Biogeochemical Processes in Denitrifying Woodchip Bioreactors and their Application in the Mining Industry

Sammanfattning: This thesis evaluates passive denitrifying woodchip bioreactors (DWBs) for the removal of nitrate (NO3-) in neutral pH mine drainage, where water is passed through a carbon-rich porous matrix (e.g. woodchips) for the reduction of NO3- to nitrogen gas. DWBs have been used for the removal of NO3- from water in various settings and are expected to operate with little maintenance for at least a decade; however, the processes controlling the emission of greenhouse gases and other undesirable by-products, as well as the magnitude and variability in NO3- removal rates and how these develop over time, are not completely understood and were the focus of this thesis.Water treatment in DWBs was investigated in laboratory-scale column tests and in a pilot-scale bioreactor installed at the Kiruna iron ore mine, northern Sweden. Denitrification was the major pathway for NO3- removal in both experimental systems. Incoming NO3- concentrations (up to 30.0 mg N L-1) were removed to below detection limits at temperatures and hydraulic residence times (HRTs) between 5-22°C and ~1.9-2.6 days, respectively, without substantial production of nitrite or ammonium (NH4+). NO3- removal was incomplete in both systems when HRTs decreased to ~1 day, and/or as temperature decreased below 5°C in the pilot-scale bioreactor, under which conditions an increased production of nitrous oxide (N2O) and NH4+ was observed (relative to the NO3- reduced).In the column tests, non-ideal flow was detected and solute transport was described using a dual-porosity model. Stagnant zones not transmitting flow did not participate in NO3- removal and the fraction of immobile water increased with increases in the advection velocity, suggesting that bioreactor performance could be enhanced by emphasizing design with low advection velocities.  The study demonstrated that dominating biogeochemical processes varied with time in the pilot-scale bioreactor. There was a decline in organic carbon export and increase in pH and alkalinity that, based on a stoichiometric mass-balance, was suggested to be the result of a change in fermentation end-products that provided a carbon source to the denitrifying community. The decline in NO3- removal rates and biogeochemical process diversity, and the preferential selection of denitrifiers with the genetic capacity for reduction to N2O, but not N2, are hypothesized to arise from the temporal development of syntrophic structures between fermenters and denitrifiers.