Nitrate removal rates in woodchip media of varying age

 

Situation: Increased anthropogenic nitrate loading can cause eutrophication in marine waters. Denitrifying bioreactors can remove nitrate from agricultural runoff, but in many cases are only practical for installation if they can be maintenance free for a number of years.  

 

Actions: In this study, nitrate removal by woodchips of various ages was tested in a laboratory under the same conditions. Four samples of woodchips were used: two that were fresh, one from a bioreactor that had been operating for 2 years, and one from a bioreactor operating for 7 years. Four different experimental runs with increasing nitrate concentrations were used. 

 

Take-home message: 

• Denitrification rates did not increase with increased concentration of nitrate, showing that nitrate was not a rate-limiting factor (reaction rate was zero-order, not first-order).
• The 7 year old media had a mean nitrate removal rate (9.1 mg N L-1 d-1) within 75% of the rate for the 2 year old media, and within 40-59% of the fresh media.
• These results were consistent with field results, showing that woodchips lose about 50% of their reactivity during their first year of operation, but then relatively stable rates persist for a number of years after.

 

For more information:

• Robertson, W.D. Nitrate removal rates in woodchip media of varying age. 2010. Ecological Engineering (36): 1581-1587.
• Email W.D. Robertson at wroberts@sciborg.uwaterloo.ca