Nitrate removal from tile drainage water: The performance of denitrifying woodchip bioreactors amended with activated carbon and flaxseed cake

Abstract

The performance of three (1.0m3 volume each) pilot-scale bioreactors, with one filled with woodchips only and the other two amended (10% v/v) with activated C and flaxseed cake, respectively, was tested under field conditions for nitrate removal from tile drainage water in Lithuania. The bioreactors were monitored for a period of 1.5 y, during which the flow rate, hydraulic retention time, water T, pH, dissolved O, nitrate, nitrite, ammonium, sulfate, and chemical O demand concentrations were measured. The results showed that the average nitrate removal efficiency in the bioreactor with no additives was 40.3%, and it was 44.1% in the bioreactor with activated C additives. In the bioreactor with flaxseed cake additives, 41.2% of the N was removed. During the cold seasonal period when the inflow T varied between +1.8 °C and +10.0 °C, activated C-amended woodchips showed higher efficiency and supported higher nitrate removal rates compared to those of other fillers. The highest nitrate removal efficiency was achieved at a pH of 7.0–7.5. The experiment demonstrated the potential for the enhancement of nitrate removal through the incorporation of activated C in denitrifying bioreactors. The addition of activated C substantially reduced the organic C losses while maintaining denitrification. Woodchips amended with flaxseed cake showed no significant difference in nitrates removal compared to the woodchips only treatment. A much higher release of organic C with no added effect on nitrate removal indicated that flaxseed cake as an external C source was more biodegradable than woodchips, but the benefit of its addition to woodchips was not proved.