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and how likely it is that they remain suspended, we do not seem to be in a position to expect that the water, after the subsidence of the deposit, will be safe to drink. We must adopt here the plan which is the safest for the community; and the effluent water should therefore be used for irrigation, or be filtered before discharge. The clear fluid is well adapted for market gardens; the plants grown as vegetables for the table are sometimes injured by irrigation with unpurified sewer water, but they thrive with the purified effluent water.
In arranging any processes for precipitation everything must be as simple as possible; there is no margin for expenditure or complicated arrangements.
Instead of using the dried deposit as manure, General Scott has proposed to make cement, and for this purpose adds lime and clay to the sewer water. The deposit contains so much combustible matter that it requires less coal to burn it than would otherwise be the case, and the saving thus effected enables (it is supposed) cement to be sold at a remunerative rate. If this should turn out to be the case, the sewage cement process has the advantage of destroying by fire everything which might be injurious in the deposit, while the effluent water, which contains rather more than two-thirds of the chlorine and three-fourths of the dissolved nitrogen, has some value as an irrigator. At present the pecuniary results of the process cannot be properly determined. General Scott also proposes to use the burnt material as manure to lime the land in some
5. FILTRATION THROUGH EARTH, CHARCOAL, ETC.
By filtration through earth is meant the bringing of sewer water upon a comparatively small area of porous soil, which is broken up and comminuted above, and is deeply underdrained, so that the sewer water may pass through the soil and issue by the drains. Mr. Dyke, in explaining the system employed at Merthyr-Tydvil' by Mr. Bailey Denton, lays down the following conditions :-There should be-1st, a porous soil; 2d, an effluent drain, not less than 6 feet from the surface; 3d, proper fall of land to allow the sewage to spread over the whole land; and 4th, division of filtering area into four parts, each part to receive sewage for six hours, and to have an interval of eighteen hours. He considers that an acre of land would take 100,000 gallons per day, though this seems a rather large amount. At Merthyr-Tydvil 20 acres of land were divided into beds, which sloped toward the effluent drain by a fall of 1 in 150. The surface was ploughed in ridges, on which vegetables were sown; the sewage (strained) passed from a carrier along the raised margin of each bed into the furrows. The effluent water was stated to be pure enough to be used for drink. Since 1872 these filter-beds, as well as 230 acres of other portions of the land, have been used as ordinary irrigation ground. The effluent water remains bright and pure. Another case of marked success with intermittent filtration is that of Kendal. The best soil for filtration appears to be a loose marl, containing hydrated iron oxide and alumina, but sand and even chalk produce excellent results. But in order that fil
On the Downward Intermittent Filtration of Sewage at Merthyr-Tydvil, by T. J. Dyke, F.R.C.S. Eng.
2 Report on Town Sewage.
tration shall be successful it is necessary that the amount of filtering material shall be large; it must not be less than 1 cubic yard for 8 gallons of sewage in twenty-four hours,' and in the case of some soils must be more. If the drains are 6 feet below the surface, then an acre will contain 9,680 cubic yards of filtering material, and at 8 gallons per yard an acre would suffice for 77,440 gallons. Crops may be grown on the land, and indeed it is desirable that they should be.
When the filters are too small, they fail to do much good; and Letheby has given analyses which prove that small filters may be nearly useless. It appears undesirable to use charcoal filters on this account, and all filtration through charcoal has been a failure. Spongy iron has been lately very strongly recommended. Carferal has also been suggested.
Filtration may be downward or upward, but the former kind is much more efficacious. Upward filtration may be said to be now abandoned.
Condition of the Effluent Water.-When 5.6 gallons of sewage were filtered in twenty-four hours through a cubic yard of earth, it was found by the Rivers Pollution Commissioners that the organic carbon was reduced from 4.386 parts to .734, and the organic nitrogen from 2.484 parts to .108 parts in 100,000. The whole of the sediment was removed. Nitrates and nitrites, which did not exist before filtration, were found afterward, showing oxidation.
By irrigation is meant the passage of sewer water over and through the soil, with the view of bringing it as speedily as possible under the influence of growing plants. For this purpose it is desirable that the sewer water should be brought to the land in as fresh a state as possible. In some cases, as at Carlisle, carbolic acid in small quantities has been added to the sewage in its flow for the purpose of preventing decomposition, and the plan appears to be effectual. The sewer water is usually warmer than the air at all times, and will often cause growth even in winter.
