How a Septic System Works

Soil-based Systems Send Effluent
into Drain Fields where Nature Takes Its Course

Septic systems are underground wastewater treatment structures, commonly used in rural areas that don’t have centralized sewer systems. They use a combination of proven technology and nature to treat wastewater from household plumbing produced by bathrooms, kitchen drains and laundry.

A typical septic system consists of a septic tank and a drain field, or soil absorption field. The septic tank digests organic matter and separates floatable matter (e.g., oils and grease) and solids from the wastewater. Soil-based systems discharge the liquid (known as effluent) from the septic tank into a series of perforated pipes buried in a leach field (drain field), leaching chambers, or other special units designed to slowly release the effluent into the soil or surface water.

Alternative systems use pumps or gravity to help septic tank effluent trickle through sand, organic matter (e.g., peat and sawdust), constructed wetlands, or other media to remove or neutralize pollutants like disease-causing pathogens, nitrogen, phosphorus, and other contaminants. Some alternative systems are designed to evaporate wastewater or disinfect it before it is discharged to the soil or surface waters.

The process

Specifically, this is how a typical septic system works:

  1. All water runs out of your house from one main drainage pipe into a septic tank.
  2. The septic tank is a buried, water-tight container, usually made of concrete, fiberglass, or polyethylene. Its job is to hold the wastewater long enough to allow solids to settle down to the bottom, forming sludge, while oil and grease float to the top as scum.
  3. Compartments and a T-shaped outlet prevent the sludge and scum from leaving the tank and traveling into the drain field area.
  4. The liquid wastewater (effluent) then exits the tank into the drain field.

How the drain field functions

The drain field is a shallow, covered excavation made in unsaturated soil. It typically has three main components: good soil, aggregate, and perforated pipe. Effluent flows from the septic tank into perforated pipes that have been laid in a bed of aggregate (crushed rock) and loam (good soil).

The perforated pipe distributes the effluent throughout this drain field, and as it trickles through the aggregate, it is absorbed into the soil. As the wastewater percolates into the soil, the process naturally removes harmful coliform bacteria, viruses and nutrients and returns the water to the water table.

Coliform bacteria are a group of bacteria that inhabit the intestines of humans and other warm-blooded animals. Coliform in water is an indicator of human fecal contamination. If the drain field is overloaded with too much liquid, it will flood, causing sewage to flow to the surface of the ground or to back up in toilets and sinks.