Performance of a Horizontal Flow Constructed Reed Bed Filter for Municipal Wastewater Treatment: The Case Study of the Prototype Installed at Gaston Berger University, Saint-Louis, Senegal

In Saint-Louis, Senegal, a constructed wetland with horizontal flow reed beds (FHa and FHb) has demonstrated significant efficacy in treating municipal wastewater. Analyzing various treatment stages, the system showed only a slight temperature variation, from an influent average of 26.3°C to an effluent of 24.7°C. Electrical conductivity decreased from 1331 mS/cm to 974.5 mS/cm post-primary treatment, with suspended solids (SS) dramatically reduced from 718.9 mg/L to 5.7 mg/L in the final effluent. Biochemical oxygen demand (BOD5) and chemical oxygen demand (COD) saw a notable decrease, from initial levels of 655.6 mg/L and 1240 mg/L to 2.3 mg/L and 71.3 mg/L, respectively. Nitrogenous compounds (N-TN) and phosphates () also decreased significantly, indicating the system’s nutrient removal capacity. Microbiological analysis revealed a reduction in fecal coliforms from 7.5 Ulog/100ml to 1.8 Ulog/100ml and a complete elimination of helminth eggs. The presence of Phragmites and Typha was instrumental in enhancing these reductions. The system’s compliance with the Senegalese standards for disposal into natural environments, WHO recommendations for unrestricted water reuse in irrigation, and the European legislation for water reuse was established. The effluent quality met the stringent criteria for various classes of agricultural reuse, illustrating the system’s potential for sustainable water management. This wetland model presents a robust solution for water-stressed regions, ensuring environmental protection while supporting agricultural needs. The study calls for ongoing research to further refine the system for optimal, reliable wastewater treatment and water resource sustainability.

Tectonometamorphic Cycles in Different Geodynamic Conditions

Formation and attenuation of crust thermal anomalies, accompanied by development of geological structures, are governed by special laws, together with metamorphism and other endogenic process, form tectonometamorphic cycle (TMC). Because of rock rheology under metamorphic conditions, the structures, concurring with metamorphism, reflect geodynamic conditions much more perfect than those of the structures of nonmetamorphosed rocks. For this reason, structural investigations open wide, sometimes unique, possibilities for geodynamic research into regional metamorphosed terrains. The TMC features under different geodynamic conditions are shown with examples of California (subduction), Himalaya and Junggar Alatau (collision), and Northwest environs of the White Sea (polycyclic development). The analysis of these units serves as basis of some general conclusions. TMC, the steady pattern of crust thermal anomalies manifestation, does not practically depend on peculiarities of metamorphism. At lower and middle crust levels, occur the high-temperature complete cycles that include following two stages. The first is represented by nappes and paragenes of parallel bedding flow, the second, by linear folds, domes and faults. At top levels of fold complexes display the low-temperature reduced cycles, expressed only by structures of the second stage. There are gradual transitions between the complete and reduced cycles. Thermal anomalies in the first stage of complete cycles devolop against the background of large horizontal displacements at contacts between interacting plates or are genetically connected with their gently sloping fault planes. Of all structural elements of the cycles, in compressional environments (nappes, linear folds, domes etc.), only the structures of the first stage of complete cycles can be considered as indications of plate tectonics. Presence of such structures at the most ancient supracrustal complexes point to existence of plate tectonics already at the early stages of geological development of the earth.

Numerical Simulation of Dilute Phase Transportation in Pulse Riser

To study the hydrodynamic characteristics of gas-solid-phase flow in a pulse riser,a dense discrete particle model considering particle collisions and solid volume fraction is used.The core-annular flow in the enlarged-diameter section of the pulse riser is described,which can be destroyed with a high superficial gas velocity.The particle trajectory crossing effect and particle-particle and particle-wall collisions in the reduced-diameter section of the pulse riser can also destroy the core-annular flow and enhance the gas-solid interaction.The solid volume fraction exhibits an S-type distribution at different solid mass rates.The distribution of axial velocity,radial velocity,and relative slip velocity is investigated by analyzing the simulated results at different pulse riser diameter ratios.A suitable pulse riser diameter ratio can improve the performance of the pulse riser.

