Water quality treatment in a SuDS management train

SuDS water quality treatment is assessed separately from attenuation, and in England the usual method is the simple index approach set out in the CIRIA SuDS Manual. It works by assigning pollution hazard indices to the proposed land use for three indicator pollutants, then assigning mitigation indices to each SuDS component in the drainage train, and checking that the components in series provide at least as much mitigation as the land use generates hazard. A drainage strategy that sizes a basin correctly and says nothing about treatment is only half a design, and lead local flood authorities return them.

For a reviewer, the question is never whether a pond is present. It is whether the arithmetic has been shown for each discharge point.

Why quantity alone is not enough

Attenuation design answers a hydraulic question: how much runoff, discharged at what rate, over what return period, with what allowance for climate change. None of that says anything about what is in the water.

Urban runoff carries suspended solids, metals from brake and tyre wear, hydrocarbons from vehicles and fuel, nutrients from landscaping, and increasingly microplastics. Discharging it untreated to a watercourse or to groundwater transfers a pollution problem rather than solving a flooding one.

National policy expects sustainable drainage systems to be designed to provide water quality benefits alongside flood risk management, and the Planning Practice Guidance on flood risk points to the same outcome. The regulatory interest sits with the Environment Agency for groundwater and for main rivers, and with the lead local flood authority as statutory consultee on major applications.

Pollution hazard levels by land use

The method starts with the contributing surface, not with the development as a whole. Each drainage sub-catchment is characterised by its use, and a hazard level is assigned from the published table for total suspended solids, metals and hydrocarbons.

The gradations run in the order you would expect. Residential roofs and roofs generally sit at the low end. Residential car parking, low traffic roads and individual driveways sit above them. Commercial yards, lorry parking, waste handling areas and high traffic roads sit higher again, and some industrial uses fall outside the simple index approach entirely and need a site-specific assessment.

Two practical points for a reviewer. Check that the sub-catchments are drawn on a plan and that each has a stated use, because an assessment that treats a mixed-use site as one catchment will usually be over-treating the roofs and under-treating the yard. And check what has been assumed about the servicing area, since that is where the highest hazard usually sits and where designers tend to be optimistic.

Mitigation indices for each component

Every SuDS component carries mitigation indices for the same three pollutants, reflecting the treatment processes it provides: sedimentation, filtration, adsorption, biodegradation and plant uptake.

Filter strips, swales, bioretention systems, permeable pavements, detention basins, ponds and wetlands all have published values, and they differ pollutant by pollutant. A component that performs well for suspended solids may perform less well for dissolved metals, which is why a single component rarely satisfies a high hazard catchment on its own.

Two rules govern the arithmetic and both are commonly misapplied. Components in series are additive, so a filter strip followed by a swale followed by a basin gives a combined index. But where two components provide the same treatment process, the guidance discounts the second contribution, because you cannot claim the same sedimentation twice. A design showing three consecutive detention features and claiming a triple index is wrong.

Building a management train that balances the indices

The train is built from the contributing surface downstream, following the prevention, source control, site control and regional control sequence.

1. Prevention first: reduce the hazard at source by limiting impermeable area, keeping servicing areas small, and covering or bunding high risk activities.
2. Source control next: permeable paving, rainwater harvesting, green roofs and filter strips that treat where the runoff falls.
3. Site control: swales and bioretention conveying and treating on the way to the outfall.
4. Regional control: a basin, pond or wetland providing final treatment and attenuation for the whole catchment.

Then tabulate. For each sub-catchment, list the hazard indices, the components in series with their mitigation indices, the discounted total, and a pass or fail for each pollutant. A reviewer should be able to follow that table without reopening the manual.

Where the total falls short, the options are to add a component, to change a component to one with better performance for the failing pollutant, or to reduce the hazard by changing the surface. Adding a bigger basin does not help if the deficiency is in metals.

Discharge to groundwater against surface water

The destination changes the test. Infiltration to ground brings groundwater protection into play, and the Environment Agency's position on infiltration is stricter where the site sits in a source protection zone or over a principal aquifer.

Infiltration of runoff from high hazard surfaces is generally not acceptable without treatment before the point of infiltration, and infiltration directly through a deep borehole or soakaway bypassing the soil profile is treated as a discharge to groundwater requiring particular care. Where the site lies in source protection zone 1, expect the starting position to be that infiltration from anything other than clean roof water will not be permitted.

Discharge to a surface water brings the receiving water quality into the assessment. A chalk stream, a site of special scientific interest or a water body already failing its objectives will attract a higher expectation, and where phosphorus is the issue the nutrient position may be a separate exercise altogether. Our note on what the lead local flood authority checks in a drainage strategy covers the consultee's wider checklist.

Where proprietary treatment is acceptable

Proprietary treatment devices, meaning manufactured separators, filters and cartridge systems, have a place and they are not a substitute for the train.

The guidance is that they may contribute where independently verified performance data supports the claimed treatment, tested under a recognised protocol rather than by the manufacturer alone. They are most defensible where space genuinely does not allow a vegetated component, for example on a tight urban site or a basement car park outfall.

Where a submission relies on a proprietary device, a reviewer should ask three questions. What independent test data supports the performance claim, and under what protocol. What maintenance regime does the performance assume, and who will carry it out. And what happens to the treatment train if the device is not maintained, which is a real question given that these units need regular servicing and are easy to forget. Long-term responsibility is the weak point, and our note on SuDS adoption and maintenance and who holds it long term sets out the arrangements.

Presenting the assessment so a consultee can check it

The best water quality sections are short and fully auditable. Include a sub-catchment plan with areas and uses, a table per catchment with hazard indices, components, mitigation indices and the result, a note of any discounting applied and why, a statement of the discharge destination and its sensitivity, and the maintenance schedule that the treatment assumes.

State the edition of the guidance you have used, because the indices have been revised and a reviewer working from a different edition will get different numbers.

What gets returned is the strategy that shows a pond on a layout and asserts that water quality is addressed. Put the arithmetic in the Drainage Strategy itself rather than in a modelling appendix nobody opens, and keep it consistent with the flood risk position set out in the Flood Risk Assessment.

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

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