CHAPTER V. THE LAND TREATMENT OF SEWAGE. Sewage farms-Only indicated where soil is suitable-Clay lands nearly useless-The theory of nitrification-Intermittent downward filtration— Irrigation proper-Stratford-on-Avon farm-Edinburgh farm-Paris farm-Berlin farm-Proper method of laying out and working a farm— Quantity of land required-Fallacy of the manurial value of sewage. Sewage Farms. Perhaps the simplest method of purifying sewage, and undoubtedly the cheapest and the best where local circumstances permit of its being carried out, is by means of land treatment, by which I mean irrigation and a certain amount of intermittent filtration. Unfortunately, however, it is not everywhere that a suitable soil is to be found, and if it can be found the price may be prohibitive, or no soil can be had at any price, a condition which is found in very narrow valleys. Nevertheless, the first inquiry to be undertaken with reference to the means of purifying the sewage of any district is to ascertain the nature of the soil and subsoil of the locality. For this purpose the author has found a short boring tool most useful. By means of a rod about four or five feet in length and half to three-quarters of an inch in diameter, made like a cheese-taster, pieces of core within three or four feet of the surface may be obtained. The geological maps are also extremely useful, but their accuracy must not be taken for granted. The best soil for the purpose of purifying sewage by irrigation is an open sandy soil, such as is met with in the Bunter sandstone, or the sandy gravels deposited in many valleys. "^ The older authorities complicated the question as to what is the best soil for purifying sewage by trying at the same time to grow a remunerative crop, but it is now more generally recognized that purification must be considered first and profit afterwards. There can be no doubt that the answer given by the great bulk of the older authorities to this particular question was the correct one-namely, a sandy or loamy soil, the soil not being of too open a nature. In fact, the soil should be of such porosity that it would absorb at the same rate that it would purify-that is to say, it should not be capable of absorbing more than 25,000 gallons per acre per diem, even if the sewage has the suspended matters first removed by means of precipitation. But if the purification alone of sewage is aimed at, and no return from crops is looked for, a much more open soil is most suitable; and if the sewage, after undergoing a rough form of precipitation, is only applied intermittently for a few hours, and the soil is given a rest for several days before any more is applied to it, a much larger quantity of sewage than this indeed, as much as 100,000 gallons per diem per acre under special circumstances can be purified. * is that more sewage will be What is generally taken by indication not to apply more The danger with open soils applied than can be oxidized. the average sewage farm as an sewage is the fact that the soil begins to absorb less readily. It cannot be too emphatically stated that when this point is reached the land is overdosed. Provided, however, that too much sewage is not applied to the land, and it is applied intermittently, the more open the soil is the better, if it is a good thickness and the subsoil is fine gravel and uniform in texture. Even blown sand on the sea-coast will thoroughly purify sewage, if it is applied in small quantities at a time and the application is stopped before the sewage reaches halfway to the land-drains. It cannot be too clearly understood that there is no relation between the quantity of sewage that can be got to pass through the soil and the quantity of sewage that can be purified; for instance, a thin soil overlying a shaly open rock will take any amount of sewage, but will not purify more than a retentive clay. The following are some of the formations the soils of which are suitable for purifying sewage: alluvial drift and gravel, Oolitic sandstone, bunter sandstone, and the magnesian limestone when sufficiently weathered, the old red sandstone, and occasionally the millstone grit. * Cf. actual working of Stratford-on-Avon intermittent filters, p. 59. The conditions under which irrigation alone should be adopted are where there is an open sandy loam or loamy gravel which can be obtained at a price not much exceeding £150 per acre. The surface of the coal measures is generally of an extremely stiff clay, which may be useful enough for brickmaking and other industrial purposes, but the area of which, necessary for sewage purification, would have to be "so great as to render land treatment impracticable." * The same remark applies to a great deal of the surface of millstone grit. The shale with which the gritstone is interstratified is quite impervious, as is evidenced by its bringing the springs in the millstone