ENGINEERED BIORETENTION FOR REMOVAL OF NITRATE FROM STORMWATER RUNOFF

Control of nitrate from urban stormwater runoff can have a significant impact on nitrate levels in local waters. One option for providing treatment to urban stormwater runoff is bioretention, a simple plant- and soil-based low impact treatment/infiltration facility. The goal of this study is to re-engineer the concept of bioretention to remove nitrate from urban runoff. Specifically, a modification to incorporate a continuously submerged anoxic zone with an overdrain is being evaluated for its capacity for nitrate removal via denitrification. Work to date has focused on selecting an electron donor and carbon source that will promote significant denitrification and be stable for extended periods of time in the subsurface. The electron donor and carbon source to be used in bioretention must be readily metabolizable, as well as low cost and readily available. Two sets of organic substrates for chemoorganotrophic denitrifying bacteria were evaluated: Set #1, alfalfa, newspaper, and leaf mulch compost; and Set #2, sawdust, wood chips, and wheat straw. An inorganic substrate for chemolithotrophic denitrifying bacteria, sulfur, was evaluated in experimental Set #3, in three configurations: large sulfur particles (2 to 2.36 mm) alone, large sulfur particles with limestone, and small sulfur particles (0.6 to 1.18 mm) with limestone. All materials were uniformly mixed with washed silica sand and transferred into 40 cm long by 6.4 cm inner diameter Plexiglas columns. A total of 4 columns were used for each experimental set, including a control column packed with washed silica sand only. The columns were seeded with the supernatant of settled secondary effluent and fed anoxic synthetic stormwater runoff. Based on the Set#1 and Set#2 experiments for the organic substrates, excellent nitrate removal was observed in columns containing alfalfa, newspaper, wheat straw, wood chips, and sawdust. However, based on total nitrogen removal and other water quality characteristics, newspaper and wood chips are the best candidates out of these sets. In Set#3, significantly better nitrate removal occurred in the column with the small sulfur particles/limestone compared to the large sulfur particles, probably as a result of the increased surface area of the sulfur available per unit volume of reactor. Further studies will be performed using the electron donors that gave the best nitrate removal efficiency and effluent quality in the experiments reported here: newspaper, wood chips, and small sulfur particles/limestone.