Membrane BioReactor (MBR) is a commonly used technology in wastewater treatment; however, fouling is the main drawback of MBR. Grundfos proposes a system that splits the technology into two parts: the biological tank and the membrane system. The latter rotates to decrease and mitigate the fouling. This paper evaluates two membrane configurations, a central and eccentric axis. The shear stress profile in a single rotating membrane was obtained through CFD modelling with different operating conditions that show that an increasing TSS or angular velocity causes an increment in the shear stress over the membrane surface. Additionally, based on the CFD results and empirical correlations, a factor α used to determine the shear stress for non-Newtonian fluids was calculated in order to predict the behavior for central and eccentric placement configuration MBRs. The velocity profile in a wagon of 36 membranes was obtained from CFD simulations, where it was evident that eccentric placement is the best option since it reduces the dead zones and increases the shear stress, which helps reduce fouling.

For the past 70 years, researchers have dealt with the investigation of odour in sewer systems caused by hydrogen sulphide formations and the development of approaches to describe it. The state-of-the-art models are one-dimensional. At the same time, flow and transport phenomena in sewers can be three-dimensional, for example the air flow velocities in circular pipes or flow velocities of water and air in the reach of drop structures. Within the past years, increasing computational capabilities enabled the development of more complex models. This paper uses a three-dimensional two-phase computational fluid dynamics model to describe mass transfer phenomena between the two phases: water and air. The solver has been extended to be capable of accounting account for temperature dependency, the influence of pH value and a conversion to describe simulated air phase concentrations as partial pressure. Its capabilities are being explored in different application examples and its advantages compared to existing models are demonstrated in a highly complex three-dimensional test case. The resulting interH2SFoam solver is a significant step in the direction of describing and analysing H2S emissions in sewers.

Use of sustainable stormwater technologies such as bioretention cells (BRCs) is gaining in popularity across the world as episodes of severe flooding are becoming more frequent due to increased urbanization, and associated costs are rising due to decaying infrastructure and insufficient flood management. The aim of this study is to use numerical modeling to expand the understanding of BRC systems across the Toronto region. There is no one universally accepted approach to designing BRC systems. Local sensitivity analysis (LSA) with the one-factor-at-a-time method and global sensitivity analysis (GSA) with factorial design were conducted to identify the most influential components of BRC design for overflow reduction. Eight different model scenarios were used in a long term simulation to determine the efficacy of Ontario's BRC design standards for meeting Toronto's runoff volume control target (RVCT) of 27 mm. LSA shows that the highest reduction in overflow can be achieved by increasing BRC surface area, the saturated hydraulic conductivity (BSM K-sat) of bioretention soil media, or BRC ponding depth. On the other hand, GSA suggests that the most effective BRC performance can be achieved by simultaneously increasing the area of BRC, BSM K-sat, and BRC storage depth. Continuous simulation results show that Ontario's minimum BRC design guideline does not meet Toronto's RVCT. However, small adjustments to the baseline design, such as a 0.4% increase in BRC surface area, a 5 cm increase in ponding depth, or a 3 cm/h increase in BSM K-sat, can reduce the number of storm events causing overflow by up to 50% and meet RVCT.

Core Ideas The volume ratio of large to small particles controls intact sand hydraulic properties. Simple model explained sand saturated hydraulic conductivity within a factor of two. The new concept was also related to effective porosity and water retention points. Soil mineral particles larger than 0.1 mm and organic matter need to be considered. The volume ratio concepts seem promising for developing pedotransfer functions. Hydraulic conductivity (Ks) and effective porosity (ϕeff) for saturated water flow are essential hydraulic properties for describing fluid and chemical transport in soil and groundwater systems. Typically, Ks is predicted by pedotransfer functions of soil texture and total porosity or ϕeff. This study shows that a more conceptual approach that uses a volume‐weighted ratio of coarser (part of the sand fraction) to finer (clay and organic matter) particles alongside total porosity could explain variations in both Ks and φeff in intact 100‐cm3 samples of 20 sandy surface and subsurface soils with <10% fines (clay + organic matter). The Ks function used was a simple power‐law function of the volume‐weighted coarse/fine particle ratio with two calibration parameters [A and pore network connectivity (PNC)]. The value of the power‐law exponent (PNC) in the calibrated function was 1.8, similar to power‐law exponents for gas diffusivity and air permeability in unsaturated soil (1.5–2). The second calibration parameter (A) probably depends on the soil classes under consideration, the Ks measurement method, and the sample scale. A sensitivity analyses showed that both Ks and ϕeff (taken as the volume content of pores larger than 30 μm, that is, drained at –10 kPa of soil water matric potential) are especially sensitive to organic matter content. Besides the water transport parameters, water retention under dry conditions was also closely correlated with the volume‐weighted fines content. Therefore, the volume ratio concept seems to be a promising platform for the development of simple, accurate functions for the hydraulic properties of coarse‐textured soils.

