Experts in filtration and separation

We advance and disseminate knowledge in the design and use of filtration and separation techniques in industry, commerce and other walks of life.


Published papers: Volume 2, Issue 5

Author(s) Title, page numbers and abstract
E.S. Tarleton and R.J. Wakeman Solid/Liquid Separation Plant Design – Equipment Selection By Computer Software (pages 4-10)

bullet Abstract

bullet Within the broad area of solid/liquid separation and plant design, the purpose of this paper is to provide guidance on what form of small scale tests and results analysis are appropriate for the selection of equipment. Through a knowledge of elementary experimental data, selection charts and an expert system approach, it is shown how interactive software can be used to identify and rank potentially suitable separation equipment. Both basic and more sophisticated examples of equipment selection are presented and additional tables indicate how ranked equipment can be further shortlisted for further investigation.

N. Pearson Selection Of Centrifuges – Why Big Samples? (pages 10-17)

bullet Abstract

bullet Why does a centrifuge manufacturer require a large sample of the slurry to be separated, whenever a meaningful “Process Guarantee” is requested? This paper endeavours to explain the commercial modelling techniques used (with their limitations) to select the size and type of machine and the validity of the “Process Guarantee”.

A. Walker Solid-Liquid SEPARATION Plant Design – Vacuum Filtration (pages 18-22)

bullet Abstract

bullet The use of vacuum to provide additional driving force for the filtration of solids from a liquid suspension is common throughout the chemical and process industries. There are certain benefits and constraints associated with vacuum filtration, as there are with pressure filters, gravity filters and centrifuges, and these need to be considered in some detail before choosing any filtration equipment. In the following paper, the technique for the selection and sizing of a vacuum filter will be discussed, as well as ways of applying this information to the system under investigation.

V.A Goldade and A.G. Kravtsov Fibrous Materials For Ultra-Fine Air Purification (pages 22-26)

bullet Abstract

bullet Electret and filtration characteristics of charged polypropylene based fibrous polymer materials were studied. Forming processes and those of additional electrification of melt-blown fibers in a corona discharge were combined in one cycle. The charge state of the fibrous plastic was analyzed using a thermal depolarization method, which involved measurement of thermally stimulated currents (TSC). Surface charge density was measured by compensation method. Filtration parameters of materials were studied, including skipping factor of oil aerosol through the sample and aerodynamic resistance.It was established that polymer electrification at the melt stage allows attainment of the highest electret charge value, even under rather low electric field intensity. The secondary melt-blown fibrous materials showed higher electret characteristics. The mechanism of fibres charging is discussed. Electret charge on polymer fibres was shown to lead to a considerable improvement of filtration rates. Owing to Coulomb forces, not only are charged particles entrapped on electret fibers, but neutral contaminants are also able to acquire a dipolar moment in the electret field.

The skipping coefficient was investigated depending upon testing time, aerosol particle diameter and filtration velocity for different kinds of samples. The best results were obtained using two-layer samples with charged and non-charged layers. Linear growth of aerodynamic resistance of the filtering materials has been recorded with filtration velocity increase. The conclusion is that among fibrous filtering materials used in fine purification systems, including respirators, the most efficient are those which combine electret and non-electret filtering layers.

A. Macías-Machín, J. Umbría and A. Lecuona New Gas Filter Using Atomised Ultrafreezing (Fua) (pages 27-28)

bullet Abstract

bullet Air pollution has harmful effects on man, animal, vegetation, materials and the biosphere. For this reason, it is very important to remove the different polluting agents from the air. Those in the form of particles can be removed by means of different devices and equipment in order to clean gaseous currents, which may be emitted into the atmosphere without danger. The removal of particles from gases can be performed by means of solid-gas separation, where filtration is the most common. However, it is important to take into consideration those processes that are inherently less polluting. Regarding new filtration techniques, a series of devices using the simultaneous removal of particulate matter and gaseous pollutants in a single collection unit have been developed. These devices, which have attracted attention due to the originality of their designs, are called Multipurpose Filters.

