Bulk Dissolved Oxygen Research Articles

Role of comammox bacteria in granular bioreactor for nitrogen removal via partial nitritation/anammox

In this study, two bioprocess models were first constructed with the newly-discovered comammox process described as one-step and two-step nitrification and evaluated against relevant experimental data. The validated models were then applied to reveal the potential effect of comammox bacteria on the granular bioreactor particularly suitable for undertaking partial nitritation/anammox (PN/A) under different operating conditions of bulk dissolved oxygen (DO) and influent NH4+. The results showed although comammox bacteria-based PN/A could achieve > 80.0 % total nitrogen (TN) removal over a relatively wider range of bulk DO and influent NH4+ (i.e., 0.25–0.40 g‐O2/m3 and 470–870 g‐N/m3, respectively) without significant nitrous oxide (N2O) production (< 0.1 %), the bulk DO should be finely controlled based on the influent NH4+ to avoid the undesired full nitrification by comammox bacteria. Comparatively, conventional ammonium‐oxidizing bacteria (AOB)-based PN/A not only required higher bulk DO to achieve > 80.0 % TN removal but also suffered from 1.7 %∼2.8 % N2O production.

Spatial distribution of mechanical properties in Pseudomonas aeruginosa biofilms, and their potential impacts on biofilm deformation.

The mechanical properties of biofilms can be used to predict biofilm deformation under external forces, for example, under fluid flow. We used magnetic tweezers to spatially map the compliance of Pseudomonas aeruginosa biofilms at the microscale, then applied modeling to assess its effects on biofilm deformation. Biofilms were grown in capillary flow cells with Reynolds numbers (Re) ranging from 0.28 to 13.9, bulk dissolved oxygen (DO) concentrations from 1 mg/L to 8 mg/L, and bulk calcium ion (Ca2+ ) concentrations of 0 and 100 mg CaCl2 /L. Higher Re numbers resulted in more uniform biofilm morphologies. The biofilm was stiffer at the center of the flow cell than near the walls. Lower bulk DO led to more stratified biofilms. Higher Ca2+ concentrations led to increased stiffness and more uniform mechanical properties. Using the experimental mechanical properties, fluid-structure interaction models predicted up to 64% greater deformation for heterogeneous biofilms, compared with a homogeneous biofilms with the same average properties. However, the deviation depended on the biofilm morphology and flow regime. Our results show significant spatial mechanical variability exists at the microscale, and that this variability can potentially affect biofilm deformation. The average biofilm mechanical properties, provided in many studies, should be used with caution when predicting biofilm deformation.

Performance of a lab-scale membrane aerated biofilm reactor treating nitrogen dominant space-based wastewater through simultaneous nitrification-denitrification

A membrane-aerated biofilm reactor (MABR) was designed for the treatment of a source-separated urine wastewater formulated to mimic conditions on an early planetary space habitation. Literature suggested that a MABR would not work effectively for nitrogen removal for this wastewater since it is nitrogen dominant (total organic carbon to nitrogen ratio < 1). The MABR studied in this study confirmed that hypothesis by recording average nitrification and denitrification efficiencies as high as ∼65 % and ∼35 %, respectively. Nonetheless, the simultaneous nitrification and denitrification performance was unexpectedly high when compared to the reactor’s low liquid velocities (0.004 cm/s) and it’s relatively high bulk dissolved oxygen (DO) levels (> 2 mg/L). Dissolved organic carbon (DOC) removal and nitrification rates were reported as high as 0.33 ± 0.12 g-C/m2-d and 0.32 ± 0.02 g-N/m2-d, respectively. Additionally, the MABR achieved total nitrogen (TN) removal efficiency of 36.5 %. Additionally, the operational parameters of hydraulic retention time, intramembrane partial oxygen pressure, recycle ratio and liquid velocity linked with bulk DO concentration were evaluated. It was observed that TN removal was significantly impacted by the variation of those parameters, but DOC removal efficiency and nitrification rate did not change significantly. This study helps to establish the efficacy of MABR systems for the treatment of nitrogen dominant, space-based wastewater.

