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Source: Adapted from Manual of Septic-Tank Practice, U.S. PHS Pub. 526, DHEW, Cincinnati, OH, 1967.

Source: Adapted from Manual of Septic-Tank Practice, U.S. PHS Pub. 526, DHEW, Cincinnati, OH, 1967.

should not be scrubbed clean. The use of septic tank cleaning solvents or additives containing halogenated hydrocarbon, aromatic hydrocarbon, or hazardous chemicals can cause carryover of solids, and clogging of absorption field as well as contamination of groundwater and should not be permitted.

An individual should never enter a septic tank that has been emptied, regardless of whether it is open or covered. Cases of asphyxiation and death have been reported due to the lack of adequate oxygen or presence of toxic gases in the emptied tank. If it should become necessary to inspect or make repairs, the tank should first be checked with a gas detector20 for oxygen and toxic gases and thoroughly ventilated using a blower, which is kept operating.

Although soap, drain solvents, disinfectants, and similar materials used individually for household purposes are not harmful to septic tank operation unless used in large quantities, organic solvents and cleaners, pesticides, and compounds containing heavy metals could contaminate the groundwater and well-water supplies and should not be disposed of in a septic tank system. Also, sanitary napkins, absorbent pads, and tampons should not be disposed of in septic systems.

High weeds, brush, shrubbery, and trees should not be permitted to grow over an absorption system or sand filter system. It is better to crown the bed and seed the area with grass. If trees are near the sewage disposal system, difficulty with roots entering poorly joined sewer lines can be anticipated. About 2 to 3 pounds of copper sulfate crystals flushed down the toilet bowl once a year will destroy roots that the solution comes into contact with, but will not prevent new roots from entering. The application of the chemical should be done at a time, such as late in the evening, when the maximum contact time can be obtained before dilution. Copper sulfate will corrode chrome, iron, and brass, hence it should not be allowed to come into contact with these metals. Copper sulfate in the recommended dosage will not interfere with operation of the septic tank. A U.S. EPA registered herbicide, or a chemical foam, is also reported to be effective.21

Common causes of septic tank system failures are seasonal high groundwater; carryover of solids into the absorption field due to use of septic tank cleaning compounds, lack of routine cleaning of the septic tank, or outlet baffle disintegration or loss; excessive water use or hydraulic overloading; settlement of the septic tank, connecting pipe, or distribution box; and improper design and construction of the absorption system, including compaction and smearing of absorption trench bottom and sidewalls.

Corrective measures, once the cause is identified, include water conservation measures such as reduced water usage, low-flow toilets and showerheads, and reduced water pressure. Other measures to consider are cleaning of septic tank and flushing out distribution lines, and installation of additional leaching lines; installation of a separate absorption system and division box or gate for alternate use with the annual resting of existing system; lowering the water table with curtain drains; discontinuation of use of septic tank cleaning compounds; replacement of corroded or disintegrated baffles; replacement or releveling of distribution box; cleaning of septic tank at least every 3 years; and disconnection of roof, footing, and area drains.

Subsurface Soil Absorption Systems

The conventional system used for treating septic tank effluent is an absorption field or leaching pit. Where the soil is not suitable for subsurface disposal, a sand filter, evapotranspiration system, modified tile field system, aeration system, system in fill, mound system, stabilization pond, or some combination may be used (these systems are discussed in later sections). In all cases, it is important to avoid compaction of trench bottoms and soil of the absorption system or construction during wet weather.

Absorption Field System

Design standards and details for absorption fields are shown in Figures 2.1, 2.2, and 2.5 and in Table 3.8. Absorption field laterals should be laid in narrow trenches (18 to 24 inches), parallel to the contour and perpendicular to the groundwater flow, preferably not more than 24 inches below the ground surface, and spaced as shown in the Figure 3.5, to provide dispersion of the septic tank effluent over a larger area and promote aerobic conditions in the trenches. The highest seasonal groundwater level should be at least 2 feet, and preferably 4 feet, below the bottom of trenches. Where laterals must be laid at a greater depth, gravel fill around the laterals should extend at least to the topsoil (see Figure 3.5). After settlement and grading, the absorption field area should be seeded to grass.

