Denitrification bioreactors – an approach for reducing nitrate loads from tile drainage water

Abstract

Excess water from agricultural areas in Lithuania is mostlyremoved by tile-drainage systems. Flowing into drains,water washes out inorganic nitrogen (mainly nitrates) compounds from the soil. The load of nutrients entering open streams results to eutrophication process that disturbs natural trophic relationships of aquatic ecosystems. To reduce the pollution of nitrate-nitrogen from drainage systems, nature-basedsolutions- denitrification bioreactors –have been developed. The operation of these biotechnologies is based on the biological nitrogen removal process. The rate of transformations of nitrates into gaseous forms depends on biological activity, inflow water temperature, pH, dissolved oxygen content, flow velocity and hydraulic retention time. Therefore, three bioreactors (1 m3each)were installed under field conditions at Aleksandras Stulginskis University, Lithuania. The bioreactors were filled with mixed woodchips along with three types of additives (10% v/v). Woodchips in denitrification bioreactors create a suitable environment for the development of heterotrophic bacteria. Nitrogen removal efficiency along with measurements of various water parameters at the inlet and outlet of each bioreactor were conducted during the period from June to December 2017 with irregular time intervals. The study showed that the average nitrate removal efficiency in bioreactor with no additiveswas 44%. The average removal efficiency in bioreactor with activated carbon additive was 48%, while in bioreactor with the flax-seed cake additive - 43% of nitrogen load was removed. During the cold seasonal period, when water temperature at the inlet varied between 2.10C, and 12.20C, the average removal efficiency of 32%, 36% and 34% was observed in bioreactor with no additives and in bioreactors with activated carbon and flax-seed cake additives, respectively.