Learn how a StormFilter cartridge system works inside a catch basin or vault, how much flow and sediment it handles, and how it compares with drop-in inserts on pollutants, cost and maintenance.
A StormFilter is a Contech underground treatment device that routes runoff through rechargeable, media-filled cartridges housed in a catch basin, manhole or vault. The cartridges trap particulates and adsorb some dissolved pollutants. To compare one with a drop-in insert, look at the pollutant fraction you must treat, the flow you must pass and the maintenance access you have.
How do the cartridges work inside a catch basin or vault?
The Stormwater Management StormFilter houses media-filled cartridges that trap particulates and adsorb pollutants such as suspended solids, hydrocarbons, nutrients and metals. Standard structures include catch basin, manhole, vault and curb inlet configurations.
The EPA's verification report on the catch basin version describes the flow path. A four-cartridge unit has a storm grate and inlet bay that acts as a grit chamber, a flow spreader that traps floatables, oil and surface scum, a cartridge bay, an overflow baffle and an outlet bay. Water infiltrates the media into a center tube. When the tube fills, a float valve opens and a check valve closes, which creates a siphon that draws water through the media. The filtered water drains to a manifold and out the discharge pipe. The verified unit used an organic granular media made from composted deciduous leaves.
How much flow and sediment does one unit handle?
A four-cartridge catch basin unit treats 60 gpm and holds 100 lb of dry sediment before the cartridges are full. The same report states each cartridge is designed for 15 gpm and 25 lb of dry sediment capacity, and that flows above the cartridge capacity bypass the media.
The arithmetic: 4 cartridges × 15 gpm = 60 gpm, which matches the unit's stated maximum treated flow. 4 cartridges × 25 lb = 100 lb of dry sediment. The StormFilter was designed to treat a maximum flow rate of 60 gpm, and flows greater than that would pass the overflow baffle to the discharge pipe, bypassing the filter media. The bypass is intentional, because it prevents catch basin backup and surface flooding, but it means a small unit on a large drainage area treats only part of a big storm. Units come in one- to four-cartridge configurations, and cartridge count and bay size change capacity, so size the unit against the flow you need to treat.
What types of catch basin filters and inserts are there?
Three format classes cover most options. Each differs in excavation, power and media.
- Cartridge vault systems, such as the StormFilter, sit in a purpose-built structure. They suit new construction or sites where excavation and maintenance access are available.
- Drop-in inserts fit an existing basin. The EPA describes inlet controls as products that capture floatable and settleable pollutants such as oil, grease, trash and coarse sediment, and notes they are typically pretreatment devices. Our Ultra Urban® Filter is fabricated to each basin's geometry and retrofits without excavation, pumps or power; see how the insert and media work together.
- Media packs such as Smart Pak® and Smart Bags place media inside vaults, channels and other structures you already have.
What do sediment-focused filters remove well?
Sediment-focused filters retain particulates, trash and floatables well. A NOAA-hosted field study of a catch basin insert reported median turbidity reductions of 81.2%, a measure of particulate-bound pollutants. The same study notes that insert effectiveness at capturing gross particulate pollutants has been demonstrated previously.
If your lab report shows TSS as the exceedance and dissolved fractions are within limits, a screen or sediment insert may be all you need.
Which dissolved pollutants do they miss?
Dissolved metals, phosphorus and hydrocarbons pass through sediment controls, and an insert needs adsorbent media to retain them. Even then, removal is partial and depends on the site's water. The NOAA study added adsorbent media to an insert and reported these field medians:
| Pollutant | Median removal, field study |
|---|---|
| Dissolved copper | 27.4% |
| Dissolved lead | 36.3% |
| Dissolved zinc | 69.3% |
| Total petroleum hydrocarbons | 45.6% |
| Turbidity (particulate-bound) | 81.2% |
These are third-party results from one study, not Smart Sponge® results, and they will not transfer to another site. They show the gap: particulates drop sharply, while dissolved copper and lead are only partly retained. Contech lists total phosphorus, soluble metals, oil and very fine particles among the pollutants its media systems target. For dissolved metals, see our page on catch basin inserts for heavy metals.
How is performance verified, and which approvals should you look for?
Ask for independent verification of your exact configuration and media. The EPA's Environmental Technology Verification report on the catch basin StormFilter is one example, and NJCAT verification is another.
Questions to put to any report:
- Does the tested unit match the cartridge count, media and structure you are buying?
- Was it a field or laboratory test, and what was the influent water like?
- Does it report dissolved fractions, or only total solids?
- What flow rate and loading were tested, and where does bypass begin?
- Is the media in the report the media in the quote?
A report verified on one media tells you little about a different media.
What do catch basin filters cost?
Ask for a quote that itemizes the pieces, because lifecycle cost depends on several things. Cartridge count sets capacity, and media type sets what dissolved pollutants you can address. Excavation for a vault is a large cost driver compared with a retrofit insert that needs no digging. Media replacement frequency and crew access then repeat every cycle. Compare quotes on total cost over the life of the unit, with media and labor included.
How often do you inspect, clean and replace media?
Plan around a 12-month media replacement interval and inspect after heavy rain. Contech states it typically designs StormFilter systems for a target filter media replacement interval of 12 months. Cartridges hold 25 lb of dry sediment each, so a high-sediment site can fill them sooner.
Neglect has predictable effects. Full cartridges treat less water, more flow goes to bypass, and untreated runoff leaves the site.
How do you choose between a StormFilter and an insert?
Choose by pollutant fraction, flow, footprint and maintenance access. Sediment and trash point to screens or inserts. Dissolved metals, phosphorus, bacteria, hydrocarbons or PFAS point to targeted media. A cartridge vault fits where flow and excavation allow, and a drop-in insert fits where retrofitting without excavation matters. Test on your own water before committing, since results depend on the site.
If you want to size a schedule and format for your basins, start with how cleaning schedules are set.