Microplastics and tyre wear in stormwater ➔ what a micron rating tells you about retention and what it cannot. Read the guide!
Key points at a glance
Someone specifies a filter for a car park or a yard drain, sees a nominal 70 µm on the data sheet, and wants to know what share of…
Three reasons, and each is sufficient on its own.
Road-borne particulate material is a mixture, and the fractions behave differently.
Why a micron figure cannot be converted into a performance
Three reasons, and each is sufficient on its own.
A nominal rating describes the material, not a measured cut-off. It says how the textile is designed. It does not say that everything larger is retained and everything smaller passes.
Retention changes during operation. A clean filter and a partly loaded filter behave differently. Material already retained forms a layer that holds finer particles than the textile alone would. The rating is a starting condition, not a steady state.
The incoming particle size distribution decides the outcome. The same filter on two different surfaces produces two different results, because what arrives is different. Without knowing the distribution at a specific location, a percentage is invented rather than measured.
What is actually in road runoff
Road-borne particulate material is a mixture, and the fractions behave differently.
| Fraction | Typical origin | Behaviour at a 70 µm textile |
|---|---|---|
| Coarse mineral, sand and grit | surface wear, winter grit | retained |
| Larger tyre and brake wear particles | traffic | largely retained |
| Fine tyre wear particles | traffic | partly retained, a share passes |
| Fine mineral dust | general | largely passes |
| Dissolved substances, metals in solution, PAH | traffic, surface | not retained at all |
Tyre wear is the largest single source of road-borne particulate plastic. A tyre wear particle filter at the gully catches the coarser end of it at the point of entry. That is a genuine source control contribution and it is also the accurate limit of the claim.
What this means for specifying a filter
Three practical conclusions.
Specify by position and servicing, not by the finest available rating. A 70 µm filter that is serviced belongs in the locations where it has a purpose. Across a whole gully stock, 100 µm that gets maintained achieves more than 70 µm that does not.
Do not accept removal percentages without a test behind them. Ask which test, on which distribution, at which loading state. If those questions have no answer, the number has no meaning.
Treat a filter insert as one measure, not as a treatment device. Where a scheme has to demonstrate performance, that requires a designed system with evidence. An insert can sit inside such a scheme.
A note on national frameworks
Requirements for the treatment of surface water runoff differ by country, and so do the parameters used to describe them. German practice, for example, works with a fines parameter and a set of technical rules that have no direct equivalent elsewhere.
What travels across all of them is the point above: a stated micron rating is a material property and not a treatment performance. Whatever framework applies to your project, that distinction is the one that matters when comparing products.
Frequently asked questions about standards and performance figures
These are the questions most often asked by designers, municipalities and operators
Is a 70 µm filter bag AFS63-compliant?
No. AFS63 is an analytical sum parameter, not a filter specification, and a nominal filter rating is not a tested separation limit. Reliable efficiencies can only come from testing under the appropriate procedure.
Can I comply with DWA-A 102 by using a filter insert?
The question cannot be answered for an individual component because the framework assesses treatment concepts for catchments rather than single products. A filter insert can form part of a concept; the designer is responsible for the assessment.
What is the difference between nominal and absolute filter ratings?
Nominal is a manufacturer’s design rating without a guaranteed cut-off. An absolute rating requires testing under a defined method. Nominal ratings are common for textile filter media used with stormwater.
Why do you not publish removal efficiencies?
Because without system-specific testing they would not be reliable. A percentage derived from a material figure alone would not withstand scrutiny. We therefore state material data, intended use and limitations instead.
For which surfaces is treatment required?
That depends on the pollutant load of the surface and the receiving-water or infiltration objective. Classification is part of the drainage design and coordination with the competent authority. There is no universal answer.
Where can I get a reliable design calculation?
From a drainage or urban water engineering consultancy. We are happy to support the technical selection of a filter insert within an existing treatment concept.
Still unsure which filter or next step fits?
Send us the medium, dimensions and application. We will help you decide whether a standard product is suitable or whether the case needs a custom solution.
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