ON SEWAGE DISPOSAL.[14]
By Professor Henry Robinson.
The outcome of several public inquiries which have taken place during the last year or two, and of much valuable data derivable from other sources, establishes, we think, a well marked advance with reference to sewage disposal; and it may be of use, as well as of interest, if we lay before this Congress the conclusions which, we conceive, are deducible therefrom. We propose to deal with the subject under the following heads: 1. Sewage disposal on land. 2. Sewage disposal by chemical treatment. 3. Sewage disposal by discharge into a tidal river, or into the sea, without treatment.
1. SEWAGE DISPOSAL ON LAND.
The object of dealing with sewage on land may be taken as twofold, namely, to purify it (which is the sanitary object), and to utilize its manurial products (which is the agricultural object). Where want of skill or where prejudice has existed, these two have not been properly separated, and the results have been in many cases unfavorable to sewage disposal on land from either of the before mentioned points of view. It has been regarded as an axiom that clay land cannot be employed to clarify sewage. This is true when it is proposed to pour the sewage on it as if the land were porous. Very recent experience, however, has led to clay land being converted from an impervious to a pervious condition, by which it has been successfully utilized. This is effected by digging out the clay to a depth of about 6 ft., burning it into ballast and replacing it in layers interposed with an occasional layer of open alluvial soil, the whole area being well drained with a free outlet for the effluent. We have successfully carried this plan out, and with this result, that whereas it was not possible previously to clarify the sewage of 100 people to an acre of clay land, the prepared filtration area has been able to continuously clarify the sewage of about 1,500 people to the acre. The cost of converting clay land into this form of filter may be taken as varying from £750 per acre to £1,000 per acre, according to local circumstances. One area which we have just completed has cost £1,000 per acre. Before sewage is passed on to these filters (or on to land) it should be strained so as to remove the larger particles. The best arrangement for this purpose is to pass the sewage upward through a straining medium (not downward), and to run the solids from the bottom of the straining tank on to a low lying piece of land for digging in as they are run out. Where such a filtration area is made to form part of a sewage farm it acts as a safety valve, and enables the land and crops to have a rest when they do not require further irrigation; at the same time the process of purification is not interrupted. If open, porous land is available for sewage purification, and if it can be drained 6 ft. deep to a good free subsoil, so that the effluent can get readily away, we find that the sewage of from 600 to 700 people can be dealt with on each acre per annum with both good agricultural and sanitary results.
In our address as President of the Engineering and Architectural Section of the Congress of this Institute at Newcastle upon Tyne, in 1882, we directed attention to the important investigation which had been conducted by Mr. R. Warrington, of Rothamsted, the result of which was to show the action which goes on in the soil when sewage is passed through it. Further information which the same observer has published since that date is of equal value, and deserves to be read by all who have to advise in regard to sewage disposal on land. The process of "nitrification" (as it is termed), which he has so fully investigated, consists in the conversion into nitrates (which serve to nourish plant life) of the organic matter in sewage. This takes place by the action of a living ferment of the bacteria family, which is created by and feeds on the impurities in sewage, and these organisms both consume the impurities and convert them into nitrates. The action of living agents thus brings about the oxidation of the organic matter in sewage, just as worms, larvæ, fungi, and insects feed on the vegetable matter in the soil, increasing the amount of nitrogenous material in it. Experience during the past year or two has proved the feasibility of preserving green crops in a succulent state by compressing them in silos, so that they can be utilized for cattle fodder in the winter. This system deserves notice in connection with sewage farming, as we are of opinion that it will prove a valuable means of getting over the well known practical difficulty which is experienced of finding a market for the large amount of green crop which is produced by sewage irrigation. In speaking of this system the term "silo" is applied to the artificial chamber or receptacle for green crops (such as grass, vetches, clover, etc.).
The term "silage" is applied to the crop thus treated, and the term "ensilage" is applied to the process of making "silage." The details of the construction of silos cannot be referred to here, beyond stating that what is required is to construct a pit or chamber either in the form of an excavation in the ground, with a brick or other lining, or by building it above the ground. The object is to enable the green crop to be deposited in an air and water tight chamber, in which pressure can be applied to the crop to compress it. This is effected in some cases by well treading the crop after it is laid in the silo, and then spreading layers of earth to about a couple of feet, and pressing the covering well down. Another way is to construct the silo with a movable covering of the exact size and shape of its interior. This cover is raised and lowered by suitable chains and rollers. After the crop is placed in the silo, the cover is lowered and weighted so that a thorough compressing is effected; the weight applied giving about 200 lb. or so per square foot of surface. Salt is sometimes added as the crop is placed in the silo. A crop thus dealt with is stored for months; when the silo is opened the fodder is found preserved, and in a state readily taken to by cattle. It is desirable to choose the site for the silos so that the fodder is preserved somewhere near the place of consumption; also to lay out the works so that as little handling as possible is required. For instance, the silo should be on sidelong ground, so that the crop can be carted and tipped at a high level, and the silage taken out for use at a lower level.
