Biofiltration is a process that occurs as water is filtered through a combination of porous media and vegetative layers (bio). Filtration media layers may be composed of mixtures of mulch, compost, soil, sand, and gravel – which also provide a growth media for plants.
These systems use the microbial, physical and chemical properties of their layered media to filter and retain stormwater – which improves water quality, removes pollutants, and slows stormwater run-off.
Stormwater biofiltration systems include systems such as bioretention, biofilters, and rain gardens – which can be designed to meet WSUD (Water Sensitive Urban Design) Standards. Retention areas allow water to pool in these systems, and slowly drain and filter through the subsequent layers.
Biofiltration is also supported by the microbial properties of the plants growing in the bioretention media. The roots of these plants maintain the permeability of the media and allow water to infiltrate between the layers.
These plants are selected for their pollution removal capabilities and sustain microbial populations that assist with stormwater treatment. Microbial action helps to breakdown pollution, sequester pollutants, and remove nitrogen.