grit to the surface. Then, with regard to the mountain limestone, Cambrian rocks and granites, the soil here is far too shallow to effect purification, often not being six inches in depth, while below the soil we come to rocks which are frequently riddled with fissures, many of which lead to underground caverns, in which the sewage would stagnate and putrefy, or to open channels connected with the nearest river. In almost every formation beds of clay are to be found, and it should not suffice to have one or two trial-holes sunk, but trial-holes should be dug on every side of the piece of land it is proposed to irrigate, and bore-holes to a depth of four feet should be made at intervals all over the site. The most unsuitable soil, and, unfortunately, one of our commonest, is clay land. It is said that clay lands can be rendered more fit for filtration by ploughing and digging in ashes, which convert the impervious surface and allow the sewage to sink through. There are in Derbyshire two farms upon which considerable sums of money have been spent in thus preparing the land, in one instance as much as £1123 being spent in lightening 14 acres of land to a depth of two feet with engine ashes. It is perfectly true that this enables the sewage to pass through the clay, but it does not lead to its purification, and where the land is a stiff clay it undoubtedly would be better to construct sewage filters. Clay lands, besides being too impermeable to permit the sewage to pass through them, are unfortunately open to another objection-viz. that in dry weather they crack and *See Interim Report of Royal Commission on Sewage Purification, 1901. E fissure, so that the sewage passes directly through the cracks to the land-drains, without undergoing any purification. Worms also leave permanent holes in clay, which last for a considerable length of time, and permit the sewage to pass down. At a small farm at Brampton in Derbyshire the sewage contained a considerable amount of dye-water, and upon a trial-hole being sunk on the said farm, which is on a stiff clay, the author found innumerable worm-holes passing directly downwards to the effluent drains, the worm-holes having their sides saturated with dye, and showing how the sewage passed away absolutely unpurified. In 1898, a Royal Commission was appointed to inquire and report on the methods of purifying sewage, and in July, 1901, the Commissioners issued an important interim report. The Commissioners gave as the reason for the appointment of the Commission and the re-consideration of the position of the authorities on this question, that "it is now contended that in many cases the land available is either of unsuitable quality, is available in quite inadequate area for effective filtration through the soil, or is obtainable only at a prohibitive cost, and it is suggested that sewage purification may, in such cases, be carried out on comparatively small areas artificially prepared." On this point the author gave evidence before the Commission with reference to the uselessness of stiff clay land, which was the only land available for the purification of sewage in many parts of Derbyshire, such as Chesterfield, Alfreton, Clay Cross, Heanor, Ilkeston, and at the County Asylum. As the result of these investigations, the Commissioners in 1901 reported "We are forced to conclude that peat and stiff clay lands are generally unsuitable for the purification of sewage, that their use for this purpose is always attended with difficulty, and that where the depth of top soil is very small, say, six inches or less, the area of such lands which would be required for efficient purification would, in certain cases, be so great as to render land treatment impracticable." Instead of attempting to lighten clay lands where this is the only land available, sewage filters should undoubtedly be adopted. The researches of Warington, Schloesing, and Muntz, and of the Massachusetts State Board of Health, have shown that the number of nitrifying organisms in soils of sewage filters rapidly diminishes from the surface downwards. Small quantities of soil taken at different depths from the surface at Rothamsted showed that in clay soils no nitrification takes place at a greater depth than eighteen inches; in the most porous soils, however, nitrification still took place at a depth of four feet from the surface. When we bear in mind the quantity of oxygen* necessary for the growth of the nitrifying organisms, these researches only confirm what one would NUMBER OF BACTERIA FOUND IN A GRAMME CE SAND. 56 58 60 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 FIG. 5.-Diagram showing NUMBER OF BACTERIA AT VARIOUS DEPTHS OF suppose to be the case from general reasoning. It is possible, Fig. 5 shows the number of bacteria per gram of sand at * About half the weight of the nitrates produced consists of oxygen. |