Vegetated BMP where stormwater is filtered through a specialized soil media and discharged via an underdrain. BMP may or may not be lined but little to no exfiltration of volumes. Outlet design requires surface ponding prior to surface outflow typically with a maximum ponding depth of 6 inches. Site designed biofiltration systems use specialized soil media ideally 18-24 inches in depth to enhance biogeochemical processes to retain and transform pollutants. Proprietary biofilter designs vary and specialized soil media may or may not be accessible for inspection. Small to moderate sized decentralized BMP accepting runoff from a single land use drainage area.