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How Does a Septic Tank Work?

Independent Drainage – the south's septic tank experts.

Septic tanks are a popular off-mains drainage solution, because they offer partial sewage treatment with no moving parts or need for electricity. But how does a septic tank work? Let’s go into more detail and uncover the simple yet powerful natural process that makes wastewater safer.


As strange as it may seem, a septic tank works by using bacteria to clean the water. You might be thinking that bacteria are harmful – which is why we have sewers (or off-mains drainage installations) to take the dirty water they live in away. And yes, the kinds of bacteria found in sewage are terrible for our health. But they’re also really useful for breaking our waste down.

Here’s how.

Inside a septic tank, you’ll usually find two chambers. All the drains in your property – from toilet flushes, washing machines and sinks – go into chamber one. This is where solid waste (toilet paper and all) slowly sinks to the bottom.

Now, the Bacteria can get to work. They start feeding on the waste and multiplying, in a process called anaerobic digestion. Anaerobic digestion means that they convert their food into energy without oxygen (unlike us humans and other animals, who use oxygen to turn food into energy).

This anaerobic process significantly breaks down the solid waste, turning it into a simple sludge of broken down material at the bottom of the first chamber. As the bacteria multiply and die off, they float to the top of the partly processed wastewater, forming a foamy scum.

As new material is flushed into the tank, the bacteria get more food to process – and the partially treated wastewater needs to move on to avoid being contaminated with fresh waste. At a certain water level, new material forces the part-treated water into the second chamber, where the process repeats and a secondary sludge, water and foam layering occurs.

This is the second to last stage of digestion. At this point, the bacteria are still there, but don’t get enough new food to multiply quick enough. This makes the water safe enough to enter a drainage field for one last process.


A drainage field installation is a network of pipes buried underground, with tiny perforations that let water trickle through.

After water leaves the septic tank, it goes the entire way through this network of pipes, slowly trickling out into a bed of stone. This slow, controlled release allows the remaining bacteria to have one last feed on the smallest particles of waste, without blooming out of control.

Finally, the processed water is absorbed into the surrounding soil, where it can evaporate and rejoin the water cycle.


Septic tanks are an excellent way to solve off-mains drainage challenges. If you want to learn more about septic tanks or need any kind of off-mains drainage support, talk to Independent Drainage. Call us now on 0800 0131301 or book a free consultation.

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