Drainfield Size & Design

Drainfield Size & Design

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

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).

graphic calculation

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.

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