Nitrate Removal in Bioreactors
Nitrate-N, dissolved N2O-N, and flow rates of bioreactors in two locations (Dundas, and Claremont, Minnesota, USA) were measured during the 2010 growing season. Drainage waters contained an average of 24 and 16 mg N L-1 of nitrate, and 21 and 9 µg N L-1 of N2O-N in Dundas and Claremont, respectively. Bioreactors reduced nitrate-N concentration in both locations (22-99% in Dundas, 6-77% in Claremont). Fujinuma, R., Venterea, R.T., Ranaivoson, A., Moncrief, J., Dittrich, M. 2011. On-site wood-chip bioreactors could reduce indirect nitrous oxide emissions from tile drain waters [abstract]. Proceedings of Nutrient Efficiency and Management Conference. MN Department of Agriculture. February 15, 2011. Abstract #5.
The object of this work was to analyze nitrate removal performance from four existing bioreactors in Iowa with particular attention paid to potential performance-affecting factors including: retention time, influent nitrate concentration, temperature, flow rate, age, length to width ratio, and cross-sectional shape. Based on a minimum of two years of water quality data from each of these four bioreactors, annual flow-weighted nitrate-nitrogen concentration reductions ranged from 12% to 75% with a mean of 43% from all sites in all years. Bioreactor and total (including bypass flow) nitrate-nitrogen load reductions ranged from 12% to 76% (mean 45%) and 12% to 57% (mean 32%), respectively. Statistical modeling showed temperature and influent nitrate concentration were the most important factors affecting percent bioreactor nitrate load reduction and nitrate removal rate, respectively. Modeling also indicated load reductions within the bioreactor were significantly impacted by retention time at three of the four reactors; a retention time effect on nitrate concentration reduction was especially evident during and after elevated drainage flow events at one of the sites. Christianson, Laura Elizabeth, "Design and performance of denitrification bioreactors for agricultural drainage" (2011). Graduate Theses and Dissertations. Paper 10326. http://lib.dr.iastate.edu/etd/10326.
Nitrate Removal in Bioreactors
Not all woodchips are created equal. To allow the good, denitrifying bacteria time to remove the nitrate from the water, bioreactors are designed based on a specific flow rate of water that the woodchips allow (that is, hydraulic conductivity of the woodchips). Laura Christianson. Woody Biomass Reactors for Nitrate in Agricultural Drainage. Iowa State University.
Bioreactors are currently being designed to treat 20% of peak flow, with a four-hour retention time within the bioreactor in Iowa. Bioreactors Take on the Dead Zone. Farm Journal Magazine. January 7 2012. Accessed at: http://www.agweb.com/article/bioreactors_take_on_the_dead_zone/
In a Minnesota study, residence times of 1 to 2 days was needed to reduce nitrate levels by 50%. Ranaivosen, et al. Bioreactor Performance in Minnesota. University of Minnesota. Minnesota/Iowa 11th Annual Drainage Research Forum. 2010.
Using chips that have many fine materials, shredded materials, dirt, and gravel can change this allowable rate of water flow, meaning the bioreactor may not work as intended. Currently chips used in bioreactor research have had the majority of the chips falling within the ¼-inch to 1-inch size range. Laura Christianson. Woody Biomass Reactors for Nitrate in Agricultural Drainage. Iowa State University.
Chips made from treated or preserved wood are not recommended because this limits the bacteria's ability to use the carbon in the wood. Also, including green material such as leaves or conifer needles is not recommended due to their relatively high nitrogen content and their potential to quickly be degraded. Laura Christianson. Woody Biomass Reactors for Nitrate in Agricultural Drainage. Iowa State University.
A number of other carbon source materials such as corn cobs, corn stalks, wheat straw, cardboard, and newspaper have been investigated, but research has recommended woody material because it provides a sustainable carbon source that lasts longer. Laura Christianson. Woody Biomass Reactors for Nitrate in Agricultural Drainage. Iowa State University.