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.@en

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.@en

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.@en