Part 3

Bituminous compounds make a slightly elastic joint. A joint in 4-inch pipe requires about 3/8 to 1/2 pound of compound and in 6-inch pipe about 1 to 1-1/2 pounds.

Sulfur-sand joints are hard and inelastic. The compound is made by mixing together equal volumes of ordinary powdered sulfur and very fine clean sand, preferably the finest quicksand, and then heating the mixture until the sulfur melts. A 4-inch joint takes about 3/4 pound and a 6-inch joint about 1-7/8 pounds of the mixture. Commercial sulfur-joint compounds also are available.

Soft pig lead or old scrap lead is suitable for lead joints on cast-iron pipe. About 3/4 pound per inch of pipe diameter is generally required for each joint. The lead is hot enough to pour when it begins to char the paddle used to skim off the impurities. When it cools it must be calked tightly to take up shrinkage. The calking should be uniform around the entire joint and should stop when the lead is tight. Heavy pounding or continued calking may crack the bell of the pipe.

It is easier to get good, joints when the pipe is in a vertical position. Therefore, two lengths of pipe are frequently joined and are then laid as a single unit in the trench. In using terra cotta pipe, this procedure may be followed only when the joint is made with a mastic compound. Cement-mortar joints cannot be used in such cases.

Before filling the trench, the sewer should be tested to detect possible leaks. Earth free from rubbish and large stones should then be tamped around and about 1 foot above the pipe.

THE SEPTIC TANK

Flow Through the Tank

Slow, undisturbed flow through the tank is necessary for the separation of solids and liquids and for bacterial action. Submerged inlets and outlets or baffle boards reduce disturbance. A submerged outlet prevents scum from passing out with the effluent.

The single-chamber tank without a siphon, shown in figure 4, is easy to build, inexpensive, and entirely satisfactory in most instances. In very tight soils or for large installations a siphon and sometimes two chambers are advisable.

Size

The tank should be large enough to retain the sewage at least 24 hours. The size should be determined by the largest number of persons that may live in the house, rather than by the number actually living there at the time the tank is built. The additional cost of a large tank over a small one is relatively little. If there is any question as to which of two sizes should be built, it is wise to choose the larger. The dimensions recommended in the table in figure 4 are based on an average production of 50 gallons of sewage per person per day.

Unusually large quantities of sewage call for a tank of large capacity. In village and suburban homes where there is less food preparation than on farms and where the number of persons is more or less fixed, slightly smaller sizes will serve. In no case should the capacity of the tank below the flow line be less than 500 gallons. A tank length of two to three times the width should be maintained, and it is advisable to provide a depth of at least 4 feet below the flow line.

Allow about 1 foot of "freeboard," or air space, above the flow line for the accumulation of gases. This space is generally vented through the soil stack of the house.

A siphon (fig. 5) with a dosing chamber is not considered necessary for a farm septic tank except for large installations (1,000 gallons or more), for those in tight soils, and where the disposal field is limited.

+---------------------------------------------------------------------------+ | CAPACITIES, DIMENSIONS, AND CONCRETE MATERIALS | | FOR SEPTIC TANKS SERVING INDIVIDUAL DWELLINGS | +---------+--------+--------------------------------+-----------------------+ |Maximum |Liquid |Recommended inside dimensions |Materials for concrete| |number of|capacity+-------+--------+-------+-------+ 1:2-1/2:4 mix | |persons |of tank |Width|Length|Liquid|Total +-------+-------+-------+ |served | in | | | depth| depth|Cement|Sand |Gravel| | |gallons| | | | | sacks| cubic | cubic | | | | | | | | | yards| yards| +---------+--------+-------+--------+-------+-------+-------+-------+-------+ |4 or less| 500 | 3'-0" | 6'-0" | 4'-0" | 5'-0" | 16 | 1-1/2 | 2-1/2| +---------+--------+-------+--------+-------+-------+-------+-------+-------+ |6 | 600 | 3'-0" | 7'-0" | 4'-0" | 5'-0" | 17 | 1-3/4 | 2-3/4| +---------+--------+-------+--------+-------+-------+-------+-------+-------+ |8 | 750 | 3'-6" | 7'-6" | 4'-0" | 5'-0" | 19 | 2 | 3 | +---------+--------+-------+--------+-------+-------+-------+-------+-------+ |10 | 900 | 3'-6" | 8'-6" | 4'-0" | 5'-0" | 21 | 2-1/4 | 3-1/4| +---------+--------+-------+--------+-------+-------+-------+-------+-------+ |12 | 1100 | 4'-0" | 5'-6" | 4'-6" | 5'-6" | 24 | 2-1/4 | 3-1/2| +---------+--------+-------+--------+-------+-------+-------+-------+-------+ |14 | 1200 | 4'-0" | 5'-0" | 4'-6" | 5'-6" | 25 | 2-1/2 | 3-3/4| +---------+--------+-------+--------+-------+-------+-------+-------+-------+ |16 | 1500 | 4'-6" | 10'-0" | 4'-6" | 5'-6" | 28 | 2-3/4 | 4-1/4| +---------+--------+-------+--------+-------+-------+-------+-------+-------+

