How Do Septic Tanks Work?

If your home already has a septic tank in place or you want to get one, appropriate maintenance for trouble free use can only be attained through understanding how it truly functions.

Read on to discover more about how septic tanks work, and comparisons to other underground water treatment alternatives.

What is a Septic Tank?

Septic tanks are large underground containers, designed to be used at properties where no connection to the mains sewer system is available, thus an “on-site” wastewater solution is necessary.

Its purpose is to store domestic sewage wastewater, separate various parts of the solids and break them down through naturally occurring bacteria.

Nowadays, septic tanks are constructed of either concrete, HDPE or GRP.

Septic tanks are usually installed in rural settings, due to properties in more remote locations being unable to have a direct connection to the mains sewage systems.

They tend to be used only where there is no suitable watercourse if nearby to allow for a sewage treatment plant to discharge the wastewater.

How Septic Tanks Work

Septic Tank Illustration

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When connected to an adjacent property, all wastewater flows directly into the septic tank.

Septic tanks typically consist of two compartments, which hold wastewater at different stages of decomposition.

In the first compartment, kitchen and bathroom wastewater flows directly from the connected property.

At this point, heavy solids sink to the bottom and settle at the tank’s base where bacteria breaks them down into sludge and gas.

Any light solids such as fat, grease, and oil however will float to the top instead, forming a separate layer of scum.

In the second compartment, the effluent wastewater will flow out into a soakaway.

The soakaway is essentially a network of underground pipes which directs the effluent away, and into the septic field.

Holes will also be present in the septic field pipes, allowing the effluent waste to distribute into the surrounding solid and gravel as it travels.

This process allows water to flow into the soil, encouraging oxygen to reach the bacteria to break it down.

At this stage, the aerobic bacteria takes care of decomposing the waste which is key to ensuring septic tanks function correctly.

This bacteria breaks down the waste, leaving the water safe enough to then percolate down into the soil.

Anaerobic bacteria thrives in this environment, which is what septic tanks are designed to do.

Like a settling pool, heavy solids sink to the base, whilst lighter solids separate and float to the top.

Finally, the integrated filter will prevent any solids from making their way into the outlet pipes.

Who Needs a Septic Tank?

If your property has no means to connect to the mains sewer lines, then a septic tank may be your best option for wastewater collection.

In most instances, a localized sewage treatment plant would be the first choice.

But if circumstances prevent this, then the only option is a septic tank.

Types of Septic Tank

Septic Tank Plumbing Connection

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Whilst all septic tanks process and breakdown sewage and wastewater in exactly the swimmer way, there are a few variants.

Most of the differences are based on size and the material the septic tank’s made from.

For instance, concrete septic tanks weigh a considerable amount.

So installation usually requires heavy lifting equipment to transport and position them into the ground.

GRP septic tanks are much lighter, so transportation and installation is far less troublesome.

Another consideration is tank shape, as well as size.

Many modern septic tanks are spherical or onion shaped.

The advantage with these types of tanks is they generally require less ground work and take up less space.

Although the hole required tends to be much deeper.

Low profile tanks on the other hand require less digging, but will take up more horizontal space.

So ultimately, the type, size and shape of septic tank you install will greatly depend on your personal circumstances and site requirements.

Sewage Treatment Plants vs Septic Tanks

In both cases, gravity is used to separate liquid and solid wastes in both septic tanks and sewage treatment plants.

But the key difference is septic tanks do not treat the effluent to the same degree as treatment plants.

Therefore, septic tank wastewater cannot be discharged into the local watercourse.

In comparison, a sewage treatment plant produces near-clean effluent waste.

This is due to a very important additional stage in the wastewater storage process.

As the solids become separated, the wastewater is subjected to increased levels of oxygen via an air pump.

This encourages increased growth in aerobic bacteria, which breaks down sewage to a greater degree.

This leaves a better quality effluent which can be discharged into a watercourse.