D. Schlegel A Filtering Screw-Conveyor Press For Finely Dispersed Sludges (pages 29-32)

bullet Abstract

bullet A new filtering screw-conveyor press for finely dispersed sludges, specifically for mineral sludges with particles in the micron range, works continuously and fully automatically and almost without an operator. A small pilot version has been tested and more than a thousand hours of endurance tests have shown that abrasion and blockages can be controlled. In the largest axial section of the screw, beginning with the inlet of the sludge into the screw-channel, the multiphase system in the channel is still fluid. A thin layer of filter cake in the gap between the inner diameter of the porous barrel and the outer diameter of the screw represents only a relatively small resistance against filtrate flow. Therefore the filtrate mass flow per unit area of the filter medium is much higher in the filtering screw-conveyor press than in the usual chamber filter press. It is envisaged that the filtering screw-conveyor press will be able to compete well against the usual filtration techniques.

C. Meisl, A. Fuchs and
G. Staudinger
Research Note: A Study Of The Separation Of Activated Sludge (pages 122-126)

bullet Abstract

bullet The treatment of organic pollution in waste water plants is carried out in two steps, namely, reaction and separation. In the aerobic basin the pollutants are converted into biomass by bacteria that use oxygen, that is pumped into the basin. This so-called activated sludge is separated from the treated water in the secondary clarifier. The separation efficiency of the secondary clarifier must be higher than 99.9%. The efficiency depends on the hydraulic flow within the secondary clarifier and on the separation properties of the sludge itself. The sludge can principally separate according to two different regimes. At a low concentration of suspended solids the single flocs settle according to the laws of sedimentation (including hindered settling). This regime is called disperse separation. The supernatant that represents the effluent is contaminated with the smaller flocs that do not settle quickly enough. If the amount of suspended solids in the water is above a critical level that is defined not only by the sludge type, but also by the operating parameters of the plant, the particles mostly, but not always, form a floc network. If this regime, which is called crosslinked separation, occurs the solids concentration in the supernatant is considerably lower.

Investigations on crosslinked separation have shown that the sludge flocs form a network that has a certain tensile strength and in which the small flocs are intercepted when the water percolates through it. As such the network works like a deep bed filter. The structure is able to impede the passage of even very small particles that would otherwise pass into the supernatant. As the density of the network increases with time, the filter performance also increases. In this way the water that passes into the supernatant is always cleaner than the supernatant itself. A thin layer of very clean water is formed directly above the sludge level.

Just above the sludge level, in the range of the thin layer, the particle concentration is lower than in the average supernatant. The transparency of the supernatant is used as a measure of the particle concentration. By using both, the transparency in the thin layer and the transparency in the supernatant, the transient transparency can be calculated and thus the level of contamination in the effluent can be predicted.

For the determination of the transparency values and the corresponding separation regime a photometric method has been developed. This paper provides measurements taken at commercial wastewater treatment plants.

Keywords: Activated sludge; deep bed filter; flocculation; photometry; sedimentation.

C.J. Richardson and V. Nassehi Finite Element Modelling Of Concentration Profiles In Flow Domains With Porous Walls (pages 127-134)

bullet Abstract

bullet Free flow regimes in domains with porous walls are a common feature in processes such as the filtration of suspensions and various hydro-environmental systems. Prediction of the fluid dynamical behaviour of these regimes is not straightforward because factors such as fluid viscosity, fluid density and wall permeability vary continuously during the flow. Notably, a portion of the carrying fluid will seep through the porous wall boundary thus depositing solid particles as it passes. In this paper an adaptive algorithm for the prediction of concentration profiles in regimes with porous walls is presented. This algorithm is based on the Galerkin finite element solution of a flow model incorporating the combined Navier-Stokes and Darcy equations and a transport model represented by the convective-diffusion equation. The stepwise development of the Galerkin finite element model is explained and the important features of the process from both the fluid dynamical and rheological points of view are discussed.

The effect of fluid seepage on the rheological behaviour of the fluid is epitomised by an equation relating fluid viscosity and volume fraction of solids carried by the flow. The numerical description of this model is completely general and can be easily extended to incorporate other factors.