When the total length of the laterals is between 500 and 1,000 linear feet, a siphon or pump should be installed between the septic tank and absorption system to distribute the sewage to all the laterals. If the lateral length is 1,000 to 3,000 linear feet, the system should be divided into two or four sections with alternating

TABLE 3.8 Suggested Minimum Standards—Subsurface Absorption Fields

Minimum Governing Distances

Item Material Size Grade To Building To Well or To Water or Property Suction Line Service Line

Line

Sewer to septic tank

Septic tank

Lines to distribution box and disposal system Distribution box

Absorption field

Cast iron for 10 ft from bldg. recommended.

Concrete or other app'd matl. Use a 1:2, 0.25:3 mix.

Cast iron, vit. clay, concrete, or composition pipe.

Concrete, clay tile, masonry, coated metal, etc.

Clay tile, vit. tile, concrete, composition pipe, laid in washed gravel or crushed stone, 0.75 in. to 2.5 in. size, min. 12 in. deep.

4 in. min. diameter recommended

Min. 750 gal 4 ft liquid depth, with min. 16 in. M.H. over inlet.

Usually 4 in. diameter on small jobs.

Min. 12 x 12 in. inside earned to the surface. Baffled.

4 in. dia., laid with open joint or perforated pipe. Depth of trench 24 ft to 30 in.

0.25 in. per ft max., 0.125 in. per ft min. Outlet 2 in. below inlet.

i in. per ft: but jg in. per ft with pump or siphon.

Outlets at same level.

in. per ft, but jï in. per ft with pump or siphon.

5 ft or more recommended

10 ft

10 ft

2ft if cast-iron pipe, otherwise 50 ft 50 ft

50 ft

10 ft

10 ft except when fill used, in which case 20 ft required

100ft

100ft

10 ft

10 ft

10 ft

10 ft (25 ft from any stream: 50 ft recommended)

Sand filter5

Leaching or seepage pit5

Chlorine contact-inspection tank

Clean sand, all passing 0.25 in. sieve with effective size of 0.30 to 0.60 mm and uniformity coefficient less than 3.5. Flood bed to settle sand.

Concrete block, clay tile, brick, fieldstone, precast.

Concrete, concrete block, brick, precast.

Send 2-lb sample to health dept. for analysis 15 days before construction.

Laterals laid on 10 ft slope in. per ft; but ^ in. per ft with pump or siphon.

50 ft

10 ft (25 ft from any stream; 50 ft recommended)

Round, square, or rectangle

2x4 in. and 2 in. liquid depth recommended

Line to pit 20 ft

Outlet 2 in. below 10 ft inlet.

150 ft plus in coarse gravel

50 ft

20 ft (50 ft from any stream)

10 ft

"Water service and sewer lines may be in same trench, if cast-iron sewer with lead-caulked joints is laid at all points 12in. below water service pipe; or sewer may be on dropped shelf at one side at least 12 in. below water service pipe, provided sound sewer pipe is laid below frost with tight and root-proof joints, which is not subject to settlement, superimposed loads, or vibration. Separate trenches are strongly recommended.

b Manual of Septic-Tank Practice, U.S. PHS Pub. 526 (1967), states that the leaching area should be increased by 20 percent where a garbage grinder is installed, and by 40 percent where a home laundry machine is also installed. It recommends that the gravel in the tile field extend at least 2 in. above pipe and 6 in. below the bottom of the pipe.

Note: A slope of per ft = 6.25" per 100" = 0.0052ft per ft = 0.52 percent.

