Popular Science Monthly/Volume 18/April 1881/The Purification of Sewer-Waters
|THE PURIFICATION OF SEWER-WATERS.|
THE sewers of Paris discharge 262,646 cubic metres of liquid matter every twenty-four hours. It is estimated that the quantity discharged will be increased before many years to 300,000 cubic metres daily. Each cubic metre of liquid contains two and a half kilogrammes of solid matter, of which one kilogramme and a half is merely in suspension. This stuff, flowing into the Seine, causes an accumulation of 110,000 cubic metres of mud in a year at the mouths of the conduits, and makes necessary for its removal an annual expenditure of nearly 200,000 francs. Even this sum is not adequate for the purpose. Far from securing the removal of the obstruction, it is not even sufficient to prevent a continued accumulation, and the muddy deposits are constantly extending farther down the river, and at the same time becoming thicker. Since 1875 they have become about a yard thick, and occupy nearly a quarter of the bed of the river from Asnières to beyond Chatou. The Seine has, moreover, been made foul, and its waters have become unfit for domestic use, poisonous to fishes, and a source of fetid emanations.
The authorities of Paris have been for many years considering measures to remove these sources of impurity from the river. As they are rich in fertilizing matters, the thought was suggested that they might be turned to good account for purposes of agriculture. It was therefore resolved to apply them to works of irrigation in the peninsula of Gennevilliers, where, passing through the thin soil of red earth underlaid with gravel, they might leave their rich manures on the arid land, and be returned to the Seine purified. Five hectares (twelve and a half acres) of land were chosen to be irrigated by the sewer-water, which was conducted in trenches around garden-beds. These lands in return produced abundant crops of the coarser vegetables. Three years afterward, in 1869, independent gardeners began to take in the sewer-water, and the demand for it increased, so that, in 1876, 115 hectares were irrigated, and in 1880 more than 300 hectares. Had the use of the sewage-water as a fertilizing material been the only condition to be fulfilled, the success might have been pronounced complete. The principal object, however, was the purification of the Seine, and in this only the most insignificant result was obtained; for the gardens were capable of taking only a minute fraction of the sewage that had to be disposed of. Complaints of bad effects upon health were increased rather than diminished, so that, in 1875, the Minister of Public Works appointed a commission to devise some means of remedying the unpleasant situation.
A plan was submitted by eminent engineers, under which it was believed the Seine could be definitely relieved of the noxious substances which were defiling it. This plan contemplated the convection, by means of new machinery and conduits which were to be constructed for the purpose, of the foul waters to the peninsula of St. Germain, where it was thought 6,300 hectares of land might be applied to the reception and disposition of them. Of this tract, 1,500 hectares f denuded and sterile land in the forest of St. Germain might be employed as a place of deposit, where the sewage that was not used in irrigation could be turned on and absorbed into the ground, for it was foreseen that the use of the water would not be uniform, and that in certain weathers and seasons the cultivators would be as anxious to keep it off from their lands as they would be at other times to draw it on. This proposition brought out a general protest. The region which it was proposed to irrigate was largely occupied with country-seats whose proprietors did not at all relish the introduction of the objectionable matter so near their homes. Additional force was given to their objections by reports that Gennevilliers had become subject to infiltration of the waters, and afflicted with fevers generated by them. The project was withdrawn for a time, but was eventually renewed, with the feature of irrigation omitted. In its modified form, it contemplated only the employment of the 1,500 hectares of the forest of St. Germain as a reservoir, on which the sewage-matter should be turned to be absorbed in the ground. This proposition aroused a more determined resistance than the others. Those who lived along the line of the proposed conduit apprehended that the foul waters might at some time be turned upon their own land; the residents of the neighborhood of the forest regarded with dismay the establishment at their doors of a vast cesspool in the shape of a tract of land which should be covered daily with nearly 200 cubic metres of water, with 60,000 or 70,000 cubic metres of nastiness in a year. The plan is, moreover, defective, for it is not capable of satisfying either of the conditions which are had in view.
The conditions which are essential to a complete solution of the problem must provide for the purification of the sewage and the preservation of the river from contamination with it, and for the utilization of the rich manures which are contained in the waters. The former condition, in fact, is imposed as an absolute necessity; while the abandonment of the second would only constitute a certain economical loss. Irrigation is really contemplated only as one of the means to the end; but it is a very inadequate means, for its successful adaptation to the chief purpose would require the employment of a larger quantity of land than it is practicable to obtain for the disposal of the sewage of a large city. A city of ten or twenty thousand inhabitants might with comparatively little trouble find the three hundred or five hundred acres in its vicinity which would be requisite for this purpose; but, with a city twice as large, the problem is more complicated, while, in a metropolis, it becomes impossible of solution.
