Stimulating in situ hydrogenotrophic denitrification with membrane-delivered hydrogen under passive and pumped groundwater conditions

A technology was developed to stimulate autotrophic biological denitrification by supplying hydrogen (H ₂) to groundwater via gas-permeable membranes. The purpose of this project was to investigate this technology at field scale, determining whether it could be successfully scaled up from the laboratory. The field site was located in Becker, Minnesota and contained high levels of NO 3- (22.8±2.0 mg/L-N) and dissolved oxygen (DO) (7±1 mg/L). Membranes installed in groundwater wells were successful in delivering H ₂ to the groundwater over the two-year operating period. Hydrogen stimulated microbial reduction of DO and NO 3-, degrading up to 6 mg/L DO and converting up to 10.0 mg/L NO 3- -N to NO 2- -N when operated passively. When recirculation pumps were installed performance in the field did not improve significantly because of mixing with more oxygenated water. However, complementary modeling studies showed that complete DO reduction and denitrification to N ₂ was possible but the zone of influence and total H ₂ demand were limiting factors. Water was recirculated in the field from downgradient to upgradient membrane-containing wells to increase the H ₂ delivery through the membrane by an increase in water velocity. The depth to groundwater (∼13.7 m) caused some water reoxygenation during recirculation, which may preclude the use of this technology at deep sites, as this makes it more difficult to install sufficient wells and control recirculation.