Denitrification bioreactors - a method for reducing nitrate from tile drainage water

Artificial drainage is a common agricultural practice in Lithuania. In this country, the total drained land area occupies 47% of the total land area and 87% of the agricultural land area. It is well known that tile drainage systems serve as transport pathways of contaminants directly from agricultural land to streams. When entering drains, water leaches nutrients (mostly inorganic forms of nitrogen), and the increased nutrient inflow into surface water bodies leads to their eutrophication. As a new technology, woodchip denitrification bioreactors for tile drainage are being investigated for practical edge-of-field nitrate (NO3) removal. This technology is based on routing tile drainage water through the bioreactors, where nitrate is used by bacteria to oxidize carbon while reducing NO3 to nitrogen gas. The rate of transformations of nitrates into gaseous forms depends on biological activity, inflow water temperature, pH, dissolved oxygen amount, flow velocity and hydraulic retention time. Therefore, to test the technology three pilot-scale bioreactors (1 m3 each) under field conditions were installed at Aleksandras Stulginskis University, Lithuania. The bioreactors were filled with mixed woodchips along with two types of additives (10% v/v) — activated carbon and flax-seed cake. The prevailing particle diameter of the woodchips varied from 1.5 to 3.0 cm. The bioreactors were fed nitrate during the study period at concentrations ranging from 4.0 to 29.1 mg N L1. Nitrate-nitrogen removal efficiency and rate along with the measurements of other water parameters at the inlet and outlet of each bioreactor were conducted with irregular time intervals. The study has shown that the average nitrate removal efficiency in bioreactor with no additives was 44% and in bioreactor with activated carbon additive - 48%. ... [et al.]Vytauto Didžiojo universitetasŽemės ūkio akademij