Classical Filtration Research Articles

Fouling mechanisms of alginate solutions as model extracellular polymeric substances

Extracellular polymeric substances (EPS) are viewed as major fouling components during filtration of biological feeds in industry. However, very few studies investigated the fouling mechanisms of polysaccharides, one major group of macromolecules in EPS. In this work, dead end unstirred filtration of sodium alginate, a model polysaccharide, was carried out and analysed using classic filtration laws and the combined pore blockage and cake formation model. It was found that the cake model appears to fit the entire range of the ultrafiltration data while the consecutive standard pore blocking model and cake model are more applicable to microfiltration membranes (0.2 m track-etched and 0.22 m PVDF membranes). The specific cake resistance (α c ) (approximately 1×10 16 m/kg) is similar to α c of bovine serum albumin (BSA) and actual EPS in MBR systems reported in the literature, and higher than that of many colloidal particles. α c increases with the increase of alginate concentration but slightly decreases with the presence of BSA in the feed solution. The alginate cake obtained during filtration features both low compressibility and low porosity despite some differences between different membranes. The lack of a full flux recovery following filtration interruption implies the existence of strong intermolecular attractive interactions within the alginate fouling layer.

Critical fouling conditions induced by colloidal surface interaction: from causes to consequences

Critical fouling conditions (CFC) are defined as the process operating conditions leading to the formation of multilayer irreversible fouling at the membrane surface. This irreversible fouling is the result of a phase transition in the accumulated matter from a dispersed phase (concentration polarisation) to a condensed phase (deposit or gel formation): the spinodal decomposition. Properties of concentrated colloid dispersions and their related phase transitions are integrated into a classical filtration mass balance via colloidal osmotic pressure, II. This then allows us to define CFC for both cross-flow and dead-end filtration. These CFC are expressed in terms of critical pairs of operating conditions: the set permeate flux/boundary layer thickness (directly linked to cross-flow velocity) in cross flow and the critical set permeate flux/filtered volume in dead end.

Filtration Properties of Oil-in-Water Emulsions Containing Solids

Summary This paper describes a laboratory study of the factors controlling the filtration and fluid-leakoff properties of emulsions containing solid particles. Such fluids are sometimes used as low-leakoff completion fluids. High-pressure/high-temperature (HP/HT) filtration tests clearly demonstrate that emulsions have the ability to reduce filtrate loss to the formation. However, the emulsion droplets tend to invade the formation and form an internal filter cake. This is evident from the liftoff pressures needed to initiate flowback and deep invasion of emulsion droplets in long-core experiments. Emulsions containing solids (CaCO3, in our case) had lower filtrate volume and higher return permeabilities than solids-free fluids. The effects of percent oil, filtration pressure, core permeability, temperature, and viscosity of the continuous phase were investigated. Filtration pressure and core permeability had major influences on the filtration properties of the emulsified completion fluids. Higher injection pressures increased internal damage and lowered the return permeability. Higher-permeability cores had higher filtrate loss. Emulsion droplets were observed in the effluent, and solid particles are needed to form a stable external filter cake. Long-core experiments showed that using emulsions containing acid-soluble solid particles have 100% return permeability after an acid squeeze, showing that the emulsion-droplet invasion depth is less than 1 in. It is shown that solids-free fluids had the highest formation damage and higher liftoff pressures. Analysis of the fluid-leakoff data indicates that emulsions containing solids do not behave as classical filtration theory predicts because of the invasion of emulsion droplets into the formation. Sizing the solid particles and emulsion droplets in completion fluids and muds according to the permeability of the rock is important in forming stable external filter cakes. This increases the return permeability and results in easier cake removal (low liftoff pressures).

On the Measurement and Evaluation of Pressure and Solidosity in Filtration

New test filtration equipment has been constructed that makes simultaneous measurement of local solid compressive pressure and local solidosity possible. Four materials were used to evaluate the new equipment. In this equipment the solidosity was measured with resolution of 1 mm in height (i.e. in the direction of the liquid flow). The experimental error was estimated to be 0.005–0.04 m 3 m −3 , depending on the properties of the slurry. Constitutive relationships for solidosity, permeability and specific filtration resistance were estimated and an overall good fit was obtained. Furthermore, it was found that the average specific filtration resistance based on the classical filtration equation could differ by as much as 20% compared to corresponding values based on local measurements.

