Onsite Septic Systems
Summary
An onsite septic system treats a building's wastewater in a tank and then in soil, using a drainfield or an alternative treatment and dispersal system. Soil depth, percolation rate, groundwater, and setbacks decide the system type and the footprint, so soil testing and the health department's rules come before the building is sited.
Cols
Scope
- Use: where no public sewer is available; about one-third of new development in the US is served by septic or other decentralized systems
- Delivery: building sewer, septic tank, then a drainfield or alternative treatment and dispersal in suitable soil
- Interface: the building drain is covered in Sanitary and Vent Piping
Terminology
- Septic tank: buried tank where solids settle and scum floats
- Drainfield / soil absorption area: trenches or beds that distribute effluent to the soil
- Percolation (perc) test: test of how fast water soaks into soil, in minutes per inch
- Reserve area: land set aside for a replacement drainfield
- Mound system: sand mound above grade for shallow soil or high groundwater
- Aerobic treatment unit (ATU): small treatment plant that adds oxygen
- Design flow: daily wastewater flow assigned to a building, often per bedroom
Types
- Conventional: septic tank and a gravel or stone trench drainfield; relatively large footprint
- Chamber: gravelless drainfield; suits high groundwater, variable flow, or sites where gravel is scarce
- Drip distribution: effluent dispersed through drip lines in the top 6 to 12 in. of soil; needs a dose tank and power
- Aerobic treatment unit: small lots, poor soil, high water tables, or sensitive water bodies; ongoing maintenance
- Mound: shallow soil, high groundwater, or shallow bedrock; needs space and maintenance
- Recirculating sand filter: high water tables or sites near water; more expensive
- Evapotranspiration: arid climates with shallow soil; may fail with heavy rain or snow
- Constructed wetland: lined cell of gravel, sand, and plants
- Cluster or community: serves two or more dwellings, usually managed by a professional
Coordination
- Soil depth: one health department requires at least 3 ft of native suitable soil beneath a gravity drainfield, and allows a mound where only 1 to 2 ft exists
- Percolation testing: several straight-sided holes, at least 4 in. in diameter, dug to drainfield depth; no standard system is allowed in one state where the average rate is faster than 5 minutes per inch; an Iowa note says alternatives suit rates under 1 or over 60 minutes per inch
- Sizing: drainfield trench area per bedroom runs from about 165 sq ft at 5 minutes per inch to about 946 sq ft at 100 minutes per inch in one extension table
- Design flow: local codes assign about 120 to 150 gal per day per bedroom
- Setbacks examples: 10 ft from any part of the system to a building, foundation, or property line in one state; 15 ft to the property line, 100 ft from a well, and 20 ft from a foundation in one county; 50 ft from a private well for the tank and 100 ft for the field in an Iowa table
- Reserve area: keep room for a replacement drainfield and protect it from paving and vehicles
- Locates: call 811 before excavating
- Interaction with wells: locate the well and septic field together; see Private Water Wells
Relocation and disconnection
- Moving a system: a new system is designed, permitted, and installed at the new location; the old system is then abandoned
- Connecting to public sewer: where a sewer becomes available, the septic tank is taken out of service and the building sewer connects to the lateral; see Sanitary Sewer Connections
- Decommissioning: abandonment steps for a tank are set by the state or county health department; check requirements before demolition
- Building over a field: do not build on or drive over a drainfield or reserve area
- Existing systems: locate, inspect, and protect existing tank and field before site work; add the abandonment of any unused tank to the demolition scope
Manufacturing and cost
Main manufacturers and suppliers
Trade association: National Onsite Wastewater Recycling Association.
Sources
- U.S. EPA: types of septic systems, https://epa.gov/septic/types-septic-systems
- Virginia Tech Cooperative Extension: building a home with a septic system, https://www.pubs.ext.vt.edu/SPES/spes-580/spes-580.html
- West Virginia: individual sewage systems rule 64-47-6, https://www.law.cornell.edu/regulations/west-virginia/W-Va-Code-R-SS-64-47-6
- Tehama County, CA: required setbacks for septic systems, https://www.tehama.gov/wp-content/uploads/2026/05/Required-Setbacks-for-Septic-Systems-Table.pdf