Sewage-water, when used for irrigation, is undoubtedly rendered innocuous. It might seem at first sight feasible to combine the two operations, so as to accomplish both objects at once. There is, however, an essential difference in the conditions required for the two solutions which makes the combination impracticable. Ten or twenty times as much land would be required to utilize the waters by irrigation as would be needed simply to absorb the foul matter and cause it to be destroyed by slow oxidation. Thus, the 1,500 hectares of the forest of St. Germain would be as efficacious, if their action merely as an absorbing medium is contemplated, as 40,000 or 60,000 hectares would be in a scheme for irrigation. The demand for the water under the operations of irrigation would necessarily be very fluctuating. None would be wanted in the winter or in hot weather. While green crops might require much during the whole season of their growth, grain-crops must be carefully protected against it as the season of their ripening approaches. What is to be done with the constantly accumulating supplies of waste water during these seasons when it can not be used?
The whole difficulty arises from the fact that the fertilizing matter which we wish to make useful is drowned in an excess of water. It is this excess which renders sewage unfit for the fertilization of cereals, crops which are too often injured by the superabundance of rain-water. And it is the same excess which makes it impossible to find sufficient surfaces on which it can all be employed in the cultivation of kitchen-garden vegetables, with whose demands its organic constituents well agree. Were this matter extracted in a dry state, it would furnish a precious element of fertilization to a large agricultural interest; and we might preserve it without difficulty and without loss, and transport it at will to apply it to crops of all kinds. At the same time, the water in which it is held could be returned clear and wholesome to the rivers, whose salubrity it is now destroying. Clearly, there can be no discrepancy in the conditions required for the accomplishment of this double object; whatever favors one side must be equally favorable to the other. The two results are absolutely concordant, and may be produced at the same time by one and the same operation, an operation which we may call decantation. The practicability of this process is established by the fact that it has been adopted and is employed with complete success in a large factory in the neighborhood of Paris.
The paper-mill of Essonnes has to deal with 10,000 cubic metres of foul water a day. For two years it has returned to the river Essonnes these 10,000 cubic metres of water clarified, while it has at the same time extracted the mud which they held, and delivered it in a solid state as manure to the agriculturists of the neighborhood.
The apparatus employed at this establishment is composed of two parts, corresponding with two very distinct phases of the operations:
First, is a series of water-tight basins, or tanks, which are used in the decantation, properly so called, of the foul waters.
Second, is arranged a series of tanks having permeable bottoms, constructed parallel with the former tanks, but on a lower level; these are destined for the drainage of the mud which is deposited in the decanting-vats.
The process is as follows: The foul waters from the factory are drawn into a single conduit from about twenty inches to two feet wide, along and over which is disposed a series of circular bucking tubs, containing lime-water, a substance which is known to be very efficient in securing the precipitation of organic matters. The tubs are provided with dashers, which keep the lime-water constantly in suspension, and with gauged faucets, which permit it to be introduced regularly in the proportion of two hundred to two hundred and fifty grammes of lime to the cubic metre of water into the current of foul water which is flowing at the foot of the tubs. A few eddies produced by means of artificial obstacles placed in the conduit secure immediately a complete mixture of the lime with the foul water. As soon as the lime-water is introduced, the waste waters almost wholly lose their offensive odor, and cease to offer the slightest danger from noxious exhalations.
The water having been thus prepared in its passage through the conduit, is distributed into ten basins of decantation, each about sixty-five feet long, twenty feet wide, and five and a half feet deep, which are arranged side by side. Each of the basins has a capacity for the decantation of a thousand cubic metres of liquid a day. The water is constantly entering at one end in each, and flowing out over the top at the other end. The rate of flow is almost imperceptible, being hardly a millimetre a second, and precipitation takes place as completely as if the water were quite still; consequently, the water goes out fully clarified. Thus a course of about sixty-five feet in length, which, at the rate of ·001 of a metre a second, represents a delay of nearly six hours in the basin, suffices to clear the water of all matters in suspension. A talus of mud is gradually formed in the bottom of the tank, which at the end of a week becomes flush with the surface of the water at the entrance-end, and just covers the bottom at the end of the outlet. The basin has now produced all the effect of which it is capable. If any more water is allowed to go through it, it will contain mud in suspension, for it is still in the act of precipitating it when it goes out. The operation must be stopped here. We close the feeding-gate of the tank, draw off the clear water that is left in it by means of a decanting-tube, and lay bare the talus of mud.
The bottom of the basin is slightly inclined in a contrary direction to the course of the water, and is provided with a large valve at the lower end. On opening this valve, the mud, which is still in a very liquid condition, is passed into a lower basin—the drainage-basin—of the same capacity as the former one, and so disposed that its upper surface is a little below the bottom of the same. The first tank may be put in operation again immediately after the mud has been drawn from it.
