There are two types of private-sewage disposal systems in use today. One is the cesspool, the other is the septic tank. Of the two, the septic tank is by far superior, and in many localities it is the only system endorsed by law.
The cesspool is considered outdated today. Its principal advantage lies in the fact that it is a simple and inexpensive installation. The system consists of a large hole in the ground that is lined on the inside with rocks or concrete blocks laid without mortar. The sewage from the house flows into this tank and the liquids filter through openings in the rocks or blocks and are absorbed into the earth. The top of the cesspool is provided with a tightly fitting concrete lid to keep out insects and vermin.
The major drawback to this type of disposal system is that it can easily contaminate wells or nearby water supplies. The liquids that are absorbed by the ground are tainted, and slowly but surely the earth surrounding the cesspool eventually becomes contaminated.
A septic-tank sewage disposal system makes it possible for farm and suburban families to enjoy the comforts and conveniences of urban plumbing. A system properly de-signed, built, and operated provides safe, convenient, and inoffensive disposal of household and human wastes. It consists of a house drain, a house sewer, a septic tank, an outlet sewer, a distribution box, and a disposal field.
The house drain carries the sewage from plumbing fixtures to the sewer line outside the house. This is made of 4" cast iron soil pipe with leaded joints and extends about 5 feet beyond the foundation's wall. It has a clean out at the point where it connects with vertical pipe from the plumbing system.
The septic tank is connected to the house drain with a line of pipe called the house sewer. If home laundry equipment or a cellar drain is installed below the level of the house sewer, a separate dry well or sump pump will be needed.
The house sewer is usually built of 6" bell-and-spigot sewer pipe. All joints should be thoroughly filled with a mortar composed of 1 part portland cement, 3 parts mortar sand, and enough water to make a plastic mix. When properly made, such joints will normally keep out roots. Sometimes, as a further precaution against root penetration, a mortar band 1" thick and 3" wide is made around the joint.
Any mortar squeezed out on the inside of the pipe and left there might cause clogging. Therefore, after each section of pipe is laid, inside joints are cleaned smooth with a swab, as shown in Fig. 2. Cast iron sewer pipe should be used for the sewer line when it is less than 3 feet under a driveway, within 50 feet of a well or suction line, within 10 feet of any drinking-water supply line under pressure, or within 5 feet of basement walls.
The recommended slope for both house drain and house sewer is 1" in 4 feet toward the septic tank. When it is necessary to give the house sewer more slope than 1/4" per foot, about 10 feet of the line next to the tank should be levelled off to slope 1" in 10 feet. This is done to reduce the rate of flow of sewage as it enters the tank.
The septic tank is the chamber which receives the raw sewage. Here bacterial action breaks down the solids into gases, liquids, sludge, and scum. The gases escape through the house sewer, passing off into the air through the stack on the roof of the house. The sludge settles to the bottom of the tank, the scum forms at the surface, and the nearly clear liquids flow out into the disposal lines and seep into the ground.It is the general opinion of sanitation authorities that a septic tank should be made large enough to hold the average amount of sewage discharged into it during a 24 hour period. Under no conditions should the tank hold less than 500 gallons. Tanks which are too small soon become overloaded with sludge and raw sewage, which then pass into the disposal field, clogging the system. For the ordinary household, a grease trap has only two bedrooms, choose the 600-gallon tank. If, on the other hand, only four persons are served and the house has four bedrooms, choose a 750-gallon tank. Increase the recommended tank size by 25%, if your kitchen is equipped with a garbage disposal unit.
Required septic-tank capacities
When choosing a site for the septic tank, select ground that slopes away from the house. It is wise to place the tank as close to the house as is safe and practical, keeping in mind that it should be 50 feet away from the source of the water supply.
In constructing a septic tank, the excavation is made. The hole is dug to a depth which permits alignment with the house sewer. The length and width of the excavation are made 1 foot larger than inside tank dimensions to allow space for the 6"-thick concrete walls.
