Den itrifi cation woodchip bioreactors with different additives (activated carbon, biochar, flaxseed cake) – an approach for reducing nitrate loads from tile drainage water

Abstract

Artificial drainage is a common agricultural practicc in Lithuania. Throughout the history of land drainage, the overall drained area (ditches plus tile drains) has totaled 30,214 km2, or 87 % of the agricultural land area. It is well known that tile drainage systems serve as transport pathways for contaminants directly from agricultural land to the streams. When entering drains, water leaches nutrient substances; therefore, the increased nutrient inflow into surface water bodies leads to their eutrophication. In Lithuania, 80 % of the total nitrogen in the streams enters from agricultural areas. This is a considerable challenge that requires new solutions. Various studies have suggested that bioreactors promoting heterotrophic denitrification may provide nitrate (NO3-N) removal from tile drainage w ater. However, most of the research have been performed at relatively high temperatures not relevant in climates where temperatures may be low (<10 °C) for extended periods. Therefore, woodchips amended with various additional carbon sources are increasingly being investigated to promote bioreactor efficiency under low temperature conditions. In this study, the performance of six pilot-scale denitrifying bioreactors, two filled with woodchips only and the other three amended (10 % v/v) with different types of additives - activated carbon, biochar, flaxseed cake, and the other one (20 % v/v) with biochar. The study has shown that the average nitrate removal efficiency in bioreactor with no additives was 44 % and in bioreactor with activated carbon additive it reached 48 %. In both bioreactors - with the flaxseed cake and with biochar (20 % v/v) additives - 45 % of nitrogen load was removed. The bioreactors were monitored for a 1,5 years period, during which the flow rate, hydraulic retention time, water temperature (T), pH. TDS, dissolved oxygen (O), nitrite, ammonium and nitrate concentrations were measured. [...]