As a general thing, the most practicable method of final disposal of partially purified sewage, is obtained by discharging the contents of the discharge tank into an underground system of distributing pipes. Such a system is shown in Fig. B, the illustration showing a plan view of the system.

If the soil is light and porous, there is no difficulty in the use of this method of disposal, but it is not so satisfactory in its results when used in other soils.

Good judgment should be used in determining the method of providing for final disposal of sewage. In the case of a moist soil, which is unfit for filtering purposes, the system mentioned above may be employed to advantage; that is, by the use of filter beds placed underground, the final filtered product being pumped out onto the surface. The underground disposal system, irrespective of the means of discharging the contents of the discharge tank into it, consists of a connection from the discharge tank into a main distributing underground pipe, from which a number of branches are taken out, the object of the piping being to distribute the liquid as evenly over the area used for disposal purposes as possible. These branch lines of pipe should be of unglazed earthenware, laid with open joints, so that through them the liquids may escape. These pipes may be laid in any way to conform to the shape of the distributing area.

Laterals may be constructed of 2-in. pipe, and it is well to allow an opening of nearly a quarter of an inch at each joint.

If such a joint is unprotected, sand will find its way into the pipe and gradually choke it up. Therefore it is well to use a thimble or collar of some sort to cover each joint. This collar may be a short piece of earthen pipe of a larger size than the pipe to be protected. Generally the branch distributing lines should be laid from 3 1/2 to 4 ft. or more apart, in order that too large an amount of liquid may not be deposited over a given area. •

The pipes should be graded, for otherwise the liquid will escape in larger quantity through joints nearer the main, and those farthest from it will have comparatively little to do. If the soil is moist or of clay, the laterals should be run farther apart than in sandy soils. Experience shows that about one to one and a half feet of porous, loose-jointed tile is necessary to properly handle a gallon of liquid, according to the nature of the soil, and for heavy soils a greater length. Therefore, in providing underground disposal for a discharge tank holding 500 gallons of liquid, from 500 to 750 ft. of 2-in. pipe would be demanded for its underground disposal.

In grading the main distributing pipe, as well as the laterals, there is one point that should be guarded against. The pitch should be very gradual, as, if much pitch is given them, the liquid will quickly flow to the farthest ends of the main and laterals, and overburden such areas, while not giving other areas a sufficient share.

Common fittings should not be used in connecting the laterals with the main, as the branch in such fittings is from the middle, and this would not allow all the liquid in the main to pass into the laterals. Special fittings are made for this kind of work, in which the branch is dropped below the center of the main fitting, sufficiently to allow all liquid in the main to escape through the branch.

Unless these fittings are used on the main, the latter should be run with open joints, in order that at each discharge of liquid the entire volume may be able to escape into the soil. In order to give perfect results, the area covered, and the length of pipe used, should be sufficient to thoroughly dispose of one discharge of liquid before another is received.

This final purifying action of filtration is the result of the action of a class of bacteria which are of entirely different character to those which do such effective work in the purifying process that goes on in the septic tank. While the latter operate out of contact with light and air, the action of the bacteria in the filtration purifying process, depends entirely on the presence of air and light.

These bacteria exist in countless numbers in the air spaces which sand and other porous substances contain, their existence in such materials depending on the fact that air is easily admitted, upon which they depend. The better a filtering medium is for its purpose, the more porous it will be found to be. As air is admitted more easily to the soil near the surface, at these points bacteria will be found in the greatest number, and as greater depths are reached, the number of these bacteria rapidly decreases until their number is insufficient to accomplish satisfactory work.

Therefore, the nearer the surface the underground distributing pipes are run, the greater the efficiency of the system. If possible, these pipes should be laid about a foot from the surface. Owing to frost, however, they must generally be laid deeper.

If areas used for underground disposal are turfed over it will be found that the turf will afford considerable protection against frost. While the bacteria in the septic tank change the complex forms of sewage into simple chemical compounds, the action of the bacteria of the sand again changes the chemical nature of the liquid, the change being from nitrites into nitrates, and resulting in chemically pure water.

When first passed through primary or contact filter beds of broken stone and gravel, the liquid is broken up, and its particles exposed to the oxidizing action of the bacteria, and the action in the sand filter is similar, although more thorough.

The entire change from sewage in the most extreme condition of contamination into pure water, is made by these simple processes, there being no outlay for expensive apparatus of any kind, or any demand for outlay in running expenses. A plant constructed on these lines may be, and often is, used for the reduction of the entire sewage of villages and small towns, which could otherwise dispose of the public sewage only with great difficulty, and doubtless far less efficiently, and with much greater expense.

The same system, on a smaller scale, may be employed for a residence, and with safety, even in thickly populated districts, for all the apparatus may be located underground, and, as already explained, its nature is such that it is in no way a menace, such as the cesspool always is.

The whole plant for a small residence may usually be located in a back yard of ordinary size.

While in many locations the close proximity of the septic tank to the house is not objectionable, in the use of it in cities certain restrictions are advisable, for it is not certain that it will receive proper attention, that leakage from the different chambers may not occur, etc.

Therefore, except in the case of houses surrounded by a considerable extent of private grounds, it should not be used in thickly populated districts unless unavoidable. Its use in such places, however, is not often called for, owing to the presence of public sewers.