Chapter 11

[10]See footnote, p. 12. For more detailed information on form and concrete work the reader is referred to U. S. Department of Agriculture Farmers' Bulletin 1480-F, "Small Concrete Construction on the Farm."

Steel reinforcement.—To stiffen the cover slab and guard against cracking, a little steel should be embedded in the concrete about 1 inch above the inside top. For this purpose a strip of heavy stock fencing is convenient and inexpensive. The line wires should be not less than No. 10 gauge (about ⅛ inch) and the stay wires not less than No. 11 gauge. The reinforcement should be cut at manholes and fastened around manhole openings. If desired a standard wire-mesh reinforcement weighing about one-third of a pound per square foot may be used. Another alternative is to use 14-inch round rods, spacing the crosswise rods 6 inches apart and the lengthwise rods 12 inches apart. Poultry netting should not be used, because of its lightness.

Sewer from tank to distribution field.—The length of this sewer depends on the situation of the field and the fall to it. The size of the sewer depends on the fall that can be obtained and the size of siphon. The table inFigure 24shows the minimum fall at which 4-inch, 5-inch, and 6-inch sewers should be laid to take the discharge of the 3-inch and 4-inch siphons specified. The line and grade should be set in the same manner as for the house sewer (seefig. 18) and the construction should be as specified under that caption.

Distribution field.—The distribution field or area is a sewage filter, and its selection and the manner of preparing it largely determine the success of subsoil disposal of sewage. As a rule farm land is not the best filtering material. It is too fine grained and fertile. Its tendency is to hold water too long, to admit insufficient air, to clog when even small quantities of sewage are applied. Hence the distribution area should be of liberal size—on the average 500 square feet for each person served. It should be dry, porous, and well drained—qualities that characterize sandy, gravelly, and light loamsoils. It should be devoid of trees and shrubbery, thus giving sunlight and air free access. It should be located at least 300 feet downhill from a well or spring used for domestic water supply. Preferably it should slope gently, but sharp slopes are not prohibitive. Subsoiling the area is always desirable.

Clay and other compact, impervious soils require special treatment. Less sewage can be applied to them, and hence it is well to have the area larger than 500 square feet per person. Clay should be subsoiled as deep as possible with a subsoil plow. In some instances dynamite has been of service in opening up the ground to still greater depth. Drainage and aeration should be further promoted by laying tile underdrains, as outlined inFigure 17and shown in more detail inFigure 29.

After the construction work the distribution areas should be raked and seeded with thick-growing grass. Grass is a safe crop; its water requirement is high, and it affords considerable protection from frost. Suitable grasses are redtop, white clover, blue grass, and Bermuda grass. The area may be pastured or kept as grass land.

Distribution system.—Poor distribution of the sewage and failure to protect the joints of the distribution tile account for most of the failures. Each flush of the siphon should be so controlled that every part of the field will receive its due proportion. The distribution tile must be so laid that loose dirt will not fall or wash into the open joints.

Different methods of dividing the flush and laying out the distribution tile are shown in Figures27and30. Layouts 1, 2, and 3,Figure 27, are suitable for flat or gently sloping areas and are planned for the shallow siphon chambers tabulated onpage 29. Layout 4,Figure 27, is suitable for steep slopes. In all four layouts use is made of one or moreVbranches (notYbranches) to divide the flow equally among the several lines.Vbranches, sometimes called breeches, should be leveled with a carpenter's level crosswise the ends of the legs, thus insuring equal division of the flow.

The size and length of distribution tile and the spacing of the lines or runs admit of considerable variation in different soils. Water sinks rapidly in gravels and sands, and hence larger tile and shorter length are permissible than in close soils. Lateral movement is slow in all soils, but extends farther in gravels and sands than in close soils. In average soils the effect on vegetation 5 feet away from the line is practically nil.

From these considerations, with the siphon dose 20 gallons per person, it is usually a safe rule to provide 50 feet of 3-inch tile for each person served and to lay the lines 10 feet apart. Such provision gives a capacity within the bore of the tile lines about equal to the siphon dose, and as some sewage is wasted at each joint a reasonable factor of safety is provided. A spacing of 10 feet will, it is believed, permanently prevent the extension of lateral absorption from line to line, provided the area is fairly well drained. As between 3-inch and 4-inch tile the smaller size costs less and is better calculated to taper the dose to small proportions. Four-inch tile is less likely to get out of alignment or to become clogged; a length of 28 feet has the same capacity in the bore as 50 feet of 3-inch.

