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Soil Evaluation

Soil is an important part of an OSST system because it treats and disposes of the septic tank effluent. Soil for a drainage field must effectively filter effluent, and not allow inhibiting consolidation or swelling of the soil. For example, dense clay soils don’t absorb or filter the effluent properly and are considered an unacceptable soil for an OSST system with an absorption drainage field.

A soil evaluation is an assessment of subsurface soil conditions at a specific site that is conducted under the supervision of a professional engineer or professional geologist. Consideration is given to the influence of the following: topography; drainage ways; terraces; floodplain; percentage of land slope; location of property lines; location of easements; buried utilities; existing and proposed tile lines; existing, proposed, and abandoned water wells; amount of available area for the installation of the system; evidence of unstable ground; alteration (cutting, filling, compacting) of existing soil profile; and soil factors determined from a soil analysis, percolation tests, and soil survey maps if available.

The evaluation involves boring test holes to a depth of 6 to 8 ft (depending on local regulations) at the site to examine the soil profile. The soil profile is examined to determine if there is existence of a limiting soil or a bedrock layer that will hinder proper absorption and filtering of wastewater in the planned area of the absorption system.

A minimum depth of unsaturated soil beneath the planned soil absorption system is required to restore wastewater before it reaches a limiting layer. A limiting layer may be an impervious soil layer, bedrock, or a high water table. If a limiting layer is within 5 to 7 ft of the surface (depending on local regulations), the site is not suitable for an absorption drainage field system. If a limiting layer is located within 2 ft (0.6 m) of the surface, the site is not suitable for a mound system. Other conditions that restrict use of a drainage field system include a ground slope of greater than 30% (30 ft drop in 100 ft, 30 m in 100 m), adverse effects on adjoining property, and a proposed location in a 100-year floodplain.

Soils can vary significantly from site to site and even at a single building site. An accurate assessment of soils at the building site is needed to ensure that an OSST system won’t fail. Depth of the soil and how rapidly it will absorb water is used to determine the suitability of the disposal area for a septic system. This information, coupled with the estimated waste water produced by the building or buildings, is used to deter mine the size of the soil absorption system.

FGR.6 A trench for a percolation test.

FGR.7 A typical soil percolation test procedure. Procedure will vary by governmental jurisdiction. Check with health authority having jurisdiction at the building site.

Soil Percolation Test

A soil percolation test is a subsurface soil test at a depth of a pro posed absorption system to determine the water absorption capability of the soil. The test identifies the soil percolation rate, the rate that water seeps through saturated soil. (See fgr.6.) The percolation test is conducted by boring a series of test holes at the site under consideration and observing water seepage rates in holes placed throughout the intended area of the drainage field. Each hole is presoaked with water in an at tempt to saturate the soil. The water seepage rate is then measured in each hole and reported as the number of minutes it takes an inch (min/in) of water to soak into the soil.

Certain soils don’t absorb or filter the effluent properly and therefore are considered unacceptable soils. Gravels tend to pass water very quickly and won’t do an adequate job of treatment before the wastewater reaches the limiting zone.

Dense clay soils accept water too slowly and will have difficulty treating all the wastewater from a building, causing the system to become overloaded.

A soil percolation test is required for any system dependent on soil absorption for effluent disposal. Many local health departments use the soil percolation test to gather soils information before locating and sizing the drainage field system.

Typically, this test must be conducted under the supervision of a professional engineer or professional geologist.

Percolation testing procedures are established by a governmental health entity, commonly a municipal, county, or state office of health. fgr.7 is a legal description of a typical procedure for testing soil percolation at a building site. Test standards vary by local requirements. For specifics, the de signer should check with the governmental entity having jurisdiction at the building site.

BOD Test

The BOD test measures the amount of dissolved oxygen organisms are likely to need to degrade wastes in wastewater. This test is important for evaluating both how much treatment wastewater is likely to require and the potential effect that it can have on receiving waters.

