Determination of nitrate removal kinetics model parameters in woodchip bioreactors

Woodchip bioreactors have emerged as a viable water management tool to reduce nitrate contamination from agricultural subsurface drainage, wastewater, and stormwater. Understanding of denitrification kinetics is critical to the design and application of field woodchip bioreactors. The denitrification process in woodchip bioreactors generally obeys a model of Michaelis-Menten type enzyme kinetics. The objective of this study was to determine Michaelis-Menten model parameters for nitrate removal in laboratory bioreactors using the fresh, composted and aged woodchips. The results showed that the maximum nitrate removal rates (V_max) were 2.09, 0.88 and 0.30 mg N/L/h, and the half saturation constants (K_m) were 2.60, 2.16 and 2.01 mg N/L for the composted, fresh and aged woodchip bioreactors at 22 °C. The V_max values decreased to 0.26 and 0.05 mg N/L/h, and the K_m values decreased to 1.74 and 1.19 mg N/L when the composted and fresh woodchip bioreactors were operated at 5 °C. Denitrification in woodchip bioreactors can be operationally defined as a zero-order reaction when treating contaminated water with nitrate much higher than the K_m values. The nitrate removal efficiency of the bioreactors followed the order of composted woodchips > fresh woodchips > aged woodchips. The average nitrate load reduction rates were 8.81-21.0, 7.36-9.78, and 2.46-3.54 g N/m³/d for the composted, fresh, and aged woodchip bioreactors at influent nitrate concentrations of 10-50 mg N/L and 22 °C. Woodchip composting before bioreactor installation can be used as a practical strategy to enhance denitrification performance of bioreactors.