A flow-through structure that uses granular media (e.g. sand or activated alumina) to actively filter stormwater to remove stormwater pollutants. Filtration is controlled by the flow rate through the media and discharge via an underdrain or outlet. Little to no volume loss occurs. May be confined space but not always. Pollutant load reductions achieved by reduced concentrations in effluent. No stormwater volume reduction occurs. Typically, moderate sized centralized BMPs but can be small decentralized BMPs treating small areas of imperviousness.