Percolation Rate/Drainfield Size

Precolation Rate

• 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.

Drainfield Size

• 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

5-10	165	330	495	660	825	990	1155	1320	1485
11-20	210	420	630	840	1050	1260	1470	1680	1890
21-30	250	500	750	1000	1250	1500	1750	2000	2250
31-40	275	550	825	1100	1375	1650	1925	2200	2475
41-50	330	660	990	1320	1650	1980	2310	2640	2970
51-60	350	700	1050	1400	1750	2100	2450	2800	3150

>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.