Modelling filtration with superimposed sedimentation

Filtration at moderate rates is often superimposed by sedimentation. This work shows that classical cake filtration equations can be complemented by an additional sedimentation term to model cake filtration measurements with significant sedimentation. Algorithms are derived for zone as well as clarifying sedimentation. In contrast to the classical filtration theory not the absolute values but the differentials of cake height and filtrate volume are set into a proportional relation. The resulting set of equations can be solved numerically. If the porosity is expressed as a function of the cake height and the Carman–Kozeny relation is employed, the model may be extended to compressible cakes. Experiments with the system ‘lactose in ethanol’ justify the model. Experimental data can be fitted much better with the new model than with equations of the classical filtration theory alone. In contrast with other filtration models that include sedimentation, no additional parameters need to be introduced, and the physical meaning of constants such as cake resistance, specific cake volume and, settling velocity is maintained.

Interpreting deposition patterns of microbial particles in laboratory-scale column experiments.

The transport and fate of microbial particles in subsurface environments is controlled by their capture (natural filtration) by sediment grains. Typically, filtration models used to describe microbe removal in porous media predict exponential decrease in microbial particle concentration with travel distance. However, a growing body of laboratory-scale column experiments suggests that the retained microbial particle profiles decay nonexponentially. The observed behavior may be attributed to the heterogeneity in the interactions between microbial particles and sediment grains, most likely due to the inherent variability in the microbial particles. This factor can be incorporated into classical colloid filtration (deposition) theory by inclusion of a distribution in the deposition rate coefficient. We show that certain distributions of the deposition rate coefficient (i.e., log-normal, bimodal, and power-law distributions) give rise to nonexponential deposition patterns. Comparisons of model predictions to experimental data indicate that the observed nonexponential deposition behavior of bacteria and virus particles may be attributed to a broad range (i.e., a power-law distribution) of microbial deposition rates. Other mechanisms such as particle release and blocking by previously deposited microbial particles are also shown to be potential sources of deviation from the classical filtration theory. Our results further suggest that monitoring fluid-phase particle concentration is insufficient for accurate characterization of the deposition and transport behavior of microbial particles in saturated porous media. Rather, the shape of the microbial particle retention profile is shown to be a key indicator of the mechanisms controlling microbial deposition and transport.

Biopolymer Recovery with the Pressure Electrofiltration and Determination of the Biopolymer Quality by MAS-NMR and FT-IR Techniques

The classical dead-end filtration of a biopolymer containing suspension is a very inefficient process, since biopolymers, like the polysaccharide xanthan, form an almost impermeable filter cake. The results of this effect are a high specific filter cake resistance and long filtration durations. For this reason, precipitation combined with centrifugation is often used to separate biopolymers from a fermentation broth. A new dead-end filtration process has been developed to overcome this problem. This process is the pressure electrofiltration, with which the filtration kinetics can be improved drastically. In the experiments performed 99 % of the polysaccharides were recovered from the solution by the pressure electrofiltration. Furthermore, the chemical structure of xanthan was examined with FT-IR and MAS-NMR spectra, before and after the pressure electrofiltration. This was carried out in order to demonstrate a possible chemical or electrochemical reaction of the biopolymer during the electrofiltration.

Transciliary filtration using the fugo bladeTM

Transciliary filtration is an approach to manage glaucoma that departs from classic filtration surgery. This approach uses the tissue ablation and noncauterizing, hemostatic capabilities of the Fugo BladeTM (Plasma Blade). A nonbleeding micropore is created, which drains aqueous from behind the iris and into subconjunctival lymphatics. No peripheral iridectomy is needed. Mitomycin and excess cautery are contraindicated. Surgical time for this procedure is approximately 5 minutes.

Characterisation and modelling of fouling in membrane bioreactors

A membrane bioreactor used for denitrification of a synthetic substrate was studied in term of membrane fouling. For standard pH and temperature conditions, subcritical conditions were defined to ensure the process stability. The stepwise method was used to determine the critical flux for the deposition of colloidal particles. Under standard physicochemical conditions, only a low and constant fouling resistance was observed if the permeate flux was maintained below the critical flux. The influence of physicochemical variations was then investigated by varying pH and temperature in the biological reactor. It was observed that, when the pH value was higher than a critical one, the membrane was rapidly fouled. This maximum admissible pH value decreased when the temperature increased. On analysing the reversible nature of fouling and the variations of ionic concentrations with the pH, the role of carbonate calcium precipitation was pointed out. By using classical filtration models, it was shown that the fouling mechanism could be the deposition of CaCO 3 particles formed in the bulk suspension by bulk crystallisation.

Comparative study of crossflow microfiltration with conventional filtration of sherry wines

The present work reports, firstly, on the application of the gel-polarisation model to the crossflow microfiltration of sherry wines and brandies, in order to calculate the model parameters. In a preliminary set of experiments, the influence of the transmembrane pressure (TP) on the filtrate flow was studied, at several feed flows. The obtained results show a constant ratio between the optimum TP (TP opt) and the feed flow of 32×10 −4 l h −1 Pa with a TP opt of 11×10 4 Pa, for a feed flow of 360 l h −1 for all products. The obtained values of D for the different products can indicate that the average size of the particles in the gel layer is from 10 to 50 nm. Secondly, it has compared crossflow microfiltration process with classic filtration processes, which are used in the standard production system. The results show that the microfiltration has a higher effectiveness than conventional filtration, except in the case of brandy, where conventional filtration is more effective.