Keywords: Porous wall; concentration boundary condition; finite element method; convective-diffusion equation; initial value problems.

H.-J. Schmid and H. Umhauer Electrostatic Precipitator Efficiency In The Fine Particle Regime (135-141)

bullet Abstract

bullet Earlier investigations on the grade efficiencies of electrostatic precipitators (ESPs) yielded significantly higher separation efficiencies for small particles (diameters less than 2 µm) than predicted by any model. Therefore, several experiments were conducted in a laboratory scale plate-type model precipitator with flat plates and smooth wires using monodisperse 1 µm particles to verify and understand this phenomenon: an optical in-situ measurement yielded local particle flux profiles. By integrating these profiles, balances for several small parts of the precipitator could be made providing deeper insight into the local particle precipitation. Furthermore, particle charge measurements were conducted on an aerosol sample taken directly from the precipitator channel. The flux profiles show the development of a characteristic profile along the duct that gets steeper towards the wall. These profiles are very similar for different operating parameters if the Deutsch-number is kept constant. On the other hand, the local balances always show a much higher separation efficiency than predicted by the Deutsch theory and even higher than that predicted by the laminar flow model. Measured particle charges are also significantly higher than predicted by common charge models.

Keywords: Air quality; CFD; electrostatic charging; electrostatic precipitation; grade efficiency; in-situ measurements; modelling; particle deposition; sub micron particles.

A.P. Koscheev, A.E. Serzhantov and A.D. Shepelev Surface Chemistry And Sorption Properties Of Chemically Modified Carbon Fibres From Polymer Blend Precursor (pages 142-146)

bullet Abstract

bullet An activated carbon fibre (ACF) material obtained by the pyrolysis of a polymer blend precursor was subjected to different chemical treatments, including electrochemical oxidation and thermal oxidation by air in order to modify its surface properties. The surface chemistry of different samples was studied by means of temperature programmed desorption. The influence of surface modification on the hydrophobic properties of ACFs, the adsorption of Cr and Pt ions from aqueous solutions and the removal of I2 from the gas phase were investigated. Electrochemically oxidised carbon fibres exhibited hydrophilic surfaces and strongly modified sorption efficiency. The adsorption capacities for Cr and Pt ions from aqueous solutions increased by two orders of magnitude after oxidation. In contrast, the amount of trapped I2 was reduced by a factor of 7. It was concluded that the adsorption of metal ions is enhanced and the interaction with I2 was suppressed by the action of carboxylic and lactone groups that are formed at the surface during treatment. The modifications of the surface chemistry of ACFs caused by interaction with oxygen and NO2 at room temperature were also studied. Keywords: Carbon fibre; functional groups; electrochemical treatment; heat treatment; adsorption.

Published papers: Volume 2, Issue 4

Author(s) Title, page numbers and abstract
A. Strehlow and M. Schmoch Comparison of techniques for electrode rapping in electrostatic precipitators (pages 5-8)

bullet Abstract

bullet Dust from industrial flue gases is often removed by electrostatic precipitators (ESP). The dust particles collected in these devices need to be cleaned off the electrodes at adequate intervals to maintain the function of an ESP. The use of hammers to rap the electrodes to clean them is common practice.In principle, there are many ways to rap the plates with a hammer as there is an almost infinite choice of locations and orientations to install the hammers and anvils. The hammers are typically tumbling hammers that are fixed to a rotating shaft or cylindrical hammers that are lifted by an electromagnetic force.

Another parameter is the suspension of the electrodes, which has to be adjusted to the chosen type of rapping system because of its influence on the oscillation intensity of the electrodes. The most suitable way to determine the efficiency of a given configuration is to measure the acceleration of the electrodes in those sections where the dust has to be removed.

Various types of rapping systems have been investigated to evaluate their efficiency. Acceleration measurements with a frequency analyser were performed on collecting electrodes as well as on different types of discharge electrodes. Since electrodes installed inside an ESP are hardly accessible most of the measurements took place on a test rig, which allowed measurements of full scale electrodes. Additional site measurements were carried out to validate the test-data.

