Designing bioretention systems to improve nitrogen removal

Background/Question/Methods

Bioretention systems effectively remove many stormwater stressors, including oil/grease, heavy metals, phosphorus, and ammonium. However, reported nitrate removal performance is highly variable. Bioretention media is typically coarse-grained with low organic matter content, which facilitates high infiltration rates but fails to provide anaerobic conditions and carbon availability necessary to promote nitrate removal by denitrification. Research at EPA’s Edison Environmental Center investigates the effects of media carbon amendments, surface area, subsurface oxygen depleted water (SODW), and hydraulic loading on nitrogen removal. A sandy media with low organic matter content was added to eight experimental, pilot-scale rain gardens above a shallow drainage layer. Double-shredded hardwood mulch was chosen as a carbon amendment to promote denitrification and added as a 20-cm layer in four of the eight pilot-scale rain gardens; the other four did not receive the mulch. Four rain gardens were constructed with an elevated outlet pipe to create an internal storage zone resulting in anaerobic conditions to promote denitrification; the other four drain freely. Pilot-scale rain gardens were constructed in tanks of two sizes to test the effects of hydraulic loading. After initial hydrologic tests, four rain gardens will be planted with turf grass and the other four with native herbaceous plants. Hydrologic tests were conducted before and after planting. Stormwater runoff collected from an adjacent parking lot was homogenized and added to the tanks at high (6 L/min) and low (3 L/min) flow rates. Effluent flow rates and volumes were measured and samples were taken for pH and speciated nitrogen analysis at first discharge and 0.5 and 2 hours later.

Results/Conclusions

Non-vegetated Phase I results showed that all the rain gardens removed nitrate-nitrite loads. Neither surface area nor presence of the hardwood mulch layer showed a significant effect on load reductions.  Introducing the SODW and hydraulic loading showed a significant effect on nitrate-nitrite removal. Rain gardens with the SODW provided a significant increase in nitrate-nitrite load reduction (73% reduction) compared to those without this design feature (9% reduction).  The difference in ammonium load reductions based on the SODW was also significant.  The rain gardens without the SODW removed 66% while the SODW rain gardens showed an increase. Nitrate-nitrite load reduction increased significantly at larger flow rate.  Ammonium increase was significantly larger at smaller flow rate.