Filtration is the physical, chemical, and biological process by which stormwater passes through a porous medium, resulting in the removal of suspended solids, particulate-bound pollutants, and, to a lesser extent, dissolved contaminants.
In stormwater management, filtration occurs when runoff is conveyed through materials such as soil, sand, gravel, compost, engineered media, or geotextiles. As water moves through these media, particles are removed through several mechanisms. Physical straining captures larger particles in pore spaces, sedimentation occurs within voids as flow velocity decreases, and adsorption allows finer particles and dissolved pollutants to adhere to media surfaces. Biological activity within the filter media can further enhance treatment by breaking down organic pollutants and facilitating nutrient uptake.
Filtration systems are commonly incorporated into practices such as bioretention cells, sand filters, infiltration trenches, vegetated swales, and permeable pavements. These systems are designed to slow flow and maximize contact time between stormwater and the filter media, which improves pollutant removal efficiency. The composition, gradation, and depth of the media are critical design factors that influence hydraulic conductivity, clogging potential, and treatment performance.
Filtration is particularly effective at removing total suspended solids, particulate phosphorus, metals, and hydrocarbons attached to sediment particles. However, its effectiveness for dissolved pollutants varies and often depends on media composition and the presence of reactive materials that promote adsorption or chemical transformation.
Maintenance is an important consideration, as accumulated sediment and debris can clog pore spaces and reduce infiltration capacity over time. Regular inspection, sediment removal, and, when necessary, media replacement are required to ensure long-term functionality. Filtration is often used in combination with other stormwater treatment processes such as sedimentation, infiltration, and biological uptake to achieve comprehensive water quality improvement.