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What breaks down sewage in a septic tank?

What is a septic tank sewage system?

Commonly used in rural areas without access to centralised sewer systems, a septic tank is essentially an underground chamber made from concrete, fibreglass, or plastic which breaks down waste matter by the process of decomposition and drainage.

Septic tank diagram

A septic system generally contains the following functional parts:

  1. A septic tank: this can be for all wastewater from a home, or for toilet discharge only A septic tank can also be a single tank or a multi-chamber septic system
  2. A disposal area: this is usually a sand mound or a trench, as you shouldn't discharge septic waste above ground
  3. A grease trap: common in the commercial food industry and used to stop oil and grease from going down the drain
  4. Drainage pipes
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How does a Septic Tank break down sewage?

A septic tank works by the simple process of biological decomposition and distribution drainage. Bacterial microorganisms break down the waste inside a septic tank which can then be safely redistributed or disposed of.

Septic Tanks and the sewage process

  1. All waste flows into the Septic tank. This can include all wastewater from a home, or toilet discharge only
  2. This waste fills most of the tank and aerobic bacteria begin breaking down the organic matter.
  3. Broken-down solids fall to the bottom of the tank creating a sludge layer of organic solids and the byproducts of bacterial decomposition.
  4. Some of the matter (ie. grease and fats) float to the top of the tank and form a scum layer.
  5. A filter prevents most solids from entering the outlet pipe.
  6. As new wastewater enters the tank, it displaces the matter that's already there. This water flows out of the septic tank and into a drain field.
  7. The drain field is made of perforated pipes buried in trenches filled with gravel.
  8. Holes in the drain septic field pipe slowly allow the watery waste to seep into surrounding gravel.
  9. Gravel around pipes allows water to flow into soil and oxygen to reach bacteria.
  10. Aerobic bacteria in gravel and soil complete the decomposition of the waste.
  11. Clean water seeps down into the groundwater and aquifer.

References

https://www.epa.gov/septic/how-septic-systems-work