Water quality treatment for surface water runoff

A surface water treatment train is the sequence of components that clean runoff before it leaves the site, and it is assessed separately from the hydraulic design that controls how fast the water leaves. Lead local flood authorities now ask for both. The method most of them expect is the simple index approach in the CIRIA SuDS Manual, which scores the pollution hazard of each contributing surface and the mitigation each component provides, and requires the mitigation to equal or exceed the hazard for three pollutant groups. Design the train alongside the layout, because the components take space at ground level and retrofitting them into a finished site plan rarely works.

Why treatment sits alongside attenuation

Attenuation answers a quantity question: the discharge rate and volume the receiving system can accept. Treatment answers a quality question: whether what arrives at the watercourse, the sewer or the aquifer is clean enough.

The two are related but not the same, and a tank that satisfies the volume requirement contributes almost nothing to water quality. That is why an underground attenuation-only strategy attracts a water quality objection even when the hydraulics are faultless. It is also why permeable paving and swales are worth more than their storage volume suggests, because they deliver both.

Receiving waters have their own targets under river basin management planning, and a scheme discharging to a watercourse already failing on a chemical or biological element will face a harder question than one discharging to a system in good condition.

Hazard and mitigation indices in practice

The simple index approach works on three pollutant groups: total suspended solids, metals and hydrocarbons. Each contributing surface is assigned a pollution hazard level according to its use, and each index is read from the current tables in the SuDS Manual rather than from memory, because the values have changed between editions.

The broad pattern runs like this. Roofs are the lowest hazard, though metal roofs and cladding raise the metals index. Residential driveways, parking courts and low traffic roads sit in the middle. Commercial yards, delivery and servicing areas, busy roads and any area where vehicles manoeuvre or park in numbers sit higher. Industrial sites handling oils, chemicals, scrap or waste sit highest, and some of those cannot be dealt with by standard components at all.

Then each proposed component is assigned mitigation indices for the same three groups, and the sum of the mitigation available in the train has to equal or exceed the hazard for each group separately. The separate test matters. A train that scores well on suspended solids and poorly on hydrocarbons has not passed, and the usual fix is a component that targets hydrocarbons specifically rather than more of the same.

Setting the train by sub-catchment

Do not average across the whole site. Split the drained area into sub-catchments by land use, assess each one, and design a train for each.

1. Draw the sub-catchments on a plan, colour-coded by hazard level, with areas stated.
2. Assign the hazard indices for each sub-catchment from the current tables.
3. Set out the proposed components in sequence for each sub-catchment.
4. Sum the mitigation indices and compare with the hazard for each pollutant group.
5. Where the total falls short, add or change a component rather than adjusting the hazard.
6. Show the point where each sub-catchment joins the main system.

That plan is the single most useful drawing in a drainage submission. A reviewer can check the argument in two minutes, and where the argument fails, it fails visibly at one sub-catchment rather than across the whole strategy.

Which components deliver which treatment

Proprietary devices are accepted where they are justified, and on a constrained urban site they are sometimes the only option. What a reviewer will ask for is independently derived performance data, the sizing calculation, and the maintenance schedule, because a full silt chamber has no mitigation value at all.

Oil separators deserve a specific note. They are designed for a defined pollutant and a defined flow, they need selecting as bypass or full retention according to the risk, and the Environment Agency's pollution prevention guidance on their use and design is the reference a reviewer expects to see cited.

Infiltration into a sensitive aquifer

Infiltration sits at the top of the discharge hierarchy and at the top of the groundwater risk hierarchy, which is an awkward combination. Where the site overlies a principal aquifer or sits within a source protection zone, the Environment Agency's groundwater protection position statements restrict what can be infiltrated and from where.

The practical rules of thumb are these. Roof water is usually acceptable. Runoff from residential parking usually needs treatment before infiltration. Runoff from industrial yards, fuelling areas and waste handling areas is generally not acceptable for infiltration at all. In the innermost source protection zone the position tightens considerably, and a discharge to ground may need an environmental permit as well as planning permission.

Check the designation before the layout is fixed, because moving from infiltration to a lined system with a controlled discharge changes both the storage volume and where it goes. Groundwater levels interact with all of this, as set out in groundwater flooding evidence and how to assess it.

Silt management before infiltration

Most infiltration systems that fail do so because of construction silt, not design error. A permeable surface or an infiltration basin receiving runoff from bare earth during the build will blind, and the permeability recorded in the completion test will bear no relationship to the design assumption.

Deal with it explicitly in the construction management arrangements: divert runoff away from infiltration areas during earthworks, install the sacrificial surface layer late, protect basin floors, and provide temporary settlement before any discharge reaches the permanent system. Then provide a sediment forebay or equivalent pre-treatment in the permanent design, with access for a machine to clear it.

Maintenance access is a design constraint rather than an afterthought. A forebay that cannot be reached by a small excavator will not be cleared.

Evidencing the treatment train in the strategy

The Drainage Strategy should contain the sub-catchment plan, the index table with hazard and mitigation totals per pollutant group, a schematic of the train showing the sequence of components and the flow path, the groundwater and receiving water context, a maintenance schedule naming the responsible party and the frequency for each component, and the adoption position.

Where the site is also in Flood Zone 2 or 3, the quality case sits in the strategy while the flood risk case belongs in the Flood Risk Assessment. Reuse systems complicate the accounting, because water taken out for reuse is water not treated by the downstream train, and that interaction is covered in rainwater harvesting and whether it counts as attenuation.

PlanScope drafts a Drainage Strategy structured to the sections a lead local flood authority looks for and flags every site-specific figure it cannot verify rather than filling it in, which is the right behaviour for index values and permeability results that have to come from the current tables and from site testing. For specialist technical reports, qualified professional input may be required.

Enter the site postcode on PlanScope to see which drainage documents the scheme is likely to need.

This is general guidance, not legal advice, so check the position with your lead local flood 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.