Figure 4.--Single-chamber septic tank. Note alternate use of baffle boards where sanitary tees are omitted at inlet and outlet.

The siphon provides intermittent discharge of effluent, which allows time for the disposal area to rest and aerate between discharges. This is more important where the discharge is nearly continuous than in small installations.

The frequency and volume of the discharge into the tile field are controlled by the sizes of the siphon and the dosage chamber. The dealer should be informed of the size of the tank and the number of persons in the household, in order that he may furnish the proper unit. A 3- or 4-inch siphon will be adequate for almost any farmhouse installation.

Construction

Most septic tanks are built of concrete cast in place, since in this way there is a minimum possibility of cracks developing. Concrete blocks, however (not cinder blocks), stone, brick, or structural tile are sometimes used. Prefabricated commercial tanks of concrete and various other materials also are available.

+--------------------------------------------------------------+ | SIPHON | +--------------------------+-----------------------------------+ |Diameter of siphon |Clearance under bell | | A - 3" or 4" | E - 2" | +--------------------------+-----------------------------------+ |Diameter of bell |Distance across U-trap | | B - 10"or 12" | F - 10" or 12" | +--------------------------+-----------------------------------+ |Bottom of outlet |Bottom of outlet | | to discharge line | to bottom of U-trap | | C - 20-1/2" to 25-3/4"| G - 12" or 13" | +--------------------------+-----------------------------------+ |Drawing depth |Height above floor | | D - 13" to 17" | H - 7-1/4"to 11-3/4" | +--------------------------+-----------------------------------+ | DIMENSIONS OF DOSING CHAMBER | +-----------------+-------------------+------------+-----------+ | Number of |Depth below | | | | persons served | discharge line| Width | Length | +-----------------+-------------------+------------+-----------+ | 4 or less | 16-1/4" to 20-1/4"| 3'-0" | 6'-0" | +-----------------+-------------------+------------+-----------+ | 6 | " | 3'-0" | 7'-0" | +-----------------+-------------------+------------+-----------+ | 8 | " | 3'-6" | 7'-6" | +-----------------+-------------------+------------+-----------+ | 10 | " | 3'-6" | 8'-6" | +-----------------+-------------------+------------+-----------+ | 12 | " | 4'-0" | 8'-6" | +-----------------+-------------------+------------+-----------+ | 14 | " | 4'-0" | 9'-0" | +-----------------+-------------------+------------+-----------+ | 16 | " | 4'-6" | 10'-0" | +-----------------+-------------------+------------+-----------+

Depending upon depth C of siphon.

Same as single chamber tank fig. 4.

Figure 5.--Typical design for a concrete septic tank with a dosing chamber and a siphon.

Masonry units should be laid in full beds of 1:3 cement mortar and the walls and floor plastered with at least a 1/2-inch coat of 1:2 mortar. Cells of concrete blocks and tile must be filled with concrete. Masonry walls are generally 8 inches thick, and care must be taken to follow inside dimensions given for concrete tanks. Directions for laying structural tile, brick, and concrete blocks can be obtained from dealers or trade associations.

Commercial tanks are suitable if they embody the essential features given in this bulletin. Capacities should be as recommended in figure 4 for concrete tanks. Proper installation and periodic servicing also are essential. Tanks badly damaged in handling should not be used. Rapid corrosion of steel tanks will result if the asphalt coating is impaired. Minor defects in precast masonry tanks may often be overcome by plastering the interior with cement mortar.

BUILDING A CONCRETE TANK

For information on making and placing concrete, see Farmers' Bulletin 1772, Use of Concrete on the Farm.