Construction of the modified system is similar to a conventional system. Intermittent or alternating dosing (siphon, pump, tipping bucket) is usually required, particularly if the total length of distributors exceeds 500 feet. Design for a relatively tight soil still uses the conventional soil percolation test but carries it beyond the 1 inch/60 minute test to a point of constant rate. Moisture loss due to evaporation and transpiration is not credited but taken as a bonus.

The following example gives the modified system design for a "tight" soil site of fairly uniform composition.


Design a subsurface leaching system for a daily flow of 300 gal. The soil test shows 0.25in./hr and a permissible settled sewage application of 0.10 gpd/ft2.

300 2

Required leaching area = = 300 ft

If trenches 3 feet wide with 18-inches of gravel underneath lateral distributors are provided, each linear foot of trench can be expected to provide 5 ft2 of leaching area. The required trench = 3,000/5 = 600 linear feet, or 8 laterals, each 75-feet long, spaced 9 feet on center. Two gravel beds, 50 by 60 feet, can also provide the leaching area needed to compensate for the loss of the sidewall trench infiltration and dispersion area (see Figure 3.7). Use an alternating dosing device. This occupies the same land area as the absorption field. Evapotranspiration can be enhanced by incorporating sand trenches or funnels in the gravel between the distributors.

Capillary Seepage Trench

Another alternative to traditional absorption field design is the use of a capillary seepage trench. The capillary seepage trench is similar to a conventional seepage trench except that it has an impermeable liner at the bottom of the trench, which extends part way up the trench's sidewalls. As a result, sewage effluent collects

Required Length Absorption Trench
FIGURE 3.7 Absorption bed for a tight soil. Curtain drains may be needed to lower groundwater level and beds may need to be crowned to shed rainwater.

along the entire length of the trench and moves both upward and horizontally by capillary action before percolating downward. This modification in trench design results in a more even distribution and a longer time of contact. Fly ash is often used as trench fill material because it allows for more rapid capillary movement of the effluent and provides an increased surface area for microbial growth. Because a capillary seepage trench does not use the bottom of the trench as an absorption area, the trenches need to be longer than a conventional absorption trench.

Raised Bed Absorption-Evapotranspiration System

If clay, hardpan, groundwater, or rock is found within 4 feet of the ground surface, sewage disposal by a conventional absorption field is not recommended. Instead, a raised or build up area can be constructed using 12 to 18 inches of porous earth having a percolation rate of at least 1 inch in 120 minutes. A sandy, loamy, gravely soil is preferred and should be approximately 70 to 80 percent (by weight) medium to coarse sand (0.25 to 2.0mm E.S.); 10 to 20 percent silt, fine sand and clay (0.25 or less mm E.S.); and not more than 10 percent gravel (2.0 mm to 7.5 cm E.S.). Preliminary percolation tests of the undisturbed soil can give an indication of its suitability. A long, narrow absorption field with fewer and longer laterals (75 to 100 feet), perpendicular to the groundwater flow, will provide greater area for underground wastewater dispersion, minimize possible groundwater mounding, and make seepage out of the toe of the feathered fill less likely.

The fill should be spread in 6-inch layers using a lightweight crawler tractor to achieve a uniform soil density without channels or holes. The fill soil should not be spread when it is wet or compacted. Also, sufficient fill should be provided so that the bottom of the absorption trenches are at least 2 feet above the highest ground water level, rock, clay, or hardpan. After soil stabilization (at least 6 months minimum), percolation tests should be run at four to six locations. The resultant percolation rates should be between 1 inch in 8 minutes and 1 inch in 31 minutes to prevent premature clogging, ensure effluent retention in the fill, and obtain maximum purification of sewage effluent before it reaches groundwater or ground surface.

An example of a raised bed system that uses both absorption and evapotranspiration is presented in Figure 3.8. In light of the uncertainties associated with these systems (e.g., uneven soil settlement and unreliability of percolation tests in fill), a conservative design is considered prudent. A fill percolation rate of 1 inch in 31 minutes, or 0.45 gpd/ft2 (EPA, Table 3.4), which would correspond with a basal area application rate of 0.14 gpd/ft2, is recommended. The basal area is the absorption field area extending 2.5 feet beyond the outer edges and ends of the distribution trenches. The absorption system should be dosed two to three times per day using a pump or siphon. The dose should be about 60 percent of the volume of the distribution lines. Intermittent operation will permit full dosage of the distribution laterals and enhance dispersion of the wastewater over the entire absorption field.

Gross leaching area

Gross leaching area

Section A-A

FIGURE 3.8 Raised bed sewage disposal system. Design basis: 300 gpd and a transvap-percolation rate of 0.45 gpd/ft2 or trench bottom (total of 335 feet, 2 feet wide) or 0.14 gpd/ft2 of system gross leaching area (72 x 31 feet). Pump or siphon distribution is usually required with dosing two or three times per day.

Section A-A

FIGURE 3.8 Raised bed sewage disposal system. Design basis: 300 gpd and a transvap-percolation rate of 0.45 gpd/ft2 or trench bottom (total of 335 feet, 2 feet wide) or 0.14 gpd/ft2 of system gross leaching area (72 x 31 feet). Pump or siphon distribution is usually required with dosing two or three times per day.

FIGURE 3.8 (continued)

The entire surface of the raised bed should be covered with at least 6 inches of topsoil, graded to enhance rainfall runoff, and seeded to grass. A diversion ditch or berm should be installed upgrade to divert surface runoff around the system. Also, a curtain drain may be needed in areas of high groundwater if the bottom of the trenches cannot be kept at least 2 feet above groundwater. If clay or hardpan is intercepted, the curtain drain trench and collection pipe should extend at least 6 inches into the impervious formation.

Septic Tank Sand Filter System

Sand filters can be used when conventional subsurface absorption systems are unlikely to function satisfactorily because of soil conditions or rock, or where space is very limited and discharge to a surface water or ditch is permissible. Settled sewage is typically distributed over the top of a sand filter bed by means of perforated, open-joint pipe as shown in Figure 3.9. The sewage is then filtered and oxidized through 24 to 30 inches of sand, on which a film of aerobic and nitrifying

Footing drain

Total loss in elevation = 2H" + 2" + 1" + 2" + 35" + 2" = 44 V A to B

Collecting sewer to y watercourse, if approved. Design as future sanitary sewer

Number of

Liquid Capacity

Area of


of Septic Tank

Sand Filter

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook

Post a comment