Floc model and intrafloc flow

Settling characteristics of nonspherical porous sludge flocs with nonhomogeneous mass distribution

The paper reports on the development of an advanced Lagrangian particle tracking model of sludge flocs that takes into account its nonspherical shape, the internal porosity and permeability, as well as the nonhomogenous mass distribution. The floc shapes, sizes and free settling velocities are determined based on the experimental measurement of settling sludge flocs originating from a wastewater treatment plant. Based on the floc shape characterization, a prolate axisymmetric ellipsoid is selected as the modelled sludge particle. In order to determine the main particle characteristics, e.g. the internal porosity, the density and the flow permeability, a Lagrangian particle tracking model is developed based on Brenner's drag model for a prolate axisymmetric ellipsoid and a buoyancy force model for a porous particle. The model is implemented for numerical simulations of the free settling process. The obtained floc characteristics are presented in the form of a two-part polynomial fitting curve, which can be used to model floc characteristics. The values of settling velocities of flocs computed by the model show very good agreement with experimental results. Futhermore, as the internal structure of a floc is seldom uniform, the nonhomogeneous mass distribution is considered, influencing the rotational and translational motions of the settling flocs. The nonhomogeneous mass distribution is introduced into the floc settling model. The parametric analyses of different barycentre offsets and shear rates are performed, and their influences on the free settling velocity are evaluated. The presented modelling approach can also be applied to flocculent settling of alum and other flocs in drinking water treatment plants. The developed Lagrangian model is suitable for use as a point source within the framework of Eulerian flow computations, and is solved as a two-phase flow model with a suitable Computational Fluid Dynamics code.

Vancomycin sorption on activated sludge Gram+ bacteria rather than on EPS; 3D Confocal Laser Scanning Microscopy time-lapse imaging

Antibiotics-bacteria interactions depend on antibiotic concentration at the scale of bacteria. This study investigates how vancomycin penetrates into activated sludge flocs and can be sorbed on the bacteria and extracellular polymeric substances (EPS). The 3D structure of flocs was imaged using EPS autofluorescence. The green fluorescent BODIPY^® FL vancomycin was introduced in a microscopic chamber containing activated sludge and penetration of vancomycin into the flocs by diffusion was observed using time-lapse microscopy. The penetration depended on the floc structure, as long and large pores could go through the whole flocs making preferential path. The antibiotic concentration into the flocs was also found to depend on the sorption rate. BODIPY^® FL vancomycin was found to bind preferentially into Gram⁺ bacteria than on EPS. The vancomycin adsorption constant on bacteria according to the linear adsorption model, Kd_bacteria was estimated to be 5 times higher (SD 2.6) than the adsorption constant on EPS Kd_EPS. These results suggest that antibiotic removal by sorption into wastewater treatment plants could change according to the amount of bacteria in the sludge. Moreover, antibiotic concentration at the scale of bacteria could be significantly higher than the concentration in the bulk solution and this should be taken into account when studying antibiotic activity or biodegradation.

Variation in water density related to pollutants removal in wastewater treatment processes and its use in explaining the working principles of the Unifed SBR

The wastewater quality of several municipal wastewater treatment plants (MWTPs) in Beijing was studied, and the water densities of different processing units were also measured during the wastewater treatment process. The results clearly showed that the water density declined from influent to effluent of the wastewater treatment process. Meanwhile, the variation in water density had good statistical correlation with the concentrations of total organic carbon, total phosphorus, suspended solids and total solids. Furthermore, the variation in water density could be used to explain the working principles of the Unifed sequencing batch reactor (SBR). Tracer tests were conducted in the Unifed SBR to investigate the hydraulic characteristics of the reactor. The experimental results showed that the variable values of water density from influent to effluent in the Fangzhuang MWTPs were greater than those caused by the temperature difference of >3 °C between the influent and the liquid in the reactor at 13 °C. Moreover, the flow regime of wastewater in the Unifed SBR was affected by the variation in water density, which may lead to stratification or a density current. Ascribed to the appearance of stratification in the Unifed SBR reactor, the water quality of the effluent could not be affected by that of the influent.

Experimental and numerical investigations of sedimentation of porous wastewater sludge flocs

The paper studies the properties and sedimentation characteristics of sludge flocs, as they appear in biological wastewater treatment (BWT) plants. The flocs are described as porous and permeable bodies, with their properties defined based on conducted experimental study. The derivation is based on established geometrical properties, high-speed camera data on settling velocities and non-linear numerical model, linking settling velocity with physical properties of porous flocs. The numerical model for derivation is based on generalized Stokes model, with permeability of the floc described by the Brinkman model. As a result, correlation for flocs porosity is obtained as a function of floc diameter. This data is used in establishing a CFD numerical model of sedimentation of flocs in test conditions, as recorded during experimental investigation. The CFD model is based on Euler-Lagrange formulation, where the Lagrange formulation is chosen for computation of flocs trajectories during sedimentation. The results of numerical simulations are compared with experimental results and very good agreement is observed.

