How does a septic tank work?

Invented in the 1860’s by John Mouras of France, the septic tank is a simple and effective means of allowing wastewater to be discharged into the ground. Septic tanks work by allowing solids to settle in the bottom of the tank and the liquid to drain out.

The first septic tanks were normally brick-built, consisting of 2 or 3 chambers. Multiple chambers make the separation of solids and liquids more effective, with separating out more liquid with each chamber the flow moves through. A crust often develops on top of the water, this happens because some solids are lighter than water and float to the surface, subsequently drying out.

Below is a simple diagram of a septic tank to give you a better idea:

Working diagram of a septic tank
















As you can see from the above diagram, baffles stop the floating solids from getting through, and out of the chamber.

How Do Modern Septic Tanks Work?

Modern septic tanks are often made from GRP (Glass Reinforced Plastic) or PE (polyethylene). Some only consist of one chamber and have removable easy to clean particle filters to help prevent suspended solids entering the drainage field.

Treatment Quality

The aim of the septic tank is really to remove solids before the water is allowed to discharge. Wastewater contains ammonia and pathogens so the water released is not suitable for discharging to a ditch or stream (ammonia is toxic to fish and plant life). The water from a septic tank must be discharged via a suitably sized drainage field.

It is sometimes said that upgrading to a sewage treatment plant is a better option for people to aid the lifespan of the soakaway and be kinder to the environment. Whilst there is merit in this, there are ways to help avoid a blocked drainage field. These are regular maintenance and good soakaway construction. If when installing a drainage field you are able to ventilate at the end of the drainage field, you will allow air flow through the pipes which will help with the growth of Aerobic (good) bacteria.

The drainage field is as important as the septic tank because further treatment occurs in the drainage field to clean the water before it gets to any underground water supply.

Here are some test results on the water prior to a septic tank, after the septic tank and then after a drainage field.

Parameter Raw waste Septic tank Effluent One foot below drainage field Three Foot below drainage filed
BOD5 (mg/L) 270-400 140-220 0 0
TSS (mg/L) 300-400 45-65 0 0
Ammonia (mg/l) 60-120 50-60 0 0
Phosphorus (mg/L) 10-40 10-30 Trace Trace
Viruses(PFU/ml) Unknown 1,000-1,000,000,000 0-1000 0
Faecal Coliform (MPU)/100ml 1,000,000-100,000,000 1000-1,000,000,000 0-100 0

As you can see, the septic tank offered little reduction in BOD5 or ammonia (some of the measure we use to determine water quality), but a large reduction of TSS (total suspended solids) most of the treatment actually occurred in the drainage field. From this, you can see the importance of the drainage field in a septic system.


Maintenance of a septic tank is simple, removing the solids once a year is a good rule of thumb for most domestic tanks and as long as you don’t put things down the toilets or drains that should not be put into a septic tank then emptying is all you should need to do. Failure to empty may result in a blocked soakaway or smells occurring.


What’s next?

If you’re considering buying a septic tank or getting your existing tank serviced, we have a number of services to make the process easier:

  • You can use our sizing guide to determine the right size septic tank for your needs.
  • If you’re looking for a tank to install, explore our septic tank range.
  • Ows Hall also offers servicing and emptying servivces – get in touch to find out how we can help.

Last Update: December 8, 2021  

January 22, 2018   9834    Septic Tanks  
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