American Society of Civil Engineers

Optimizing Bio-retention Design to Improve Denitrification in Commercial Site Runoff

by William F. Hunt, III, P.E., (Extension Specialist, Department of Biological and Agricultural Engineering, N.C. State University, Box 7625, Raleigh, NC 27695-7625), Albert R. Jarrett, P.E., (Professor, Department of Agricultural & Biological Engineering, Pennsylvania State University, 239 Agricultural Engineering Bldg, University Park, PA, 16802), and Jonathan T. Smith, P.E., (Extension Engineer, Department of Biological and Agricultural Engineering, N.C. State University, Box 7625, Raleigh, NC 27695-7625)
Section: Urban Hydrology & Draingage, pp. 1-10, (doi:

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Document type: Conference Proceeding Paper
Part of: World Water & Environmental Resources Congress 2003
Abstract: Due to recent fish kills along the Mid-Atlantic coast, new regulations require all future land disturbing activities — including the urbanization or development of land — to be environmentally friendly. Consequently, stormwater practices are being designed to remove nutrients, particularly nitrogen and phosphorus, before runoff reaches streams. One stormwater practice, bio-retention, is gaining popularity in commercial development because it can easily be sited in required parking lot medians; however, the current design methods used for bio-retention do not reduce nitrate-nitrogen levels sufficiently, as they are designed and constructed to have aerobic drainage conditions throughout the entire soil profile. For nitrate-nitrogen to be converted to nitrogen gas, an anaerobic zone is necessary. This research determined the effect of an anaerobic layer within bio-retention on nitrate-nitrogen levels in stormwater effluent. Two principal methods will be used to measure nitrate-nitrogen reduction. First, 10 bio-retention microcosms were constructed. Each microcosm was designed with the ability to set a variable anaerobic layer. Synthetic stormwater runoff was be applied to each of the microcosms, and the effects of an anaerobic drainage layer, the retention time of stormwater, the thickness of the anaerobic layer, and rainfall frequency were measured with respect to pollutant reduction. Using the results from the laboratory experiment, 10 field bio-retention areas were designed and installed in North Carolina. The field sites primarily tested the impact of an anaerobic zone in the bio-retention unit. The required soil depth of bio-retention is also examined. Results from the laboratory and field experiments will be used to adjust the current bio-retention design guidelines in North Carolina and, potentially, other Mid-Atlantic states. Initial results indicate that the drainage configuration using the sandy loam soil tested is not a significant factor in nitrogen reduction. However, regardless of the design, TN, NO3, and NR4 removal rates exceeded 80% in two separate laboratory studies.

ASCE Subject Headings:
Stormwater management
Best Management Practice
Nonpoint pollution