Scientific Memoirs by
great. It more nearly reproduces the natural conditions under which polluted water is
purified, and with a highly concentrated sewage the results are better in the first and
also in the subsequent stages.* It is true that if a certain body of sewage is to be passed
through a septic tank and an upward filter bed in one day, the latter must be nearly
twice as large as the former (on account of the space occupied by the coarse filtering
material), but it is equally true that a quantity of concentrated sewage that can be
sufficiently hydrolysed in the upward filter bed would require at least two days' sojourn in
a septic tank, i.e., the septic tank would require to be double (or more) the capacity of
the daily flow of sewage, so that there is I believe an advantage in point of cost of con
-struction in the case of the upward filter bed. In the excellent septic tank installation (by
Mr. Silk) in connexion with the Presidency Jail, Calcutta, the tank itself has a capacity of
720 cubic feet, or equal to about 4,600 gallons of fluid; into this tank was passed daily
the excreta of 220 persons, that of each diluted with 3 gallons of flushing water. This may
be taken to equal at most 750 gallons of sewage, so that it would appear that the sewage
was allowed a little over 6 days to pass through the tank before it reached the filters. On
the plan I propose and which is estimated on the plan of the apparatus carefully tested
in the Simla experiments, the upward filter bed which takes the place of the septic tank
will only be of a size to allow of the whole body of the sewage of a regiment to pass
through in one day; it is obvious that there will be a great gain in the lessened cost of
construction, though it is certain that in the Calcutta tank a very much larger body
of sewage could be treated with the good results attained under the present arrangements.
It may be taken as a practical rule that the strength (concentration) of the sewage
determines the size of the hydrolytic portion of the apparatus. With a weak English
sewage with high dilution, the tank must be of a capacity to hold one day's flow; where
dilution is lower and the sewage proportionately more concentrated, the tank must be of a
capacity to allow of a longer sojourn of the sewage therein in order to obtain the due
solution and transmutation of the excess solids in suspension: and it is precisely in the
demonstrated facts of the superior hydrolytic power of the upward filtration bed that we
can afford to provide less unit accommodation in this form of apparatus, when we have to
deal with highly concentrated sewage. The one objection that may be raised is the smell
that is alleged to arise from an open bed; let this be put to the practical test. The writer
can aver from experience that while there might be some "nuisance" from this cause in
a crowded site, yet effluvia are only perceptible if one stands over the upward tank, and that
this "nuisance" disappears at a distance of a few yards. The apparatus, as proposed, would
be from three to five hundred yards from the nearest barrack on the site, and it might be
fenced in to prevent a near approach. The water seal on top of the tank prevents the issue of
anything but innocuous gases, and even this could be obviated by a top layer of charcoal or
fine soil on a wire screen. But the "effluvium nuisance" is imaginative as proved by
ample experience in the working of many apparatus attached to country houses and insti
-tutions in England. We permit our regimental lines to be dotted over with latrines and
filth-carts in close proximity to the barracks, and there is absolutely no valid reason
against placing a biological installation at a far greater distance from the inhabited site on
this account. I have good grounds for the belief that the upward filtration bed will be
* See Kenwood and Butler, Congress of the Sanitary Institute, Birmingham, 1898.
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