Design Flow, Absorption Area, and Trench Geometry
Bedrooms to design flow, perc time to the application rate, the absorption area in square feet, the trench length and runs, the stone and cover, the tank size, and the setbacks most county rules write.
The county rule line: bedrooms set the design flow, the perc time sets the application rate in gallons per square foot a day, and the absorption area is flow divided by rate. Trenches run 12 to 36 in wide, level, with 6 in of stone over the pipe and 2 ft of cover.
How the math works
- Design flow: the county design-flow table runs 150 gpd for one bedroom, 200 for 2, 250 for 3, 350 for 4, 450 for 5, and 550 for 6, with fixture add-ons on a large home, a jetted tub, a softener regeneration, a disposal, and a guest wing. Design flow is the number the whole system is sized on, and a home with a 280 gpd flow has 280 gpd of water going to the soil.
- Application rate: the perc test time in minutes per inch maps to the absorption rate in the county table: 2.0 gpd per sq ft at 5 minutes and under, 1.5 at 5 to 10, 1.0 at 10 to 20, 0.8 at 20 to 30, 0.6 at 30 to 40, 0.5 at 41 to 50, and nothing accepted over 50 minutes on a gravity bed. The rate is a soil property, the perc test is run in the soil condition the permit is filed in, and the rate drives the area.
- Absorption area: design flow divided by application rate is the absorption area in square feet, and the area is the trench bottom. A 3 bedroom, 2 bath home at 280 gpd on a 24 minute perc is 350 sq ft, which is 350 ft of 12 in trench, 234 ft of 18 in trench, 175 ft of 24 in trench, and 117 ft of 36 in trench. The same home at the 250 gpd table rate without the bath add-on is 313 sq ft, 313 ft of 12 in trench, and 209 ft of 18 in trench.
- Trench geometry: trench width 12 to 36 in, level runs within 1 in per 8 ft of run, 3 ft center spacing between the runs, 6 in of clean 3/4 in stone over the pipe, fabric over the stone and down the walls, 2 ft of cover, run length to the county limit (100 ft on 12 in, 50 ft on 24 in, 30 ft on 36 in in most tables), and a distribution box on a gravity bed with the runs level.
- Tank, dosing, and setbacks: 1,000 gal on 1 to 3 bedrooms, 1,500 on 4, 2,000 on 5 and 6 in the county table, which is about 3.5 days of storage on a 280 gpd design flow, a dosing siphon at 1 gal per 10 ft of trench, a check valve at the house on a pumped bed, and the setback line: 10 ft from a building, 5 ft from a property line, 10 ft from a pool, 20 ft from a tree, 50 ft from a potable well, 100 ft from a stream, and 2 ft of cover over the pipe.
How big does a septic drain field need to be?
Design flow divided by the application rate is the absorption area, and the area is trench footage. The default 3 bedrooms and 2 baths is 280 gpd of design flow, a 24 minute perc is 0.8 gpd per sq ft, so 350 sq ft of absorption area: 350 ft of 12 in trench, 234 ft of 18 in trench, and 117 ft of 36 in trench.
What does a perc test tell you?
The perc time in minutes per inch is how fast the soil takes water, and the county table turns it into the application rate in gallons per square foot a day: 5 minutes and under takes 2.0 gpd per sq ft, 10 to 20 takes 1.0, 20 to 30 takes 0.8, 41 to 50 takes 0.5, and over 50 minutes is not a gravity bed. The test is run at the depth the bed goes in, in the condition the permit is filed in, and a perc test in a dry August soil on a high water table site is not the test the permit needs.
How deep does a septic trench need to be?
2 ft of cover over the pipe is the common line, with the trench dug to the pipe invert, 6 in of clean 3/4 in stone over the pipe, the fabric over the stone and down the walls, and the topsoil fill. A driveway and a parking area over a bed go to a plate, a slab, and a shield, and a shallow rock site goes to a mound with 2 to 4 ft of fill.
How big a septic tank do I need?
The county table is 1,000 gal for 1 to 3 bedrooms, 1,500 gal for 4, and 2,000 gal for 5 and 6. On a 280 gpd design flow a 1,000 gal tank is about 3.5 days of storage, and pumping at the 25 to 33 percent sludge volume is about 12 to 24 months on a household flow.