Irrigation.—The disposal of sewage by irrigation is by far the best method now within reach. Two methods are in use, viz, surface and subsoil irrigation. The first in its most complete form consists in carrying the liquid sewage to a piece of ground set apart for the purpose and carefully underdrained. The sewage is allowed to flow over the ground in shallow channels. The fluid slowly disappears in the soil and enters the drains as comparatively pure water, which may be allowed to flow into a stream. For villages this is the best means of disposing of sewage. Those who as village officials may be interested in this method will find plans of such sewage farms, together with faithful accounts of their operation and the results obtained, in the annual report of the State Board of Health of Massachusetts for 1892, page 559, and same report for 1893, page 563. Suggestions for its application to country houses are given farther on.
For isolated rural homes, or village homes commanding a certain amount of ground around the house, the liquid sewage from water-closets, the kitchen and chamber slops may be disposed of by the simple means of subsoil irrigation, first described by Mr. Moule and subsequently elaborated by Colonel Waring.
The system as used at present in its most successful form consists, outside of the house, of the following parts (see [fig. 6]):
Two adjoining water-tight receptacles of brick. One of these receives the sewage from the house and is intended to act as a settling chamber for the coarser particles, paper, etc. This communicates with the second receptacle, which receives from it the fluid sewage. This chamber is called the flush tank and is provided with a siphon. When the fluid has reached a certain level, the siphon is set in operation and discharges the contents of the chamber at one time into the subsoil pipes.
Fig. 7.—Subsurface irrigation of sewage: a, absorption tiles (Gerhard's "The Disposal of Household Wastes"); b and c, lines of absorption tiles showing their relation to flush tank (From Waring's "Sewerage and Land Drainage").
From the second cistern a system of subsoil pipes laid over a treeless piece of ground, preferably a lawn, receives and discharges the sewage into the ground. These pipes should consist of porous tiles, 2 inches in diameter and about 1 foot long, laid from 8 to 16 inches beneath the surface of the ground, and with a gentle inclination of 2 or 3 inches for every 100 feet. The tiles should have open joints not less than one-fourth of an inch wide. They are laid upon earthen gutters and the joints are protected above by caps from being clogged with earth. The intermittent discharge of the liquid sewage is quite essential to the successful working of this system. If the sewage is allowed to dribble away into the pipes certain portions of these will become supersaturated with fluid and others will not receive any; the purification of the sewage in the soil is thereby rendered imperfect. The discharge of a large quantity of fluid at one time, besides scouring the system of pipes, fills it more uniformly and distributes the work to all parts of the subsoil system.
The successful construction of such a plant requires the services of someone familiar with it, and it is therefore not necessary for me to do more than call attention to it here as a highly recommended system for homes, especially in villages, where the proper amount of land is procurable and where the sewage must be disposed of in a manner both inoffensive and safe. In any case the soil of such land must be porous, not clayey and retentive. Those who wish to familiarize themselves with the details will find descriptions in the Sanitary Engineer for 1883, page 530, by Philbrick; in "The Disposal of Household Wastes," by Gerhard, and in "Sewerage and Land Drainage," by Waring. The entire plant is said to cost $200 to $300, the annual expenditures for cleaning, repairs, etc., about $10.