Rural Hygiene

by Henry N. Ogden

Surface application on land.

Two methods of operation may be pointed out. The sewage (and this is the simplest method of disposal possible) may be brought to the upper edge of a small piece of ground, usually sowed to grass, and allowed merely to run out over the surface of the ground. There should be, however, some method of alternating plots of ground, one with another, so that the sewage is turned from one to the other every day. Each plot will then have one day's application of sewage and one day's rest, and this would complete the disposal, were it not for the interference of rain and cold. The winter season practically puts a stop to this method of treatment, and rainy weather reduces the power of the soil to absorb sewage. For these two reasons, it is desirable to have one plot in reserve, or three in all, and the area of each plot should be based on the amount of sewage contributed. For a family of ten persons using twenty-five gallons of water per day the total area provided should be one tenth of one acre, or an area seventy feet square divided into three plots. Figure 67 shows six beds arranged to care for the sewage of a public institution in Massachusetts. As a guide to the amount of land needed, it will be safe to provide at the rate of one acre for each forty persons where the soil is a well-worked loam but underlaid with clay. The effect of this irrigation on the grass will be to induce a heavy, rank growth which must be kept down by repeated cutting or by constant grazing. Both methods are practiced in England, and it may be said in passing that no injury to stock from the feeding of such sewage-grown grass has been recorded. The grass cut from such areas (and the cutting is done every two weeks through the whole summer) is packed into silos and fed to cattle through the winter with advantage. Or, if grazing is resorted to to keep the grass down, the herd is alternated with the sewage from one field to the other, so that the bed which has received sewage one week is used for pasture the next week, and the number of head which can thus be fed is astonishing. In order to secure an even flow of sewage over such grass land as is here contemplated, there must be a gentle slope to the field, and the ditch or drain bringing the sewage to the field should run along its upper side. Openings from the drain, controlled by simple stop planks, are provided at intervals of about ten feet, and no attention is needed further than the opening and closing of these admission gates.

Fig. 67.—Sewage beds.

Another method of applying sewage to the surface of the ground is to lead it in channels between narrow beds on which vegetables have grown. These beds are made about eight feet wide with two rows of root crops, such as turnips or beets, set back about two feet from the edge. The beds are made by properly plowing, the channels between the beds being back-furrowed. Here, again, the principle of intermittent application is essential, and the area to be provided is the same as already given for the surface irrigation. Three beds should be provided, as before; but, in general, no provision need be made for carrying off the sewage at the lower end of the beds, since it may be safely assumed that all of the sewage will be absorbed by the soil. Of course, a sandy soil will absorb more water than a clay soil, and if the soil is entirely clay, it is not suitable for such treatment. Sewage passed over the surface of clay soil, however, will, in the course of a few months, so modify the clay as to convert it into a loam, and in this way increase its absorptive power.

When possible, it is desirable to have a plot of plowed ground over which the sewage may pass before reaching the beds, so that the grosser impurities may be left behind and harrowed in or plowed under. If proper regard is paid to intermittent application, no danger from odors need be feared, and the repeated plowing in will increase immensely the fertility of the soil. Nor need one be afraid that all of the manurial elements will be left behind on this plowed ground. About two thirds of the organic matter in sewage is in solution, and this will be carried onto the beds just as if passage over the plowed ground had not occurred.

Artificial sewage beds.

In order to secure a higher rate of discharge of sewage through the soil it is best to arrange an artificial bed which shall be made of coarse, sandy material which will allow a rate of at least 10 times that already given. The best material out of which to make such an artificial bed is a coarse sand; that is, a sand whose particles will not pass through a sieve which has 60 meshes to the inch and which would pass through a sieve of 10 meshes to the inch. Such an ideal sand will purify sewage at the rate of 50,000 gallons per acre per day, or an acre will take care of the sewage of at least 1000 persons. This means that it is necessary to provide about 50 square feet for each person in the family, or a family of 10 persons could have all the sewage taken care of on an area 25 feet square. The same principle of intermittency of application, however, must be observed by dividing the bed into three parts, so that the sewage may be alternated from one bed to another. Practice has indicated that it is better to shift from bed to bed about once a week and to deliver the sewage onto each bed intermittently; that is, to discharge a bucketful at a time with short intervals between, rather than to allow a small stream to flow continuously onto a bed. Such a bed should be about 3 feet deep, as already stated, and preferably should have light concrete side walls and bottom, as shown in the sketch (Fig. 68). Ordinarily, the surface of the sand will be level, and the dose of sewage applied to the bed will cover it a fraction of an inch deep, and in the course of an hour or so will disappear into the sand and reappear in the underdrains as clear water.