To perform the test, wastewater samples are placed in BOD bottles and are diluted with specially prepared water containing dissolved oxygen. The dilution water is also seeded with bacteria when treated wastewater is being tested.

The amount of dissolved oxygen in the diluted sample is measured and the samples are then stored at a constant temperature of 68°F (20°C). Common incubation periods are 5, 7, or 20 days, with 5 days (or BOD5) the most common. At the end of the incubation period, the dissolved oxygen is measured again. The amount that was used (expressed in milligrams per liter) is an indication of wastewater strength.

Coliform Test

The coliform test determines whether wastewater has been adequately treated and whether water quality is suitable for drinking and recreation. Because they are very abundant in human wastes, coliform bacteria are much easier to locate and identify in wastewater than viruses and other pathogens that cause severe diseases. Therefore, coliform bacteria are used as indicator organisms for the presence of other, more serious pathogens. Some coliform are found in soil, so tests for fecal coliform are considered the most reliable.

However, tests for both total coliform and fecal coliform are commonly used.

There are two methods for determining the presence and density of coliform bacteria. The membrane filter (MF) technique provides a direct count of colonies trapped and then cultured. The multiple tube fermentation method provides an estimate of the most probable number (MPN) per 100 milliliters from the number of test tubes in which gas bubbles form after incubation.



Flow Rate Use gal/min L/min Load Adult or child - - 75 gal/day 284 L/day Infant - - 100 gal/day 379 L/day Clothes washer 5 19 30 to 50 gal/load 114 to 189 L/load Dish washer 2 8 7 to 15 gal/load 26 to 57 L/load Garbage disposal 3 11 4 to 6 gal/day 15 to 23 L/day Kitchen sink 3 11 2 to 4 gal/use 8 to 15 L/use Shower or bath tub 5 19 25 to 60 gal/use 95 to 227 L/use Toilet (per flush) 3 11 4 to 7 gal/use 15 to 26 L/use Lavatory 2 8 1 to 2 gal/use 4 to 8 L/use



Wastewater Flow Rate Per Day per Person, Unless Noted Otherwise Type of Establishment gal L Airports, per passenger 3 to 5 11 to 19 Assembly halls, per seat 28 Boarding schools 50 189 Boarding schools 100 378 Bowling alleys, per lane 75 284 Churches (large with kitchens), per sanctuary seat 5.7 22 Churches (small), per sanctuary seat 3 to 5 11 to 19 Country clubs, per locker 25 95 Dance halls 28 Day camps 25 95 Day workers at schools and offices 15 57 Drive-in theaters, per car space 519 Factories (per shift-exclusive of industrial wastes) 25 95 General hospitals 150 568 Hotels (with connecting baths) 50 189 Hotels (with private baths-2 persons per room) 100 378 Kitchen wastes from hotels, camps, boarding houses, and so on, serving three meals per day 10 38 Laundries (coin operated), per machine 400 1514 Luxury residences and estates 150 568 Marinas Flush toilets, per fixture per hour 36 136 Urinals, per fixture per hour 10 38 Wash basins, per fixture per hour 15 57 Showers, per fixture per hour 150 568 Motels with bath, toilet, and kitchen wastes, per bed space 60 227 Motels, per bed space 50 189 Nursing homes 75 284 Public institutions (other than hospitals) 100 378 Public picnic parks (bathhouse, showers, and flush toilets) 10 38 Public picnic parks (toilet wastes only) 5 19 Restaurants (toilet and kitchen wastes per unit of serving capacity 25 95 Rooming houses 40 151 Schools (toilet and lavatories only) 15 57 Schools (with above plus cafeteria and showers) 35 132 Schools (with above plus cafeteria) 25 95 Service stations First bay 1000 3780 Each additional bay 500 1892 Service stations, per vehicle served 10 38 Stores, per toilet room 400 1514 Subdivisions or individual homes 75 284 Swimming pools and beaches 10 38 Trailer parks or tourist camps (with built-in bath) 50 189 Trailer parks or tourist camps (with central bathhouse) 35 132 Work or construction camps 50 189




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