WW-00583 Reviewed 2009
Roger E. Machmeier
Copyright © 2013 Regents of the University of Minnesota. All rights reserved.
Successful design of an on-site sewage treatment system depends on an dequate site evaluation. The percolation test is only a small part of a complete site evaluation. Refer to Locating On-Site Home Sewage Treatment Systems, FO-0797, for the complete site evaluation procedure. Image
Suitable soil is the key to providing adequate on-site sewage treatment. Soil that is too coarse will not do a good job of moving nutrients and bacteria. Loam or clay loam soil will do an excellent job of nutrient and bacteria removal but will require a relatively large soil treatment area. Seasonal saturation of a soil will cause sewage to back up if the trenches are constructed too deep.
Evaluate the soil texture (sand, loamy sand, sandy loam, loam, silt loam, clay loam, etc.) for every foot of depth or at least where a change in soil texture occurs (Figure 1). Record this data on a log sheet of the boring hole. If you encounter seasonally saturated soil or an impervious layer (rock or clay) at a depth of 3 feet or closer to the ground surface, the area is not suitable for a subsurface soil treatment unit. A sewage treatment mound, however, could be installed at such a location if other factors were suitable.
Seasonal saturation of soil is indicated by a gray color together with red streaks or splotches and is called mottling. The bottom of the drainfield trench should be located no closer than 3 feet from the mottled soil. Even though the hole may be dry when you make the soil boring, the soil will be saturated during wet conditions and during the operation of the sewage treatment system. The system will fail in clay soil or inadequately treat sewage in coarse soil.
The lawn area required for the soil treatment unit depends on the percolation rate of the soil (soil texture) and the amount of sewage discharged by the proposed or existing residence. Refer to your local sanitary code for required setbacks from buildings, property lines, water supply wells, etc. Take at least four soil borings in each soil texture in order to locate lawn area of adequate size.
The auger or posthole digger is likely to smear the soil along the sidewalls of the test hole. Therefore, the bottom 12 inches of the sidewalls and the bottom of the hole should be scratched or scarified with a sharp, pointed instrument such as a knife. Nails driven into a 1 x 2-inch board as shown in Figure 3 will do a good job of scarifying the hole to provide an open, natural soil into which water may percolate. Remove all loose soil material from the bottom of the test hole. Add 2 inches of one-fourth to three-fourths inch gravel to protect the bottom from scouring when water is added. The gravel can be contained in a nylon mesh bag as shown in Figure 4 in order to be removed after the test is performed and used for additional percolation tests.
Carefully fill the percolation test hole with clear water to a depth of at least 12 inches above the soil bottom of the test hole. Use a hose to prevent the water from washing down the sides of the hole or add the water directly into the percometer as shown in Figure 6. A 6-inch diameter hole requires about 1.5 gallons per foot of depth.
Sandy soil containing no clay does not swell. The percolation test may proceed immediately if the 12 inches of water seeps away in 10 minutes or less. The percolation test procedure for sandy soils is described under step 6C.
For prolonged soil soaking, keep the 12-inch depth of water in the hole for at least four hours, and preferably overnight. Add water as necessary. You may use an automatic siphon or valve to maintain the 12-inch water depth (Figure 5). A valve made from the carburetor of a small engine is shown in Figure 7.
Calculate the percolation rate for each reading (see sample field notes). When three consecutive percolation rates vary by no more than 10 percent, use the average value of these readings to determine the percolation rate for the test hole. Percolation rates determined for each test hole should be averaged in order to determine the design percolation rate. For reporting the percolation rate, worksheets showing all measurements and calculations should be submitted with the site evaluation report. You can reproduce the blank form on the back page of this folder for use in recording percolation test data.
Note that a percolation test should not be run where frost exists in the soil below the depth of the proposed sewage treatment system.
The daily amount of sewage wastes must be estimated in order to size the soil treatment unit. For residences, the daily amount of sewage flow is based on the number of bedrooms and the type of residence. A luxury, three-bedroom house likely will generate more sewage than a more modest house. Sewage flows for different types of houses can be estimated from Table 1. Using a large sewage flow provides a factor of safety in sizing the soil treatment unit. Also consider future house expansion.
|Table 1. Sewage flows and soil treatment areas|
| aType I: The total floor area of the
residence divided by the number of bedrooms is more than 800 sq. ft. per
bedroom, or more than two of the following water-use appliances are installedautomatic
washer, dishwasher, water softener, garbage disposal, jaccuzi or humidifier
Type II: The total floor area of the residence divided by the number of bedrooms is more than 500 sq. ft. per bedroom, and no more than two of the water-use appliances are installed.
Type III: The total floor area of residence divided by the number of bedrooms is less than 500 sq. ft. per bedroom, and no more than two of the water-use appliances are installed.
Type IV: No toilet wastes flow into sewage treatment system.
|Table 2. Soil treatment areas in square feet|
| a For trenches only, the bottom areas
may be reduced if more than 6 inches of rock is placed below the distribution
pipe; for 12 inches of rock below the distribution pipe the bottom areas
can be reduced by 20 percent; a 34 percent reduction for 18 inches; and
a 40 percent reduction for 24 inches.
b Soil is unsuitable for standard soil treatment units. Refer to information on mounds and alternative systems.
To illustrate how to use Table 1, determine the amount of trench bottom area required for a three-bedroom, type I dwelling. The soil percolation rate, as measured by the percolation data presented on page 7, is 22.3 MPI. From Table 1, a three-bedroom, type I dwelling is estimated to generate 450 gallons of sewage per day. The trench bottom area required for a percolation rate in the range of 16 to 30 is 1.67 square feet per gallon of waste per day.
Thus, the total required bottom area is 1.67 X 450 = 750 square feet for trenches with 6 inches of rock below the distribution pipe. If 12 inches of rock is used as recommended, the trench bottom area can be reduced by 20 percent (see footnote a, Table 2). The required trench bottom area is then 0.80 X 750 = 600 square feet.
The trench bottom area can be reduced by 34 percent for 18 inches of rock below the distribution pipe and by 40 percent for the maximum rock depth of 24 inches. As rock depth increases the required trench bottom area decreases, because more soil is exposed along the trench sidewall and a greater liquid depth increases the flow through the trench bottom.
The minimum trench width is 18 inches; the maximum width is 36 inches. Using 36-inch wide trenches in the above example, total trench length with 12 inches of rock below the distribution pipe is 200 lineal feet (600 ÷ 3). It is recommended that the 200 lineal feet should be divided into two or more trenches. The sewage effluent should be distributed between the trenches by means of drop boxes.
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