Experimental Investigation of Local Efficiency Variation in Fibrous Filters

Classical filtration models consider filter media as homogeneous and use an arbitrary inhomogeneity factor to account for the differences between theory and experiments. The variations in media-packing density have been experimentally observed and seen to be significant. There is minimal experimental information on the effect of these variations on filter performance. In this paper, local efficiency variations in commercial fibrous filters have been obtained for varying operating conditions. A filter scanner was developed to measure local filter efficiencies, and the features of the scanner are described in detail. The variations in local efficiencies are seen to have a Gaussian distribution, and the operating conditions and media behavior influence the variability in the local efficiency data. Theoretical modeling can be used to obtain two-dimensional packing density distribution data from the local efficiency measurements.

Superfluidity of Aqueous Solutions of Mechanically Activated Quartz in Capillary Porous Bodies

Water permeation through porous beds of ground quartz is not explicable in terms of the classical filtration theory: as the filtration rate is reduced, the water permeability of these beds grows monotonically to a doubled value, while the theory predicts that it must be invariable in the range of filtration rates examined. This phenomenon is a direct consequence of the mechanical activation (solubilization) of quartz during grinding. The superfluidity phenomenon is also observed with specially prepared aqueous solutions of mechanically activated quartz and porous bodies formed from chemically inactive powders. It is demonstrated that the superfluidity of water in beds of mechanically activated powders is due to the formation of such solutions during filtration.

Gas sparged cross-flow microfiltration of biologically treated wastewater

A novel technique was tested for reducing tubular mineral membrane fouling by injecting gas into a cross-flow stream. The injected gas is thought to form complex hydrodynamic conditions inside the microfiltration module, which increase the wall shear stress, preventing the membrane fouling and enhancing the microfiltration mass transfer. The experimental study was carried out with biologically treated wastewater filtered through a tubular inorganic membrane (Carbosep M14). The flux, monotonously increasing with gas velocity, was more than tripled. New dimensionless quantities of shear stress number and resistance number were developed by generalisation of the dimensional analysis already carried out for the steady state flux of classical unsparged cross-flow filtration. A unique formalism allowed then interpreting the experimental results of both classical diphasic filtration and sparged filtration. The main limiting mass transport process was due to the solid content.

Modelling module of the intelligent control system for the variable volume pressure filter

Artificial Intelligence methods like expert systems, fuzzy logic and neural networks have proved to be excellent tools for the control of mineral processes. This technology is currently being embedded directly into process equipment like flotation cells and dewatering filters. This paper presents the modelling module for a pressure filter. The modelling module of the intelligent system predicts filtration using the two-stage hybrid model. The first stage model is based on a numerical model for compressive cake filtration and the second stage model is the identified grey-box model based on the classical filtration model. The filtration parameters for the compressive cake filtration model were obtained from laboratory tests. The parameters for the classical filtration model are defined during filtration using the recursive least square identification method. The two-stage hybrid model of the on-line support system was tested in a full-size filter at a pilot plant.

The use of electrode probes in determinations of filter cake formation and batch filter scale-up

A well controlled apparatus was used to obtain new data for constant pressure cake formation in a batch leaf filter which could be fitted with electrode type sensing probes. For aqueous suspensions of calcite and talc, the influence of the intrusive electrodes on cake formation was quantified with respect to filtration parameters. As the electrode length and diameter were altered such that the projected cross-sectional area occupied up to 2.6% of the filter area, the average specific cake resistance (as determined by classical filtration theory) was observed to increase by up to 49% and the average cake porosity decreased by up to 21 %. When the electrodes were small relative to the dimensions of the filter cell, their influence on macroscopic cake properties was low. The additional information which electrical resistance measurements provide is discussed in relation to the results and observations by previous researchers and the benefits to filter test analysis and batch filter calculations are highlighted. For moderately compressible cake filtrations it is postulated that the incorrect assessment of cake porosity can typically account for an ∼12 % error in scale-up calculations for batch filters.

