Denitrification woodchip bioreactor two-phase column study: Evaluation of nitrate removal at various hydraulic retention times and effect of temperature on denitrification rates
Michelle L Soupir
Denitrification woodchip bioreactors, a relatively new technology for edge-of-field treatment of subsurface agricultural drainage, are showing great potential for nitrate removal. However, additional research is needed to evaluate potential adverse environmental impacts of bioreactor installation, as well as determine nitrate removal efficiency under varying conditions. A two-phase denitrification bioreactor column study was conducted, with phase 1 as a room temperature study at a mean temperature of 21.5deg;C, and phase 2 as a controlled temperature study at 10°, 15°, and 20°C. This two-phase triplicate column study investigated (1) the potential release of total organic carbon,(2) nitrate removal at 2, 4, 8, 12, and 24 hour hydraulic retention times(HRT), (3) nitrate removal at 10, 30, and 50 mg L-1 influent nitrate concentrations, and (4) nitrate removal efficiency at a 12 hour hydraulic retention time at 10°, 15°, and 20°C. The columns were packed separately for both phases of the study, with marked differences in woodchip weathering characteristics due to outdoor storage before purchase. A notably higher release of total organic carbon was observed during the first 25 days of phase 1 compared to the initial flush observed during phase 2. These results suggest weathering woodchips before bioreactor installation may mitigate the potential environmental impact of introducing a new carbon source into a system. During phase I, percent nitrate removal was observed to increase with increased hydraulic retention time. However, nitrate load removal per day did not follow the same trend. The highest nitrate load removal was observed at 2 and 12 hour HRTs, with statistically identical at 2 hour and 12 hour HRTs. There was no significant difference in load removal between the target 4, 8, and 24 hour HRTs. Additionally, there is no significant difference in load removal between 18 and 24 hour HRTs. The effect of influent concentration on nitrate load removal and concentration reduction was evaluated during both the room temperature and controlled temperature phases of the study. Comparison of mean nitrate removal for various influent nitrate concentrations showed lower nitrate removal with a lower nitrate concentration of 10 mg -1, with similar nitrate removal at influent concentrations of 30 and 50 mg -1. During the temperature controlled phase of the study, temperature was held to 10°, 15°, and 20°C with a relatively steady achieved HRT of 12.81 ± 0.74 hours. Nitrate removal showed a stepped increase with temperature. A Q10 factor of 2.28 determined from the change in nitrate removal rates from 10-15deg;C provided a reasonable estimation of nitrate removal rates with changing temperature for this study.