Impact of coexistence of sludge flocs on nitrous oxide production in a granule-based nitrification system: A model-based evaluation

A common operational status of granule-based reactor is the inevitable coexistence of sludge flocs. Such hybrid system could have a profound impact on nitrous oxide (N2O) production in nitrifying process. In this work, a mathematical model is employed to evaluate the key role of the coexistence of sludge flocs on N2O production in a granule-based nitrifying system for the first time, by considering both nitrifier denitrification and hydroxylamine oxidation pathways. The modelling results show that the N2O production gradually decreases with the increase of the percentage of sludge flocs in the total biomass (10–60%). More N2O is tended to be generated in sludge flocs which has lower N2O production capacity compared to granular biomass, thus lowering the total N2O production. The relative contributions of two N2O production pathways are only affected by bulk dissolved oxygen (DO) for the sludge flocs in the hybrid system, whereas those are affected by both bulk DO and the fractions of sludge flocs for the granular biomass. The results reveal a substantial effect of the coexistence of sludge flocs on N2O production in granule-based nitrifying process, which should not be ignored in future design and operation.

Assessment of operational conditions towards mainstream partial nitritation-anammox stability at moderate to low temperature: Reactor performance and bacterial community

This study aimed at assessing the performance and microbial community in a granular one-stage partial nitritation-anammox sequencing batch reactor (PNA-SBR) subjected to temperature transition from 25 to 15 °C without biomass acclimation. The PNA-SBR was operated by controlling the oxygen transfer rate (OTR) according to the ammonium loading rate (ALR), which resulted in micromolar (µM) bulk dissolved oxygen (DO) concentration. The applied strategy proved to be feasible to operate the one-stage PNA-SBR at mainstream conditions because it was possible to control nitritation according to anammox rate. Nitrogen removal rate (NRR) of 330.24 ± 25.36 mg N·L−1·d−1 was achieved at 25 °C. Nitratation control by µM bulk DO limited the NO3−production:NH4+removed at 0.28 ± 0.04. No instability was experienced by decreasing the temperature to 15 °C, but removal rates were adapted to the resulting anammox activity, which decreased at low temperature. After temperature transition, nitratation was kept controlled and the NO3−production:NH4+removed molar ratio remained at 0.33 ± 0.05, although anammox activity deteriorated and higher nitrate production was obtained. Sequencing analysis revealed the dominant bacterial groups in the microbial community that clustered within the phyla Planctomycetes, Proteobacteria, Chloroflexi, and Bacteroidetes. Temperature drop only affected bacterial abundance, but the main bacteria involved in nitrification and anammox processes did not change during the study. Candidatus Kuenenia was the main anammox genus. Moreover, the presence of bacterial groups associated with heterotrophic metabolism indicates denitrification might be supported by the release of dissolved organic carbon due to bacterial lysis, and lower nitrate effluent concentration could be reached in PNA reactors.

Tuning the Deoxygenation of Bulk-Dissolved Oxygen in Copper

Using synchrotron-based ambient-pressure X-ray photoelectron spectroscopy, we report the tuning of the deoxygenation process of bulk dissolved oxygen in copper via a combination of H2 gas flow and elevated temperature. We show that a critical temperature of ∼580 °C exists for driving segregation of bulk dissolved oxygen to form chemisorbed oxygen on the Cu surface, which subsequently reacts with hydrogen to form OH species and then H2O molecules that desorb from the surface. This deoxygenation process is tunable by a progressive stepwise increase of temperature that results in surface segregation of oxygen from deeper regions of bulk Cu. Using atomistic simulations, we show that the bulk-dissolved oxygen occupies octahedral sites of the Cu lattice and the deoxygenation process involves oxygen migration between octahedral and tetrahedral sites with a diffusion barrier of ∼0.5 eV.

Effects of inoculum type and bulk dissolved oxygen concentration on achieving partial nitrification by entrapped-cell-based reactors.

An entrapment of nitrifiers into gel matrix is employed as a tool to fulfill partial nitrification under non-limiting dissolved oxygen (DO) concentrations in bulk solutions. This study aims to clarify which of these two attributes, inoculum type and DO concentration in bulk solutions, is the decisive factor for partial nitrification in an entrapped-cell based system. Four polyvinyl alcohol entrapped inocula were prepared to have different proportions of nitrite-oxidizing bacteria (NOB) and nitrite-oxidizing activity. At a DO concentration of 3 mg l(-1), the number of active NOB cells in an inoculum was the decisive factor for partial nitrification enhancement. However, when the DO concentration was reduced to 2 mg l(-1), all entrapped cell inocula showed similar degrees of partial nitrification. The results suggested that with the lower bulk DO concentration, the preparation of entrapped cell inocula is not useful as the DO level becomes the decisive factor for achieving partial nitrification.