The effect on growing plants, but especially on Italian rye-grass, is very great; immense crops are obtained, although occasionally the grass is rank and rather watery. For cereals and roots it is also well adapted at certain periods of growth, as well as for market vegetables when the viscid parts are separated. When the sewer water permeates through the soil there occur-1st, a mechanical arrest of suspended matters; 2d, an oxidation producing nitrification, both of which results depend on the porosity and physical attraction of the soil; and, 3d, chemical interchanges. The last action is important in agriculture, and has been examined by Bischof, Liebig, Way,' Henneberg, Warrington, and others. Hydrated ferric oxide
The Rivers Pollution Commissioners give a smaller amount, viz., 5 gallons per cubic yard; but some of their experiments seem to show that we must increase the amount. For example, the soil at Beddington was found by them to have a remarkable power of nitrification up to the extent of 7.6 gallons per cubic yard in twenty-four hours. But when this rate was doubled nitrification ceased, and the soil became clogged. The best soil experimented on (Dursley soil), containing 43 of silica and 18 of oxide of iron, purified 9.9 gallons in twenty-four hours per cubic yard. But as few soils would be so good, the limit of 8 gallons is selected in the text.
On the application of sewage to land many works have been published. Dr. Corfield's work on the Treatment and Utilization of Sewage, 2d edition, and the Report of the Committee of the British Association, 1872, give the best summary of the subject up to date of publication. Also the Report of the Committee on Town Sewage, 1876.
Journal of Royal Agricultural Society, vol. xi.
4 Chemical News, May, 1870. Warrington's paper gives a good résumé of the subject, and many original experiments, and can be consulted for full details.
and alumina absorb phosphoric acid from its salts, and a highly basic compound of the acid and metallic oxide is formed. They act more powerfully than the silicates in this way. The hydrated double silicates absorb bases. Silicates of aluminum and calcium absorb ammonia and potassium from all the salts of those bases, and a new hydrated double silicate is formed, in which calcium is more or less perfectly replaced by potassium or ammonium. Humus also forms insoluble compounds with these bases. Absorption of potash or ammonia is usually attended with separation of lime, which then takes carbonic acid.
The soil must be properly prepared for sewage irrigation; either a gentle slope, or a ridge with a gentle slope on each side of about 30 feet wide,' with a conduit at the summit, or flat basins surrounded by ridges, are the usual plans. The sewer water is allowed to trickle down the slope at the rate of about 8 feet per hour, or is let at once into the flat basin. The water passes through the soil, and should be carried off by drains from 5 to 6 feet deep, and thence into the nearest water-course.
The sewer water should reach the ground in as fresh a state as possible; it is usually run through coarse strainers to arrest any large substances which find their way into the sewers, and to keep back the grosser parts which form a scum over the land; it is then received into tanks, whence it is carried to the land by gravitation, or is pumped up. The "carriers" of the sewer water are either simple trenches in the ground, or brick culverts, or concreted channels, and by means of simple dams and gates the water is directed into one or other channel as may be required. Everything is now made as simple and inexpensive as possible-underground channels and jets, hydrants, hose and jets, are too expensive, and overweight the plan with unnecessary outlay.
The amount of land required is, on an average, 1 acre to 100 persons; this is equal to a square of 70 yards to the side, and will take 2,000 gallons in twenty-four hours.
The sewer water is applied intermittently when the plants are growing; but in winter it is sometimes used constantly, so as to store up nourishment in the soil for the plant-growth in the spring."
The amount of sewer water which can be applied will vary with the kind of ground, the amount of rain, and the season of the year. In the year ending 1871, it appears that, on the Lodge farm at Barking, 622,324 tons of sewage were applied to 163 acres (nearly), or about 3,800 tons per In the sixteen months ending December, 1872, the average quantity
1 This was the arrangement of Mr. Hope's farm at Romford.
2 See an interesting paper on the "Utilization of the Sewage of Paris," by Sandford Moore, B.A., Assist.-Surgeon, 4th Dragoon Guards (Medical Times and Gazette, June, 1870). In the summer "arrosage " is practised; the land is ploughed in furrows and ridges, and the water is allowed to flow into the furrows, and not allowed to wet the vegetables which are planted on the ridges. In winter "colmatage" is had recourse to; the ridges are levelled and the entire surface is submerged under sewage water. The sewers of Paris receive only a small part of the solid excreta (though most of the urine), but the fluid is highly fertilizing. Precipitation with alum was also formerly had recourse to in Paris, but has now been abandoned.
For detailed information see the Report of the Prefecture of the Seine, Sur l'Assainissement de la Seine. An abstract is given in the Annales des Ponts et Chaussées, and is translated by R. Manning, M.I.C.E. (E. & F. N. Spon), 1876. Similar works are in process at Berlin, and are described in the same paper. At Brussels, the Senne, during its passage through the city, is no longer used as the main sewer, and although the sewage is still poured into it at a lower point, it will ultimately be disposed of by irrigation.
was 3,342 tons per acre annually, On the most porous part of the farm as much as 960 tons have been applied in twelve hours.'
