Temperature effects on wastewater nitrate removal in laboratory-scale constructed wetlands

Constructed wetlands may be used for removal of high nutrient loads in greenhouse wastewater prior to discharge into the environment. Temperature affects both the physical and biological activities in wetland systems. Since nitrification and denitrification are temperature-dependent processes, effluent nitrate concentrations will fluctuate due to changes in air and wetland temperature. In a cold climate, constructed wetlands can function in a temperature-controlled, greenhouse environment year-round. This work evaluates four temperature treatments on nitrate removal rates in five planted and five unplanted laboratory-scale wetlands. Wetlands were supplied with a nutrient solution similar to the fertigation runoff solution (100 PPM nitrate-N) used in greenhouse crop production. A first-order kinetic model was used to describe experimental nitrate depletion data and to predict nitrate removal rate constants (k) in the wetlands planted with Iris pseudocoras. The negligible removal in unplanted wetlands was thought to be due to lack of carbon source in the fertigation solution. Between 18 and 23°C in planted systems, k increased from 0.062 to 0.077 h^-1, appeared to peak around 30°C (k = 0.184 h^-1), but decreased at 38°C (k = 0.099 h^-1). Based on the Arrhenius equation, k was a first-order exponential function of temperature between 18 and 30°C in planted systems. Quantification of temperature effects on planted and unplanted laboratory-scale constructed wetlands can be used to enhance the design and management of wastewater treatment wetlands.