InterNACHI Septic key terms

A watertight buried vessel that receives household wastewater and separates it into a floating scum layer, a clarified middle liquid, and a settled sludge layer before discharging effluent to the absorption area.

The partially treated liquid that leaves the septic tank after solids have separated, still carrying dissolved and suspended pollutants that the drainfield soil must further treat.

The layer of fats, oils, grease, and light solids that floats at the top of tank contents and is retained by the outlet baffle to keep it out of the drainfield.

The dense solids that settle to the tank bottom and are digested slowly by anaerobic bacteria, accumulating until the tank requires pumping.

A fitting at the tank inlet that slows incoming flow and deflects it downward, reducing disturbance of the scum layer and improving solids separation.

A fitting on the tank outlet that draws effluent from the clarified middle zone and blocks floating scum and bottom sludge from escaping to the drainfield.

A small chamber that receives tank effluent and divides it among drainfield laterals, intended to spread flow evenly across the absorption area.

One of several perforated pipes in a conventional absorption area that distributes effluent along a trench or bed so the soil can infiltrate and treat it.

Washed gravel or crushed stone placed around and over perforated distribution pipe in a drainfield to store effluent and provide void space for flow and air exchange.

A permeable fabric laid over drainfield aggregate or chambers to keep backfill soil from migrating into the stone voids while allowing liquid and air to pass.

Controlled release of a measured volume of effluent to the drainfield, often by pump or siphon, so the soil rests between doses rather than receiving continuous trickle flow.

A compartment or separate tank that collects effluent and houses a pump and float controls to dose the drainfield or lift sewage to a higher elevation.

A switch in a pump or dosing chamber that triggers an audible or visual warning when liquid rises above normal operating level, signaling pump failure or high flow.

An alarm panel or device that warns occupants of abnormally high liquid level in a tank or chamber, allowing prompt action before sewage backs up into the dwelling.

The soil-based absorption and treatment area, also called the leach field, where effluent infiltrates through unsaturated soil and is renovated by physical, chemical, and biological processes.

The total soil area, including trench bottoms and sidewalls, that receives and infiltrates effluent from the distribution system in a subsurface sewage treatment system.

A field measurement of how quickly water drops in a test hole in saturated soil, used to estimate the soil's ability to accept effluent and size the absorption area.

The time in minutes for water level in a test hole to fall one inch, conventionally expressed in minutes per inch and used to derive the allowable application rate.

The relative proportions of sand, silt, and clay particles in soil, which govern pore size, permeability, and the suitability of a site for subsurface effluent disposal.

A vertical cross-section of soil horizons exposed in a test pit, examined to identify restricting layers and the depth of suitable unsaturated soil below the drainfield.

Patches of contrasting color in soil, typically gray with orange or brown, indicating periodic saturation and fluctuating water table that limits effluent treatment.

The highest elevation that groundwater reaches during the wettest season, which must remain a minimum distance below the drainfield to preserve unsaturated treatment soil.

Any layer such as bedrock, clay hardpan, or a water table that restricts downward water movement and therefore defines the usable depth of soil at a site.

The required horizontal separation distance between a septic component and a well, water body, property line, building foundation, or slope, set by regulation to protect health and stability.

The pre-construction assessment of soils, topography, water table, setbacks, and space that determines whether and where a septic system can be installed.

A biological mat of bacteria and organic matter that forms at the soil interface beneath a drainfield, treating effluent but also progressively reducing infiltration capacity.

The volume of wastewater applied per unit area of absorption surface per day, which must stay within the soil's capacity to avoid ponding and failure.

A narrow excavated channel filled with aggregate and containing distribution pipe, forming the basic repeating unit of a conventional subsurface absorption system.

A capped vertical pipe extending from a tank baffle or drainfield lateral to grade, allowing measurement of levels and observation of flow without excavation.

A clear tube and check-valve sampler used to core through tank contents and measure the thickness of the scum and sludge layers for pumping decisions.

Removal of accumulated septage from a tank by a licensed pumper, performed when solids occupy roughly a quarter to a third of tank liquid depth.

The mixture of liquid and solids removed from a septic tank during pumping, requiring disposal at an approved treatment or receiving facility.

A tank, vault, or pit with limited entry and exit and potential for oxygen deficiency or toxic gas, requiring testing and attendant procedures before entry.

A toxic, flammable gas with a rotten-egg odor produced by anaerobic decomposition in tanks; it deadens smell at higher concentrations and can be fatal in confined spaces.

Reversal of flow that sends sewage into the building through fixtures or floor drains, usually caused by a blockage, saturated drainfield, or full tank.

Wastewater appearing at ground surface above or near the absorption area, a clear indicator that the soil can no longer accept the applied hydraulic load.

Informing occupants about what may be flushed or drained, water conservation, protection of the drainfield, and the need for periodic inspection and pumping.

Reducing indoor water use through efficient fixtures, prompt leak repair, and spreading laundry loads, which lowers hydraulic loading and extends drainfield life.