Treatment train stages and the pollution hazard index

Water quality in a drainage strategy is assessed separately from volume, and the test is not a calculation so much as a comparison. The suds treatment train pollution index method works by assigning a pollution hazard level to each contributing surface, reading off hazard indices for suspended solids, metals and hydrocarbons, then selecting SuDS components whose mitigation indices for those same three determinands add up to at least the hazard figures. If the sum falls short for any one determinand, the train fails, and adding a bigger attenuation tank does not help.

Most refused or deferred drainage strategies fail on hydrocarbons from car parks and on the absence of any treatment stage at all for an impermeable yard.

Why water quality is assessed separately from quantity

Attenuation answers a flood question: how much water leaves the site and how fast. Treatment answers a pollution question: what is in the water when it leaves.

A site can discharge at greenfield run-off rate into a watercourse and still cause harm, because the first flush off a car park carries hydrocarbons, tyre and brake wear metals, and sediment. National planning practice guidance on flood risk expects drainage proposals to address water quality as well as volume, and the lead local flood authority and the Environment Agency both comment on it as consultees.

The sequencing in the SuDS management train is prevention, then source control, then site control, then regional control. Prevention means not creating the pollution: enclosing a fuelling area, sweeping a yard, roofing a bin store. It is the stage most strategies skip, and it is free.

Interception is the related idea that the first few millimetres of rainfall, which carry most of the pollutant load, should be captured and not discharged at all in summer conditions. Designing for interception does more for water quality than any single component.

Hazard levels by land use

The method assigns each contributing area a pollution hazard level, in bands running from very low through low and medium to high, according to what happens on the surface.

The broad pattern is consistent across the guidance. Residential roofs sit at the bottom. Individual driveways, residential car parks and lightly trafficked roads sit low. Commercial yards, delivery areas, public car parks and most roads sit in the medium band. Sites where vehicles are maintained, refuelled, dismantled or parked in volume, along with waste handling and haulage, sit at the top.

Take the index values themselves from the current edition of the SuDS Manual rather than from a summary, because they are published as a table with defined determinands and they get misquoted. The figures are not interchangeable between editions.

Two practical points about classification.

Sub-catchment the site properly rather than applying one hazard level to the whole development. A scheme with 60 per cent residential roof and 40 per cent delivery yard has two trains, not one averaged train, and treating it as a single catchment either over-designs the roof drainage or under-designs the yard.

Be honest about the yard. A delivery area used by articulated vehicles twice a day is not a residential car park, and classifying it as one is the single most common reason a strategy is challenged at consultation.

Mitigation indices for each component

Each SuDS component carries its own mitigation indices for suspended solids, metals and hydrocarbons, and components in series are additive up to the point where the guidance caps the benefit.

Vegetated and filtering components perform best across all three determinands. Swales, filter strips, bioretention areas, rain gardens, permeable pavement with a geotextile and sub-base filtration, constructed wetlands and ponds with permanent water all remove sediment and sediment-bound metals, and the biologically active ones deal with hydrocarbons.

Hard-engineered storage performs worst. An underground attenuation tank with a flow control gives you nothing for water quality, which is why a strategy built around a tank and a hydrobrake usually has an index deficit.

Proprietary treatment devices are acceptable where there is independent performance evidence for the determinands in question, and the evidence has to be specific to the device and the loading rate. Vague manufacturer claims do not substitute for tested removal efficiencies.

One frequent error. A full retention or bypass oil separator is spill containment, not routine treatment, and the regulators do not accept it as a treatment stage in the index method. Fit one where spill risk requires it and still provide the treatment train.

Building a train that passes

Work in this order.

1. Divide the site into sub-catchments by surface type and use.
2. Assign a hazard level to each sub-catchment and record the hazard indices for the three determinands.
3. Decide what prevention and interception can achieve before any component is selected.
4. Select components in sequence, starting at source, and sum their mitigation indices per determinand.
5. Check each determinand separately against the hazard index for that sub-catchment.
6. Record the outcome in a table in the drainage strategy, sub-catchment by sub-catchment.

The checking step is where strategies fall over, because a train that comfortably clears the suspended solids figure can fail on hydrocarbons. Hydrocarbons are the binding constraint on almost every commercial scheme.

Where there is a deficit, the cheapest fixes are usually a filter strip or swale ahead of an existing component, converting part of the car park to permeable paving with a filter layer, or adding a bioretention cell at the low point. Where space is absent, a proprietary device with evidence is the fallback.

Car parks, yards and fuel delivery areas

Car park drainage is the standard case and it is still done badly. The workable pattern is permeable paving in the bays with conventional surfacing on the aisles draining to a linear bioretention strip, which keeps the treatment at source and keeps the maintenance visible.

Service yards need more. A yard used for loading drums or handling liquids should be separately drained, with the clean roof water taken out of the system upstream so it does not have to be treated. Keep the yard catchment small.

Fuel delivery and transfer areas should be roofed where possible, bunded, and drained to a sealed system or to foul with the sewerage undertaker's consent rather than to surface water. Infiltration under a fuelling area is not acceptable.

Groundwater changes the calculus. Infiltration of run-off from a medium or high hazard surface is normally unacceptable without treatment ahead of the infiltration component, and in the most sensitive source protection zones infiltration of anything other than clean roof water is likely to be refused. Where groundwater is already a site issue, the interaction with flood risk is worth reading alongside evidencing groundwater flooding in a flood risk assessment.

Interactions with the Environment Agency position

The Environment Agency comments on water quality where a discharge affects a watercourse or groundwater, and its published groundwater protection position statements set out what it will and will not accept. Those positions are more restrictive than the index method in some situations, and where they conflict, the position statement governs.

Three recurring Agency points. Infiltration must not be used to dispose of contaminated run-off. An adequate unsaturated zone is needed beneath an infiltration component. And discharges from potentially contaminated land need the contamination position resolved before the drainage design is fixed.

Where the outfall is to an ordinary watercourse, the lead local flood authority also controls the physical works, and that consent is separate from planning. The procedure is covered in land drainage consent for works to an ordinary watercourse, and it is worth starting early because the two timetables rarely align.

Schedule 3 to the Flood and Water Management Act 2010, which would introduce a separate SuDS approval regime in England, remains uncommenced. As at October 2026 the position should be checked before describing it as current law.

Showing the assessment on a drawing

A drainage strategy that passes consultation shows the water quality case graphically rather than asserting it in a paragraph.

Put a sub-catchment plan in the document, coloured by hazard level, with areas in square metres. Annotate each train on the plan in sequence, so the reader can follow a drop of water from the gully to the outfall. Add a schedule setting out, for each sub-catchment, the hazard indices, the components, their mitigation indices and the comparison outcome.

Then say who maintains each component, how often, and how access is achieved. A bioretention cell behind a locked gate with no maintenance route is a component the adopting body will refuse.

A Drainage Strategy structured this way, with the water quality section as a standalone chapter rather than a paragraph in the attenuation calculation, is what the lead local flood authority is looking for, and it sits alongside the Flood Risk Assessment rather than inside it on most sites. For specialist technical reports, qualified professional input may be required, and a drainage engineer should check the index comparison before submission.

To see which drainage and flood documents your site is likely to need, enter your postcode on PlanScope and work through the guided questions.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.