Fluid dynamics is used for fouling mitigation in membrane bioreactors (MBRs), whereby a proper understanding of the fluid dynamics is of great interest. The influence of fluid dynamics has led to the use of computational fluid dynamics for optimizing MBR systems. In this work, a model has been validated for flat sheet membranes, with use of the Eulerian multiphase method. The model is validated against a comparable setup where the liquid velocities are measured with a laser Doppler anemometer (LDA). Furthermore, the Eulerian multiphase approach is validated against the more numerical direct volume of fluid (VOF) approach with sludge properties for the liquid, resulting in an error between the models of less than 2% for the wall shear stresses. The VOF model further showed that the horizontal components contribute significantly to the total wall shear stresses. The model has been applied to a full-scale setup for studying the effect of deflecting membranes as deflections have been seen in production. Minimizing the deflection of the membrane sheets was crucial to achieve a good operating condition as a deflection of 2 mm in a setup with a gap of 7 mm decreased the wall shear stresses with as much as 40% on average on the specific membrane surface.

Several experiments have been conducted in order to improve the understanding of the wastewater drag and the wall frictional force acting on the headspace air in gravity sewers. The aim of the study is to improve the data basis for a numerical model of natural sewer ventilation. The results of the study shows that by integrating the top/side wall shear stresses the log-law models for the air velocity distribution along the unwetted perimeter resulted in a good agreement with the friction forces calculated by use of the Colebrook-White formula for hydraulic smooth pipes. Secondly, the water surface drags were found by log-law models of the velocity distribution in turbulent flows to fit velocity profiles measured from the water surface and by integrating the water surface drags along the wetted perimeter, mean water surface drags were found and a measure of the water surface drag coefficient was found.

Membrane bioreactor (MBR) systems are often considered as the wastewater treatment method of the future due to their high effluent quality. One of the main problems with such systems is a relative large energy consumption, compared to conventional activated sludge (CAS) systems, which has led to further research in this specific area. A powerful tool for optimizing MBR-systems is computational fluid dynamics (CFD) modelling, which gives researchers the ability to describe the flow in the systems. A parameter which is often neglected in such models is the non-Newtonian properties of active sludge, which is of great importance for MBR systems since they operate at sludge concentrations up to a factor of 10 compared to CAS systems, resulting in strongly shear thinning liquids. A CFD-model is validated against measurements conducted in a system with rotating cross-flow membranes submerged in non-Newtonian liquids, where tangential velocities are measured with a Laser Doppler Anemometer (LDA). The CFD model is found to be capable of modelling the correct velocities in a range of setups, making CFD models a powerful tool for optimization of MBR systems.

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Liquids with non-Newtonian properties are presented in many engineering areas, as for example in membrane bioreactors where active sludge exhibits shear thinning properties. Therefore, the ability to determine the rheology's dependence on shear is important when optimising systems with such liquids. However, rheometers capable of determining the viscosity are often expensive and so a cheaper alternative is constructed with this exact capability. Using the principle of rotating rheometers, a low-cost rheometer was built to determine the rheology of Newtonian and non-Newtonian liquids. The general principles and background assumptions and the physics are described. The rheometer was calibrated by comparison with measurements conducted on a Brookfield viscometer for Newtonian liquids. For validation measurements on non-Newtonian liquids, xanthan gum solutions were made and compared with measurements on the Brookfield viscometer and with values from other sources. Furthermore, the effect of excluding the different shear rates in the system is discussed and good practice hereto is given.

