Foul drainage with no mains sewer: what to show

Before a council will accept a package treatment plant on a rural site, it wants evidence that connection to a public sewer is not reasonably practicable. That is the first question, not the last, and applications routinely fail because the applicant started with the plant specification and never demonstrated why the mains option was ruled out. The drainage hierarchy in Approved Document H is the framework the case officer applies, and a Foul Sewage Assessment that works through it in order is far more persuasive than one that begins with a product brochure.

Establishing that connection is not feasible

The hierarchy runs public sewer first, then a package treatment plant or septic tank discharging to ground through a drainage field, then a package treatment plant discharging to a watercourse.

To rule out the first option you need evidence, not assertion. Write to the sewerage undertaker, describe the development, and ask for a position on the nearest point of connection, the capacity available, and the cost and practicality of a connection. Their reply is the document that settles it.

Distance alone is not decisive. A connection 200 metres away across your own land is a different proposition from one 200 metres away across a third party's field where no easement exists. Set out the physical route, the land ownership, the levels, and whether pumping would be needed.

Cost is relevant but it is not a trump card. A council will not accept a mains connection being ruled out simply because it is more expensive than a treatment plant, and the assessment should deal with proportionality honestly.

Package treatment plants and septic tanks

The two are not interchangeable and the distinction has mattered since the beginning of 2020.

A septic tank provides settlement only. The liquid leaving it still has a high organic load, so it can only discharge to ground through a properly designed drainage field, never directly to a watercourse. Existing septic tanks discharging to surface water had to be replaced or upgraded under the rules that took effect on 1 January 2020.

A package treatment plant provides biological treatment, so its effluent is clean enough to discharge either to ground or, where the rules allow, to a watercourse.

For any new installation, specify a plant certified to the relevant European standard for small wastewater treatment systems, sized for the population equivalent of the development with a sensible allowance for future occupancy. Undersizing is the commonest technical fault and it shows up as odour and poor effluent quality within a year.

Discharge to ground against discharge to water

Discharge to ground through a drainage field is preferred, because it gives a further stage of treatment as the effluent passes through the soil and it returns water locally.

It requires ground that will accept the flow. That is established by percolation testing, and the result governs the size of the drainage field. Testing follows the method in Approved Document H and the relevant British Standard, with trial holes dug to the proposed invert level, pre-soaked, and then timed over repeated fillings. Do not test in frost, during prolonged heavy rain, or in a drought, because none of those conditions represents the design case.

Groundwater level matters as much as percolation rate. The drainage field needs an unsaturated zone beneath it throughout the year, which on a high water table site is the reason discharge to ground fails.

Discharge to a watercourse is the fallback, and it needs a watercourse with flow throughout the year. A ditch that is dry in summer is not an acceptable receiving water.

The rules that govern small discharges

Most domestic scale discharges in England operate under general binding rules rather than a permit. The rules set volume limits, currently expressed as a maximum daily volume for discharges to ground and a different maximum for discharges to surface water, together with conditions about the equipment used, its installation, maintenance and the location of the discharge.

The main conditions are practical:

1. The plant must be certified to the relevant standard and installed to the manufacturer's instructions.
2. It must be maintained and desludged by a registered carrier, with records kept.
3. The discharge must not be in a groundwater source protection zone one, nor too close to a well, spring or borehole used for drinking water.
4. New discharges generally have to be registered, and the registration passes to a new owner on sale.
5. The discharge must not cause pollution.

Where any of those cannot be met, or the volume exceeds the limits, the answer is an environmental permit rather than a redesign of the drainage field. Permits take months and carry application and annual charges, so establish which route applies before the planning application rather than after.

Nutrient neutrality is a further layer in affected catchments. A new discharge in one of those catchments will need mitigation, and that is a scheme-level problem rather than a drainage detail.

Percolation testing and drainage field design

The test produces a value that feeds directly into the drainage field area calculation, so it is worth doing properly and worth doing early.

Approved Document H sets out separation distances that govern where the field can go. The field has to stand clear of buildings, of any watercourse, and of any water supply source, and there has to be sufficient unsaturated ground beneath it. Check the current figures in the Approved Document rather than working from memory, because the layout implications are substantial on a small plot.

An illustrative example. A barn conversion on a half acre plot tests at a slow percolation rate on heavy clay. The drainage field needed would occupy most of the garden and sits too close to the boundary ditch. The scheme switches to a treatment plant discharging to the ditch, which has year-round flow, and the applicant registers the discharge under the general binding rules. Two site visits and a test, rather than a redesign after refusal.

Where testing shows the ground will not work and there is no watercourse, the remaining options are a sealed cesspool, which needs regular emptying and is expensive to run, or a pumped connection to the mains. Both are worth pricing before the design is fixed.

What to submit with the application

Put the whole chain of reasoning in one document so that the case officer and the environmental health officer can follow it without asking questions.

Include the sewerage undertaker's written response on mains connection, the percolation test results with trial hole locations, dates and the method used, the drainage field design or the outfall details with the receiving watercourse identified, the plant specification and certification, the population equivalent calculation, and a statement of which regulatory route applies with the reasoning.

A site plan showing the field or outfall position against the separation distances does more work than several pages of text. Cross-reference the drainage strategy for surface water, because foul and surface water have to be dealt with separately and a combined arrangement will be resisted.

The technical detail on plants and the rules is covered further in the notes on package treatment plants and the general binding rules and on unsewered sites, plants and permits.

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