Fig. 27.—Methods of laying distribution system: Methods 1, 2, and 3 for flat or gently sloping land; method 4 for steep slopes (see alsoFigure 30);A, direction of slope;B, contour of field;C, sewer from tank, preferably size 5 inch, though 4 or 6 inch may be used, depending on the fall and the size of the siphon (see table,fig. 24);D,V-branch set to divide the flow exactly;E, reducer, to 4 inches;F, ⅛ bend, 4-inch;G, increaser, from 4 inches;H, increaser, 3 to 4 inches;I, reducer, 4 to 3 inches;J, distribution tile, 3-inch;K, distribution tile, 4-inch

Good-quality drain tile in 1-foot lengths or second-quality sewer pipe in 2-foot lengths may be used. The lines are generally laid in parallel runs, but may be varied according to the topography. Layouts 1, 2, and 3,Figure 27, for flat or gently sloping land, run with the slope; layout 4, for steep slopes, runs back and forth along the contour in a series of long flat sweeps and short steep curves. The grade of the runs and sweeps should be gentle, rarely more than 10 or 12 inches in 100 feet. In layouts 1, 2, and 3,Figure 27especially, it is desirable that the last 20 feet of each run should be laid level or given a slight upward slope, thus guarding against undue flow of sewage to the lowest ends of the system.

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 3 to 6 feet near the Canadian line. Where frost goes down 5 to 7 feet, it is better to lay the tile at moderate depth and cover the runs with hay, straw, or leaves weighted down, removing the covering in the spring.

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 inFigure 28. The lower end of each run should be closed with a brick or flat stone; or, what is better, an elbow orTbranch 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 inFigure 29. It is not always possible to run the underdrain in lines between the distribution lines as shown in Figures17and29, 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 Figure16or29.

Figure 30shows 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 withTorYbranches; 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 inFigure 30. 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.

Fig. 28.—Four methods of protecting open joints in distribution lines—an all-important work. Sketches show cross-section and longitudinal views; the depth from the surface of the ground to the top of the tile is about 10 inches

1.A, Subsoiled 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.

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.

Fig. 29.—-Close soils should be deeply subsoiled and underdrained. Porous, well-drained, air-filled soil is absolutely necessary.A, Subsoiled ground;B, 3 or 4 inch distribution tile;C, depth variable with the climate, 1¼ to 3½ feet;D, 4-inch underdrain;E, depth such as would prepare land for good crop production, generally 3½ to 4 feet;F, stone or other coarse material;G, gravel grading upward to coarse sand;H, loose soil

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.

Fig. 30.—Two systems of distribution on steep slopes—use of diverting box.A, Direction of slope;B, contour of field;C, 4, 5, or 6 inch sewer from tank;D, diverting box;E, 3-inch or 4-inch distribution tile

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 doseof 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 inFigure 31. The switch should be turned frequently, certainly as often as is necessary to prevent saturation or bogginess of either area.

Fig. 31.—Two simple types of sewage switch.A, Sewer from tank;B, switch box;C, cover;D, blade or stop board (in the left-hand box the direction of flow is controlled by placing the blade in alternate diagonal position; in the right-hand box the stop works in iron guides cast integral with a short piece of light-weight pipe set in the masonry; if desired the guides may be wood, fastened to the masonry with expansion bolts);E, sewer to distribution area;F(right-hand box), alternate position of outlets or additional outlets if required

A complete installation.—The general layout and working plans of a complete installation built in 1915-16 are shown inFigure 32. 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, cheating 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. Levelswere taken and a contour plan prepared to serve for laying out the plant and establishing the grades.

Fig. 32.—A complete installation for a large rural home. General layout on a contour plan and construction drawings. Note abandonment of old cesspool near the well and garden and removal of sewage to a lower and safer location in the pasture, where the treatment is subsurface distribution, aided by numerous filter wells about 4 feet deep filled with coarse gravel. Note that sludge is removed from the bottom of the settling chamber by opening the gate on the sludge drainClick on image to view larger size

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 groundand 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 cleared 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 inFigure 21contains about 1,000 bricks and is estimated to cost $60 complete. The septic tank shown inFigure 23for 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 inFigure 23(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 per foot of pipe and drain tile

(Approximate retail prices, Washington, D. C., February, 1928)

Extra heavy cast-iron soil pipe

The cost of cast-iron fittings may be roughly estimated as follows; Bends, 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:

1. State, town, and whether in or near an incorporated municipality.

2. Usual number of persons to be served.

3. Average daily consumption of water in gallons.

4. Kind and depth of well, depth to water surface.

5. Character of soil, whether sandy, gravelly, loamy, clay, or muck.

6. Condition of soil as to drainage.

7. Character of subsoil.

8. Character of underlying rock and, if known, its depth below the surface.

9. Depth to ground water at both house and field where sewage is to be distributed.

10. Minimum winter temperature and approximate depth to which frost goes.

11. Number and kind of buildings to be connected with the sewer.

12. Number and kind of plumbing fixtures in each building.

13. Whether plumbing fixtures are to be put in the basement.

14. Depth of basement floor below ground.

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.

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