Abstract
For the application of biofilm processes, a better understanding of nitrous oxide (N2O) formation within the biofilm is essential for design and operation of biofilm reactors with minimized N2O emissions. In this work, a previously established N2O model incorporating both ammonia oxidizing bacteria (AOB) denitrification and hydroxylamine (NH2OH) oxidation pathways is applied in two structurally different biofilm systems to assess the effects of co- and counter-diffusion on N2O production. It is demonstrated that the diffusion of NH2OH and oxygen within both types of biofilms would form an anoxic layer with the presence of NH2OH and nitrite ( ), which would result in a high N2O production via AOB denitrification pathway. As a result, AOB denitrification pathway is dominant over NH2OH oxidation pathway within the co- and counter-diffusion biofilms. In comparison, the co-diffusion biofilm may generate substantially higher N2O than the counter-diffusion biofilm due to the higher accumulation of NH2OH in co-diffusion biofilm, especially under the condition of high-strength ammonium influent (500 mg N/L), thick biofilm depth (300 μm) and moderate oxygen loading (~1–~4 m3/d). The effect of co- and counter-diffusion on N2O production from the AOB biofilm is minimal when treating low-strength nitrogenous wastewater.
Highlights
The destruction of stratospheric ozone layer has become a significant environmental issue in 21st century, which is contributed largely by nitrous oxide (N2O)[1]
All of the substrates are supplied into the biofilm from the bulk liquid, while in membrane aerated biofilm reactors (MABR), the oxygen and other substrates are dividedly provided to the biofilm base through a gas-permeable membrane and surface of the biofilm from the bulk liquid, respectively (Fig. 1)
Since there is no gas stripping process in bulk liquid, the effluent N2O concentration could be considered as the total N2O production by the ammonia oxidizing bacteria (AOB) biofilm
Summary
The destruction of stratospheric ozone layer has become a significant environmental issue in 21st century, which is contributed largely by nitrous oxide (N2O)[1]. Electron mediators (Mred as the reduced form and Mox as the oxidized form) were used to model the electron transfer from oxidation to reduction in biochemical reactions This two-pathway model has been evaluated using the experimental data from several highly different cultures performing nitritation and/or nitrification, respectively and successfully applied for prediction of N2O production from a step-feed full-scale wastewater treatment reactor[15,16]. All of the substrates are supplied into the biofilm from the bulk liquid, while in membrane aerated biofilm reactors (MABR), the oxygen and other substrates are dividedly provided to the biofilm base through a gas-permeable membrane and surface of the biofilm from the bulk liquid, respectively (Fig. 1) The former is known as the co-diffusion biofilm and the latter is termed as the counter-diffusion biofilm[18,19]. This work aims to reveal the key difference of N2O production from the co- versus counter-diffusion nitrifying biofilms and the associated underlying mechanisms of N2O production by AOB from the co- and counter-diffusion biofilms using the two-pathway N2O15
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