How a Septic Tank Really Works

09 February 2017 by CSG

You may not have given much thought to the way your septic tank works – which is fine as long as it is working – but knowing just a little can help you ensure that it remains in good order for many years to come.

Okay, here’s the really basic information, which most people already know:

• Human waste contains harmful bacteria and can be a means of spreading viruses. Throughout human history – and in developing countries today – the source of some of the greatest threats to life has come from diseases, such as cholera and typhoid, which are transmitted via human waste.
• Most houses or buildings with waste facilities like toilets discharge their waste directly into the main system of sewerage drains allowing the immediate removal of sewage to a place where it can be treated.
• A relatively small proportion of properties are not sited closely enough to the network of drains and so have to discharge their waste in other ways. The most common alternative is to use a septic tank.
• The septic tank’s main purpose is to receive substances such as human waste and hold them such that most of the resultant matter can be allowed to soak away into the surrounding area in a state which is less hazardous to the local environment.

So far, so good but this tends to be where, for most people, the knowledge ends. As we do with so many areas of technology, it’s tempting to see it as a ‘magic box’ that just does what it’s designed to do. How then does it actually work?

The process requires little more than time and what we may call ‘natural processes’ in a sealed environment, which ensures that there is no contamination of the wrong matter.

Essentially, the waste will sort into three states. It just needs to be given enough time to allow it to happen, unhindered. The three states are:

• Solids – As they are denser, gravity dictates that they will settle at the bottom, where they will continue to decompose, which means break down further until they leave a dense sludge.
• Liquors – As the solids become denser, the liquid matter separates from it. The more solid separation that occurs, the more safely it can be returned to the surrounding area.
• Scum – The crust is made up mostly of floating fats, oils and grease (and food). This matter collects at the surface of the liquid and should not be discharged with the liquid.

The design of the tank is such that, having enabled the separation of the liquor from the sludge, it allows the liquid matter just beneath the surface (the ‘cleanest’ bit, without the scum) to percolate back into the soil around the tank, the ‘soakaway’ area. Here the cleansing process continues, as the soil itself naturally removes coliform bacteria, viruses and nutrients from the effluent or liquid waste.

For this reason, it’s necessary to see a septic tank as merely the first stage in a process and not the whole solution to the problem of waste processing. Equally, the availability of a suitable soakaway area is just as important as the tank itself.

As the whole process relies on natural decomposition and the power of the soil as a way to treat harmful substances, problems can occur if the waste it treats contains too many chemicals, biological agents or bleaches and with our temperate climate the anaerobic digestion rate is so slow that a septic tank functions much more as a sedimentation tank.

What happens to the three states of matter over time?

With an appropriate level of soakaway area, the liquids will continue to percolate into the soil and harmlessly back into the ecosystem. The chief threat to this may be after periods of extreme wet weather. If ground is already soaked with rainwater, it may lose the capacity to accept effluent, which may bring it to the surface or congest the system, leading to a ‘back-up’ of waste. This problem should never occur as long as the correct soakaway parameters were considered when the septic tank was first installed. Even so, it’s advisable to have a healthy suspicion about this threat whenever there is a sustained period of extremely wet weather.

The scum will remain trapped in the tank as the barrier pipe allows the dispersal of the liquid while stopping the scum or crust entering the soak-away.

Eventually, the level of sludge will build up and begin to compromise the ability of the septic tank to do its job. For optimum efficiency, we advise you to have your septic tank desludged regularly in accordance with variables such as how many people live in your household – as this may require you to have it serviced more frequently.
You might have wondered, at the beginning of this blog post, why on earth you’d ever need to know the inner workings of something that many people may feel is an area best left unexplored but there are many reasons why it’s a good idea that you give some thought to the humble septic tank that spends its life anonymously doing the worst of jobs, hidden away underground.

A little knowledge on the part of every septic tank owner should ensure that it continues to work perfectly – but as many unfortunate people may attest, it’s only when a septic tank stops doing its job as well as it should that it becomes truly appreciated!