Enhancing denitrification using a novel in situ membrane biofilm reactor (isMBfR)

The insufficient supply of electron donor in surface water contaminated with nitrate leads to its incomplete reduction in natural or constructed wetlands. Although the addition of organic matter (represented as chemical oxygen demand, COD) can stimulate N removal by denitrification, direct supplementation of COD creates unacceptable risks to effluent quality. An alternative for stimulating denitrification is supplying hydrogen gas (H₂) as an inorganic electron donor. We evaluate an innovative means to do H₂-based denitrification of surface waters in a wetland setting: the in-situ membrane biofilm reactor (isMBfR), in which H₂ is delivered to a biofilm of denitrifying bacteria on demand based on the presence of nitrate. We carried out a proof-of-concept study in which an upper "photo zone" and a lower "MBfR root zone" were combined to remove nitrate and COD from simulated surface water. Employing mass-balances for H₂, COD, nitrate, and oxygen, we documented nearly complete removals of nitrate and COD, except when the H₂ supply was intentionally shut off. All nitrate removal was accomplished in the "MBfR root zone," where H₂ delivery supplemented the COD supply (as needed) and provided the large majority of electron equivalents to reduce nitrate to N₂.