Septic Metrics

How is a septic system designed?

Updated . Based on the sources listed below. Expert review pending.

Short answer

A septic system is designed around the site. A designer or health department evaluates the soil, slope and depth to water or rock, estimates daily flow from the number of bedrooms, then sizes the tank and soil treatment area. Local rules decide what's allowed, and a permit is normally needed before construction.

A site plan beside a soil test boring through two soil layers

Most homeowners assume septic design starts with the tank. It actually starts with what's under the yard, because the soil does much of the treatment.

What are the steps in septic system design?

  1. Check the rules. Your local health department explains what's allowed and what needs a permit.
  2. Evaluate the site. Someone assesses the soil, slope, flood risk and available space.
  3. Test the soil. Borings, and in some places percolation tests, show how water moves through it.
  4. Estimate daily flow. This is usually based on bedrooms. Indiana, for example, uses 150 gallons per bedroom per day.
  5. Choose the system type. A conventional system needs suitable soil. Other sites may need a mound or aerobic system.
  6. Size the tank and field. Our tank size and drain field tools show how published rules work.
  7. Get the permit and build. Where your rules require it, the work is then inspected.

Why does soil matter so much?

The University of Minnesota calls adequate site evaluation the key to successful design, and its guidance shows why. Coarse soil moves nutrients and bacteria poorly. Loam and clay loam remove them well, but need larger areas. Clay and seasonally saturated soils fail.

Depth matters too. The same guidance says saturated soil or an impervious layer within 3 feet of the surface rules out a standard subsurface system, although a mound may still be possible.

What is a perc test?

A percolation test measures how fast water drops in a soaked hole, usually in minutes per inch. It's useful, but the University of Minnesota stresses that it's only one part of a full site evaluation.

What else affects the design?

  • Slope. NC State Extension warns that gullies and steep slopes can make installation difficult.
  • Flooding and wetlands. Wet or flood-prone land can rule a site out.
  • Wells. NC State Extension says your well and your neighbors' wells should be at least 100 feet from the system. Rules vary, so check yours.
  • A repair area. It also advises keeping space in reserve for a future replacement field.
  • Bedrooms. A permit may cap the number of bedrooms, or affect where a house, driveway or pool can go.

Quick answers

Can I design my own septic system?

It depends on local rules. Many places require a licensed designer or official approval, so ask your local health department first.

What should I check before buying land?

NC State Extension suggests walking the lot, checking the soil survey and applying for a septic permit before you commit.

Rules vary by location

Design rules, setbacks and who's allowed to prepare a design all vary by state and county. Our state contacts finder can help you start.

Sources