How sewage treatment works at a sewage works
When you flush a toilet or drain a sink on the mains, the used water runs into a public sewer and travels to a sewage treatment works run by your water company. There it passes through a fixed series of stages before it is clean enough to release. The stage names below are the ones Thames Water uses in its guide to the sewage treatment process.
- Screening. Large objects and grit are taken out first, so they do not damage the pumps and equipment further down the line.
- Primary treatment. The water sits in big settlement tanks. Solids sink to the bottom and are drawn off as sludge.
- Secondary treatment. Air is pumped through the water in aeration lanes, and bacteria feed on the harmful material and break it down.
- Final treatment. One last settlement tank lets the remaining particles drop out. Some works add sand filtering at this point.
- Sludge treatment. The sludge from earlier stages is broken down by anaerobic digestion, or dried and burned, to generate energy. The leftover sludge cake is used as fertiliser.
- Testing and release. The treated water is tested against Environment Agency standards before it goes back into a river.
On the mains, all of this is the water company’s job. You pay for it through the sewerage charge on your water bill, and you never see the sludge. Off the mains, the picture is very different, and the responsibility moves to you.
Sewage treatment at a home that isn’t on the mains
A home with no sewer connection has to treat its own waste water on the property. It does that with one of three systems, and each one leaves something behind that a tanker has to collect.
- Septic tank. An underground tank where the solids settle out as sludge. The liquid flows on to a drainage field, which gov.uk describes as a series of pipes with holes laid in trenches, and the soil finishes the job. Read more on our septic tanks page.
- Sewage treatment plant. A small plant that treats the waste water further than a septic tank, often using a pump or an air supply to help the bacteria work. It still produces sludge that needs removing.
- Cesspit. A sealed holding tank with no outflow at all. Nothing is treated; everything stays in until a tanker takes it away. See cesspit emptying for how that service runs.
Not sure which you have? Start with your water bill. If there is no sewerage charge on it, the water company is not taking your waste. Your deeds or the survey from when you bought the house may also mention private drainage. If the bill and the paperwork disagree, ask your water company to confirm before you book anything.
Whichever system it is, the basic job is the same. The solids build up, and they have to be pumped out before they cause a problem.
Septic tank or sewage treatment plant: what’s the difference?
People say “the tank” for all three systems, but they treat waste in different ways and they are allowed to send the liquid to different places. This is how they compare under the gov.uk general binding rules for discharges to the ground and to surface water.
| Septic tank | Treatment plant | Cesspit | |
|---|---|---|---|
| How it treats | Settles the solids as sludge | Treats the water further, often with a pump or air supply | Holds the waste; treats nothing |
| Where the liquid goes | A drainage field in the ground | The ground at up to 2 m³ a day, or a surface water at up to 5 m³ a day | Nowhere; it is tanked away |
| Emptying | Sludge at least once a year or per the manufacturer’s instructions | Same as a septic tank | The full tank, by tanker, as it fills |
The big practical difference is the outlet. Under the general binding rules a septic tank may not discharge straight into a stream, river or ditch. If yours does, gov.uk says you must fix it: connect to the public sewer, replace the tank with a small sewage treatment plant, or install a drainage field that meets the ground rules. You should do this within a reasonable timescale, usually 12 months. A treatment plant, by contrast, can discharge either to the ground or to a surface water within the limits above.
The rules for private sewage treatment in England
If your system meets the general binding rules, you do not need to apply for anything. These are the four points that matter in practice, each taken from the gov.uk guidance.
1. Stay within the limits
A discharge to the ground must be 2 m³ (2,000 litres) a day or less (gov.uk ground discharge rules), through a septic tank or small treatment plant and a drainage field. A discharge to a surface water must be 5 m³ (5,000 litres) a day or less (gov.uk surface water discharge rules), and only from a small treatment plant. Above those limits, or where you cannot meet the rules, you apply for a permit.
2. Have it desludged
The sludge must be removed before the tank reaches maximum capacity: “at least once a year” or in line with the manufacturer’s instructions. The company that takes it away must be a registered waste carrier. Source: gov.uk ground discharge rules.
3. Keep it in good order
The system should be maintained by a competent person. If it is not in good working order, you must have it repaired or replaced. Soakaways, wells and boreholes installed after December 2007 are not permitted as a way of discharging. Source: gov.uk ground discharge rules.
4. Tell the buyer when you sell
If you sell the property, you must tell the new owner in writing that a sewage discharge is in place. Source: gov.uk surface water rules.
This page covers England. Wales, Scotland and Northern Ireland have their own regulators: Natural Resources Wales, SEPA and DAERA.
Who empties a septic tank or treatment plant?
Gov.uk is direct about it: the company you use to dispose of your waste sludge “must be a registered waste carrier”. Before anyone puts a hose in your tank, run this two-minute check.
- Ask for the number. Ask the company for its waste carrier registration number.
- Look it up. Search the number on the gov.uk register of waste carriers and check the name matches and the registration is in date.
- Note it down. Write down the number and the date you checked, and ask for a waste transfer note when the sludge leaves.
Our register lists 275 companies, each checked against the Environment Agency register by registration number. 275 held an in-date registration when we last checked on 06 March 2026. When you ask for quotes on this page, your request goes to registered carriers near you, so the first check is already done. You can still look the number up yourself.
For plants specifically, see our page on emptying a treatment plant. For typical prices and what pushes them up, read the septic tank emptying cost guide. The fairest route to a good price is two or three quotes for the same job, which is exactly what the form above gets you.
Find a registered firm near you
Pick your region or county to see the registered carriers listed there, with their registration details and contact information.
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