Spatial distribution of deposited bacteria following Miscible Displacement Experiments in intact cores

Miscible displacement experiments were performed on intact sand columns ranging from 15 to 60 cm in length to determine whether bacterial deposition varies at the centimeter scale within aquifer sediments. A 1‐pore‐volume pulse of radiolabeled cell suspension was introduced into the columns followed by a 2‐pore‐volume flush of artificial groundwater. The columns were then drained and dissected along the axis of flow. At ∼1‐cm intervals, nine samples were removed for the enumeration of sediment‐associated bacteria. Concentrations of sediment‐associated (deposited) bacteria varied by up to 2 orders of magnitude in the direction perpendicular to flow demonstrating that bacterial deposition cannot be described mechanistically by a single rate coefficient. Incorporation of a distribution of sediment size and porosity values into Monte Carlo simulations indicates that physical heterogeneities are only partially responsible for the observed variability in deposited bacteria. A simple first‐order model (classic filtration theory) adequately described the average spatial distribution of bacteria with depth within the 15‐cm column. For the longer columns, however, the average concentration of deposited bacteria did not decrease exponentially with depth. A second‐order model, modified to include an influent suspension of bacteria consisting of two subpopulations with separate sticking efficiencies (dual‐alpha population), was required to describe the observed decreases of deposited bacteria with depth. A sensitivity analysis was performed with a first‐order dual‐alpha model to understand the effects of an influent suspension with two subpopulations of bacteria on the decrease of deposited bacteria with flow path length. Numerical simulations show that even for small fractions (0.01) of nonsticky bacteria, the decrease in deposited bacteria may deviate substantially from the exponential decrease expected from colloid‐filtration theory. Results from experimental as well as numerical studies demonstrate the importance of column dissections for understanding bacterial deposition in saturated porous media.

Modelling Module of the Intelligent Control System for the Variable Volume Pressure Filter

Artificial Intelligence methods like expert systems, fuzzy logic and neural networks have proved to be excellent tools for the control of mineral processes. This technology has been currently embedded also directly into the process equipment like flotation cells end dewatering filters. This paper presents the modelling module of the intelligent control system for the variable volume pressure filter. The modelling module of the intelligent system utilises the real-time methods for the prediction of the filtration behaviour; one of them based on the new filtration model. The classical filtration model will also be tested and the recursive least square identification method. Finally the simulation results and the long-term testing results at the pilot plant will be presented.

Yeast suspension filtration: flux enhancement using an upward gas/liquid slug flow-application to continuous alcoholic fermentation with cell recycle.

This study deals with the use of an upward gas/liquid slug flow to reduce tubular mineral membrane fouling. The injection of air into the feedstream is designed to create hydrodynamic conditions that destabilize the cake layer over the membrane surface inside the filtration module complex. Experimental study was carried out by filtering a biological suspension (yeast) through different tubular mineral membranes. The effects of operating parameters, including the nature of the membrane, liquid and gas flowrates, and transmembrane pressure, were examined. When external fouling was the main limiting phenomenon, flux enhancements of a factor of three could be achieved with gas sparging compared with single liquid phase crossflow filtration. The economic benefits of this unsteady technique have also been examined. To investigate the possibility of long-term operation of the two-phase flow principle, dense cell perfusion cultures of Saccharomyces cerevisiae were carried out in a fermentor coupled with an ultrafiltration module. The air injection allowed a high and stable flux to be maintained over 100 h of fermentation, with a final cell concentration of 150 g dry weight/L. At equal biomass level, a twofold gain in flux could be attained compared with classical steady crossflow filtration at half the cost.

Predicting the performance of pressure filters

As a step towards removing heuristics from the design and specification of filters, it is shown how accurate experimentation and modelling can be utilised to predict the performance of a pressure leaf filter over a range of experimental conditions. An experimental apparatus combining the principles of mechatronics with a dead-end pressure leaf filter is outlined, along with sample data which outline its versatility. Experimental data obtained from the apparatus are compared with predictions given by classical filtration theories incorporated within flexible computer simulations. Batch experimental results obtained at constant pressure with aqueous suspensions of calcite, talc and zinc sulphide are shown to match well with theoretical predictions of the important design parameters cake height and cumulative volume of filtrate, particularly when cake compressibility is lower. Constant pressure data are further used to predict constant flow filtrations where the applied pressure is manipulated via computer software.

A new approach to variable pressure cake filtration

The widespread reliance on heuristics for the design and specification of particle/fluid separation devices has prompted a new, mechatronic based approach to dead-end pressure filtration. A computer driven experimental apparatus was used to facilitate proportionally controlled filtrations over a range of pressure/flow regimes whilst maintaining inherent suspension properties. Preliminary results from constant flow experiments using distilled water and aqueous suspensions of calcite are presented where the air pressure within the filter is controlled through a combination of flow and pressure transducers and an electronically adjusted pressure regulator. The effects of controller type, controller gain, sampling time and set point flow on system response are shown to illustrate what can be achieved using mechatronics. Predictions of constant flow filtration experiments are made using data from constant pressure tests and reasonably good agreement is achieved. A simulation, based on classical filtration and control theories, is also presented and shown to compare well with the filtrate flow responses observed from the filtration apparatus.