A catch basin insert filter is a device fitted beneath a storm drain grate to intercept pollutants before they leave the inlet. Here is what it captures, how to judge fit and bypass handling, and what drives maintenance.
A catch basin insert filter is a device suspended from or placed inside a storm drain inlet frame to intercept trash, sediment and pollutants before stormwater continues through the municipal or industrial drainage system. It sits below the grate, inside the catch basin structure itself, so it treats runoff at the point of entry rather than further downstream. What it captures and how well it performs depends on the media inside it and how it is sized to the site.
What Does a Catch Basin Insert Filter Actually Capture?
The short list is trash, debris, sediment, and in many designs, oil and grease. A basic insert is largely a gravitational and screening device: stormwater passes through a basket or frame, coarse sediment and floatables settle out or get caught in the mesh, and treated flow continues to the outlet pipe. Inserts built with an oil-absorbent component add a second mechanism, pulling free-phase hydrocarbons out of the flow as it passes through.
That combination of screening and oil absorption is why catch basin inserts are commonly specified as a stormwater pollution prevention Best Management Practice, or BMP, particularly where the goal is 40 CFR 122.26 compliance for municipal and industrial stormwater discharges. The regulation sets the general requirement for permit holders to control pollutants in runoff; the insert is one tool used to meet it, usually paired with a broader site plan rather than standing alone.
What an Insert Can and Cannot Remove
Independent evaluation work on catch basin inserts, including testing coordinated through King County, Washington, found that insert performance varies by what the device targets. The same evaluation found that catch basin inserts did not remove significant amounts of pollutants tied to silt- or clay-sized particles, and showed no meaningful ability to remove dissolved metals such as copper, lead or zinc, or total phosphorus bound to very fine sediment.That distinction matters when you are choosing a device. A filter sized for coarse sediment and floating debris is not the same product as one built to address dissolved phosphorus or metals in stormwater. Abtech separates these jobs by media: standard Smart Sponge® is hydrophobic media that binds hydrocarbons, Smart Sponge® HM targets dissolved metals such as lead, zinc, copper, arsenic and selenium along with phosphorus, and Smart Sponge® Plus is antimicrobial media aimed at bacteria including E. coli and coliform. Matching the media to the pollutant of concern, rather than assuming one insert format handles every fraction, is the central decision in specifying a device.
How Do You Judge Whether an Insert Fits Your Inlet?
Catch basins are not standardized across municipalities or industrial sites — frame shapes, grate dimensions, sump depth and curb inlet geometry all vary. A frame-mounted insert that works in one basin may not seat correctly in another, which leaves gaps for untreated flow to bypass the treatment surface entirely. Abtech's Ultra Urban® Filter catch basin insert is custom-fabricated to the basin's own shape, size and design rather than sold as a single universal frame, which is intended to reduce the fit gaps that let flow skip the filtration surface. Where the insert is built from stainless steel, it carries a 20-year warranty on that stainless steel construction.
How Does the Insert Handle Flow and Bypass?
Every catch basin insert needs a way to handle flow that exceeds its treatment capacity, commonly during heavy storm events. Properly designed inserts include a high-flow bypass so that once the device reaches its design flow rate, excess stormwater can pass without resuspending or washing out material already captured. In some manufactured inserts, the overflow route is not a true bypass — water still contacts the treatment media before overflowing — which can compromise treatment at higher flows due to short contact time and the risk of flushing previously trapped pollutants back into the flow. When reviewing an insert, ask specifically how overflow is routed and whether peak-flow events risk washing out what the device has already captured.
Frequently Asked Questions
Can one catch basin insert remove sediment, oil and dissolved metals all at once?
Not from a single generic media. Sediment and floatables are handled by screening and settling, oil and grease by an absorbent component, and dissolved metals or phosphorus by a media engineered specifically for that fraction, such as Smart Sponge® HM. Several media types can be combined within one system when a site's runoff carries more than one pollutant class.
Do catch basin inserts need power or pumps to work?
Passive insert formats, including the Ultra Urban® Filter, are designed to treat flow using gravity and the properties of the media itself, without pumps or power. That is different from skid-mounted systems used for higher-volume polishing applications, which are installed with pumps, pressure and flow controls.
How do I know if an insert will bypass untreated water during a storm?
Ask the manufacturer specifically how the overflow route is designed. A true bypass diverts excess flow around the treatment media once the design flow rate is reached; some designs route excess water through the treatment area first, which can reduce contact time and risk washing out previously captured pollutants at peak flow.
Will a catch basin insert meet my NPDES or MS4 permit requirement on its own?
An insert is typically one Best Management Practice within a broader stormwater pollution prevention plan, not a standalone compliance solution. The specific contaminant driving your permit condition — trash and debris, oil and grease, dissolved metals, bacteria or phosphorus — determines which media and format combination is appropriate, which is why a site assessment precedes a product recommendation.
How often does a catch basin insert need to be cleaned or replaced?
Maintenance frequency depends on sediment and debris loading at the site and how quickly the oil-absorbent component reaches capacity. Field evaluations have shown that inserts maintained on a regular schedule hold onto a larger share of their original oil and grease removal performance than units left unserviced, which is why maintenance access should be reviewed before an insert is specified, not after it is installed.
If you are weighing which media and format fit a specific inlet or discharge point, schedule a site assessment with Abtech to review the contaminant, the flow and the inlet geometry before specifying a system.