Flow-regime transitions in fluidized beds of non-spherical particles

Fluidized beds frequently involve non-spherical particles, especially if biomass is present. For spheri- cal particles, numerous experimental investigations have been reported in the literature. In contrast, complex-shaped particles have received much less attention. There is a lack of understanding of how par- ticle shape influences flow-regime transitions. In this study, differently shaped Geldart group D particles are experimentally examined. Bed height, pressure drop, and their respective fluctuations are analyzed. With increasing deviation of particle shape from spheres, differences in flow-regime transitions occur with a tendency for the bed to form channels instead of undergoing smooth fluidization. The correlations available in the literature for spherical particles are limited in their applicability when used to predict regime changes for complex-shaped particles. Hence, based on existing correlations, improvements are derived.

Experimental investigation on flow asymmetry in solid entrance region of a square circulating fluidized bed

To study the influence of back feeding particles on gas-solid flow in the riser, this paper investigated the flow asymmetry in the solid entrance region of a fluidized bed by particle concentration/velocity measurements in a cold square circulating fluidized beds （CFB）. The pressure drop distribution along the riser and the saturation carrying capacity of gas for Geldart-B type particles were first analyzed. Under the condition of u0 = 4 m/s and Gs = 21 kg/（m^2 s）, the back feeding particles were found to penetrate the lean gas-solid flow near the entrance （rear） wall before reaching the opposite （front） wall, thus leading to a relatively denser region near the front wall in the bottom bed. Higher solid circulation rate （u0 =4 m/s, Gs = 33 kg/（m^2 s）） resulted in a higher particle concentration in the riser. However the back feeding particles with higher momentum increased the asymmetry of the particle concentration/velocity profile in the solid entrance region. Lower air velocity （u0 =3.2 m/s） and Gs =21 kg/（m2 s）, beyond the saturation carrying capacity of gas, induced an S-shaped axial solid distribution with a denser bottom zone. This limited the penetration of the back feeding particles and forced the flnidizing air to flow in the central region, thus leading to a higher solid holdup near the rear wall. Under the conditions of uo = 4 m/s and Gs = 21 kg/（m^2 s）, addition of coarse particles （dp= 1145 μm） into the bed made the radial distribution of solids more symmetrical.

Gas-solid flow in a high-density circulating fluidized bed riser with Geldart group B particles

We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve and increasing the solid inventory in the storage tank, a high solid circulation rate and a solid holdup above 0.075 throughout the riser were simultaneously achieved. At a solid-to-gas mass flux ratio of approximately 105, flow transitioned from fast fluidization to a dense suspension upflow. In the axial direction of the riser, solid holdup had an exponential profile, increasing with increasing solid circulation rate and Jot decreasing superficial gas velocity. From the riser＇s center to its wall, the solid holdup increased markedly, exhibiting a steep parabolic profile. Increasing the solid circulation rate increased the radial non-uniformity of the solid concentration, while increasing the superficial gas velocity had the opposite effect, In our dense circulating fluidized bed riser, Geldart group B particles had similar slip characteristics to Geldart group A particles,

3D full-loop simulation and experimental verification of gas-solid flow hydrodynamics in a dense circulating fluidized bed

Because of their advantages of high efficiency and low cost, numerical research methods for large-scale circulating fluidized bed （CFB） apparatus are gaining ever more importance. This article presents a numer- ical study of gas-solid flow dynamics using the Eulerian granular multiphase model with a drag coefficient correction based on the energy-minimization multi-scale （EMMS） model. A three-dimensional, full-loop, time-dependent simulation of the hydrodynamics of a dense CFB apparatus is performed. The process parameters （e.g., operating and initial conditions） are provided in accordance with the real experiment to enhance the accuracy of the simulation. The axial profiles of the averaged solid volume fractions and the solids flux at the outlet of the cyclone are in reasonable agreement with experimental data, thereby verifying the applicability of the mathematical and physical models. As a result, the streamline in the riser and standpipe as well as the solids distribution contours at the cross sections is analyzed. Computational fluid dynamics （CFD） serves as a basis for CFB modeling to help resolve certain issues long in dispute but difficult to address experimentally. The results of this study provide the basis of a general approach to describing dynamic simulations of gas-solid flows.

