Drainfield Size & Design
Percolation Rate/Drainfield Size
- An important soil characteristic, the percolation rate, measures how long it takes water to drop one inch in a saturated hole dug in soil.
- Fast: 1 inch in 3 minutes (sandy soil)
- Slow: 1 inch in 48 minutes (clay soil)
- If it takes less than 5 minutes for the water to drop 1 inch in a saturated hole, the effluent will move too rapidly to be treated properly, such as in sandy soil.
- If it takes more than 60 minutes for the water to drop one inch, the effluent will not move fast enough, and effluent might surface. This may occur in clay soil.
- The size of the drainfield is based on the number of bedrooms and soil characteristics, and is given as square feet. Nebraska Department of Environmental Quality (NDEQ) has determine the square feet of drainfield trench required. The table below is taken from NDEQ Title 124 - Rules and Regulations for the Design, Operation, and Maintenance of On-site Wastewater Treatment Systems.
- For example, the minimum required for a three bedroom house with a mid range percolation rate of 25 minutes per inch is 750 square feet.
Square Feet of Drain Field Trench Required for Single Family Dwelling
|Number of Bedrooms||1||2||3||4||5||6||7||8||9|
|Perc Rate in Minutes Per Inch||200 gpd||300 gpd||400 gpd||500 gpd||600 gpd||700 gpd||800 gpd||900 gpd||1000 gpd|
<5 Systems must be designed with a 12 inch loamy sand liner that would have a percolation rate of 15 to 20 minutes per inch and shall be designed at the 11-20 minute per inch level
>60 Systems must be designed by a professional engineer. Construction Permit Needed
017.02 The required square footage for establishments shall be determined by the following equation: The daily design flow divided by (Five divided by the square root of the percolation rate).
017.03 Absorption area for a bed shall be calculated by determining the required square footage for a trench and multiplying the area by the factor from Table 14.2.