There are several types of stormwater filters, including cartridge filters, media filter drains, sand filters, and gravel filters.

Water Quality Benefit: As water soaks through filter materials, pollutants are physically trapped, chemically neutralized, or biologically recycled back into the environment. Filters prevent particles of dirt and other pollutants, including hydrocarbons, fertilizers, and metals like zinc and copper (which are toxic to fish), from being released into creeks and lakes.

Cartridge filters are cylinder-shaped containers filled with special material that traps particles and absorbs pollutants. Cartridges are used in vaults or tanks. Just like an air filter in a car, cartridge filter materials need to be replaced to remove pollutants and maintain their ability to treat runoff.

Sand and gravel filters use layers of sand, gravel, and/or rock to trap and strain particles out of the water. By slowing the water down, most of the sediment, small particles, and some of the pollution can settle out into the crevices of the sand and gravel. Sand filters are better at removing pollution than gravel filters because they have more surface area to collect pollutants. The smaller the gravel size, the smaller the particles that are removed. Pollutants stick to the sand particles where chemical and biological processes break them down.

Media filter drains (MFDs) use special materials, or media, that target tough-to-capture pollutants, such as dissolved metals and nutrients. MFD media can be used inside a trench or vault, or along a shoreline. The media is made of tiny washed rocks, slightly larger than grains of sand, with perlite, dolomite, and gypsum added to it. The perlite absorbs water and expands, physically trapping particles of pollution. Next, dolomite and gypsum chemically react with water to trap nutrients including phosphorus, nitrogen, and potassium. Then, with the help of naturally occurring microbes, nutrients are recycled back into the environment.