Rotating ceramic membrane discs were fouled with lab-scale membrane bioreactors (MBR) sludge. Sludge filtrations were performed at varying rotation speeds and in different concentric rings of the membranes on different sludge concentrations. Data showed that the back transport expressed by limiting flux increased with rotation speed and distance from membrane center as an effect of shear. Further, the limiting flux decreased with increasing sludge concentration. A model was developed to link the sludge concentration and shear stress to the limiting flux. The model was able to simulate the effect of shear stress and sludge concentration on the limiting flux. The model was developed by calculating the shear rate at laminar flow regime at different rotation speeds and radii on the membrane. Furthermore, through the shear rate and shear stress, the non-Newtonian behavior of MBR sludge was addressed. © 2013 American Institute of Chemical Engineers AIChE J 60: 706–715, 2014

The Limfjord is the largest Danish estuary and is connected to both the North Sea in the west and the Kattegat in the east. The connection to the North Sea was formed in 1825 by a storm surge, and has since been kept open partly artificially. The debate about the climate changes and thereby the increased risk of flooding in the estuary has revitalized the discussion whether this connection should be closed. In this paper, it is shown by numerical simulation that the establishment of a storm surge barrier across Thyborøn Channel can significantly reduce the peak water levels in the central of the fjord. The reduction is obtained by blocking the ingoing flow with a sluice in due time before the storm surge peaks in the North Sea. In order to avoid problems with reduced water quality and salinity, the water exchange should be controlled by only keeping the sluice open for ingoing currents for the rest of days during the year. Depending on the effective cross-sectional area of the sluice, the depth-averaged salinity in the Limfjord remains status quo for cross-sectional areas of 500 m2, whereas the salinity increases with up to 1.5 PSU for larger openings.

Gas-Newtonian liquid two-phase flows (TPFs) are presented in several industrial processes (e.g. oil-gas industry). In spite of the common occurrence of these TPFs, the understanding of them is limited compared to single-phase flows. Various studies on TPF focus on developing empirical correlations based on large sets of experimental data for void fraction, which have proven accurate for specific conditions for which they were developed limiting their applicability. On the other hand, few studies focus on gas-non-Newtonian liquids TPFs, which are very common in chemical processes. The main reason is due to the characterization of the viscosity, which determines the hydraulic regime and flow behaviours of the system. The focus of this study is the analysis of the TPF (slug flow) for Newtonian and non-Newtonian liquids in a vertical pipe in terms of void fraction using computational fluid dynamics (CFD) and comparing this directly with experimental measurements and empirical relationships found in literature. A vertical tube of 3.4 m with an internal diameter of 0.1905 m was used. The two-phase CFD model was implemented in Star CCM+ using the volume of fluid (VOF) model. A relatively good agreement was found between the experimental measurements, the CFD results and the empirical relationships. In terms of void fraction for Newtonian and non-Newtonian liquids, the empirical correlations perform much worse than the CFD simulations, errors of 48 and 25%, respectively, against the experimental data. This shows that CFD can be used to predict void fraction relatively well for comparison against empirical correlations and they can be used for design and scale-up processes.

Rheological behaviour is an important fluid property that severely impacts its flow behaviour and many aspects related to this. In the case of activated sludge, the apparent viscosity has an influence on e.g. pumping, hydrodynamics, mass transfer rates, sludge-water separation (settling and filtration). It therefore is an important property related to process performance, including process economics. To account for this, rheological behaviour is being included in process design, necessitating its measurement. However, measurements and corresponding protocols in literature are quite diverse, leading to varying results and conclusions. In this paper, a vast amount of papers are critically reviewed with respect to this and important flaws are highlighted with respect to rheometer choice, rheometer settings and measurement protocol. The obtained rheograms from experimental efforts have frequently been used to build viscosity models. However, this is not that straightforward and a lot of errors can be detected with respect to good modelling practice, including fair model selection criteria, qualitative parameter estimations and proper model validation. These important steps are however recurrently violated, severely affecting the model reliability and predictive power. This is illustrated with several examples. In conclusion, dedicated research is required to improve the rheological measurements and the models derived from them. At this moment, there is no guidance with respect to proper rheological measurements. Moreover, the rheological models are not very trustworthy and remain very "black box". More insight in the physical background needs to be gained. A model-based approach with dedicated experimental data collection is the key to address this.