The runs should be laid no deeper than necessary to give clearance when plowing and prevent injury from frost. Ten inches of earth above the top of the tile is sufficient generally throughout the southern half of the United States and 18 inches generally in the North, but if the field is exposed or lacks a thick heavy growth of grass the cover should be increased to 2½ or 3 feet near the Canadian line. What is better, the tile in all instances may be laid with a 10-inch cover and in cold weather the runs may be covered with hay, straw, or leaves weighted down, which may be removed in the spring.

1. A, Subsoil ground; B, 3 or 4 inch drain tile; C, strip of tarred paper about 6
inches wide and extending three-fourths the distance around the tile, allowing sewage
to escape at the bottom; D, coarse sand, gravel, broken stone or brick, slag, cinders, or
coke, the coarsest material placed around the tile (where the ground is naturally very
porous and well drained, special filling in the trench may be omitted); E, natural soil.

2. Drain tile covered with a board laid flat, leaving the entire joint open.

3. Drain tile laid in stoneware gutter pieces and the joint covered with stoneware
caps; gutter and cap pieces are inexpensive commercial products; their radius is longer
than that of the outside of the tile, thus leaving open most of the joint space; the gutter
aids in keeping the tile in line.

4. Vitrified sewer pipe with hubs facing downhill; the spigot end should be centered
in the hub with a few small chinks or wedges.

Making the joints of the distribution tile demands especial attention. For a short distance on the upper end of each run the tile should be laid with ends abutting; the joint opening should be increased gradually to one-eighth inch and this increased to one-fourth in the last 20 feet of the run. All joints should be protected against the entrance of loose dirt. Four methods are shown in figure 30. The lower end of each run should be closed with a brick or flat stone; or, what is better, an elbow or T-branch may be placed on the end and vented above the surface of the ground, improving the flow of sewage, the ventilation of pipes, and the aeration of the soil.

If the distribution tile must be laid in clay or other close, poorly drained soil, special treatment is necessary. A common method is to subsoil and underdrain the area thoroughly, as shown in figure 31. It is not always possible to run the underdrain in lines between the distribution lines as shown in figures 19 and 31, but it is a desirable thing to do, as the sewage must then receive some filtration through natural soil.

In some instances it is sufficient to lay the distribution tile on a continuous bed, 8 to 12 inches thick, of coarse gravel, broken stone, or brick, slag, coke, or cinders and complete the refill as shown in figure 18 or 31.

Figure 32 shows two other methods of controlling the flow on steep slopes and diverting proper proportions to the several lateral distributors laid along the contour of the field. This work can not be effected properly with T or Y branches; the flow tends to shoot straight ahead, comparatively little escaping laterally. To overcome this difficulty recourse is had to diverting boxes, of which two types are shown in figure 32. These boxes involve expense, but permit inspection and division of the flow according to the needs. They may be built of brick, stone, concrete, or even wood.

Type 1 consists of a single box, into which all the lateral distributors head. It will be noted that the laterals enter at slightly different elevations, the two opposite the inlet sewer being the highest, the next two slightly lower, and the next two the lowest. This staggering of the outlets, in a measure, offsets the tendency of the flow to shoot across and escape by the most direct route.

Type 2 calls for one or more diverting boxes, according to the number of lateral distributors, and readily permits of wasting sewage at widely separated elevations and distances. The outlet pipes enter the box at slightly different elevations, for the reason already stated. With either type, should the outlets not be set at the right elevations, partial plugging of the holes and a little experimenting will enable one to equalize or proportion the discharges.

Sewage switch.—The clogging of filters and soils after long-continued application of sewage has been previously referred to. It is, therefore, desirable to arrange the distribution system in two units with a switch between them, so that one area may drain and become aerated while the other is in use. This procedure is especially desirable where the soil is close and the installation of considerable size. It adds to the life and effectiveness of the distribution area and permits use of a plant in case it is necessary to repair, extend, or relay the tile in either unit.

Arrangement in two units does not necessarily mean doubling the amount of tile and the area required in a single field. However desirable that may be, expense or lack of suitable ground will often prevent. With open sands and gravels and the assumed siphon dose of 20 gallons per person, 15 to 20 feet of 4-inch tile in each unit for each person will usually suffice. With more compact soil it is advisable to more nearly double the requirements previously described. Two simple types of switch are shown in figure 33. The switch should be turned frequently, certainly as often as is necessary to prevent saturation or bogginess of either area.

A complete installation.—The general layout and working plans of a complete installation built in 1915-16 are shown in figure 34. The plant is larger than those heretofore considered, and involves several additional features. The settling chamber below the flow line has a capacity of 1,000 gallons, and on a basis of 40 gallons per person per day would serve 25 people.

