| dc.description.abstract | 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. | |