Numerical simulation of flow hydrodynamics of struvite pellets in a liquid–solid fluidized bed

Phosphorus recovery in the form of struvite has been aroused in recent decades for its dual advantages in eutrophication control and resource protection.The usage of the struvite products is normally determined by the size which is largely depended on the hydrodynamics.In this study,flow behavior of struvite pellets was simulated by means of Eulerian–Eulerian two-fluid model combining with kinetic theory of granular flow in a liquid–solid fluidized bed reactor（FBR）.A parametric study including the mesh size,time step,discretization strategy,turbulent model and drag model was first developed,followed by the evaluations of crucial operational conditions,particle characteristics and reactor shapes.The results showed that a cold model with the mesh resolution of 16 × 240,default time step of 0.001 sec and first order discretization scheme was accurate enough to describe the fluidization.The struvite holdup profile using Syamlal–O＇Brien drag model was best fitted to the experimental data as compared with other drag models and the empirical Richardson–Zaki equation.Regarding the model evaluation,it showed that liquid velocity and particle size played important roles on both solid holdups and velocities.The reactor diameter only influenced the solid velocity while the static bed height almost took no effect.These results are direct and can be applied to guide the operation and process control of the struvite fluidization.Moreover,the model parameters can also be used as the basic settings in further crystallization simulations.

Numerical investigation of flow through vegetated multi-stage compound channel

This paper addresses the problem of the renormalization group k turbulence modeling of a vegetated multi-stage compound channel. Results from Micro acoustic Doppler velocimeter（ADV） tests are used with time and spatial averaging（doubleaveraging method） in the analysis of the flow field and the characterization. Comparisons of the mean velocity, the Reynolds stress, and the turbulent energy distribution show the validity of the computational method. The mean velocity profile sees an obvious deceleration in the terraces because of vegetation. Secondary flow exists mainly at the junction of the main channel and the vegetation region on the first terrace. The bed shear stress in the main channel is much greater than that in the terraces. The difference of the bed shear stress between two terraces is insignificant, and the presence of vegetation can effectively reduce the bed shear stress.

SIZE DISTRIBUTIONS OF COARSE SAND BED LOAD IN TURBULENT FLOW FIELD

Putting forth effort on analyzing the action of turbulent flow on the sand particles on rvier bed surface,in this paper, the interdependent relation between the size distribution of coarse sand bed load and flow strength is studied.A theoretical calculation formula obtained gives good results in accordance with the field data.

A comparative assessment of empirical and lattice Boltzmann method-based drag models for simulation of gas-solid flow hydrodynamics in a bubbling fluidized bed

In simulations of fluidized beds using computational fluid dynamics （CFD）, the description of gas-solid flow hydrodynamics relies on a drag model to account for the momentum transfer between gas and solid phases. Although several studies of drag models have been published, there have been few investigations of the application of lattice Boltzmann method （LBM）-based drag models to bubbling fluidized bed simu- lations. In the present study, a comprehensive comparison of empirical and LBM-based drag models was carried out to assess the performance of these models during simulations of gas-solid flow hydrodynam- ics in a bubbling fluidized bed. A CFD model using the MFIX code based on the Eulerian-Eulerian approach and the kinetic theory of granular flow was used to simulate a 2D bubbling fluidized bed with Geldart B particles. The simulation results were validated by comparison with experimental data. Statistical anal- ysis of the results shows that LBM-based drag models can reliably model gas-solid flow hydrodynamics in a bubbling fluidized bed.

EXPERIMENTAL STUDY ON BED FORMS AND FLOW RESISTANCE OF LIGHT－WEIGHT MATERIALS WITH DIFFERENT DENSITIES

Experiments were conducted to investigate bed forms and flow resistance of light-weight sediment in an open channel flow. Three different synthetic materials of specific gravity 1. 055, 1. 46, each with uniform sizes D50 for 1. 25mm, 1. 05mm, 1. 40mm were used. Some conclusions were obtained from the resultS of these experiments and the data of other reliable sources[1, 2, 3, 4]. They indicate that the grain resistence is greatly affected by D50, and bed form resistances is the function of the downstream slope and the height of the dune. As well as natural sand, Y is not only the function of Y', but also affected by the relative roughness Rb/D50 and the size of the sediment.