The side-walls of the drainage-basin are of tight masonry, but the bottom is made as permeable as possible. For this purpose a floor of scoria is prepared and provided with a series of pipes which lead the water out into a collector. The arrangement is admirably adapted to the purpose for which it is designed, and quite obviates the difficulty of drying the mud fast enough to enable it to be taken away at a profit, which has prevented the success of all previous efforts at purification by deposition. The mud which is spread on the permeable bottom does not soil the scoria, but leaves it perfectly clean, while the water flows clear from the end of the drains. After two or three days, according to the weather, the mud will appear cracked at some points, and finally over the whole surface. After a week it will have acquired consistency enough to be cut with a shovel. A cart is then brought into the basin, and after a few hours it is emptied and is ready to receive a new charge of mud. The scoria not having been soiled, requires no cleaning, and will be as ready for use even after the end of ten operations as at the beginning.
The drained mud is carried in the shape of large lumps to an open yard, where it is dried in the air without giving forth any odor. It contains about seventy-five, per cent, in weight of water at the time it leaves the basin, but the amount of water is reduced after two or three months of exposure to not more than fifteen or twenty per cent.
These operations are of the simplest character, and involve nothing cumbrous. The whole system, with its decanting and drainage tanks, its open yard and the necessary roads, occupies a surface of not more than two hectares, or five acres, for the effective purification of 10,000 cubic metres of water every twenty-four hours. Paris has to get rid of thirty times as much foul water as the Essonnes paper-mill, or 300,000 instead of 10,000 cubic metres a day. The system practiced at Essonnes would, therefore, have to be applied on a scale thirty times as large to be adapted to the needs of Paris. Can it be made to succeed on such a scale? What is there to prevent it?
No difficulty is offered by the composition of the sewer-waters. We have procured a quantity of water from the great collector of Asnières, and have subjected it to the same treatment that is given the waste water at Essonnes. On adding to it lime-water in the proportion of 250 grammes of lime—yes, even in the smaller proportions of 200 and 175 grammes—to a cubic metre of water, a complete precipitation was promptly produced. At the end of four or five hours the water became clear and limpid.
The extent of land required to conduct the operations of decantation, drainage, and drying, on the scale demanded by the city of Paris, seems formidable at the first sight, but it is not really so. As we have seen, the whole system at Essonnes occupies only two hectares, or five acres, of land. For the city of Paris thirty times as much land, sixty hectares, only 150 acres, would be needed to give room for all the apparatus and all the manipulations; that would be a small tract compared with the 1,500 hectares, or 3,750 acres, on which it is proposed to establish a nuisance in the forest of St. Germain.
The scheme will compare favorably with any other that has been proposed, in the cost of constructing and operating the works. The tanks, occupying six hectares, or fifteen acres, would Lave to be built up with walls of solid masonry, at an expense of about 1,200,000 francs. A similar sum may be added for the installation of the roads, ways, carts, and other accessories, making a total of about 2,500,000 francs, or $500,000—a mere drop in the budget of the city of Paris, The expense of labor at Essonnes is twenty francs a day; at thirty times as much, it would be 600 francs, or $120, a day at Paris. Add to this the expense of carrying away the manure, in case no return is derived from sales, and the price of the daily supply of lime, estimated at the highest probable figures, and the whole daily expense of operation at Paris rises to 3,000 francs, or $600. The year's aggregate of these daily expenses, with the interest on the cost of original establishment (which we now place, to cover all possible additions, at double the amount of the estimate we have just made), gives the total cost of the application of the Essonnes system to the purification of the sewer waters of Paris at 1,345,000 francs, or about $260,000, a year. The cost of managing the proposed works for the absorption of the sewage in the forest of St. Germain is estimated at 2,120,000 francs, or about $403,000, a year, showing a difference of more than $140,000 a year in favor of the plan of desiccation as pursued at Essonnes.
The difference in favor of this plan is much greater than appears from these figures, for no account has been taken of the probable economic value of the desiccated mud as manure. By actual analysis this mud has been found to contain from eleven to fifteen grammes of nitrogenous substances, and from twenty to twenty-five grammes of phosphate of lime, per cubic metre. The whole deposit from a year's sewage of Paris would contain nitrogenous matters enough to suffice for the fertilization of 75,000 acres of land. It is certain that a sale would be gradually found for this valuable matter, the proceeds of which, estimated eventually to amount possibly to 1,500,000 francs a year, would in the end more than defray the entire cost of maintaining the system of extraction.
- Translated and abridged from the "Revue des Deux Mondes."