. Fes. 2, 1872. THE BUILDING NEWS. 83
THE BUILDING NEWS. ——— LONDON, FRIDAY, FEBRUARY 2, 1872.
THE LIME PROCESS CONSIDERED
WITH REFERENCE TO THE PRE-
SENT STATE OF THE SEWAGE
QUESTION.
pie employment of lime for the clarifica-
tion of sewage water was, inall probabi-
lity, suggested by what is knownas ‘ Clark’s”
process for softening water. Carbonate of
lime (the usual source of hardness) is
soluble in pure water to the extent only of
about two grains per gallon, though it is found
freely dissolved in whatare termed ‘ calcareous
waters,” owing to the presence in them of
carbonic acid, which acts asasolvent. When
this gas is expelled (as, for instance, when
the water is boiled), the carbonate of lime
held in solution through its agency is de-
posited, resulting, in the case of steam boilers,
in the formation of the so-called ‘ scale.”
The plan of softening water devised by Dr.
T. Clark consists in the addition to it of a
slight excess of milk of lime, which, by com-
bining with the free carbonic acid, occasions
a precipitate of carbonate of lime. This
precipitate carries down with it the carbonate
of lime previously held in solution by the acid,
in the form of fine crystals, together with any
colouring or organic matter which may happen
to be present. The water is thus rendered,
in the course of a few hours and at a very
trifling expense, perfectly bright and clear.
Sewage water, which may be regarded as
water containing from 50 to 150 grains per
imperial gallon of mineral and organic impuri-
ties, lends itself admirably to such a system
of purification as that which has just been
described. On the addition to sewage of from
two to forty-two grains per gallon, according
to its strength (by which we imply the
amount of the impurities in it), of slaked
lime, a bulky precipitate is formed, which
entangles the floating particles of organic
and solid matter, and as it gradually settles,
leaves the supernatant water perfectly clear,
and to a considerable extent, deodorised. The
above treatment constitutes what is known
as the lime process, and this plan of dealing
with sewage, in spite of Dr. Clark, has
been several times patented, and, partly
perhaps on account ofits simplicity, and partly
because of its cheapness, seems to have been
always a favourite one with sewage experi-
menters. It has more than once been
carefully investigated by Royal Commis-
sioners, and the relative advantages and
disadvantages arising fromits use have formed
the subject of considerable differences of
opinion.
It may be as well, before going any further,
to glance briefly at those printed reports
upon this process which are ayailable,
among the more important of which we may
name that of Messrs. Hofmann and Witt,
forming Appendix 1 to the Report on the
Metropolitan Drainage, 1857; Professor
Way’s Report on the Deodorisation of
Sewage, being Appendix No. 6 to the
Second Report of the Sewage of Towns
Commissioners, 1861, and the remarks upon
the treatment of sewage with lime; vol. 1,
p. 52 et seq. of the first report of the Rivers’
Pollution Commission, 1870. In connection
with these we may examine also the Report
of Dr. Letheby, forming Appendix No.
4 to the Report by Messrs. Bidder, Hawksley,
and Bazalgette on the Main Drainage, 1858.
When, in 1856, certain referees were ap-
pointed to investigate the main drainage of
the metropolis, it was deemed essential at the
outset of the inquiry to ascertain the value
of sewage, and the practicability of utilising
it. Dr. Hofmann and Mr. Witt were accord-
ingly requested to furnish a report upon the
subject, which they did under date of July
Ist, 1857, and the information thus obtained
may be said to have laid the foundation for all subsequent estimates not only of the value, but also of the possibility of suc- cessfully utilising sewage. Among other processes for the treatment of sewage, these gentlemen paid considerable attention to the plans adopted at Leicester and Tottenham involving the use of lime, and for several reasons, which we may now proceed to ex- amine, they rejected it as unsuitable for the metropolis. It is almost unnecessary that we should here pause to explain that we possess in sewage a very valuable manure, and that all the precipitation processes aim not only at the remoyal from the sewage water of its im- purities, but also at the production from the solid residue of a saleable manure. Now when Messrs. Hofmann and Witt came to examine the dried residue from the lime pro- cess, they found that in lieu of being a valu- able manure the deposit contained only one-twelth part of the fertilising materials present in one ton of guano, and hence they argued that the farmer would have to cart on to his land and spread 12 tons of the lime manure to produce an effect equivalent to that obtained by employing one ton of guano. It was very easy from this to prove that beyond an area of two miles from the works the extra expense of cartage and spreading would do away with all profit on this manure, When, further, it was shown, firstly, that in the effluent water, after the liming, there re- mained two-thirds of the valuable ingredients of the sewage; secondly, that neither the lime nor any other precipitant removed the whole of the soluble organic matters, which were especially liable in hot weather to undergo putrefaction; and, lastly, that the estimates of the probable cost of the process were based upon insufficient data, it may readily be imagined that their opinion was unfavourable. They, however, admit, in an extract from their report printed as a foot-note to page 22 of the report of the referees—which is not, strange to say, to be found in their report as given in the appendix—that ‘‘ If we were asked to select one of the processes as particularly calculated to furnish satisfactory results, we should certainly give the preference to the lime process.” Inthe course of their obser- vations, they state that the quantity of the soluble organic matters found in the river after admixture with the sewage is very minute, and they give no reasons for their subsequent statement and the one upon which they ground their principal objection to this process—namely, ‘that the fluid run off from the sewage deposit might very seriously scription quite tallies with what takes place when the lime used is not, if anything, slightly in excess; a fine granular precipitate is then thrown down, which does not clear the water, and settles only after many hours. But we have other data to go upon in forming our judgment—namely, the amount of the preci- pitate they obtained.