CFD simulation of gas-solid flow patterns in a downscaled combustor-style FCC regenerator

To investigate the gas-solid flow pattern of a combustor-style fluid catalytic cracking regenerator, a laboratory-scale regenerator was designed. In scaling down from an actual regenerator, large-diameter hydrodynamic effects were taken into consideration. These considerations are the novelties of the present study. Applying the Eulerian-Eulerian approach, a three-dimensional computational fluid dynamics （CFD） model of the regenerator was developed. Using this model, various aspects of the hydrodynamic behavior that are potentially effective in catalyst regeneration were investigated. The CFD simulation results show that at various sections the gas-solid flow patterns exhibit different behavior because of the asymmetric location of the catalyst inlets and the lift outlets. The ratio of the recirculated catalyst to spent catalyst determines the quality of the spent and recirculated catalyst mixing and distribution because the location and quality of vortices change in the lower part of the combustor. The simulation results show that recirculated catalyst considerably reduces the air bypass that disperses the catalyst particles widely over the cross section. Decreasing the velocity of superficial air produces a complex flow pattern whereas the variation in catalyst mass flux does not alter the flow pattern significantly as the flow is dilute.

Numerical prediction of flow hydrodynamics of wet molecular sieve particles in a liquid-fluidized bed

The Eulerian-Eulerian framework was used in the numerical simulation of liquid hydrodynamics and particle motion in liquid-fluidized beds. The kinetic theory of granular flow, which accounts for the viscous drag influence on the interstitial liquid phase, was used in combination with two-fluid models to simulate unsteady liquid-solid two-phase flows. We focus on local unsteady features predicted by the numerical models. The solid fraction power spectrum was analyzed. A typical flow pattern, such as core annular flow and particle back-mixing near the wall region of liquid-solid fluidized beds is obtained from this calculation. Effects of the restitution coefficient of particle-particle collisions on the distribution of granular pressure and temperature are discussed. Good agreement was achieved between the simulated results and experimental findings.

Circulating turbulent fluidization-A new fluidization regime or just a transitional phenomenon

While circulating fluidized bed （CFB） reactor has many advantages over the more conventional turbulent fluidized bed （TFB） reactor, it does at least have one significant shortcoming-the rather dilute solids volume concentration in CFB reactor gives rise to less ideal reaction intensity. On the other hand, while having higher reaction intensity, TFB reactor has one fatal drawback of particle back-mixing, making it not suitable for certain reactions such as catalytic reaction where the catalyst requires frequent regeneration. This paper describes some key issues in the development of a circulating turbulent fluidized bed （CTFB） reactor that combines the advantages of both TFB and CFB, that is, to have the high reaction intensity as in TFB but and also to have a suppressed solids back-mixing as in CFB due to a continuous net upflow of solids flux through the bed. Experimental results show enough evidence to suggest that a new fluidization regime is formed, the characteristics of which appears to be distinct from those observed in a regular TFB and from those in either the bottom or the upper sections of regular CFB and/or high-density CFB （HDCFB）. Fundamentally, the difference is that particle-particle interaction （collision） dominates the motion of particles in CTFB and TFB, while gas-particle interaction （drag force） is the key element that determines the two phase flow in CFB including HDCFB.

Influence of artificial ecological floating beds on river hydraulic characteristics

The artificial ecological floating bed is widely used in rivers and lakes to repair and purify polluted water. However, the water flow pattern and the water level distribution are significantly changed by the floating beds, and the influence on the water flow is different from that of aquatic plants. In this paper, based on the continuous porous media model, a moveable two-layer combination model is built to describe the floating bed. The influences of the floating beds on the water flow characteristics are studied by numerical simulations and experiments using an experimental water channel. The variations of the water level distribution are discussed under conditions of different flow velocities（ v= 0.1 m/s, 0.2 m/s, 0.30 m/s, 0.4 m/s）, floating bed coverage rates（20%, 40%, 60%） and arrangement positions away from the channel wall（ D= 0 m, 0.1 m, 0.2 m）. The results indicate that the flow velocity increases under the floating beds, and the water level rises significantly under high flow velocity conditions in the upstream region and the floating bed region. In addition, the average rising water level value（ARWLV） increases significantly with the increase of the floating bed coverage rate, and the arrangement position of floating beds in the river can also greatly influence the water level distribution under a high-flow velocity condition（v ？0.2 m /s）.

