What is the purpose of a septic tank? - Cotterill Civils

What is the purpose of a septic tank?

If you’ve always lived in a house that’s been connected to the mains sewage network, you’ve probably never heard of a septic tank.

However, septic tanks are vitally important for buildings where no public sewer is available.

Let’s take a closer look at what a septic tank is and how it works. 

What is a septic tank?

A septic tank is an onsite wastewater treatment system (OSWTS). It utilises natural processes and proven technology to treat wastewater from your bathroom, kitchen drains, and laundry appliances.

At most properties, wastewater drains into the local sewer – but this isn’t the case with septic tanks. Instead, it’s collected in an underground watertight container that’s made of plastic, fibreglass, or concrete.

Inside a septic tank, the compartments and T-shaped outlet prevent sludge and scum from leaving the tank and emanating into the drain field area.

How does a septic tank work?

A septic tank is typically connected with two pipes: an inlet pipe and an outlet pipe.

The inlet pipe is used to transport wastewater from the house to the septic tank. It’s then stored inside the tank, long enough for the solid and liquid waste to separate from each other, before exiting via the outlet pipe.

The outlet pipe removes the pre-processed wastewater from the septic tank, slowly dispersing it back into the soil and drain field.

Inside the tank, the wastewater separates into three layers - scum, wastewater, and sludge.

The top layer (scum) is usually oil, grease, and other fatty deposits. These will float above the wastewater and waste particles in the tank.

The bottom layer consists of particles that are much denser than the water and form a layer of sludge.

Bacteria will gradually eat away the sewage – reducing it to almost nothing.

The liquids in the tank will drain through a soakaway, and as this drains out, the effluent material remains in the tank.

The sludge at the bottom will need to be removed regularly, as part of general maintenance.

What are the pros and cons of a septic tank?

There are plenty of advantages, and disadvantages, to owning and using a septic tank. It’s important to know what these are so you can make an informed decision as to whether a septic tank is right for you.


  • Owning a septic tank means you don’t need to pay premiums to the local water board for a sewer connection, which ultimately saves you money.
  • With a septic tank and soakaway, no water is transferred directly into the local watercourse before it’s treated in the ground, allowing you to be environmentally friendly.
  • Using a septic tank not only creates less pollution, but also helps to reduce your carbon footprint, as the energy costs in processing mains sewage are extortionate.
  • When you run a septic tank, you’re more likely to start using your toilets and sinks more responsibly – such as not flushing harmful chlorine-based products, which can seriously damage septic tanks.


  • For a septic tank to function efficiently, without leaking sewage into your garden, it requires periodic maintenance and care.
  • Septic tanks can be sensitive. Flushing anything other than pee, poo, and toilet paper down your loo is likely to cause a blockage.
  • On average, a septic tank will need pumping every 1-2 years. However, if you maintain it properly (by topping up the bacteria every month and checking the sludge levels at least twice a year), you can decrease the number of pump-outs required.
  • It can get pretty smelly! After all, you’ll be dealing with raw sewage. 

How do you maintain a septic tank?

To prolong the lifespan of your septic tank, you need to make sure that it’s maintained correctly.

Most people assume that these systems need flushing and pumping out regularly – but this is rarely the case. For the most part, it depends on what you flush, when you flush it, and how much you flush.

One of the best ways to maintain your septic tank is to ensure that the living conditions inside the tank are perfect for bacteria. This is because the bacteria will break down the solid waste, causing it to sink to the bottom, allowing liquids to separate and drain through the outlet pipe with ease.

It’s also important to avoid disposing of things like baby wipes and other products that contain harmful chemicals down the toilet and sink, as these tend to cause blockages.

Arranging regular inspections for your septic tank is a sensible idea and will ensure that it operates at its best. During these assessments, it’s worth measuring the sludge level. Ideally, the tank should be no more than 30% sludge. Anything more than this, and you run the risk of harmful deposits flowing out of the tank – resulting in serious soakaway problems.

Ready to order a septic tank?

Here at Cotterill Civils, we are a leading supplier of septic tanks and other sewage solutions. With a variety of different types and sizes to choose from, we’re confident that you’ll find something to suit your sewage needs.

Be sure to browse the range in full. To reveal the options most suitable for your requirements, select your preferred brand and tank size. Once you’ve identified the perfect septic tank, add it to your basket and check out. It’s as simple as that.

Alternatively, if you have any questions regarding septic tanks, don’t hesitate to get in touch with the team on 0121 351 3230

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