Cesspits vs Septic Tanks

A cesspit is essentially a storage tank for effluent.

Unlike septic tanks, they do not treat the effluent at all.

Therefore, they need regular emptying otherwise they can become full and start to overflow, which could be disastrous.

So essentially, a cesspit is the most basic of wastewater storage options, whilst a sewage treatment plant is the best, leaving septic tanks sitting in the middle.

Since a cesspit is literally a storage tank for wastewater and sewage, it must be regularly maintained, emptied and cleaned.

How Long Does a Septic Tank Last?

A properly maintained septic tank should last several decades without major problems.

Carefully scheduled pumping must be carried out from time to time, however it’s important to do this at the right time to prevent damage to the system.

Ground conditions and the material fabric of the tank will determine its lifespan.

As a general rule, steel structured septic tanks last between 15 to 20 years, whilst plastic or GRP septic tanks last much longer, around 30 years or more.

Concrete tanks are the most durable, which should last for at least 40 years or longer.

Do Septic Tanks Harm the Environment?

A septic tank should not cause any major harm to the environment if it’s correctly designed and installed, of a suitable size to suit the property and is regularly maintained.

If an unsuitable septic tank has been installed, or maintenance has been neglected, it could easily contaminate local surface and groundwater.

In such cases, thai could cause environmental damage and become a serious health hazard.

For this reason alone, a septic tank should only ever be designed and installed by a fully qualified drainage and pumping professional.

Do Septic Tanks Need Soakaways?

Plastic Soakaway

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All septic tanks require a soakaway or drainage field.

This is where the effluent wastewater is treated.

Unlike sewage treatment plants, septic tanks do not treat the effluent whilst inside the tank.

The main bacterial breakdown actually happens in the soakaway.

More importantly, since January 2020 drainage fields are the only permitted discharge option, so must be installed at the time of fitting the septic tank.

It is now against the law to allow discharge to flow directly into any watercourse.

So if installing a drainage field is not possible, then fitting a septic tank is simply not an option.

In such cases, a sewage treatment plant would be more suitable.

Otherwise, you’ll have to pump all your wastewater directly to the main sewage system.

Septic Tank Maintenance Responsibilities

As a property owner or tenant, it is your sole responsibility to ensure the septic tank on your premises is emptied, maintained and cleaned as necessary.

You must check the discharge quality is not harmful to the environment, and that safe and responsible disposal is carried out.

Regular pumping should be done to remove sludge and scum from the septic tank, to ensure continued usage.

If a tank fails, the environmental harm caused and cost of rectifying any damage could be substantial.

Signs of Failed Septic Tanks

If you suspect your septic tank has failed, there’s a few telltale signs to look out for including:

  • Water is draining much slower
  • Bad smells and surface sewage present
  • Puddles, pools or vegetation growth in the drain-field
  • Backing up in the tank or thel plumbing system
  • Air or gurgling sounds coming from pipework

If you suspect a failure has occurred, you must act fast!

Any delays could cause serious problems.

If solids flow into the soakaway, this could lead to substantial environmental damage, and deem the entire septic tank system to have failed.

When Should Septic Tanks Be Emptied?

Sewage Pumping Truck

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In most cases, septic tanks should be pumped around once per year maximum.

Pumping removes the build up of sludge and scum.

But by doing so, this slows down the bacterial process that breaks down deposits.

So your tank should only be emptied at the correct time.

It could be that your tank doesn’t need pumping very often, so it greatly depends on the amount of waste running through the system, as well as the tank size.

Seeking professional help is always advised to give you a clearer indication of how often you should empty your septic tank.

Generally, a septic tank should be emptied if you notice the floating scum level at the top is more than 3 inches deep.

If so, contact a local pump company to empty your septic tank.


We hope this article has given you a much clearer understanding of how septic tanks work.

Just remember, there are alternatives if you cannot install a septic tank on your property.

But if you do already have one, it’s imperative that you get it regularly pumped, cleaned and maintained to prolong its usable life.