Having noticed the subject of diminishing the amount of street mud, and of conveying the same into the sewers, as well as that portion of the house filth which it may be practicable to discharge into them, we have next to notice the mode of further disposing of the matter thus lodged in the sewers. The practice has hitherto been (in a great degree) to accumulate the filth in cesspools, and at intervals of five to ten years to open the sewers by breaking into them, or to get access by man-holes left for that purpose, and then drawing out the semiliquid contents of the cesspool by means of a windlass and buckets; but in the Finsbury division the surveyor, Mr. Roe, has had the merit of introducing a very superior, less expensive, and less offensive mode of operation. Finding that the surface-water did not generally enter the sewer in sufficient quantity and with sufficient force to carry off the more solid contents, he contrived, by sluices or flush-gates, to dam up the water to a certain height, and then, by opening the same, to obtain a force of water sufficient for the purpose; and the working of this new plan is said to be highly satisfactory, the filth being prevented accumulating in the sewers, and, as a necessary consequence, then choaking their feeders, the house drains; and in this manner also the filth is removed at less expense, and without any annoyance and noxious effluvia which attend the old practice.
Mr. Roe has adopted also side entrances to the sewers instead of man-holes, for the inspection and repair of the flushing apparatus. The chief expense beyond the first cost of Mr. Roe’s plan is the attendance of a person to open the flush-gates; but it is probable that some contrivance may be found by which the pent-up water on reaching a certain point may be able to open its own gate. It has, however, occurred to me, in respect to this mode of flushing off the filth, that, instead of damming up the water in the sewers, and forming them into reservoirs, the purpose might be more easily and more effectually performed by accumulating the surface-water from the gutters into reservoirs before entering the street or district drains; a greater head, or force of water, might thus be obtained, while the sewers themselves would always be open and free from the obstruction of the sluices and pent-up water.
In respect to the final deposit of the filth of London and other great towns, it does seem a pity that so much valuable manure should be lost to the land, and be discharged into rivers to their contamination and obstruction, if any practicable and innocuous plan can be hit upon to avoid the alternative.
One plan has been suggested of receiving the contents of the sewers into pits, and then by means of steam-power and a sufficient supply of water, forcing the matter of the sewers in a diluted state through iron pipes into the country, and then applying it to irrigate the land in the same liquid state. It would require much calculation to form an accurate estimate of the cost and profit of such an undertaking; but there can be no doubt if the matter was so applied it would prove exceedingly valuable in enriching the land to which it was applied, as we may judge from what has occurred in the vicinity of Edinburgh from a similar kind of irrigation.
At Edinburgh, however, the liquid manure being conveyed from the town for a distance in open ditches or sewers exposed to the sun and atmosphere, it undergoes such a fœtid decomposition as to render the operation no common nuisance to the public. But such effects would not occur (at least, to the same extent) if the matter was conveyed in close pipes with a plentiful effusion of water; and it is known that animal carcases, when kept constantly exposed to fresh supplies of water, do not suffer corrupt decomposition, but are changed into a fatty matter. It seems necessary also that a certain degree of heat and exposure of surface should be present to originate and promote fœtid decomposition, as it is well known that in the pits in Paris, where so many dead bodies were thrown, the result was not a fœtid decomposition, but a change of the animal matter into adipocire, a comparatively inoffensive substance; we may therefore expect that the discharge of the contents of the sewers in pipes, excluded (as they would be) from heat, and copiously charged with water, would be comparatively free from noxious exhalations.
Another plan of reserving the contents of sewers for the purpose of manure would be to continue the sewers to some distance from the town, and then to discharge them into a series of covered catch-pits, allowing the water to filter off after depositing the solid particles; when the first series of pits were deemed sufficiently charged, the sewers might then be discharged into a subsidiary series, until the matter in the first had become sufficiently consistent for cartage. Upon the first plan of proceeding, the liquid manure could only be applied to land quite near at hand, and fit for irrigation; on the second plan the manure might be conveyed to a distance and applied to arable land.
It will be evident the great importance of applying such quantities of manure as the sewers supply to useful purposes, but it is no less evident that no system can be introduced to effect the object until preceded by satisfactory experiments of cost, profit, and efficiency. Some localities might offer facilities for one mode of action, and some for another; and it is much to be desired that parties may be induced to make experiments on both plans; and it is to be noted that though the expense of raising the liquid manure from pits or tanks would be necessary in some cases, yet in many situations no such operation would be necessary.
On the subject of purifying the air of sewers, and of preventing the escape of foul air by any crevices or chinks, I have heard of the ingenious contrivance of erecting a tall chimney and connecting it by a pipe with the crown of a sewer, for the purpose of creating such a draft up the chimney as would occasion an indraft at any leak that might occur in the ramified mass of drains of the district to which the chimney belonged. It appears to me that there are some objections to this plan; but not being acquainted with all the details, I shall avoid entering into any controversy on the subject, further than stating that I should rather propose an opposite process for purifying the air of sewers. I would recommend that they should be kept excluded as much as possible from external heat and ascending columns of air; but at the same time I would endeavour by all means to send down as copious a flow of water as practicable; and in London, though the Thames water may not be proper for domestic purposes, it would be sufficiently pure for watering the streets and cleansing the drains; and the supply being inexhaustible, its application would only be limited by the cost, of steam power and iron pipes.
In conclusion, I have to express my obligations to yourself for the useful hints you have afforded me on several of the subjects above treated of, and to say I shall be happy if any of the observations I have submitted should prove conducive to the ends in view.
I remain, dear Sir,