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Septic tank

What is a septic tank?

Septic tanks are secure systems for storing and subsequently disposing of fecal waste. A simple decantation and sedimentation process eliminates any solids in the wastewater, thus preventing them from contaminating the soil or freshwater sources.

Septic tanks are very common in rural areas that aren’t on a municipal sewer network. Concrete or cement, fiberglass, plastic or high-density polyethylene, and steel can be used to build one. There are currently also prefabricated tanks that can be used as a temporary solution.

How does a septic tank work?

To create a septic tank, a small well is dug, and a tank is built inside it. The wastewater generated in a house, group of houses, public toilets, rural school, or any other building that’s far away from a sewer system can be drained into it. There are one-chamber and two-chamber septic tanks, and each type works differently.

Single-chamber septic tanks:

Gray wastewater (from kitchens, bathrooms, or laundry rooms) and black wastewater (from latrines or bathrooms) flow into the tank through a pipe. There, they separate and settle out, with content floating to the surface or sinking to the bottom, forming sludge and sediment. The sediments trapped in the tank are digested and reduced by anaerobic fermentation, which causes the formation of gases like carbon dioxide (CO2) and ammonium (NH4).

Two-chamber septic tank: 

During the decantation and sedimentation process in the first chamber, the separated effluents flow into a second chamber. These waters are not completely purified since they still contain bacterial germs; they have gone through a pre-treatment process that lets about 30% of carbon pollution be eliminated. This means that, under certain specific conditions, it is possible to evacuate the water from the second chamber to ceramic drains, cesspools, irrigation fields, sand beds, or mini purification stations that use bacterial filters. However, this is prohibited in many countries to prevent contamination, foul odors, and other complications. 

How is a septic tank’s operation optimized?

  • Periodically vacuuming out accumulated debris. Only half of the solids decompose and disappear. This is why it’s important to calculate the correct size for the septic tank, according to the number of people who use it, the amount of liquid processed daily, the water’s contamination level, and other factors. 
  • Situating it at a minimum depth of one meter to ensure adequate silt and foam separation. A 20- to 50-centimeter upper free zone must also be incorporated.
  • Preventing the entry of rainwater, as excess water can have a negative effect on the active bacteria (microorganisms) inside the tank.
  • Installing a ventilation point so that the gases produced by bacterial activity can escape. 

What are the main advantages and disadvantages of a septic tank?


  • It offers effective pre-treatment before the water purification phase. 
  • It is inexpensive. 
  • It doesn’t produce foul odors.
  • It prevents the appearance of pests, such as flies and mosquitoes.
  • It has a service life of 10 to 20 years.


  • It uses a large amount of water (at least thirty liters of water per person per day) to evacuate waste from toilets.
  • The water that comes out of the septic tank has not been treated, so it must be connected to another treatment system (an absorption field or even an infiltration trench in the ground).
  • Construction and maintenance require specialized labor.
  • If not properly treated, the waste from a tank may cause health problems.

What is the future of septic tanks?

Although more and more homes are connected to a sewage system, septic tanks are still a practical solution in countries where water is a limited natural resource or where installing sewage systems hasn’t been possible yet.

In recounting its experience fighting Ebola in Africa, the Action Against Hunger Foundation highlighted the importance of having wastewater treatment systems that are cheap and easy to set up to fight pandemics in the future. In the future, it appears that septic tanks will continue to be a viable option for improving wastewater treatment systems around the world. 

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