Determination of major biogeochemical processes in a denitrifying woodchip bioreactor for treating mine drainage

•Woodchip bioreactor able to remove NO3− in mine and process water.•Increased production of NH4+, NO2−, and N2O relative to NO3− reduced at <5 °C.•Denitrification, sulfate reduction, DNRA, and fermentation were the major processes.•Biogeochemical diversity declined with time with joint changes in pH and alkalinity.•Decline may be triggered by a change in carbon substrate with time.

At the Kiruna iron ore mine in northern Sweden, mine drainage and process water contain elevated concentrations of nitrate (NO3−) from the use of ammonium nitrate fuel oil explosives. In order to investigate the treatment capacity of a denitrifying woodchip bioreactor technique for the removal of NO3− through denitrification, a bioreactor was installed at the mine site in 2015 and operated for two consecutive years. Neutral-pH mine drainage and process water containing 22 mg NO3−-N and 1132 mg SO42− (average) was passed through the bioreactor which was filled with a reactive mixture of pine woodchips and sewage sludge, at treatment temperatures ranging between 0.8 and 17 °C. At bioreactor temperatures above ∼5 °C, NO3− removal proceeded to below detection limits (0.06 mg N L−1) without substantial production of nitrite (NO2−), ammonium (NH4+), nitrous oxide (N2O), or methane (CH4). The relative production of NH4+ and N2O to the NO3− reduced increased as bioreactor temperatures decreased below ∼5 °C. Based on the resultant changes in alkalinity and pH from the production of bicarbonate (HCO3−) and carbonic acid (H2CO3), a stoichiometric mass balance model indicated that denitrification, nitrate reduction to ammonium (DNRA), sulfate reduction, and fermentation were the major biogeochemical processes controlling pH, alkalinity and nitrogen, sulfur and carbon concentrations in the system. It is suggested that fermentation changed from being mainly butyrate producing to acetate producing with time, triggering a decline in biogeochemical process diversity and leaving denitrification as the sole major electron accepting process.

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