For many years sewage had been discharged through two 4-inch sewers to a cesspool in the rear of the house. The proximity of the well made it unsafe, and the overflow of the cesspool dribbled over the low portion of the garden and barnyard, creating nuisance.

The first step was to make borings with a soil auger in the pasture 400 or 500 feet from the house. The borings showed a heavy clay soil to a depth of about 4 feet, underlaid with a sandy stratum only a few inches in thickness. It was decided to locate the distribution area in the pasture and to aid the seepage of sewage by digging numerous filter wells through the clay to the sandy stratum. Levels were taken and a contour plan prepared to serve for laying out the plant and establishing the grades.

The septic tank is built in one corner of the barnyard, and a 5-inch sewer connects it with the old 4-inch sewers to the cesspool. All sewer pipe joints were poured with a flexible jointing compound. The settling chamber is of hopper shape at the bottom, and a 4-inch sludge drain with gate provides for the gravity removal of sludge. The lower end of the sludge drain is above the surface of the ground and 9 feet below the flow line. The end is protected by a small retaining wall, and the sludge is readily caught in barrels and hauled out on the land for burial. The outlet is low enough to drain the settling chamber completely. If it is desired merely to force out the sludge, the drain may be brought to the surface under a head of 3 to 5 feet, discharging the sludge into a trench or drying bed, to be applied later to the land. A 2-inch waste pipe about mid-depth of the settling chamber permits drawing off the clearer portion of the sewage to the siphon chamber and from thence through another 2-inch waste pipe into the 6-inch sewer leading to the distribution field.

The 4-inch siphon has a drawing depth of 33 inches, and as the siphon chamber is 4 feet wide by 6 feet long the dose is about 500 gallons. The siphon cost $35. The 6-inch sewer to the switch box falls about 6 inches in 50 feet. The distribution field was thoroughly subsoiled, and about 800 feet of 3-inch tile was laid in each unit. At intervals of 25 feet along the distribution trenches 6-inch holes were dug through the clay stratum with a posthole digger. These holes were filled with stone and constitute the filter wells previously mentioned. All tile lines are surrounded with stone and coarse gravel, and the ground has been trimmed to give a uniform cover of 12 inches. All work was done by day labor in a thorough manner. As the men were doing other work at the same time the actual cost is not known, but it is believed the installation cost about $700.

Cost data.—Reliable cost figures are difficult to estimate. Labor, materials, freight, haulage, and other items vary greatly in different localities. The septic tank shown in figure 23 contains about 1,000 bricks and is estimated to cost $60 complete. The septic tank shown in figure 25 for 5 persons is estimated to cost $135; for 10 persons, $170; for 15 persons, $240; for 20 persons, $280. In Maryland, in 1916, the cost of installing a septic tank similar to that shown in figure 25 (for 5 people), including 86 feet of 5-inch house sewer (55 feet of cast-iron pipe passing a well, and 31 feet of vitrified pipe) and 214 feet of second-quality 4-inch sewer pipe in the distribution area, was as follows:

The quotations in the following table will be found useful in making estimates of cost:

Cost of pipe and drain tile.

(February, 1921.)

The cost of cast-iron fittings may be roughly estimated as follows: Bends, one to one and one-half times the price of straight pipe; T-branches, two times the price of straight pipe; reducers, average of the prices of straight pipe at each end. The cost of clay bends, T-branches, reducers, and increasers may be roughly estimated at four times the price of straight pipe.

Operation.—Attention must be given to every plant to insure success. Unusual or excessive foulness should be investigated. No chemicals should be used in a septic tank; garbage, rags, newspaper, and other solids not readily soluble in water should be kept out of sewers and tanks. The plant should be inspected often, noting particularly if the siphon is operating satisfactorily. If scum forms in the settling chamber it should be removed, and the sludge should be bailed or pumped out yearly. Frequently tanks are not cleaned out for three or four years, resulting in large quantities of solid matter going through to the distribution system and clogging it. Clogging may occur in the tile or in the adjacent soil. In either case the tile should be dug up, cleaned, and relaid. In some cases it has been found advantageous to relay the tile between the former lines. When sewage is applied to fairly porous land at the slow rate here recommended and the plant is well handled the tile lines should operate satisfactorily for many years. Liming heavy soils tends to loosen and keep them sweet.

Field data.—As a basis for outlining or designing a suitable installation the following data should be known:

A plan to scale or a sketch with dimensions showing property lines, buildings, wells, springs, and drainage outlets should be furnished. The direction of surface drainage should be indicated by arrows. The slope of the land (vertical fall in a stated horizontal distance) should be given or if possible a contour plan (showing lines of constant elevation) should be furnished.