Rural Hygiene

by Henry N. Ogden

Fig. 68.—Sewage beds.

In cold weather a thin sheet of sewage spread out over the surface of the sand would freeze before penetrating the bed; therefore, in the winter time, it is usual to furrow the beds; that is, dig furrows across the beds 2 or 3 inches wide at the bottom and about 10 inches deep, so that in the bottom of these furrows the sewage may be, partly at least, protected against frost. It has been found that, if sewage is discharged intermittently,—that is, in bucketfuls into such furrows,—the beds open and allow the filtration of the sewage. To be sure, the purification effected in cold weather is not quite that accomplished in warm weather, but the results are sufficiently satisfactory, and no nuisance ensues.

Subsurface tile disposal.

The other method of distributing sewage over land is by means of draintile placed in shallow trenches, so that the sewage may leach out into the soil through the open joints of the pipe. These draintiles receive the sewage intermittently, and by the constant rush of water are presumably filled throughout their length. The sewage then gradually works out of the joints into the surrounding soil, and the pipes are empty and ready to receive another dose when next delivered.

Two essential points must be considered in the successful operation of such a plant: the grade of the tile and the length of the tile.

The grade of the tile must be properly adjusted to the porosity of the soil; that is, in open, porous, and gravelly soils a grade must be steeper than in loamy and dense soils. The reason is manifest. In a gravel soil, the sewage is at first rapidly absorbed, so that as the sewage goes down the pipe line the first joints take up the water and deliver it to the soil, where it disappears, and probably no flow reaches the end of the line at all. This means that the soil surrounding the first joints does the work which the entire pipe line was intended to do and thus becomes overworked. When overworked, the soil always refuses to do anything, so that when the succeeding joints take up the sewage and in their turn become overworked, the line is useless. If, on the other hand, the grade had been steep enough to carry the sewage down the pipe line gradually so as to secure a uniform distribution, then the same or approximately the same amount of sewage would be taken out of the pipe at each joint, securing a long life for the system. In loamy soil, on the contrary, there is not the same absorption at the joints, and so on a steep grade there is the tendency for all the sewage to follow down the pipe line to the lower end and there escape to clog the soil and thus spoil the system. As a general average, it may be said that the proper grade for such a subsurface distribution pipe line in a fairly good sandy loam should be 5 inches in 100 feet; less than this as the loam becomes clay and more as the loam becomes gravel.

The other essential point for the successful operation of this method of distribution is to provide a proper length of pipe for the number of persons contributing sewage. The soil itself will absorb about the same amount as when the sewage is spread over the surface, so that a family of ten persons would require, as before, an area about 70 feet square. The pipe lines may be laid in different sections, provided the different lines of pipe are not nearer together than 10 feet. On an area 70 feet square there would be, therefore, 7 lines of pipe each 70 feet long, or 490 lineal feet of pipe in all, or 49 feet per person. The writer generally allows 40 feet in well-cultivated soil as a reasonable length of pipe for each person in the family. If the soil is sandy, this may be reduced one half, but need not be increased under any conditions, since a soil requiring a greater length of pipe than 40 feet per person would be so dense as to be unfit for use. To properly arrange the lines of pipe on a sloping ground requires careful study of the inclination of the ground and of the relation of direction of lines of pipe to slope. Usually the slope of the ground is greater than the 5 inches per 100 feet just referred to, but by laying out the lines of pipe across the slope instead of with it any grade desired may be obtained. Nor is it necessary that these lines of draintile be run in straight lines; they may very properly follow the curving slope, the proper grade being always carefully maintained.

Fig. 69.—Plan of subsurface irrigation field.