Except in loose soils, the walls of the excavation can serve as the outside form, if made straight and true. The corner assembly shown permits end and side panels to be removed without damage. This is desirable when the form lumber is to be re-used. Before the form is set in the hole, the faces should be mopped with oil to prevent the concrete from sticking.
The assembled form can be suspended in the excavation from 2”x4” crosspieces. Metal - covered boxes on the end panels provide notches into which inlet and outlet Ts are mortared. The form is held in the correct position by spacer blocks about 6" long between form faces and the walls of the excavation. These are removed as the concrete is placed.
The floor is covered first to a thickness of 6". Begin placing concrete in the walls while concrete in the floor is still soft to insure a watertight bond. Construction should be continuous to avoid seams or joints. Distribute the concrete around the walls in layers of uniform depth so the form will not be pushed out of position. The form can be easily shoved out of place if one section of wall is placed more rapidly than another.
As the concrete is deposited it should be tamped and spaded along the form faces to obtain a dense, watertight wall. Care should be taken to work the concrete around the boxes on the ends of the form. The top of the tank should be struck off carefully to provide a smooth, even support for the concrete cover slabs.
Formwork is removed after the concrete has hardened. If properly built and assembled, the form will come off readily without damage. After the form is removed and the inlet and outlet Ts are mortared into the notches provided, cover the concrete with burlap and wet it down. This aids in curing and increases the strength of the concrete.
The cover sections can be cast on any level surface. The slabs are made 3 1/2" thick, 12" wide, and long enough to reach across the tank. Each slab is reinforced with three 3/8" round bars spaced 3" apart, and placed about 1" above the bottom. Handles, consisting of rings or bent reinforcing bars, can be embedded in the tops of the slabs while the concrete is still soft. These facilitate handling the heavy slabs.
After the tank has been covered with the slabs, seal the joints between slabs with roofing cement. One or more of the slabs can be removed to clean out the tank; the joints should be carefully resealed after the slabs are replaced.
The septic tank is connected to the distribution box with a line of pipe called the outlet sewer. The line is usually built of 6" bell-and-spigot sewer pipe with well mortared joints. The precautions pertaining to bends, root-proofing, and watertight construction explained under House Sewer also apply to the outlet sewer. The best fall for the outlet sewer to the distribution box is 1" in 25 feet.
The distribution box is a small tank which distributes the liquids from the septic tank to the disposal lines. This box helps equalize the flow into the disposal lines and permits inspection of the sewage liquids.
The circular box, made from a section of concrete pipe, may be used when only three outlets and one inlet are required. Best operation results when the inlet opening is about 6" and the outlets are about 4" above the floor level. Since the distribution box should distribute the liquid evenly in the outlet lines, take care to get the outlets all on the same level. A precast concrete lid can usually be obtained from the manufacturer of the pipe.
The disposal field
The disposal field consists of two or more lines of drain tile laid with open joints in prepared trenches. The sewage liquid flows from the distribution box into the tile lines, where it seeps out through the open joints into the gravel fill. Here it is decomposed by bacteria, which complete the disposal process. These bacteria require air.
Thus it is advisable to provide in the trench a porous bed, such as gravel or crushed stone, in which the bacteria can do their work effectively. Some soils are sufficiently open and porous so that a special fill will not be needed around the tile lines. Sewage is not necessarily rendered harmless in the disposal field; therefore the field should be located a safe distance away from wells and water supply lines.
A disposal field must have adequate capacity to take care of the sewage. Consequently, the number and length of the lines and the width of the trenches are determined by the amount of flow. This gives the required length of trench per bedroom for various soil types and trench widths. The total length of trench required for the disposal field is found by multiplying the appropriate figure in the table by the number of bedrooms in the house. For example, a three-bedroom house with a disposal field in sandy loam and with 2-feet wide trenches will require 60 feet of trench per bedroom or a total of 180 feet of trench.
