Foul Water vs Surface Water Drainage: What's the Difference?

Foul water is used water from toilets, sinks, showers, baths and appliances, and it must go to a foul or combined sewer or a treatment system. Surface water is rain from roofs and paved areas, and it should go to a soakaway first, then a watercourse, then a sewer. Getting them the wrong way round pollutes rivers or overloads sewers. If you are adding drainage for an extension or new building, our sewer connections service makes sure each pipe goes to the right system.

This guide explains foul, surface and combined systems, why misconnections matter, how to tell which drain is which, and who is responsible for putting a wrong connection right.

What is foul water?

Foul water is used water from a building. Approved Document H defines it as waste water which includes "waste from a sanitary convenience, bidet or appliance used for washing receptacles for foul waste" or "water which has been used for food preparation, cooking or washing".

In a house that means toilets, basins, baths, showers, kitchen sinks, washing machines and dishwashers. It carries sewage, food, fats, soap and detergent, so it has to be treated at a sewage works before it reaches a river.

What is surface water?

Surface water is rain that falls on roofs, driveways, patios and other hard surfaces. It reaches the drains through gutters, downpipes, yard gullies and channel drains. It is comparatively clean, although it picks up silt, leaves, oil and anything spilled on a drive.

Where must foul water and surface water go?

Approved Document H sets an order of priority for each, and you only move down the list where the option above is not reasonably practicable.

So foul water heads for the public sewer if it can, while rainwater should soak into the ground if the ground allows. Our guide to soakaways explains how that works, and our guide to how a septic tank works covers homes off the mains.

What is the difference between separate and combined sewers?

A separate system has two sets of pipes: one for foul water going to treatment and one for surface water going to a watercourse. A combined system uses one pipe for both, and everything goes to the sewage works.

Approved Document H says that where sewers operate as combined systems, the sewerage undertaker "can allow surface water to discharge into the system if the sewer has enough capacity". It also notes that combined and rainwater sewers "are designed to surcharge" in heavy rain. That is why water companies prefer rainwater kept out of the sewer wherever possible.

What is a drain misconnection?

A misconnection is a pipe connected to the wrong system. CIWEM, the water and environmental management institution, describes it as "when plumbing is accidently discharged into the wrong sewer; for example, if a dishwasher was plumbed to a surface water sewer".

Anglian Water describes two kinds:

• Foul into surface water. The most common kind. If toilets, sinks or washing machines are connected to the surface water system, the dirty water goes straight into rivers instead of to treatment.
• Surface into foul. Rainwater emptying into the foul sewer fills it to capacity, which can lead to sewage backing up into homes, flooding out of manholes, or storm overflows discharging early.

How common are misconnections, and are they illegal?

They are common and, where separate sewers exist, unlawful. The Environment Agency says "an estimated 1 in 5 homes have misconnected drains" and that misconnections "pollute 9% of bathing waters and 15% of rivers".

Section 106 of the Water Industry Act 1991 says that where separate public sewers are provided, nobody is entitled to discharge "foul water into a sewer provided for surface water", or surface water into a foul sewer "except with the approval of the undertaker".

The responsibility sits with the owner. Anglian Water says property owners are responsible for fixing misconnections even if these were in place before they owned the property, and warns of a possible fine of up to £50,000. CIWEM also says a misconnection "is the responsibility of the property owner".

What causes misconnections?

Most start with a well-meant job done without checking which drain is which:

• a washing machine or dishwasher moved to a utility room or garage and plumbed into the nearest outside gully;
• an extension bathroom or downstairs toilet connected to a rainwater pipe;
• a kitchen sink waste redirected during a refit;
• downpipes from an extension roof connected into the foul drain;
• a new drive gully tied into the nearest chamber without checking what it carries.

How can you tell which drain is foul and which is surface water?

Use the checklist below, working from the easiest check to the most reliable.

Dye tests and CCTV are the reliable checks, especially before building work or buying a house. Our guide to a drain survey when buying a house explains what a pre-purchase survey covers.

Can surface water affect your water bill?

It can. Severn Trent, which serves most of Nottinghamshire, Derbyshire and Leicestershire, says that if all the surface water from your property drains to a soakaway or another private system and none of it enters the public sewer, you may be able to have the surface water drainage charge removed. You still pay the highway drainage charge, and Severn Trent needs evidence.

What if you are adding a new connection?

Section 106 gives you the right to connect your drains to the public sewer, but you must give the sewerage undertaker notice first. The new pipe has to go to the right sewer: foul to foul, surface to surface or a soakaway, and into a combined sewer only where the water company agrees. Across our area the sewerage undertaker is Severn Trent for most of Nottinghamshire, Derbyshire and Leicestershire, and Anglian Water for most of Lincolnshire. If the extension sits near a shared pipe, read our guide to building over a drain as well.

When to call a professional

Call a drainage engineer if you suspect a misconnection, the water company or council has written to you about one, you're planning an extension or conversion that adds new drainage, or a downpipe or gully overflows in heavy rain. Moving a washing machine waste is easy to get wrong without knowing where the gully goes.

A CCTV drain survey from £149 + VAT, with dye testing where needed, shows where each pipe goes. Re-routing a misconnected pipe or laying new runs is quoted by our sewer connections and groundworks teams; groundworks start from £500 + VAT. The final price depends on the job, and you get a fixed quote first.

We cover Nottingham, Nottinghamshire, Derbyshire, Leicestershire and Lincolnshire. Call 07300 600 950 or ask for a quote.

Sources

• Approved Document H: Drainage and waste disposal (gov.uk)
• Water Industry Act 1991, section 106 (legislation.gov.uk)
• Environment Agency: one small action in your home could help clean up beaches
• Anglian Water: how to avoid connecting to the wrong drain
• CIWEM: drain misconnections
• Severn Trent: surface water drainage