Nitrate Removal from Tile Drainage Water – Laboratory Tests Using Denitrification Bioreactors
Tile drainage systems introduced in agricultural areas significantly alter water and nutrient balance and increase water pollution via accelerated nitrates leaching from the soil. In order to reduce this negative effect, new ways and measures to reconstruct tile drainage systems are being searched for. One of the possible solutions of this problem is installation of denitrification bioreactors at the outlets of the systems. Their presence is based on biological removal of nitrate-nitrogen from tile water under anaerobic conditions.
Three denitrification bioreactors imitating tile drainage systems were created in the Drainage Laboratory of the Water Resources Engineering Institute at Aleksandras Stulginskis University, Lithuania. Bioreactor containers were filled with deciduous, coniferous and mixed wood chips. Water of various nitrate-nitrogen concentrations was supplied to the bioreactors from specially equipped tanks creating different flow and water retention conditions in the bioreactors.
Laboratory scale tests show that, during the first three decades (30 days) starting from implementation, biochemical processes stabilise in bioreactors and the most effective nitrate-nitrogen transformations taking place in the water are reached. Then nitrate-nitrogen compounds are rapidly converted into gaseous forms of nitrogen. The effectiveness of these transformations largely depends on biological activity (denitrifying bacteria), which is dependent on water temperature, dissolved oxygen amount and water retention time. An experimental study showed that the average nitrate-nitrogen removal from water was 54% in the bioreactor filled with deciduous wood chips, 60% in the bioreactor filled with conifer wood chips, and 63% in the bioreactor filled with mixed wood chip.
The obtained results suggest that denitrification bioreactors can be applied in tile drainage systems as an edge-of-field measure to reduce water pollution in drained agricultural areas.