Note: All parts of disposal and treatment system shall be located above groundwater and downgrade from sources of water supply. The architect, builder, contractor, and subcontractor shall establish and verify all grades and check construction. Laundry and kitchen wastes shall discharge to the septic tank with other sewage. Increase the volume of the septic tank by 50 percent if it is proposed to also install a garbage grinder. No softening unit wastes, roof or footing drainage, surface water or groundwater shall enter the sewerage system. Where local regulations are more restrictive, they govern, if consistent with county and state regulations.

FIGURE 3.5 Typical arrangement and details for absorption field disposal system.

feed to each section. When lateral length exceeds 3,000 linear feet, it is advisable to investigate a secondary treatment process, although larger absorption systems can operate satisfactorily if the site and soil permeability are suitable to disperse effluent and prevent groundwater mounding.

The bottom of the absorption lateral trenches should be practically level to prevent sewage from running out the end of a trench or onto the ground surface. Laterals for absorption fields of less than 500 feet in total length, without siphon or pump, should be laid on a slope of ^ inch/feet or 3 inches/50 feet. When siphons or pumps are used, the laterals should be laid on a slope of 3 inches/100 feet. Hydraulic loading rates should be kept between 0.25 and 0.5 gpd/ft2.

Leaching Pit

Leaching pits, also referred to as seepage pits, are used for the disposal of settled sewage from septic tanks where the soil is suitable and a public water supply is used or where private well-water supplies are at least 150 to 200 feet away and at a higher elevation. Leaching pits work like a vertical absorption field, although they lack the areal extent of such fields. Pits are usually 10 to 20 feet deep and 6 to 12 feet in diameter. The bottom of the pit should be at least 2 feet, and preferably 4 feet, above the highest groundwater level. If this cannot be ensured, lateral absorption fields should be used. In special instances, where public water supply is available, suitable soil is found at greater depths, and groundwater can be protected, pits can be dug 20 to 25 feet deep or more, using precast perforated wall sections.

As part of the pit design, soil percolation tests should be conducted at mid-depth, at changes in the soil profile, and at the bottom of the proposed leaching pit. According to the 1980 EPA Design Manual, the weighted average of these tests should be used to obtain a design figure. Soil strata whose test results exceed 30 minutes per inch should not, however, be used in calculating the effective absorption area. Hydraulic loading rates for leaching pits should generally be kept between 0.4 and 0.8 gpd/ft2, although the EPA Manual allows for up to 1.2 gpd/ ft2 depending on the results of the percolation tests and the soil type present. The effective leaching area provided by a pit is equal to the vertical wall area of the pit below the inlet. Credit is not usually given for the pit bottom. A leaching pit is usually round to prevent cave-in. If precast perforated units are not used, the wall below the inlet is drywall construction—that is, laid with open joints, without mortar. Fieldstones, cinder or stone concrete blocks, precast perforated wall sections, or special cesspool blocks are used for the wall construction. Concrete blocks are usually placed with the cell holes horizontal. Crushed stone or coarse gravel should be filled in between the outside of the leaching pit wall and the earth hole. Table 3.9 gives the appropriate sizes for circular leaching pits and Figure 3.6 provides design details.

Since leaching pits concentrate pollution in a small area, their use should generally be avoided where the groundwater is a drinking water source. For this reason, use of such pits is frequently discouraged by regulatory agencies in favor of more diffuse systems, such as absorption fields.

Cesspool

Before septic tanks were widely used, sewage from individual dwellings was frequently discharged to cesspools. Cesspools are covered, open-joint or perforated walled pits that receive raw sewage. Their use is not recommended where the groundwater serves as a source of water supply. Many health departments

TABLE 3.9 Sidewall Areas of Circular Seepage Pits (ft2)a

Seepage6

Pit Thickness of Effective Layers Below Inlet (ft)

Seepage6

Pit Thickness of Effective Layers Below Inlet (ft)

TABLE 3.9 Sidewall Areas of Circular Seepage Pits (ft2)a

Diameter (ft)

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