Nitrate removal from tile drainage water using woodchip denitrification bioreactors

Abstract

The obtained results suggest that denitrification bioreactors can be applied in tile drainage systems as edge of field measure to reduce water pollution in drained agricultural areas in Lithuania. However, more investigations both at the laboratory scale and under field conditions are needed to quantify the long-term performance of these bioreactors in order to optimize the N03 removal rates for various flow conditions. In Lithuania, artificial drainage is a common agricultural practice. It remains one of the most extensively drained countries in the world. The total drained land area occupies 47% of the country's land area and 86% of agricultural land area, of which 87% is tile-drained. Although the introduction of drainage has improved the quality of agricultural land, the benefits of drainage were associated with many alterations in the local environments. Tile drainage systems significantly alter water and nutrient balance and increase water pollution via accelerated nitrates (N03) leaching from the soil. As new technology, woodchip denitrification bioreactors for tile drainage are being investigated for practical edge-of-field N03 removal. This "nature-driven" technology is based on routing tile drainage water through woodchip bioreactors where nitrate is used by bacteria to oxidize carbon while reducing N03 to nitrogen gas. Therefore, the main objective of this research was to determine the efficiency of N03 removal from drainage water using woodchip denitrification bioreactors. Three denitrification bioreactors imitating tile drainage systems were created in Drainage laboratory of the Water Resources Engineering Institute at Aleksandras Stulginskis University, Lithuania. Bioreactors containers were filled with deciduous, coniferous and mixed wood chips. Laboratory and field tests have shown that the average N03 removal efficiency from water in the bioreactor filled with deciduous woodchips was 66%, with conifer woodchips - 68%, and with mixed woodchips filler - 67%. The average nitrogen removal rates were 1.49, 1.82 and 1.50 g irf3 h"1 at 3 .0 b hydraulic retention time (HRT), respectively.