Package treatment plants and the general binding rules

If a new house or a converted barn cannot reach a public foul sewer, the usual answer is a package treatment plant discharging under the general binding rules for small sewage discharges. Those rules let you discharge treated effluent without an environmental permit, provided the volume stays within the limits and you meet a list of conditions on siting, treatment standard and maintenance. The limits are 2 cubic metres a day where you discharge to ground through a drainage field, and 5 cubic metres a day where you discharge to a surface water such as a stream or ditch.

Councils will expect you to show which route you are using before they grant permission, and the drainage design usually has to come before the layout is fixed.

When mains connection is genuinely unavailable

The hierarchy matters, because a council can and does refuse a private system where a mains connection is achievable. Connection to the public sewer sits at the top. A package treatment plant discharging to ground comes next. Discharge to a surface water comes after that. A cesspool, which stores everything and is emptied by tanker, is the last resort.

Evidence of unavailability means a written response from the sewerage undertaker, not an assertion about distance. Ask for a pre-development enquiry and a budget estimate for connection, including any requisition of a new sewer under the Water Industry Act 1991. Keep the correspondence, because it is the document the case officer will look for.

Septic tanks are a separate category and they are no longer acceptable where the discharge goes directly to a surface water. Existing tanks discharging that way had to be replaced or upgraded, and a new installation of that kind is not an option.

The general binding rules and what they allow

The rules sit under the Environmental Permitting (England and Wales) Regulations 2016 and are published as guidance by the Environment Agency, in two versions, one for discharge to ground and one for discharge to surface water. Meeting them removes the need for a permit but does not remove the obligation to comply.

The conditions that catch people out are these. The plant must be installed and operated in accordance with the manufacturer's instructions, and it must meet the relevant British Standard for small wastewater treatment plants, which for a package plant is the certification under BS EN 12566-3. The discharge must be domestic sewage only. The system must not cause pollution. And you must not be in an area where a permit is required regardless of volume.

Registration requirements have changed more than once. Check the current gov.uk guidance for whether your discharge must be registered with the Environment Agency, and in which circumstances, before you assume the rules apply without further steps.

Discharge to ground against discharge to surface water

Discharge to ground is the preferred route wherever it is reasonably practicable, and the rules say so. The effluent passes through a drainage field into the unsaturated soil, which provides further treatment.

Discharge to a surface water is permitted where discharge to ground is not reasonably practicable, and it comes with a higher volume allowance and a higher treatment expectation. You will need the right to discharge into the watercourse, which is a private law question about riparian ownership and easements, and where the outfall is on a main river you will also need consent from the Environment Agency. Our note on flood risk activity permits for works near a main river covers that side of it.

A permit rather than the general rules is required where you exceed the volume limits, where the site is in a groundwater source protection zone 1, or where the discharge is in or near a designated site such as a site of special scientific interest. Check the constraint before you buy the plant.

Percolation testing and drainage field sizing

The drainage field is where most private systems fail, and the test is simple enough to do badly.

A percolation test follows the method in Approved Document H2 and BS 6297. Dig a test hole of specified dimensions in the position of the proposed field, fill it with water and let it seep away overnight to saturate the surrounding soil, then refill and time how long the water takes to fall from 75 per cent to 25 per cent of the effective depth. Divide that time in seconds by the drop in millimetres to get the percolation value Vp, and repeat the test at least three times across the field area.

The floor area of the drainage trench is then calculated as A equals p times Vp times 0.25, where p is the number of persons the system serves. This is an illustrative statement of the published method rather than a design.

Two results end the conversation. A Vp below 15 seconds per millimetre means the soil drains too fast to provide treatment. A Vp above 100 means it drains too slowly. In either case a drainage field is not acceptable and you need a different solution.

Siting rules apply as well. The field must be a stated minimum distance from watercourses and permeable drains, from any groundwater abstraction point, and from buildings. Check the current figures in Approved Document H2 and design with margin, because a field pushed to the boundary is a field that cannot be replaced.

Evidence a council expects with the application

Expect the environmental health or drainage officer to want a foul drainage assessment setting out the hierarchy, the evidence that mains connection is unavailable, the proposed system and its certification, the percolation test results with dates and locations, the drainage field size and position shown on a scaled plan, and the discharge point.

Where the site is close to a designated habitat or in a nutrient sensitive catchment, the requirement goes further and a nutrient budget may be needed. That is a different exercise and it can take months, so establish early whether your catchment is affected.

On larger sites the same material belongs in a Foul Sewage Assessment, sitting alongside the Drainage Strategy that deals with surface water. Keep the two separate in the documents even where one consultant writes both, because different officers read them.

Maintenance obligations that run with the property

The rules place continuing duties on the operator, which in practice means the householder.

Desludging at the frequency the manufacturer specifies, usually annually, by a registered waste carrier, with the receipt kept. Servicing by a competent person and a record of it. Repair of any fault without delay. And a duty to tell the buyer, in writing, when the property is sold, along with a description of the system and its maintenance requirements.

That last obligation is worth mentioning to clients at design stage, because a system that is awkward to access becomes a problem at every sale. A tanker needs to get within hose reach of the plant, which is typically a stated maximum distance from a hardstanding, and designing that access in at the outset costs nothing.

If the discharge causes pollution, the general binding rules stop applying and the Environment Agency can require a permit or take enforcement action. The rules are permissive, not a shield.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

If you are working out the drainage evidence for a rural site, enter your postcode on PlanScope to see which reports it is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.