Effects of three types of amendments in woodchip-denitrifying bioreactors for tile drainage water treatment

Abstract

Excess nutrient inflow from tile-drained agricultural fields significantly contributes to the pollution of surface waters in Lithuania. Therefore, two parallel experiments, each using three pilot-scale woodchip-denitrifying bioreactors, were conducted for 2.5- and 1.5-year periods to test whether activated carbon (C) and flaxseed cake (both 10% v/v) and biochar (10% v/v and 20% v/v made of deciduous wood) additives, respectively, can enhance the removal of nitrate-nitrogen (NO3-N) and phosphate-phosphorus (PO4-P) from tile drainage water and reduce C losses from bioreactors.Higher NO3-N removal efficiencies and rates were achieved in bioreactors amended with activated C (10% v/v) and biochar (20% v/v) than in those amended with woodchips alone. Bioreactors amended with 10% v/v flaxseed cake and 10% v/v biochar showed no significant differences from those amended with woodchips alone. The ability to enhance NO3-N removal by the addition of activated C and biochar was particularly evident at lower temperatures (≤10.0 °C). Moreover, activated C has proven to be an effective additive capable of substantially reducing C losses and supporting the lowest C/N (i.e., carbon loss to N removal) ratios. To a lesser extent, these capabilities were also characteristic of biochar-amended woodchips, whereas the addition of flaxseed cake resulted in the highest C/N removal ratios. These experiments also revealed that woodchips, alone or amended with flaxseed cake or activated C, are a suitable media that can allow PO4-P removal. However, the addition of biochar demonstrated large releases of PO4-P and other inorganic elements. Biochar itself was found to be a potential PO4-P source that poses a risk of large startup and over time releases.Although the addition of activated C under non-N-limiting conditions resulted in undesirable sulfate-sulfur (SO4-S) reduction (this occurred in all study bioreactors), the experiments demonstrated that the addition of activated C can be used to design a more efficient bioreactor with prolonged longevity and higher NO3-N and PO4-P removal capacities. Owing to the risk of adverse environmental effects (i.e., decreased pH, large exports of inorganic elements or organic C), the application of biochar or flaxseed cake additives in woodchip-denitrifying bioreactors proved to be limited for tile water treatment.