Movement of Nitrogen and Phoshorus through a Bi-Phasic Rain Garden Ecosytem.

Rain gardens are a relatively new best management practice (BMP) to treat urban storm water runoff while also providing for an esthetically pleasing landscape. They have been shown to reduce water runoff, but not always the reduction of pollutants, especially N and P. Using a novel bi-phasic (i.e. aerobic and anaerobic) design, replicated rain gardens were constructed to collect natural or simulated storm water runoff from a concrete pad. The rain gardens were designed for a water volume of 3.1 m³ (110 ft³) that was based on a 1-hour storm with a 10-year return frequency rainfall of 44.7 mm (1.76 in). From collection points in both the anaerobic and aerobic phases, aliquots of water were collected at intervals following a rain event and analyzed for total N (TN), ammonia-N (NH₃^-N), nitrate-N (NO₃^-N) and total P (TP). Concentration changes that occurred in the water as it entered and then flowed through the different parts of the rain gardens were measured to determine the removal efficiency of N and P. Results revealed that the combination of an aerobic and anaerobic zone in a bi-phasic design was more effective in removing nitrogen and phosphorus than if a single aerobic or anaerobic phase was maintained.