Systematic evaluation of nitrate and perchlorate bioreduction kinetics in groundwater using a hydrogen-based membrane biofilm reactor

To evaluate the simultaneous reduction kinetics of the oxidized compounds, we treated nitrate-contaminated groundwater (∼9.4 mg-N/L) containing low concentrations of perchlorate (∼12.5 μg/L) and saturated with dissolved oxygen (∼8 mg/L) in a hydrogen-based membrane biofilm reactor (MBfR). We systematically increased the hydrogen availability and simultaneously varied the surface loading of the oxidized compounds on the biofilm in order to provide a comprehensive, quantitative data set with which to evaluate the relationship between electron donor (H2) availability, surface loading of the electron acceptors (oxidized compounds), and simultaneous bioreduction of the electron acceptors. Increasing the H2 pressure delivered more H2 gas, and the total H2 flux increased linearly from ∼0.04 mg/cm2-d for 0.5 psig (0.034 atm) to 0.13 mg/cm2-d for 9.5 psig (0.65 atm). This increased rate of H2 delivery allowed for continued reduction of the acceptors as their surface loading increased. The electron acceptors had a clear hydrogen-utilization order when the availability of hydrogen was limited: oxygen, nitrate, nitrite, and then perchlorate. Spiking the influent with perchlorate or nitrate allowed us to identify the maximum surface loadings that still achieved more than 99.5% reduction of both oxidized contaminants: 0.21 mg NO3-N/cm2-d and 3.4 μg ClO4/cm2-d. Both maximum values appear to be controlled by factors other than hydrogen availability.