Hydrodynamic behavior of magnetized fluidized beds with admixtures of Geldart-B magnetizable and nonmagnetizable particles

This work focuses on the hydrodynamic behavior of admixtures of Geldart-B magnetizable and non- magnetizable particles in a magnetized fluidized bed. The applied magnetic field was axial, uniform, and steady. In operating the beds, the magnetization-LAST mode was adopted under which four distinct flow regimes exist： fixed, magnetized-bubbling, partial segregation-bubbling, and total segregation-bubbling, The operational phase diagram was drawn to display the transitions between flow regimes in an intuitive manner. Only in the magnetized-bubbling regime could the magnetic field reduce the bubble size and improve fluidization quality. In the segregation-bubbling regimes, fluidization quality deteriorated as segregation developed. The segregation of the binary mixture was quantitatively studied by observing pressure drops in the local bed. Reasons for the improvement in fluidization quality as well as the occur- rence of segregation were analyzed. Furthermore. the flow regime transition under magnetization-LAST operation mode was different from that under magnetization-FIRST mode. The magnetically stabilized bed （MSB） flow regime, which could be easily created under magnetization-FIRST mode, could no longer be achieved under magnetization-LAST mode. With the admixture, the MSB was proved to be a metastable equilibrium state. Under the magnetization-LAST mode, the admixture bed reached directly the stable equilibrium state-bubbling with segregation.

Experimental and numerical investigation of liquid-solid binary fluidized beds： Radioactive particle tracking technique and dense discrete phase model simulations

Liquid-solid binary fluidized beds are widely used in many industries. However, the flow behavior of such beds is not well understood due to the lack of accurate experimental and numerical data. In the current study, the behavior of monodisperse and binary liquid-solid fluidized beds of the same density but dif- ferent sizes is investigated using radioactive particle tracking （RPT） technique and a dense discrete phase model （DDPM）. Experiments and simulations are performed in monodisperse fluidized beds containing two different sizes of glass beads （0.6 and I mm） and a binary fluidized bed of the same particles for vari- ous bed compositions. The results show that both RPT and DDPM can predict the mixing and segregation pattern in liquid-solid binary fluidized beds. The mean velocity predictions of DDPM are in good agree- ment with the experimental findings for both monodisperse and binary fluidized beds. However, the axial root mean square velocity predictions are only reasonable for bigger particles. Particle-particle interac- tions are found to be critical for predicting the flow behavior of solids in liquid-solid binary fluidized beds.

Removal of hydrophobic volatile organic compounds with sodium hypochlorite and surfactant in a co-current rotating packed bed

A co-current flow rotating packed bed was applied to remove volatile organic compounds（VOCs） by sodium hypochlorite（Na Cl O） and surfactant（sodium dodecyl benzene sulfonate,SDBS） from air stream. Xylene was used as a model VOC herein. The effect of p H,concentration of Na Cl O and SDBS solution, liquid flow rate, gas flow rate and rotational speed on xylene removal efficiency and overall mass transfer coefficient（KGa） were discussed. Then, a correlation for KGa of the co-current rotating packed bed was proposed by fitting the experimental data of KGa and independent variables of liquid/gas ratio,rotational speed, p H, Na Cl O concentration and treatment time, which was in good agreement with the experimental data（the deviation ≤ ± 30%）.

Predicting minimum fluidization velocities of multi-component solid mixtures

Employing well-established mixing rules for mean properties, appropriate expressions are derived for predicting minimum fluidization velocities of multi-component solid mixtures in terms of mono- component values for the velocity and the bed voidage at incipient fluidization. Based on flow regime and the mixing level of constituent species, it is found that these relationships differ significantly from each other, whether related to size-different or density-different mixtures. For mixed beds of size-different mixtures, the effect of volume contraction is accounted for by the mean voidage term, which is absent for segregated beds. Incorporating the volume-change of mixing leads to values of the mixture minimum fluidization velocities even lower than corresponding values for segregated bed, thus conforming to the trend reported in the literature. Size-different mixtures exhibit flow regime dependence irrespective of whether the bed is mixed or segregated. On the other hand, the mixing of constituent species does not affect the minimum fiuidization velocity of density-different mixtures, as the difference in the expres- sions for a segregated and a mixed system is rather inconsequential. Comparison with experimental data available in the literature is made to test the efficacy of the minimum fluidization velocity expressions derived here.