What Is a Septic Tank and How Does It Work

A septic tank is a buried tank that takes all the waste water from a house that is not on the mains sewer. Inside it, the solids sink to the bottom as sludge, fats and grease float to the top as a crust, and the liquid in the middle flows out to a drainage field in the ground, where the soil finishes cleaning it. Bacteria living in the tank break down part of the sludge, but not all of it, so the tank has to be emptied by a tanker, usually once a year.

That makes a septic tank different from the two other systems you find on homes off the mains. A cesspool stores everything and lets nothing out, so it fills and needs emptying every few weeks. A sewage treatment plant does far more cleaning, using air and a motor or blower, so its water is clean enough to go to a stream. A septic tank sits in between, and the drainage field is as much a part of it as the tank. If yours is due a visit, our septic tank emptying page explains what happens on the day.

How a septic tank works

Most septic tanks have two chambers, though older ones can have one and some have three.

• The first chamber takes everything from the house. The heavy solids settle to the bottom, the lighter fats and scum rise to form a crust, and bacteria that live without air begin to break the solids down.
• The second chamber takes the clearer liquid from the middle of the first. Any fine solids still carried in settle out here.
• The outlet lets the settled liquid leave for the drainage field, through a pipe set below the crust and above the sludge so neither should escape.

Nothing in the tank moves except the water, and there is no power, motor or pump unless the field sits higher than the tank. That is a septic tank's great advantage, and it is why many of them run for decades.

The drainage field

The liquid that leaves a septic tank is not clean. It has had its solids taken out, but it still carries bacteria and dissolved waste. The drainage field finishes the job. It is a set of perforated pipes laid in gravel-filled trenches under the garden, which let the liquid seep slowly into the soil, where bacteria in the ground treat it before it reaches the groundwater.

Older properties often have a soakaway instead, a single pit filled with rubble. Those do far less treatment and are not acceptable for a new system.

A field only works where the soil drains at the right speed and sits well above the winter water table. Heavy clay drains too slowly and fissured chalk too fast, which is why so many properties on the Weald clay around Smarden and Biddenden were built with a cesspool instead. Replacing a failed drainage field explains what a field has to pass and what happens when one gives up.

Septic tank, cesspool or treatment plant

People use the words interchangeably, but the three systems are different, and which one you have decides what you must do.

• A cesspool, or cesspit, is a sealed tank with no outlet. It needs emptying every four to eight weeks, depending on the size of the tank and how many people live there.
• A septic tank settles the waste and lets the liquid out to a drainage field. It needs emptying about once a year.
• A sewage treatment plant cleans the waste with bacteria fed by air, so its water can go to a stream or ditch that flows all year. It needs power, servicing and emptying.

Cesspools vs septic tanks and what is the difference between a treatment plant and a septic tank compare them in more detail.

The rules for septic tanks in England

In England a septic tank that discharges to ground has to meet the General Binding Rules, set by the Environment Agency, or have an environmental permit. The rules cover how much it can discharge, what it must be built to, how it is looked after and what happens when you sell. A few points matter to every owner.

• A septic tank must never discharge to a ditch, stream or other watercourse. One that does has to be put right as soon as possible, by connecting to the sewer, adding a drainage field or replacing it with a treatment plant. Can a septic tank discharge to a ditch explains each fix.
• The sludge must be removed before the tank reaches its capacity, and the rules say that should be at least once a year or as often as the manufacturer advises. The company that takes it away must be a registered waste carrier.
• When you sell, you must tell the buyer in writing that the property has a septic tank, describe it and pass on its maintenance records.

The building regulations set the rules for a new tank. Approved Document H says a septic tank should be at least 7 metres from the habitable parts of a building and within 30 metres of where a tanker can park, and its example maintenance notice asks for emptying at least once every 12 months. Where the binding rules cannot be met, the Environment Agency's guidance on the permits you need for a septic tank explains the next step.

How to tell if your home has a septic tank

If you have no sewerage charge on your water bill, your property is probably not on the mains, and it has a septic tank, a cesspool or a treatment plant. To work out which, look for these.

• Covers in the garden, usually one or two square or round lids within 30 metres of where a vehicle can reach. A treatment plant often has a small housing on the lid, or a control panel nearby, and you may hear a hum.
• An outlet. A septic tank or plant has a pipe leaving it. A cesspool has none.
• How often it is emptied. Every few weeks points to a cesspool. Once a year points to a septic tank.
• The paperwork. The seller's notes, a waste transfer note from a past empty, or the deeds may name the system.

If you have just moved in and nobody can tell you, book an empty and an inspection. We can find the covers, tell you what the system is and trace where it discharges. If you are buying, a septic tank survey gives you all of that in writing before you exchange.

Looking after a septic tank

A septic tank depends on the bacteria inside it, so most of looking after it is about what goes in.

• Keep wipes, sanitary products, nappies, cotton buds and fat out of it. They do not break down and they block the outlet or the field.
• Go easy on bleach, disinfectant and antibacterial cleaners, which kill the bacteria.
• Spread out heavy water use, so the tank has time to settle between loads of washing.
• Keep cars and heavy machinery off the tank and the drainage field.
• Have it emptied once a year, and have it serviced at the same time so small faults are caught early.

Never lean into or climb down into an open septic tank. The gases inside can overcome a person very quickly, and the Health and Safety Executive treats a tank as a confined space. Our septic tank servicing covers everything that needs a look inside.

Signs something is wrong

• Slow drains, gurgling or toilets that are slow to flush
• A smell of sewage around the tank, the drains or the garden
• Wet, green or spongy ground over the drainage field
• Sewage backing up into the house
• Water standing in the tank above the outlet when it is lifted

Any of these is worth a call. A tank that is simply full is a quick job, and a blockage our blocked drains and tanks service can clear, but a field that has been flooded with sludge for months may need replacing. The most common causes of septic tank failure goes through what usually goes wrong and why.

The cost of owning one

When this guide was first written, in November 2025, emptying and servicing typically cost between £150 and £300 a visit, so ring for a current price. How much does a septic tank cost to install, empty and run covers installation, repairs and replacement as well.

Getting your septic tank looked after

A septic tank is a simple system that can run for decades, as long as the sludge is taken out before it reaches the drainage field and the field is protected from things that clog it. Know where your tank and field are, keep its records, and book an empty once a year. We look after septic tanks right across Kent and East Sussex.

KP Cesspool Services has over twenty years' experience of emptying, servicing and installing septic tanks, and you can read our reviews on Checkatrade. Call 01233 720725, email kpcesspoolservices@hotmail.com, or contact us to book an empty.