Condition of the Effluent Water after Irrigation.
When the sewer water passes over and not through the soil, it is often impure, and even suspended matters of comparatively large size (such as epithelium) have been found in the water of the stream into which it flows. It requires, therefore, that care shall be taken in every sewage farm that the water shall not escape too soon. Dr. Letheby,' whose authority on such a question no one can doubt, rated the cleansing power of soil much lower than the Rivers Pollution Commissioners or the Committee of the British Association, and his analyses make it at any rate quite certain that the proper purification of the sewer water demands very careful preparation of the ground in the first instance, and constant care afterward. But the chemical evidence of the good effect of irrigation is too strong to admit a doubt to exist, as may be seen from the table given by the Rivers Pollution Commissioners.'
The results are much better than those of any chemical precipitant, although they are not quite so good as the downward filtration plan.
' Mr. Morgan's Report, quoted in Food, Air, and Water, December, 1871. 2 The Sewage Question, 1872, pp. 3-27.
3 The standard of purity which effluent water should have has not yet been fixed. That proposed by the Rivers Pollution Commissioners, which is based on the method of analysis proposed by Dr. Frankland, and which is not yet universally admitted, was as follows:
Standard of Rivers Pollution Commissioners. Maximum of Impurity permissible in 100,000 parts by weight of the liquid.
A certain degree of acidity or alkalinity is also ordered not to be surpassed. In the discussions on the Public Health Bill in the House of Commons, this standard, which had been embodied in the Bill, was struck out, and the standard is left to be hereafter determined. (No standard is given in the Rivers Pollution Act of 1876.) The objection to the plan is not merely the doubt about the substances represented by organic carbon or nitrogen, but also because the standard does not take into consideration the volume of water into which the foul water flows. The Thames Conservancy Commissioners adopt a standard for effluent sewage as follows:
Do Sewage Irrigation Farms affect the Public Health or Public Comfort?
That sewage farms, if too near to houses and if not carefully conducted, may give off disagreeable effluvia, is certain; but it is also clear that in some farms this is very trifling, and that when the sewer water gets on the land it soon ceases. It is denied by some persons that more nuisance is excited than by any other mode of using manure. As regards health, it has been alleged that these farms may-1st, give off effluvia which may produce enteric fever, or dysentery, or some allied affection; or, 2d, aid in the spread of entozoic diseases; or, 3d, make ground swampy and marshy, and may also poison wells, and thus affect health.
The evidence of Edinburgh, Croydon,' Aldershot, Rugby, Worthing, Romford, the Sussex Lunatic Asylum,' is very strong against any influence in the production of typhoid by sewage farms' effluvia. On the other hand, Dr. Clouston's record of the outbreak of dysentery in the Cumberland Asylum is counter-evidence of weight, and so is one of the cases noted by Letheby,' of typhoid fever outbreak at Copley, when a meadow was irrigated with the brook water containing the sewage of Halifax.
The negative evidence is, however, so strong as to justify the view that the effluvia from a well-managed sewage farm do not produce typhoid fever or dysentery, or any affection of the kind. In a case at Eton, in which some cases of enteric fever were attributed to the effluvia, Dr. Buchanan discovered that the sewer water had been drunk; this was more likely to have been the cause.
With regard to the second point, the spread of entozoic diseases by the carriage of the sewer water to the land was at one time thought probable, though as solid excreta from towns have been for some years largely employed as manure, it is doubtful whether the liquid plans would be more dangerous. The special entozoic diseases which, it is feared, might thus arise, are Tape-worms, Round worms, Trichina, Bilharzia and Distoma hepaticum in sheep. Cobbold's latest observations show that the embryos of Bilharzia die so rapidly that, even were it introduced into England, there would be little danger. The Trichina disease is only known at present to be produced in men by the worms in the flesh of pigs which is eaten, and it is at least doubtful whether pigs receive them from the land. There remain, then, only Tape-worms and Round worms for men and Distoma hepaticum for sheep to be dreaded. But with regard to these, the evidence at present is entirely negative; and until positive evidence is produced, this argument against sewage irrigation may be considered to be unsupported.
The third criticism appears to be true. The land may become swampy, and the adjacent wells poisoned, and disease (ague and perhaps diarrhoea and dysentery) be thus produced. But this is owing to mismanagement, and when a sewage farm is properly arranged it is not damp, and the wells do not suffer.
1 Carpenter, various papers and essays on this subject drawn from the experience of Croydon Sewage Farm.
Dr. J. W. Williams, Brit. Med. Journal, May 11, 1872.
3 The Sewage Question, p. 190.
There is no ague or any other disease traceable to the sewage irrigation at Craigentinny, near Edinburgh.