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The temperature response of nitrate removal in denitrification beds

The addition of reactive nitrogen (Nᵣ) to agricultural systems has helped crop production match human population growth. However, the addition of Nᵣ comes at a cost to environment in the form of ozone destruction, habitat degradation and biodiversity loss. Denitrification beds represent an effective method for the removal of Nᵣ from a range of wastewaters and groundwater with high nitrate (NO₃¯) concentrations. Beds are lined containers filled with a carbon (C) source to enhance denitrification: the conversion of NO₃¯ to unreactive dinitrogen (N₂). In general, the rate of NO₃¯ removal in denitrification beds increases with increasing temperature. However, the temperature response of NO₃¯ removal in beds is poorly constrained as other controlling factors (e.g. NO₃¯ concentration and C source availability) can obscure the effect of temperature. The objective of this study was to measure the rates of NO₃¯ removal in three denitrification beds as temperature changed seasonally. The beds were located in the North Island of New Zealand and were loaded with NO₃¯ from wastewater from a hydroponic glasshouse (Karaka), domestic effluent from a campground (Motutere) and wastewater and domestic effluent from a research station (Newstead). Water samples were collected from wells installed along the length of each bed every month and were analysed for NO₃¯ concentration by ion chromatography. Rates of NO₃¯ removal were calculated using the change in NO₃¯ concentration and the flow rate. The temperatures of the beds were also measured at each sampling. Nitrate concentrations declined along the length of each denitrification bed and rates of NO₃¯ removal were calculated to average 3.6, 4.3 and 1.7 g N m¯³ day¯¹ for Karaka, Motutere and Newstead, respectively. The rates of removal increased with increasing temperature at Karaka and Motutere and the Q₁₀ values (the factor by which the rate of removal increased for a 10 °C increase in temperature) were calculated as 4.1 and 2.2 for Karaka and Motutere, respectively. The rates of NO₃¯ removal and Q₁₀ values were similar to those reported in previous studies of denitrification beds both in New Zealand and overseas. However, the rate of NO₃¯ removal at Karaka was less than the rate of removal of 7.6 g N m¯³ day¯¹ previously measured at Karaka in a study 5 years ago. Similarly, the temperature response at Karaka was higher than the Q₁₀ of 2 reported in this previous study at Karaka. The decrease in removal and increase in Q₁₀ may have been due to a decline in C source quality. There was no evidence of an increase in the rate of NO₃¯ removal with temperature at Newstead, with a Q₁₀ calculated as 1.0. The denitrification bed had been recently installed and was in a start-up phase. It was likely that the pretreatment system, in particular the nitrifying component responsible for converting ammonium (NH4+) in the effluent to NO₃¯, was not functioning effectively which resulted in low NO₃¯ concentrations entering the bed at Newstead. Nitrate was depleted within the beds at Motutere and Newstead which indicated that the rates of removal were NO₃¯ limited and that the temperature response may not have been adequately measured. This study confirmed that the rate of NO₃¯ removal increased with increasing temperature in the denitrification beds at Karaka and Motutere. The temperature response of NO₃¯ removal was similar to the response reported in previous studies of denitrification beds. However, additional research is required to further constrain the range of Q₁₀ values from which future denitrification beds can be designed to optimise NO₃¯ removal. Whether Q₁₀ values increase as wood chips age and C quality decreases also requires further investigation.
Type of thesis
Carter, A. M. (2014). The temperature response of nitrate removal in denitrification beds (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/8696
University of Waikato
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