K-J. Schröder, T. Boger, K. Drury, G. Lachut and P. Makris A New Generation of Ceramic Membranes with Excellent Space and Cost Efficiency (pages 9-11)

bullet Abstract

bullet Ceramic honeycomb monoliths have been used for more than 25 years as catalyst supports for automobile catalytic converters. Corning Incorporated, the world leader in the production of ceramic catalytic converter supports, is now utilizing this honeycomb technology to produce extruded mullite (3Al2O32SiO2) honeycombs to obtain highly compact and cost effective ultrafiltration (UF) and microfiltration (MF) ceramic membranes for liquid filtration applications. Conventional alumina ceramic membrane products are well known for their high strength, chemical durability and long operating lifetime but have always been significantly more expensive than polymeric membranes.

The use of a high packing density honeycomb support incorporated by Corning reduces the cost per unit membrane area as well as associated systems costs to levels that are competitive with tubular polymeric membranes. Like conventional alumina membranes, these honeycomb-based filters offer the ability to withstand extreme use conditions such as pH extremes, high temperatures and exposure to oxidants and solvents. In this paper the function and design of these new membranes are described. The economic and operational benefit of units incorporating these new membranes are discussed.


bullet Abstract

bullet A combination of ultrafiltration, nanofiltration and bipolar electrodialysis for the recovery of lactic acid is presented. The method simplifies so far known methods involving membrane technology and increases the recovery of lactic acid. Optimal operating conditions for the nanofiltration were investigated. It was found that the chemical composition has a great influence on lactic acid transport in the membrane. Also, the performance of electrodialysis was evaluated. It was found to correspond closely to the theoretical expectations.


bullet Abstract

bullet In recent years there has been a tendency toward the use of increased molecular weight flocculants for sewage sludge dewatering. Such ‘ultra high molecular weight’ flocculants engender dose efficient dewatering performance compared with lower molecular weight analogues. Image analysis of sewage sludge flocs confirms the theory that a larger mean floc size is formed by higher molecular weight entities at a given flocculant dose. The use of image analysis is a useful analytical tool for the study of sewage sludge flocculation by synthetic polyelectrolytes.


bullet Abstract

bullet Pretreatment refers to process steps that are required before the membrane separation in order to enhance or maintain the membrane performance over an acceptable duration of operation. The types of pretreatment required to reduce the potential for membrane fouling (when pretreatment is used as a technique for the management of fouling) depend upon both the process stream and the membrane type. Some chemical pretreatment operations themselves (e.g. chlorine addition) may lead to membrane damage, in which cases a further process is required to remove the offending chemical before it reaches the membrane modules. Pretreatment is designed to extend the lifetime of the membrane, prevent fouling of the membrane and maintain the performance (i.e. rejection and recovery) of the system.

M. Middlebrooks and M. Baumgartner Airborne Molecular Contamination (AMC) Control and the Effect of Filter Media Technical Parameters (pages 23-28)

bullet Abstract

bullet This paper reviews the different adsorption mechanisms for AMC control (gas phase filtration) and explains the influence of different filter media technical parameters (e.g. adsorbent grain size, impregnation, filter configuration depending on media selection) on performance characteristics such as efficiency, life time and pressure drop. Technical test results are mainly explained with examples of clean room applications according to the SEMI F21-95 standard.

K. Weber and W. Stahl Influence of an electric field on filtration in a filter press (pages 88-93)

bullet Abstract

bullet The combination of mechanical and electrical filtration seems to be an effective method to enhance the filtration kinetics, because the electrokinetic effects are enhanced with increasing particle surface. The effect of electrophoresis decreases the migration velocity of the particles. The electroosmosis moves the diffusive layer of water around the particles. Thus, it promotes the filtration flow. In this way the electrokinetic effects of electroosmosis and electrophoresis improve filtration in one direction. By the addition of an electric field to a filtration in a two-sided filter-cell the electrokinetic effects enhance the filtration on the one side and impair the filtration on the other side. However, experiments have shown that even this inhomogeneous distribution of the filtrate leads to an acceleration of the filtration kinetics.

