The H₂-based membrane biofilm reactor (H₂−MBfR) is an emerging technology for removal of nitrate (NO₃⁻) in water supplies. In this research, a lab-scale H₂−MBfR equipped with a separated CO₂ providing system and a microsensor measuring unit was developed for NO₃⁻ removal from synthetic groundwater. Experimental results show that efficient NO₃⁻ reduction with a flux of 1.46 g/(m²⋅d) was achieved at the optimal operating conditions of hydraulic retention time (HRT) 80 min, influent NO₃⁻ concentration 20 mg N/L, H₂ pressure 5 psig and CO₂ addition 50 mg/L. Given the complex counter-diffusion of substrates in the H₂−MBfR, mathematical modeling is a key tool to both understand its behavior and optimize its performance. A sophisticated model was successfully established, calibrated and validated via comparing the measured and simulated system performance and/or substrate gradients within biofilm. Model results indicate that i) even under the optimal operating conditions, denitrifying bacteria (DNB) in the interior and exterior of biofilm suffered low growth rate, attributed to CO₂ and H₂ limitation, respectively; ii) appropriate operating parameters are essential to maintaining high activity of DNB in the biofilm; iii) CO₂ concentration was the decisive factor which matters its dominant role in mediating hydrogenotrophic denitrification process; iv) the predicted optimum biofilm thickness was 650 µm that can maximize the denitrification flux and prevent loss of H₂.

Wastewater treatment plants can be significant sources of nitrous oxide (N₂O), a potent greenhouse gas. While our understanding of N₂O emissions from suspended-growth processes has advanced significantly, less is known about emissions from biofilm processes. Biofilms may behave differently due to their substrate gradients and microbial stratification. In this study, we used mathematical modeling to explore the mechanisms of N₂O emissions from nitrifying and denitrifying biofilms. Our ammonia-oxidizing bacteria biofilm model suggests that N₂O emissions from biofilm can be significantly greater than from suspended-growth systems. The driving factor is the diffusion of hydroxylamine, a nitrification intermediate, from the aerobic to the anoxic regions of the biofilm. The presence of nitrite-oxidizing bacteria further increased emissions. For denitrifying biofilms, our results suggest that emissions are generally greater than for suspended-growth systems. However, the magnitude of the difference depends on the bulk dissolved oxygen, chemical oxygen demand, and nitrate concentrations, as well as the biofilm thickness. Overall, the accumulation and diffusion of key intermediates, i.e. hydroxylamine and nitrite, distinguish biofilms from suspended-growth systems. Our research suggests that the mechanisms of N₂O emissions from biofilms are much more complex than suspended-growth systems, and that emissions may be higher in many cases.

Nitrous oxide (N₂O) is a potent greenhouse gas that can be formed in wastewater treatment processes by ammonium oxidizing and denitrifying microorganisms. While N₂O emissions from suspended growth systems have been extensively studied, and some recent studies have addressed emissions from nitrifying biofilms, much less is known about N₂O emissions from denitrifying biofilm processes. This research used modeling to evaluate the mechanisms of N₂O formation and reduction in denitrifying biofilms. The kinetic model included formation and consumption of key denitrification species, including nitrate (NO₃ ^-), nitrite (NO₃ ^-), nitric oxide (NO), and N₂O. The model showed that, in presence of excess of electron donor, denitrifying biofilms have two distinct layers of activity: an outer layer where there is net production of N₂O and an inner layer where there is net consumption. The presence of oxygen (O₂) had an important effect on N₂O emission from suspended growth systems, but a smaller effect on biofilm systems. The effects of NO₃ ^- and O₂ differed significantly based on the biofilm thickness. Overall, the effects of biofilm thickness and bulk substrate concentrations on N₂O emissions are complex and not always intuitive. A key mechanism for denitrifying biofilms is the diffusion of N₂O and other intermediates from one zone of the biofilm to another. This leads to zones of N₂O formation or consumption transformations that would not exist in suspended growth systems.