Stormwater filtration at the gully: three tasks, three answers
Road, yard and car park gullies share a construction: a grating, a catch basket beneath it, then the connection onward. What changes is what you are trying to keep out, and there are three distinct answers.
The three are not interchangeable. The first two are textile inserts for an existing basket. The third replaces the basket with a bucket, because pellets need a container as well as a filter.
Key points at a glance
Stormwater filtration at this point is source control. A road runoff filter sits in the basket and holds material before it travels on, while surface water filtration in…
A stormwater inlet filter is a source control measure. It holds solids at the point of entry, before they reach a sewer, a watercourse or an infiltration structure.
Every filter in a drain restricts flow as it loads. That is not a defect, it is the mechanism, and it has one consequence that decides whether these…
Three different jobs at the same drain
| Task | Product | Rating |
|---|---|---|
| General solids: sand, grit, leaves, debris | gully bag | nominal 100 µm |
| Finer road-borne particles, tyre wear | microplastic filter bag | nominal 70 µm |
| Plastic pellets on industrial sites | pellet retention system | nominal 70 µm in a steel bucket |
Stormwater filtration at this point is source control. A road runoff filter sits in the basket and holds material before it travels on, while surface water filtration in the wider sense covers everything between the gully and the outfall. In North American terminology the same component is a storm drain catch basin filter.
What storm drain inlet filters can do
A stormwater inlet filter is a source control measure. It holds solids at the point of entry, before they reach a sewer, a watercourse or an infiltration structure.
That is a real contribution and it is worth stating precisely. These are not tested treatment devices with a demonstrated removal efficiency, and we do not present them as such. They retain solid particles above their rating. They do not retain dissolved substances, hydrocarbons in solution or heavy metals.
Where a scheme has to demonstrate a treatment performance, that belongs to a designed system with evidence behind it. A filter insert can sit inside such a scheme as one measure among several.
The maintenance point, stated plainly
Every filter in a drain restricts flow as it loads. That is not a defect, it is the mechanism, and it has one consequence that decides whether these products help or hurt.
A stormwater drain filter needs a servicing routine or it becomes the problem it was fitted to prevent. A bag in reasonable condition passes water. A bag left in place for a season may not, and a surcharged gully in a storm is worse than an unfiltered one.
We do not publish intervals, because catchment, surroundings and season decide. What we recommend is putting the filter into the same cycle as the gully itself, and adding a check after the first heavy rain that follows a long dry spell, which is when the surface delivers most.
Choosing a rating for a stock of gullies
For an estate, a depot or a site with many gullies, the practical approach is not to fit the finest available everywhere.
Standardise on 100 µm across the stock. As a general stormwater sediment filter it takes the bulk of the material at the lowest servicing burden. Then identify the locations where finer retention has a specific purpose, usually a defined surface with known traffic discharging to something sensitive, and fit 70 µm there. That gives you a regime that gets maintained, which is worth more than a specification that does not.
The technical background on ratings, and why a micron figure cannot be read as a removal efficiency, is set out in the guide.
Frequently asked questions about filters in road and yard gullies
Most of these questions come from municipalities, maintenance depots and industrial sites
100 or 70 microns – which is better?
Neither is better in every situation. A 100 µm bag retains sand, sediment and leaf material and offers higher flow. A 70 µm bag also targets finer road-borne particles but loads more quickly. Choose according to the objective and the maintenance routine you can realistically provide.
Will a filter bag fit every catch basket?
The flexible bag for catch baskets adapts to many round baskets. The microplastic filter bag is available in several lengths for common gully-pot forms. If the geometry differs, compare the actual dimensions.
What is the difference from a catch basket?
A basket is a rigid perforated component and mainly holds coarse debris. A textile insert routes the water through filter media and retains much finer material. The two are not mutually exclusive; an insert can be used inside an existing basket if the dimensions suit it.
Do the filters retain tyre wear particles?
Partly. Tyre wear occurs over a broad particle-size range; the fraction above the effective retention range can be captured while finer fractions pass through. A percentage removal claim is not possible without measurement at the actual site.
What happens during heavy rain?
A loaded filter reduces flow, and at very high inflow water can pass over the edge of the basket. The insert does not replace hydraulically adequate drainage. A suitable maintenance routine is the effective countermeasure.
Do I need a different filter for plastic pellets?
Not necessarily a different micron rating, but a different construction. In the pellet retention systems, the textile is fixed to the bucket so that the water is forced through the fabric. That is the decisive difference for light, buoyant pellets.
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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