Mechanism and active sites for the partial oxidation of methanol to formaldehyde over an electrolytic silver catalyst

In situ Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and fixed-bed microreactor experiments and were used to examine the operating mechanism of an electrolytic silver catalyst in the titled reaction. Methanol oxidation studies were performed at atmospheric pressure, temperatures between 448 and 1073 K and CH 3OH/O 2 feed stoichiometries of 1.5–4.0. Particular attention was given to the role of surface chemisorbed and bulk-dissolved oxygen species in the catalytic cycle, and the influence of oxygen speciation on catalyst selectivity to formaldehyde. The results indicate that the product distribution of CH 3OH oxidation over electrolytic silver catalysts relates intimately to the nature of the oxygen species formed on the catalyst, which is discussed within a framework comprising two distinct surface chemisorbed atomic oxygen species (denoted O α and O γ) and a bulk-dissolved species (denoted O β). The selectivity to formaldehyde was highly dependent on the surface ratio O γ/O α, both of which increased with temperature from 548 to 923 K and CH 3OH/O 2 feed ratio from 1.5 to 2.25. An optimum formaldehyde yield of 84.3% was obtained during microreactor testing of the silver catalyst, under conditions close to those employed industrially (temperature = 923 K, molar ratio CH 3OH/O 2=2.25, GHSV=1.25×10 5 h −1). SEM revealed that pronounced thermal and catalytic etching of the silver catalyst occurred during CH 3OH oxidation at 923 K. Based on these results, a scheme was developed for the interaction of CH 3OH and O 2 with electrolytic silver catalysts under conditions of industrial formaldehyde manufacture. This work clarifies the role of the O α, O β and O γ species in methanol oxidation on silver surfaces and provides a fundamental basis for future formaldehyde yield optimisation.

The Role of Subsurface Oxygen in the Silver-Catalyzed, Oxidative Coupling of Methane

The silver-catalyzed, oxidative coupling of methane to C2 hydrocarbons (OCM) is shown to be an extremely structure-sensitive reaction. Reaction-induced changes in the silver morphology lead to changes in the nature and extent of formation of various bulk and surface-terminating crystal structures. This, in turn, impacts the adsorption properties and diffusivity of oxygen in silver which is necessary to the formation of subsurface oxygen. A strongly bound, Lewis-basic, oxygen species which is intercalated in the silver crystal structure is formed as a result of these diffusion processes. This species is referred to as Oγ and acts as a catalytically active site for the direct dehydrogenation of a variety of organic reactants. It is found that the activation energy for methane coupling over silver of 138 kJ/mol is nearly identical to the value of 140 kJ/mol for oxygen diffusion in silver measured under similar conditions. This correlation between the diffusion kinetics of bulk-dissolved oxygen and the reaction kinetics of the oxidative coupling of methane to C2 hydrocarbons suggests that the reaction is limited by the formation of Oγ via surface segregation of bulk dissolved oxygen. Catalysis over fresh silver catalysts indicates an initially preferential oxidation of CH4 to complete oxidation products. This is a result of the reaction of methane with surface bound atomic oxygen which forms preferentially on high-index terminating crystalline planes. Reaction-induced facetting of the silver results in a restructuring of the catalyst from one which initially catalyzes the complete oxidation of methane to COx and water to a catalyst which preferentially catalyzes the formation of coupling products. This represents an extremely dynamic situation in which a solid-state restructuring of the catalyst results in the formation of a Lewis-basic, silver–oxygen species which preferentially catalyzes the dehydrogenation of organic molecules.

High temperature partial oxidation reactions over silver catalysts

The exceptional catalytic activity of silver for a number of partial oxidation reactions has been known for nearly a century. Despite the widespread use of silver in heterogeneous catalysis, there still remain unresolved questions about the mechanistic details of reaction. The ethylene epoxidation and formaldehyde synthesis reactions are the two industrially-relevant reactions which have received, by far, the most attention. The importance of these reactions cannot be underestimated. Both ethylene epoxide and formaldehyde serve as primary chemicals for a wide variety of materials which find use in an enormous number of products. There is, therefore, a great scientific and economic motivation for understanding these reactions and for unraveling the secret to the exceptional catalytic activity of silver. This contribution in honor of Prof. W. Sachtler’s 75th birthday summarizes recent results obtained in our laboratory on the formaldehyde synthesis reaction and the oxidative coupling of methane as a laboratory test reaction. The article shows that the formation of bulk-dissolved oxygen species is critical to activation of the silver for reaction. Both the chemical composition and the bulk morphology of the catalyst are strong functions of the gas-phase composition and reaction temperature. Similar apparent activation energies determined for oxygen diffusion and oxidative coupling of methane suggest that the rate-limiting step of the oxidative coupling of methane over silver is the diffusion of bulk-dissolved oxygen to silver (1 1 1) surfaces at which oxygen rapidly reacts with methane.