II. SEWAGE DISPOSAL BY CHEMICAL TREATMENT.
In the last edition of our book on "Sewage Disposal," in speaking of precipitation we said that "the purification of sewage by chemicals has been the subject of misapprehension, owing to the extravagant advantages which have been claimed for the system by its advocates." This is even more true now than it was two years ago, inasmuch as in the recent scheme for dealing with the sewage of the Thames Valley chemical treatment per se was relied on to produce from the sewage of a future population of 350,000 an effluent at all times fit to be discharged at one point into the river Thames above London; but the Parliamentary Committee rejected it. One part of the report of this Committee deserves attention, when speaking of sewage treatment by chemicals. It is as follows: "Your committee believe that in these cases the process of filtering the chemically purified effluent through earth ought, if possible, to be adopted, which was not provided for in the scheme under their consideration." This opinion is exactly in accordance with our experience, and is that which we have held throughout. It is at the root of the whole matter, because efforts are made by those interested in chemical processes to attain as high a standard of purity as possible with the attendant heavy expense of chemicals. Experience shows that it is impossible at all times and seasons to be sure of a constant and uniformly high standard of purity, and that chemical works should be supplemented by a filtration area, however small. The addition of this, however, enables a lower standard of effluent from the precipitation tanks to be admissible, and this can be attained with very simple and inexpensive chemicals.
In the course of our practice we have had to advise as to the majority of the processes, and to design the works for their being carried into operation. We have found that the cost of such works complete varies from 0.091 to 0.166 pound per head of the population, and that the average cost of the works at several towns which we have been connected with is 0.123 pound per head. This figure may be conveniently followed by that of the cost of treatment, which we find varies from 0.036 to 0.110 pound per head per annum, and an average of several places gives 0.06 pound per head per annum. The above figures apply only to places where the very highest standard was sought to be attained, but our more recent experience leads us to modify the arrangement of the works and the cost of treatment, so as to rely on filtration of the effluent as an important factor. We estimate that under these conditions the cost of the works complete would be about 0.075 pound per head, and the cost of treatment 0.04 pound per head per annum. The disposal of the sludge has always been a difficulty in these works, but this is now overcome in two ways: either by digging it into the ground, as is done at Birmingham now, or by pressing it into cakes in filter presses. It is found at Birmingham that one ton of sludge with 90 per cent. of moisture is produced from 1,000 people. There the lime process is used. We have found that about one ton to 2,000 people is produced where a salt of alumina or iron is used with the lime. At Birmingham the sludge is dug into the land adjoining the works, and it is found that one square yard of land will take one ton of sludge with 90 per cent. of moisture once in three years, which results in three yards of land being required to be provided for each ton of sludge. This system of digging in sludge is successfully carried out as regards freedom from nuisance. Where land is not available to dig in the sludge, it is necessary to make it portable for removal and disposal away from where it is produced. This is best effected by filter presses. Appliances are made for this purpose, by which the sludge is pressed to a consistency of about 50 per cent. of moisture. The cost of effecting this is about 0.007 pound per head per annum. It is found in practice that where the sludge is produced by straining the solids from sewage before passing it on to land for purification, it requires a little lime to enable the press to work well. About two barrow loads of lime for each ton of pressed sludge suffices.
It has been thought that the cost of precipitation would be covered, and even a profit gained, by the sale of the sludge. This hope, however, is not nearer realization now than it was in the time, now gone past, when chemical processes were relied on to turn sewage from a profitless into a profitable commodity. There is, consequently, less justification now than there was at that time for adopting a precipitation system for sewage disposal. It is entirely a question of carefully considering the engineering and financial points involved, regardless of the sanguine representations of interested or enthusiastic advocates of any particular system. As the estimated manurial value of the sludge which is precipitated from sewage by the addition of chemicals does not seem to be capable of realization, we think that probably the reason may be found in the fact that the chemicals arrest that process of decomposition which is essential to the conversion of the organic matters into nitrates for vegetation to utilize.