Septic tanks, treatment plants and cesspools

Most homes in the UK send their waste water to a public sewer, and the water company deals with it from there. Many rural homes, and some on the edge of towns, do not. Their toilets, sinks and showers drain to a private system in the garden or a nearby field: a septic tank, a small sewage treatment plant, or a cesspool.

If you have one, it is yours to look after, and the rules about what it may release into the ground or a stream apply to you. This page explains the three main types in general terms, what goes wrong, and where to find the rules. It is not legal advice.

How to tell if you are on mains drainage

Signs that a property is not on a public sewer include:

• no sewerage charge on the water bill, or a reduced one;
• a large lid, or several, in the garden or a field, away from the house;
• a tank that is emptied by a tanker every so often;
• a note in the property's purchase documents or the seller's information.

If you are not sure, ask your water company whether the property is connected to its sewer. A drainage firm can also trace where the drains go.

The three main types

Septic tank. An underground tank, usually with two chambers. Solids settle to the bottom as sludge, grease floats as a crust, and bacteria break down some of the waste. The partly treated liquid flows out to a drainage field or soakaway, where it filters through the soil. Some older tanks discharge to a ditch or stream.

Small sewage treatment plant. A tank that actively treats sewage, usually with a powered pump or blower adding air so bacteria can work faster. The liquid it releases is cleaner than from a septic tank. It needs electricity, regular servicing and sometimes spare parts. Electrical faults are for a qualified electrician or the maker's engineer. We do not give wiring instructions.

Cesspool. A sealed tank that holds everything and releases nothing. It must be emptied by a tanker, often far more frequently than a septic tank, because nothing leaves it.

Who is responsible

GOV.UK's guide Septic tanks and sewage treatment plants: what you need to do says you are responsible for (the "operator" of) a septic tank or treatment plant if you own the property that uses it, if your property shares it with others (jointly responsible), or if you have an agreement with the owner that makes you responsible, for example in a tenancy agreement. It also says there are different rules if you have a cesspool, and different rules in Scotland, Wales and Northern Ireland.

That page applies to England. If you live elsewhere in the UK, look for the guidance from your own environmental regulator or government rather than relying on the English rules.

The discharge rules

A septic tank or treatment plant releases liquid into the ground or into surface water. That release is regulated. In England, the Environment Agency publishes "general binding rules" for small sewage discharges. Its guide General binding rules: small sewage discharge to the ground explains which rules apply depending on when the discharge started, and what to do if you cannot meet them. There is a separate guide for discharges to surface water. The guidance also says you need the other permissions that apply, such as building regulations approval and, where necessary, planning permission.

We do not summarise which rules apply to your system or whether you need a permit. That depends on details such as where the liquid goes, when the system was installed or changed, and its size. Read the guidance for your nation, and ask the regulator or a drainage specialist if you are unsure. If you are buying or selling a property with a private system, raise it with your conveyancer early.

Emptying and servicing

Septic tanks and cesspools need emptying by a licensed waste carrier with a tanker. How often depends on the size of the tank, how many people live there and what goes into it. The maker of a treatment plant will give a service schedule. Keep records of every empty and service. They help if you sell, and they help an engineer work out what has gone wrong.

Signs a system needs attention include slow drains throughout the house, gurgling, sewage smells in the garden, very green or soggy ground over the drainage field, and toilets backing up. Drain smells and blocked toilet cover the household end; if several fittings are slow at once, the problem is likely further down.

What not to put down the drain

A septic tank relies on bacteria. Things that harm them, or that they cannot break down, cause trouble:

• wet wipes, even those labelled flushable, sanitary products, nappies and cotton buds;
• cooking fat, oil and grease;
• large amounts of bleach, disinfectant or drain chemicals;
• medicines, paint and solvents;
• food waste from a waste disposal unit, unless the system's maker allows it.

Spread laundry loads through the week rather than doing many in one day, and fix running toilets and dripping taps, because extra water can flush solids out before they settle.

Lids, chambers and safety

Septic tanks and cesspools can contain harmful gases and deep liquid. Do not enter a tank or a chamber, and keep lids secure, especially where children or pets play. Inspection chambers and manholes explains what is reasonable to lift and when to stop. For drains between the house and the tank, see outside drains. Repeated blockages may call for a CCTV drain survey.

Who to call

For emptying, a licensed tanker service. For blockages, a drainage firm. For a failing drainage field, a cracked tank or a replacement, a drainage specialist who knows private systems and the rules. Hiring a plumber covers getting quotes and checking credentials.

How often should a septic tank be emptied?

There is no single answer. It depends on the tank, the household and what goes into it. Ask an emptying firm to assess it and follow the maker's guidance for a treatment plant.

Can I use bleach if I have a septic tank?

Small amounts in normal cleaning are commonly used, but large amounts can harm the bacteria the tank depends on. Use cleaning products sparingly and avoid pouring chemicals down the drain.

What is the difference between a septic tank and a cesspool?

A septic tank partly treats waste and releases liquid to the ground or water. A cesspool is sealed and releases nothing, so it has to be emptied much more often.