Solid Composition Constituents of Amount aa originally |Supernatant) Separated. present. Fluid. Organic Matter. 88:76 40°34 48-42 Mineral 7173 55°68 15°65 Total... | 160-09 | 96:02 6£07
N.B.—These 64-07 grains doubtless contain the greater
part of the 20 grains ef lime.
From 160-09 grains of matter in solution
and suspension, a precipitate is obtained of
only 64:07 grains on the addition ef the 20
gvains of lime. Further, it seems to us very
remarkable that although 20 grains of mineral
matters have been added to the sewage, we
have only 15-65 grains in the precipitate, and
no increase has taken place in the total im-
purity ; for on adding together the grains
per gallon present in the supernatant fluid
and in the precipitate 96-02 + 64-07, we still
get 160-09 grains. Here a very probable
error, and one which makes a vast difference
to all the subsequent calculations founded
upon this table, occurs tous, The analysis
is headed ‘ Effect of Treatment with Lime
(deduced from comparison of analyses of the
original sewage with the analysis of the
supernatant fluid after separation of the
deposit).” From this title it is manifest
that as their results are mere deductions from
two independent analyses, the figures 64-07
grains, which are evidently obtained by sub-
tracting the 96-02 grains found in the effluent
water from the 160-09 grains ascertained to
have been present in the sewage before
treatment, do not make any provision for the
20 grains of Dorking lime which were used
for the purpose of precipitation. The 96-02
grains in the effluent water should really have
been deducted from 160-09 4 20 grains, giving
a residue of 84:07 grains in lieu of 64-07.
This latter value is much more like the truth,
for if we contrast the analyses of Messrs.
Hofmann and Witt with those of other ob-
servers, we find that in every case the amount
of the precipitate, even when a proportionally
much smaller quantity of lime is used, is
affect the river.” From a careful consideration of their ex- periments, we have little hesitation in affirming that they did not give the lime process a fair trial, though their results, as far as they go, are otherwise very satisfactory and conclusive. The sewage water selected by them for the purpose of precipitation by means of lime and taken from the Northumberland sewer, was very concentrated ; that is, in one gallon, or 70,000 grains, it contained no less than 160-09 grains of suspended and dissolved matters. To each gallon of sewage water 20 grains of ‘‘common gray Dorking lime” were added, after being carefully slaked. Now, Dorking lime is burnt from the lower chalk,and rarely contains less than 10 per cent. of silica, iron, and alumina, and frequently much more. If we assume that the lime used by Messrs. Hofmann and Witt had this per- centage of clay. we find that they would thus get only 18 grains of pure lime capable of taking hold of the acids in the sewage. For this purpose the quantity named would be wholly insufficient, and the precipitation would therefore be incomplete. ‘That such was the case there can be but little doubt, for we are told immediately afterwards that after the lapse of three hours ‘‘ the supernatant fluid was still opalescent,” and that ‘‘the whole of the suspended matter had not been com- pletely removed by the process.” ‘This de- |
always over 50 per cent. of the total impurities in the sewage. In the table on next page we have taken the results given in the reports of the various Royal Commissions. and the engineers con- sulted by the Metropolitan Board of Works, and so arranged the various calculations as to compare with the amounts obtained by Messrs. Hofmann and Witt. The only case in which lime has been used in considerable excess— namely, to the extent of 42 grains per gallon, is that given upon the anthority of Mr. William Higgs, in the Appendix No. 4 B. to the report of the engineers before alluded to. The effect of this quantity of lime upon sewage taken from the same place, and but slightly richer than that experimented upon by the former observers, was the separation from it of a precipitate amounting to no less than 164-4 grains per gallon, leaving little more than two-sevenths of the solid impuri- ties in the supernatant water in lieu of the residue of three-fifths shown in the case of the previous experiment. sy good fortune we possess a_ third series of investigations upon the same sewage conducted for the Sewage of Towns Com- missioners by Professor Way, which will be found at page 69 of their second report. The sample of sewage water operated upon by him, and taken only a year later, is much weaker than the former ones, containing as it
