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What is a septic tank and how does it work? - Your Septic Tank Guide

What is a septic tank?

You’ve probably heard the name before, but what is a septic tank?

A septic tank is a water-tight underground chamber used to collect any wastewater produced by a property or collection of properties. Water from sinks, washing machines, toilets, showers etc can all be collected by a septic tank.

Water leaves a property and makes its was to the septic tank via drainage pipes. On the way to the septic tank, the wastewater will usually pass through inspection chambers and/or manholes.

Having manholes and inspection chambers are a vital part of a septic tank system as they allow for regular emptying and inspection, should they need any routine maintenance.

There are various different types of septic tanks that can be used, depending on the use case if each property.

Brick-built Septic Tanks

A brick-built septic tank usually uses two main chambers. Once the wastewater fills the first chamber, the second then fills with the surplus volume.

On the sides of the tank, there is an inlet pipe on one side, and outlet pipe on the other. These will be either T-pipes of dip pipes depending on the age of the tank.

Once the waste water has passed through both chambers, it it released out of the outlet pipe into either a soakaway or drainage field.

Brick-built septic tanks are generally the more traditional type of septic tank but are still just as effective as modern day septic tanks. The inner workings of the septic tank is explained below.

Glass Reinforced Plastic Septic Tanks

Also known as GRP septic tanks and sometimes onion septic tanks; these work in a similar way to the aforementioned brick-built septic tanks. Despite differences in design, the principles of how these work remain the same.

GRP tanks contain a fitting in the top of the septic tank called a buffer. This prevents the back flow of solids and fats back into the drainage pipe and instead acts as a sieve to hold on to the solids and allow just the water to flow out the pipes adjacent to the buffer.

a universal tankers member of staff emptying a septic tank in front of a vacuum tanker

How Does a Septic Tank Work?

Despite what the name may suggest, septic tanks do not actually provide any treatment to wastewater, but instead aid in the separations of solids and liquids. After wastewater has entered a septic tank, it is separated into one of three areas (these areas can also be known as layers or zones).

There is the top, middle and bottom layer.

Top layer

This is where the least dense matter settles. Waste such as fats, oil and grease (FOG waste) sit here as they have not yet broken down. The top layer is often referred to as the crust.

Middle layer

The middle layer is the least viscous section of a septic tank. When the waste flowing into the tank has been broken down and solids separated, what is left is the free-flowing waste that settles in this section of the tank. As a result, this section contains the wastewater without any remaining solids.

It is from the middle layer of the septic tank that the wastewater leaves and flows into the soakaway system or drainage field (more on this later).

Bottom layer

The bottom layer is where the most dense waste settles. Matter known as sludge sinks to the bottom of the tank over time. This is the matter (along with the crust) that needs to be removed regularly from the tank in order the for tank to keep functioning without issues.

Soakaway system

A soakaway is common drainage system used to disperse water from a septic tank into the surrounding area. A regular soakaway consists of of a hole dug into the ground filled with and combination of coarse stone, rubble and plastic crates. Having the hole filled with these materials makes a soakaway the perfect holding area for large volumes of liquid, giving adequate time for the liquid to disperse into the surrounding ground.

Soakaway systems are highly effective at allowing the flow of liquid out of a septic tank, whilst also ensuring the surrounding ground doesn’t get overly saturated with large volumes of water.

Drainage field

Similar to a soakaway, a drainage field serves a similar function for a septic tank. A drainage field allows water from the septic tank to drain via perforated pipework, into the ground surrounding the pipe.

This managed flow of liquid allows for consistent dispersion of water into the adjacent subsoils. This prevents the ground surrounding a septic tank from getting oversaturated whilst ensuring the septic tank drains a sufficient volume of liquid allowing it to function and work effectively.

Regular Emptying

If the sludge (and crust) is not removed from a septic tank, blockages may occur resulting in costly repair bills. Signs that a septic tank is full include...

Water pooling

When a septic tank is full, excess water can flow over into the nearby drainage areas.

Slow drainage

If your drains are flowing a lot slower than usual, this could be the sign of a full septic tank and potentially even of a blockage in the drainage system.

Excessive odours

As the dense sludge starts to fill a tank, the smell of the waste can start to escape the tank. Not only is this unpleasant, but a sign that the tank should be emptied soon.

Sewer backup

This is one of the most unpleasant ways to find out your septic tanks needs servicing. In addition to this, allowing a septic tank to get to this level can often result in a costly repair bill. When a septic tank is full and there is no space in the tank, waste no longer has anywhere to go, so will backup the drainage system and eventually back into the house.

When this happens, the drains nearest to the tank will be the first to show signs of backup. If sewer backup occurs, you need to get your septic tank emptied immediately.

a universal tankers member of staff emptying a septic tank

Why are septic tanks used?

Septic tanks are commonly used when there is no access to mains sewerage. This is often in rural locations where access to mains sewers is not even an option.

Septic tanks collect all liquid produced from a property or collection of properties. With there being no connection to a wider network of drainage, it is a necessity for septic tanks to be emptied on a regular basis.

How often do I need to empty my septic tank?

The intervals at which you need to empty a septic tank is entirely dependent on amount of usage and the size of the tank. As a general rule, domestic septic tanks need to be emptied annually. However as previously mentioned, this is entirely dependent on the size of the tank and the amount of waste going into it. Often, larger tanks have a regular empty every 3-5 years.

You need to ensure all up-to-date rules and regulations are followed in regards to your septic tank. Failure to comply with the correct regulations may result in an unlimited fine. We’ve compiled a handy guide outlining the rules and regulations which is available to download HERE.

Our Septic Tank Services

Here at Universal Tankers, our services include emptying of a wide range of septic tanks, whether that be domestic or commercial. We also offer a regular reminder service should you require this service to make sure you never get any of the signs of a full septic tank discussed previously. Get in touch with our team team for any enquiries.


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