Where your system sends its treated wastewater — to a watercourse or to the ground — decides which rules apply and, often, which type of system you’re allowed to have. Here’s the difference in plain terms.

Discharging to the ground

“To the ground” means the effluent soaks away through a drainage field: perforated pipes buried in trenches, where the soil provides the final stage of treatment.

Under the General Binding Rules, a discharge to ground must:

• be no more than 2 cubic metres per day;
• pass through a drainage field built to BS 6297;
• not be within a groundwater Source Protection Zone 1;
• keep the required distances from watercourses, boreholes, buildings and boundaries.

Both septic tanks and treatment plants can discharge to the ground this way.

Discharging to a watercourse

“To a surface water” means a pipe discharges to a stream, river, ditch or canal.

Under the binding rules, a discharge to a watercourse must:

• come from a sewage treatment plant — not a septic tank;
• meet the treatment standard BS EN 12566;
• be no more than 5 cubic metres per day.

This is the crux of the 2020 rule: a septic tank discharging straight to a watercourse is not allowed and must be upgraded — usually to a treatment plant, or by adding a drainage field so it discharges to the ground instead. See the General Binding Rules guide.

Which do I have?

If you’re not sure whether your system discharges to the ground or a watercourse — which is very common with older properties — our free compliance checker helps you work it out and tells you what the rules mean for you.

When you need a permit instead

If your discharge is above the volume limits (2 m³/day to ground, 5 m³/day to surface water), or you’re in a sensitive location such as a Source Protection Zone, the binding rules don’t cover you and you’ll need a bespoke permit from the Environment Agency.

Sources: GOV.UK — discharge to the ground · GOV.UK — discharge to a surface water