Maximum Rates of Nitrate Removal in a Denitrification Wall

 

Situation: : Denitrification walls (a type of denitrifying bioreactor) can effectively reduce nitrogen (N) inputs to receiving waters by enhancing denitrification, microbial process of converting nitrate to N gases, through the mixing of organic matter into the subsoil below the watertable. However, in order for the use of denitrification walls to be more widely adopted, designers of these systems need more information on the nitrate-N removal capacity.  

 

Actions: The objective of this study was to determine upper rates of denitrification in an existing denitrification wall where nitrate-N concentrations were not limiting the process. This study was conducted at the Bardowie dairy farm, Cambridge, North Island, New Zealand. Changes in microbial biomass, available carbon, and nitrate concentrations in well water and soil water samples were measured to determine factors controlling denitrification.  

 

Take-home message: 

• Despite dosing of with high nitrate-N inputs in the field, denitrification remained limited by nitrate-N concentration, leading to conservative estimates of N removal. Groundwater nitrate-N was likely preferentially flowing through areas of high hydraulic conductivity.
• Estimated nitrate-N removal rates in field testing were much higher than those in laboratory testing.
• The multitude of designs, carbon substrates, concentrations, and operating conditions for denitrification walls makes it difficult to estimate one nitrate-N removal rate that will be applicable to all locations.
• Direct field testing is recommended to estimate performance of denitrification walls rather than extrapolation of laboratory data.

 

For more information:

• Schipper, L.A., G.F. Barkle, M. Vojvodic-Vukovic. 2005. Maximum Rates of Nitrate Removal in a Denitrification Wall. Journal of Environmental Quality (34): 1270-1276.
• Email Dr. Louis Schipper at schipper@waikato.ac.nz