Trenches more than 100 feet long should not be used. At least two disposal lines should be installed, even though less than 100 feet of disposal line is required. Then, should one of the lines become temporarily waterlogged, the other line can still function. On fairly level ground the disposal lines may be laid to form a closed loop, but on a hillside the disposal lines are laid to follow the contour.
A sealed line about 6 or 8 feet long should connect each disposal tile line with the distribution box. This watertight connection helps keep the area around the distribution box from becoming waterlogged. The remainder of each disposal line consists of 4", farm drain tile laid with 1/4" to 1/2" joint openings and with a slope of 2" to 4" per 100 feet.
To obtain a sure, uniform slope, the tile is often laid on a grade board. Where feasible, the depth of the tile line below ground should average 18". Gravel, crushed stone, or cinders are placed around and over the tile line to provide a porous bed. A layer of untreated building paper or a 2" thickness of straw is placed over the porous material to prevent loose earth backfill from clogging the bed.
Sludge gradually accumulates on the bottom of the tank and scum collects on the surface of the liquids, reducing space for the liquids. Sludge and scum should not be permitted to accumulate to a combined depth of more than 18". The average septic tank requires cleaning once every two or three years. A check made at the distribution box every year will show whether more frequent cleaning is necessary. If sludge comes into the box, the tank needs attention. Cleaning can be done with pumps or long-handled scoops. Because sludge often carries disease germs, it should be buried.
The purpose of a seepage pit is to aid in the disposal of liquid wastes. It is a covered pit with open-jointed lining and usually supplements tile drains in getting rid of effluent matter from septic tanks or laundry wastes. The seepage pit should be placed at least 100 feet from the source of water supply, 10 feet from property lines, and 20 feet from any buildings.
Tightly jointed sewer pipe should be used in connecting it with tile drains. The seepage pit should be at least 3 feet in diameter and over 6 feet in depth. At least 1 foot of coarse gravel should line the bottom of the pit. Brick, stone, block, or similar material should be used to line the sides. This lining should be at least 4" thick and sealed above the inlet pipe. The upper section of the lining is usually drawn in to decrease the size of the cover. The cover should be sunk 1 to 2 feet below the ground and be made of concrete 4" thick.
The capacity of a seepage pit is determined by its size plus the ability of the soil to absorb water. If a seepage pit is connected to a septic tank, its capacity should equal that of the tank. Table 4 shows the approximate size of pit required by different-sized homes with varying types of soil.
These are used to dispose of non-sewage waste. They are covered pits with an open-jointed lining through which drainage from roofs, areaways, or basement floors can seep.
A large dry well resembles a seepage pit in size and construction. It does not, however, have to be curbed but can be filled entirely with coarse gravel or crushed stone. A small dry well to serve an individual drain can be constructed easily. It should be 50 feet from the source of water supply, 20 feet from any disposal field, and 10 feet from the building foundation. It is made by sinking a 3-foot length of 15" or 18" vitrified clay or cement pipe.
The pipe is then filled with either coarse gravel or crushed stone, and a line is brought in from the downspout. The top of the well should have either a concrete cover or one of wire netting over which topsoil is packed.
As was said above, the cesspool is a poor method of disposing of waste and is forbidden by law in many states. It consists of a covered pit into which raw waste is discharged . The liquid waste is taken off by seepage while the pit retains the sludge. Cesspools are particularly dangerous in areas where ground water is less than 20 feet from the surface and on properties where shallow wells are in use. They should be used only when construction of a septic tank is impossible.
The cesspool should be at least 3 feet in diameter and should be deep enough to reach porous earth. The sides of the cesspool should be lined with an open-joint curbing of brick, stone, or block, and sealed above the inlet pipe. Since solids tend to fill the cesspool quickly, it should be connected to a seepage pit or a drainage trench which can take care of liquid overflow. The overflow pipe should be located 6" to 12" lower than the inlet pipe.