Hydrologic and Water-Quality Evaluation of a Rapid-Flow Biofiltration Device

Manufactured treatment devices (MTDs) are increasingly installed to treat pollutants from stormwater in urban areas, but few peer-reviewed studies have assessed their field-scale performance. The Filterra biofiltration unit is a box filter combining high-flow engineered media with potential for biological treatment from a planted tree. A Filterra device was monitored for its impact on hydrology and water quality of runoff from a 0.10-ha impervious watershed in Fayetteville, North Carolina, for 22 months. Undersizing of the system and clogging of the media bed likely caused 22% of runoff to bypass the system. Although peak flows were significantly reduced (p<0.0001) by a median 57% (likely owing to parking lot detention), postdevelopment peak flows were not consistently reduced to predevelopment flows. The Filterra provided stormwater treatment by significantly reducing event mean concentrations (EMCs) and total pollutant loadings for sediment (total suspended solids and suspended sediment concentration), nutrients (total phosphorus, total nitrogen, total ammoniacal nitrogen, and total Kjeldahl nitrogen), and zinc. Median EMC removal efficiency ranged from 35 to 97% and median loading efficiency was 39–80% for these pollutants. The device did not reduce nitrate/nitrite, total dissolved phosphorus, or copper. Nutrient concentrations of stormwater discharged from the Filterra generally met “good” water-quality thresholds established for the state of North Carolina and EMC targets established by the state of Washington for emergent MTDs. Clogging susceptibility and frequent maintenance of the Filterra unit were notable. Overall, results suggest the Filterra is an effective MTD for treating stormwater from small impervious watersheds.