An image-based method for obtaining pore-size distribution of porous media

This purpose of this investigation is to develop a new method for obtaining pore-size distribution (PSD) and other pore parameters from inner structure images. This method can be summarized into four key procedures to realize its functionality. A MATLAB program was composed to make the method applicable for image analysis. Image tests on wastewater biofouling layer samples showed the validity and feasibility of the method for analyzing intrinsic porous structures. The structural information obtained by this method can facilitate the understanding of transport phenomena, e.g., water flow and species diffusion inside porous structures.

The effect of influent temperature variations in a sedimentation tank for potable water treatment--a computational fluid dynamics study

A computational fluid dynamics (CFD) model is used to assess the effect of influent temperature variation on solids settling in a sedimentation tank for potable water treatment. The model is based on the CFD code Fluent and exploits several specific aspects of the potable water application to derive a computational tool much more efficient than the corresponding tools employed to simulate primary and secondary wastewater settling tanks. The linearity of the particle conservation equations allows separate calculations for each particle size class, leading to the uncoupling of the CFD problem from a particular inlet particle size distribution. The usually unknown and difficult to be measured particle density is determined by matching the theoretical to the easily measured experimental total settling efficiency. The present model is adjusted against data from a real sedimentation tank and then it is used to assess the significance of influent temperature variation. It is found that a temperature difference of only 1 degrees C between influent and tank content is enough to induce a density current. When the influent temperature rises, the tank exhibits a rising buoyant plume that changes the direction of the main circular current. This process keeps the particles in suspension and leads to a higher effluent suspended solids concentration, thus, worse settling. As the warmer water keeps coming in, the temperature differential decreases, the current starts going back to its original position, and, thus, the suspended solids concentration decreases.

Advective flow in spherical floc

Numerous structural models of flocs, such as homogeneous model or radially-varying model, were proposed in literature for predicting the extent of advective flow on the intrafloc transport processes. This work probed the three-dimensional structure of original and chemically flocculated wastewater flocs using the fluorescence in situ hybridization (FISH) and the confocal laser scanning microscope (CLSM) techniques, from which the spherical mesh model on real floc structure was constructed. Simulation results revealed that if an average characteristic of sludge floc, such as porosity or drag force correction factor of sludge floc is of concern, both homogeneous or radially-varying models may be able to apply, particularly for those flocs that were closely compacted. However, the detailed flow patterns inside real floc are much more tortuous than those of the homogeneous or radially-varying models. If local hydrodynamic environment within the floc is of interest, then only the complicated structural model with real floc could be applicable.

Oxygen diffusion in active layer of aerobic granule with step change in surrounding oxygen levels

High biomass density and large size limit the transfer of dissolved oxygen (DO) in aerobic granules. In the literature, the oxygen diffusivity is often employed as an input parameter for modeling transport processes in aerobic granules. The interior of an aerobic granule was observed to be highly heterogeneous. In this work, the distributions of extracellular polymeric substances (EPS) and cells in the interior of phenol-fed and acetate-fed granules were built up using a five-fold staining scheme, combined with the use of a confocal laser scanning microscope (CLSM). The steady-state and transient DO with step changes in surrounding DO levels at various depths were measured in the granules using microelectrodes. Cells were probed in a surface layer of thickness 125-375 microm. A marked fall in DO was also noted over this surface layer. No aerobic oxidation could occur beneath the active layer, indicating the oxygen transfer limit. Fitting the steady-state and transient DO data over the active surface layer yielded apparent diffusivities of oxygen were (9.5+/-3.5)x10(-10)m(2)s(-1) for the phenol-fed granule and (3.5+/-1.0)x10(-10)m(2)s(-1) for the phenol-fed granule. These values were lower than those adopted in models in the literature.

Size-dependent anaerobic digestion rates of flocculated activated sludge: role of intrafloc mass transfer resistance

The anaerobic digestion rate for flocculated sludge has been considered to be lower than that of original sludge, particularly in the later stages of digestion; attributed this relatively slower rate to the increased mass transfer resistance for reactants through the large flocs after flocculation. This study confirmed that methane production was retarded by flocculation. The structure of the floc was identified with fluorescence in situ hybridization (FISH) and a confocal laser scanning microscope (CLSM) technique. To verify the mass transfer resistance induced by flocculation, microsensors were applied to assess the response of oxygen concentration distribution inside the flocs that are subjected to sudden changes in ambient oxygen levels. Response time for the electrode at a floc's center was five times greater than the response time in original sludge flocs. Although the effective diffusivity of oxygen in the floc increased by 2.3 times after flocculation, the increased size of the flocculated floc was the major contributor to the total mass transfer resistance to oxygen.