The Correlation of Subsurface Oxygen Diffusion with Variations of Silver Morphology in the Silver–Oxygen System

Silver undergoes pronounced morphological changes following high-temperature treatment in various gas atmospheres. SEM analysis shows that high-temperature treatment in oxygen leads to pronounced facetting of the silver surface.In situXRD shows a strong texturing of the polycrystalline bulk and an expansion of the unit cell resulting from the dissolution of oxygen. Two distinctly different forms of subsurface oxygen have been identified and the influence of morphological changes on the mechanism leading to their formation clarified. The first of these species is Oβ. It is bulk-dissolved oxygen which diffuses via an interstitial mechanism through low-resistance diffusion paths such as grain boundaries and open crystalline planes. Temperature programmed desorption spectroscopy analysis reveals that high-temperature oxygen pretreatment results in the formation of crystalline surfaces exhibiting different oxygen diffusion barriers. At elevated temperatures, pure thermal reordering dominates over oxygen-induced restructuring of the silver bulk and surface. The near-surface region is, therefore, comprised primarily of close-packed crystalline planes at elevated temperatures. The barrier for interstitialcy diffusion is overcome at temperatures in excess of 923 K, allowing bulk-dissolved, atomic oxygen to (Oβ) segregate into these low-indexed planes. This likely occurs via an interstitialcy diffusion mechanism where oxygen substitutes for silver atoms in the lattice. This oxygen species is referred to here as Oγ. Analysis of ISS depth profiling of silver foils treated under various conditions shows that the diffusion coefficient is a positive function of the oxygen concentration. This is a direct result of the oxygen-induced recrystallization of the silver resulting in a lowered barrier to diffusion.In situXRD shows that oxygen located in octahedral holes of silver preferentially diffuses in the [110] direction. This requires the anisotropic displacement of silver atoms. The resulting strain culminates in a slight expansion of the silver unit cell and the preferential growth of crystals in the shape of needles. STM imaging of the surface shows the macroscopic silver facets to be composed of smaller, columnar crystallites, thus confirming the validity of the model based on the XRD data.

Prediction of oxygen limitation in an aerobic biofilm reactor

Abstract This study is aimed at developing a determining model for predicting the critical bulk dissolved oxygen (DO) concentration for onset of oxygen limitation in a biofilm reactor with respect to a given bulk substrate concentration. Oxygen and substrate profiles within biofilms were examined through a numerical analysis of a mathematical biofilm model for evaluating the relationship between these two concentrations. It has been found that the critical bulk DO level (Cb) is directly proportional to the bulk substrate concentration (5 b ), which can be expressed as C, < 0.065 Sb for the onset of oxygen limitation. This oxygen limitation determining model has been verified with experimental results with a submerged RBC reactor.

Effect of catalyst particle size on conversion of methyl alcohol to formaldehyde in fluidized beds of silver and copper catalysts

The exceptional catalytic activity of silver for a number of partial oxidation reactions has been known for nearly a century. Despite the widespread use of silver in heterogeneous catalysis, there still remain unresolved questions about the mechanistic details of reaction. The ethylene epoxidation and formaldehyde synthesis reactions are the two industrially-relevant reactions which have received, by far, the most attention. The importance of these reactions cannot be underestimated. Both ethylene epoxide and formaldehyde serve as primary chemicals for a wide variety of materials which find use in an enormous number of products. There is, therefore, a great scientific and economic motivation for understanding these reactions and for unraveling the secret to the exceptional catalytic activity of silver. This contribution in honor of Prof. W. Sachtler’s 75th birthday summarizes recent results obtained in our laboratory on the formaldehyde synthesis reaction and the oxidative coupling of methane as a laboratory test reaction. The article shows that the formation of bulk-dissolved oxygen species is critical to activation of the silver for reaction. Both the chemical composition and the bulk morphology of the catalyst are strong functions of the gas-phase composition and reaction temperature. Similar apparent activation energies determined for oxygen diffusion and oxidative coupling of methane suggest that the rate-limiting step of the oxidative coupling of methane over silver is the diffusion of bulk-dissolved oxygen to silver (1 1 1) surfaces at which oxygen rapidly reacts with methane.