Effect of short-range guiding forces on cell movement patterns in a high-density population of flexible cells.

<p>(A) Initial configuration. Cells are aligned, but oriented randomly. (B). Final configuration of a population of non-guided cells at 3 h. (C). Final configuration of a population of cells with active following at 3 h. Color indicates speed of individual cells (colorbar at the bottom, μm·min⁻¹).</p

Effect of short-range guiding forces on cell movement patterns in a low-density population of flexible cells.

<p>(A) Initial random cell configuration. (B-E) Final configuration of a population at 6 h (B) non-guided cells, (C) cells with passive following, (D) cells with active following, (E) cells with head-to-tail adhesion. In (A-E) color indicates speed of individual cells (colorbar at the bottom left, μm·min⁻¹). (F) Strain energies due to cell overlap in circular aggregates of cells with active following (colorbar at the bottom right, J).</p

Effect of long-range guiding forces on cell movement patterns in a low-density population.

<p>Initial configuration of cells is random, as in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004213#pcbi.1004213.g002" target="_blank">Fig 2A</a>. (A) Final configuration of a population of rigid cells at 6 h. Color indicates speed of individual cells, μm·min⁻¹. (B) Strain energies due to cell overlap in population of rigid cells, J. (C) Final configuration of a population of flexible cells at 6 h. Color indicates speed of individual cells (see colorbar in (A), μm·min⁻¹).</p

Short-range guiding forces between two cells in the model.

<p>Only the leading pole (“head”) of one cell (left) and the trailing pole (“tail”) of another cell (right) are shown. For clarity, the distance between the head and the tail of interacting bacteria is exaggerated. Numbering of line segments <b><i>Q</i></b> that connect adjacent particles on the same bacterium is shown for the case of engine direction <i>k</i>^e = 1 (see text and [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004213#pcbi.1004213.ref034" target="_blank">34</a>] for notation and a detailed explanation of collision resolution algorithm). <i>W</i> is cell width, and <i>d</i> is the distance between head and tail particles of interacting bacteria.</p

Hydroxylamine Diffusion Can Enhance N₂O Emissions in Nitrifying Biofilms: A Modeling Study

Wastewater treatment plants can be significant sources of nitrous oxide (N₂O), a potent greenhouse gas. However, little is known about N₂O emissions from biofilm processes. We adapted an existing suspended-growth mathematical model to explore N₂O emissions from nitrifying biofilms. The model included N₂O formation by ammonia-oxidizing bacteria (AOB) via the hydroxylamine and the nitrifier denitrification pathways. Our model suggested that N₂O emissions from nitrifying biofilms could be significantly greater than from suspended growth systems under similar conditions. The main cause was the formation and diffusion of hydroxylamine, an AOB nitrification intermediate, from the aerobic to the anoxic regions of the biofilm. In the anoxic regions, hydroxylamine oxidation by AOB provided reducing equivalents used solely for nitrite reduction to N₂O, since there was no competition with oxygen. For a continuous system, very high and very low dissolved oxygen (DO) concentrations resulted in lower emissions, while intermediate values led to higher emissions. Higher bulk ammonia concentrations and greater biofilm thicknesses increased emissions. The model effectively predicted N₂O emissions from an actual pilot-scale granular sludge reactor for sidestream nitritation, but significantly underestimated the emissions when the NH₂OH diffusion coefficient was assumed to be minimal. This numerical study suggests an unexpected and important role of hydroxylamine in N₂O emission in biofilms

Analysing the effects of the aeration pattern and residual ammonium concentration in a partial nitritation-anammox process

<p>A mathematical model was used to evaluate the effect of the aeration pattern and ammonium concentration in a partial nitritation-anammox sequencing batch reactor with granular and flocculent sludge. In the tested conditions, model results indicate that most of the aerobic ammonium oxidation potential would occur in the bulk liquid, with 70% of the ammonium-oxidizing bacteria (AOB) biomass in suspension rather than in granules. The simulated granular sludge consisted predominantly of anammox bacteria with AOB present in the outer layer of the granule (50 μm AOB layer, accounting for 3% of the granule weight). Simulation results indicated that when granules do not contain any AOB, the amount of granular biomass required to achieve the same level of nitrogen removal would strongly increase (in the simulated conditions, by a factor of three) due to anammox inhibition by oxygen. This underlines the importance of a small fraction of AOB present in the granular anammox sludge. The aeration pattern had an important impact on the nitrogen removal: a better performance was suggested for continuous aeration (90% N-removal) than for intermittent aeration (68–84% N-removal). Anammox inhibition during the periods of high oxygen concentration was identified as the main reason for the lower nitrogen removal in the intermittently aerated system. With increasing oxygen concentration, a higher residual (effluent) ammonium concentration was needed to assure nitrite-oxidizing bacteria repression in the system. This study contributes to further understand the complexity of a reactor with both granular and flocculent sludge and the impact of operation conditions on reactor performance.</p

Effect of heterotrophic growth on autotrophic nitrogen removal in a granular sludge reactor

<div><p>This study deals with the influence of heterotrophic growth on autotrophic nitrogen removal from wastewater in a granular sludge reactor. A mathematical model was set-up including autotrophic and heterotrophic growth and decay in the granules from a partial nitritation-anammox process. A distinction between heterotrophic bacteria was made based on the electron acceptor (dissolved oxygen, nitrite or nitrate) on which they grow, while the nitrogen gas produced was ‘labelled’ to retrieve its origin, from anammox or heterotrophic bacteria. Taking into account heterotrophic growth resulted in a lower initial nitrogen removal, but in a higher steady state nitrogen removal compared with a model in which heterotrophic growth was neglected. The anammox activity is related with the fact that heterotrophs initially use nitrite as electron acceptor, but when they switch to nitrate the produced nitrite can be used by anammox bacteria. Increased anammox activity in the presence of heterotrophs, therefore, resulted in a marginally increased N₂ production at steady state. Heterotrophic denitrification of nitrate to nitrite also explains why small amounts of organic substrate present in the influent positively affect the maximum nitrogen removal capacity. However, the process efficiency deteriorates once the amount of organic substrate in the influent exceeds a certain threshold. The bulk oxygen concentration and the granule size have a dual effect on the autotrophic nitrogen removal efficiency. Besides, the maximum nitrogen removal efficiency decreases and the corresponding optimal bulk oxygen concentration increases with increasing granule size.</p></div

Biofilm development and the dynamics of preferential flow paths in porous media

<div><p>A two-dimensional pore-scale numerical model was developed to evaluate the dynamics of preferential flow paths in porous media caused by bioclogging. The liquid flow and solute transport through the pore network were coupled with a biofilm model including biomass attachment, growth, decay, lysis, and detachment. Blocking of all but one flow path was obtained under constant liquid inlet flow rate and biomass detachment caused by shear forces only. The stable flow path formed when biofilm detachment balances growth, even with biomass weakened by decay. However, shear forces combined with biomass lysis upon starvation could produce an intermittently shifting location of flow channels. Dynamic flow pathways may also occur when combined liquid shear and pressure forces act on the biofilm. In spite of repeated clogging and unclogging of interconnected pore spaces, the average permeability reached a quasi-constant value. Oscillations in the medium permeability were more pronounced for weaker biofilms.</p></div