there would seem to be but a small margin for 

 error; the quantities of detritus, however, have been very 

 differently stated by the various authorities. From the 

 most reliable analyses of the London sewage, taken at all 

 periods of the day and night, and in many different parts 

 of the metropolis, there appears to be a tolerable 

 unanimity in assigning the ratio of the organic to the 

 mineral ingredient of the suspended matters to be as 1 is 

 to 2. After a period of settlement it is found that the 

 proportion is, by the subsidence of the heavier mineral 

 particles, exactly reversed, as the larger portion of these 

 valueless components of sewage impurities rapidly subside, 

 entangling with them about -Jth of the organic matters in 

 suspension. General Scott proposes, therefore, a double 

 system of tanks. The first set would consist of a series 

 of shallow catch pits, in which the sewage will only be 

 brought to a state of partial repose, and in which it will 

 part with about four-fifths of the solid mineral matters 

 and one-fifth of the organic matter. In the second set of 

 tanks, in which more time will be given for the settlement 

 of the matters in suspension, the sewage will be deprived 

 of nearly all the remaining suspended impurities, namely, 

 one-fifth of the mineral, and four-fifths of the organic 

 matters. If we assume the gross weight of the organic 

 matters at 50,000 tons per annum, the mineral ingredients 

 will, according to the analyses quoted by General Scott, 

 equal ico.ooo tons, and the total of 150,000 tons thus 

 obtained, is, in reality, a very low estimate of the amount 

 of the suspended matters in London sewage. These 

 matters, General Scott is of opinion, he could roughly 

 separate in his tanks thus: — In the detritus tanks he 

 would obtain 80,000 tons of mineral matters, together 

 with 10,000 tons of organic matters ; in the second set of 

 tanks he would expect to find about 20,000 tons of mineral 

 matters mixed with about .|o,coo tons of organic matters. 

 The exact percentage composition of this latter sludge 

 would, he believes, after studying and comparing many 

 analyses and valuations, be somewhat as follows : — 



Organic matter (without nitrogen) 66'5o 



Nitrogen ... 3'50 



Phosphoric acid 2 "So = tribafic calcic phosphate... 6 '07 



Potash 



Sand and inert mineral matter 



1-25 



22-68 



In the debate which took place after the paper, Dr. 

 Frankland, while admitting General Scott's process to be 

 "worthy of trial," took exception to this estimate, and 

 maintained that his experience was " that after the separa- 

 tion of detritus from London sewage, the maximum per- 

 centage of organic matter was 63, whilst the minimum was 

 2 1, the average being 39J-, and these high percentages were 

 obtained under exceptionally favourable circumstances, 

 because, in the collection of these samples of sewage, 

 little or none of the so-called detritus was mixed with it 

 at all." He further stated that "he did not think it 

 would be safe to calculate on more than 33 per cent, of 

 organic matter in the dried sludge." This question of 

 the possibility or otherwise of effecting a separation more 

 or less perfect, of the mineral from the organic elements 

 of the sludge lies at the root of General Scott's proposals, 

 and while giving all due weight to Dr. Frankland' s high 

 authority, we are compelled to admit that General Scott's 

 figures, many of them based on the analyses of Dr. 

 Frankland himself, seem to point in the opposite direction 

 to that pointed out by Dr. Frankland, as concerns the 

 relative proportion of the mineral and the organic matters 

 after settlement. 



The question to be decided is, admitting the composi- 

 tion of the sewage solid to be in the first instance 2 

 mineral to 1 organic, can we reduce this proportion to 

 2 organic to 1 mineral, by bringing the sewage to a state 

 of quiescence in tanks? This could be tried on a suffi- 

 ciently large scale to settle the point at issue in a very 



short time, and as it is a question which to a great extent 

 depends upon the result of actual experiment on a large 

 scale, it is certainly one for the officers of the Metropolitan 

 Board of Works to decide. 



Passing over the theoretical values of the deposits, 

 based upon their contents in nitrogen, phosphoric acid, 

 and potash, which General Scott has dealt with very 

 carefully, we come to the question of deodorising the 

 sludge and its preparation as a manure. For the former 

 purpose the employment of slaked lime is advocated, used 

 in the small quantity of only '66, or less than 1 per cent, of 

 the total weight of the sludge. This slaked lime, made into 

 milk of lime by the addition of water, is to be thoroughly 

 incorporated with the sewage deposit, and a sufficient 

 amount of crude superphosphate is then to be added, in 

 order nearly, but not quite, to neutralise the lime. A 

 crystalline precipitate of phosphate of lime is thus formed 

 in the sludge, which greatly aids in the drying of the 

 compound, or, to put it more correctly, facilitates the 

 extraction of the water. Some of those who took part in 

 the debate doubted whether General Scott, in his estimate 

 of 20s. per ton on the dried material, which included the 

 cost of chemical treatment, had made a sufficient allow- 

 ance for the great labour and difficulty which would have 

 to be incurred in drying the sludge for use as a manure. 

 Dr. Voelcker, who pointed out that " he had gone very 

 carefully into the figures in the paper, and was very glad 

 to find that General Scott had avoided those exaggera- 

 tions which frequently disfigured calculations of this 

 kind," quoted some observations he had made tending to 

 show that sewage sludge parted with water with extreme 

 difficulty, though he admitted that after treatment with 

 lime and phosphoric acid such sludge would dry with 

 greater rapidity. In the various forms of filter presses 

 now largely used for drying clay slip and expressing pre- 

 cipitates, very great improvements have recently been 

 effected, and it has been stated on good authority that it 

 becomes possible by their use to reduce the moisture in 

 such materials as low as 50 per cent. There still remains, 

 however, a large proportion of water to expel, and, as 

 Dr. Voelcker stated, this can only be accomplished by 

 means of artificial heat. 



The question of the cost of drying sludge is one which 

 possesses many features of interest, and the entire subject 

 would be one well worthy of the special consideration of 

 the Society of Arts at their annual conference on the 

 treatment of sewage. We should like to have devoted 

 more time to the calculations of General Scott of the 

 theoretical value of the three chief fertilisers present in 

 sludge, viz., nitrogen, phosphoric acid, and potash, as also 

 to the expense of preparing soluble phosphoric acid, con- 

 ceming which latter point Dr. Voelcker threw out some 

 valuable suggestions during the discussion, but we must 

 now conclude. We entirely agree with General Scott in 

 his denunciation of the folly and imprudence of continuing 

 to cast raw sewage into the Thames ; he has certainly 

 pointed out a way of greatly abating the present evil, and 

 as the plan he advocates could be tried upon a sufficient 

 scale at an almost nominal expense, we feel justified in 

 urging with Dr. Frankland that this should be done, and 

 we cordially echo his concluding observation, "that the 

 Board of Works have no right to look for a profit in 

 getting rid of the objectionable matter. If they can suc- 

 ceed in doing it without a loss or at a cost not greater 

 than that involved in dredging it out of the river again, it 

 ought to be done ; because if sewage mud is deposited in 

 the river there must be an obstruction to navigation, 

 besides the putrefaction of organic matters which, when 

 deposited on the banks of a tidal estuary, become very 

 offensive, especially in warm weather." 



So far as one can judge from the facts adduced by 

 General Scott, his scheme promises to be more efficient 

 for the ends aimed at than any hitherto proposed, and 

 certainly it seems to us that the great scientific principle; 



