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Sanitation · Choose and design blackwater treatment · sanitation-7

Design a conventional septic tank and drainfield

A septic tank settles solids and floats grease, then clarified effluent flows to a drainfield where soil bacteria treat it. It is the most common onsite system where soil is suitable.

How

  1. Have the system designed to your authority's sizing rules from your design flow and soil results.
  2. Understand the parts: a watertight tank with inlet and outlet baffles, a distribution device, and trenches or beds of gravel or chambers in the soil.
  3. Chamber (gravelless) trenches are an option where gravel is scarce or groundwater is high, according to US EPA.
  4. Plan access: risers from the central manhole and inspection ports to the surface (Oregon State Extension recommends this), and a clear vehicle route for the pump truck.
  5. Keep roof drains, driveways and paths off the drainfield, and keep the reserve area clear.
  6. In England, new drainage fields should meet BS 6297:2007 and new small treatment plants BS EN 12566.

Safety: Septic tanks produce toxic and flammable gases. Never enter a tank, and keep flames away from openings.

Sources

  1. Decentralized Systems Technology Fact Sheet: Septic Tank Soil Absorption Systems (EPA 932-F-99-075). US Environmental Protection Agency, accessed 2026-11
  2. Types of Septic Systems. US Environmental Protection Agency, accessed 2026-11
  3. Septic Tank Maintenance (EC 1343). Oregon State University Extension Service, accessed 2026-11
  4. General binding rules: small sewage discharge to a surface water. UK Government (Environment Agency, England), accessed 2026-11
  5. What to Expect When Getting Your Septic Tank Pumped (SPES-574). Virginia Cooperative Extension, Virginia Tech, accessed 2026-11

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