Tutorial - How does a septic system work?

A Typical Septic System

A typical septic system includes a building sewer (vented through roof), septic tank, distribution box, and a soil absorption system.  Soil absoprtion systems are also referred to as drain fields or leach fields and may be configured as beds, trenches, pits, chamber systems, etc.

Figure 1 – Typical Septic System Components


Septic Tank

The septic tank physically separates solids and scum from the waste stream.  Heavier particles fall to the bottom of the tank.  Lighter particles (fats, oil & grease) float to the top of the tank. The septic tank outlet baffle allows flow to exit from the mid-level of the tank, causing a large portion of the solids to be captured in the bottom of the tank.  The outlet baffle also extends above the normal liquid level to capture and hold floatables in the top section of the tank. 


Distribution Box (D-box)

Effluent from the septic tank typically flows to a distribution box.  The distribution box (D-box) splits and distributes the flow in equal amounts to various parts of the leaching facility.

Note: some small systems may not include a distribution box; the effluent from the septic tank may flow directly to a single leach trench or single leach pit. 

Soil Absorption System (SAS)

Drain fields & other leaching facilities are types of soil absorption systems. The soil absorption system has two purposes:

  • It disperses the flow over a large area.  This allows the liquid to drain into the soil at a reasonable rate.
  • It provides further treatment of the wastewater by passing the flow through a naturally developed "bio-mat."  The biomat filters or breaks down pollutants and kills pathogens.


System Ventilation

The building's plumbing must be vented to prevent a build-up of dangerous (sometimes explosive) gases within the building plumbing system. Venting a septic system is required to remove noxious gases from septic system components, and to prolong the life of the septic system.  Roof vents commonly provide venting for both the plumbing and the septic system. Poor septic system venting is often quite noticeable at the septic tank outlet and the D-box where gases (like hydrogen sulfide) combine with moisture to create acids that degrade the concrete. 


See also Vents.

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