Experiments using a pilot scale filter press confirmed the results of the laboratory tests and also demonstrated faster kinetics. This kinetic acceleration could even be improved by a new electrode arrangement, where an additional electrode, operated as the anode, was inserted between the filter plates, while the electrodes behind the filter cloths were operated as the cathodes. In this way the kinetic of a pressure filtration experiment using a filter press and a kaolin suspension at a pressure of p = 10 bar and a voltage of U = 120 V was accelerated by 66 %.

Keywords: Electrofiltration; electrophoresis; electroosmosis; filter press; filtration kinetics.

J. Dueck, D. Purevjav and Th. Neesse Surface Force Effects on Filter Cake Characteristics (pages 94-98)

bullet Abstract

bullet In order to validate a filtration model that accounts for the particle interactions within filter cakes, filtration experiments with a quartz suspension were carried out. The bonding energy of the particles was varied using the electrolytes NaCl, MgCl2 and AlCl3 via the different valencies of the counter ion. The measured filter cake porosities and permeabilities show a remarkable correspondence to measured zeta-potentials and adhesive forces.

Despite some differences between the experimental results and the model equations, the principal validity of the theoretical expectations was confirmed. The model considerations can be helpful in providing a deeper understanding of the influence of electrolytes and how they act as coagulants in fine particle filtration. Keywords: Coagulation; filter cake; particle-particle-interaction; permeability; porosity; specific bonding energy.

A. Kavouras and G. Krammer Determination of solid hold up in jet filters from periodic continuous operational data (pages 99-105)

bullet Abstract

bullet The cleaning of gas dust filters is often incomplete. In this study experiments using a pilot plant scale fabric filter system were carried out where the filter cake was removed from the fabric by jet pulses. In general the jet pulses do not remove the entire filter cake from the exposed area and the entire filter area is not exposed to the jet pulses. The filter cake, which is not torn off the filter by the jet pulses, remains unchanged on the filter medium. As a result of the incomplete filter cleaning, the distribution of the gas flow across the filter area can be highly uneven. As a consequence, the rise in the pressure drop over time throughout a filtration cycle is not strictly linear. A pressure drop versus time curve that deviates from linearity can indicate an uneven distribution of solids across the filter area. Due to the mal-distribution of the solid load over the filter area, the jet pulsed filter cannot be described as a uniform, single filter area. A simple transient two-zone filtration model is employed to estimate the solid hold up of a jet pulsed filter from easily accessible data (solid mass stream to the filter, gas volume stream, pressure drop curve and pressure drop coefficient of the filter cake). The model considers the mal-distribution of the solid over the filter area through division of the entire filter area into two discrete segments. The model results are validated with experiments.

The hold up of the dust on the filter cake, as well as its distribution over the filter area are important pieces of information for the modelling of chemical reactions in the filter cake.

Keywords: Two-zone filtration model; fabric filtration; hold up; jet filters; filtration modelling.

A. Schiel, A.P. Weber, M. Katzer and G. Kasper Precipitation of submicron aerosol particles at high temperatures by electrostatic means (pages 106-108)

bullet Abstract

bullet The thermal charging of submicron particles for model aerosols of TiO2 and SiO2 as well as milled slag at 1600°C has been studied with the aim to achieve electrostatic gas cleaning at high temperatures. The charging state of the particles was determined by evaluation of the Deutsch model for precipitation in an electric field applied to a parallel plate capacitor. The size dependent particle separation efficiency for the precipitation was determined. A dependence of the charging state on the particulate material was found in relation to the work functions of TiO2 and SiO2. The milled slag was considerably less charged compared to the pure metal oxides and thus was less well precipitated. For all materials the precipitation was found to be rather independent of the particle size, which could be a favourable feature for industrial application.

Keywords: Air quality; coal combustion; electrostatic charging; electrostatic precipitation at high temperatures; submicron particles; fly ash; high temperature gas cleaning.


bullet Abstract

bullet The deposition of monodisperse spherical aerosol particles in a dust loaded model filter due to inertial impaction and direct interception is considered. A regular lattice of parallel cylinders covered with coaxial porous permeable shells that simulate the layers of deposit on the fibres was used as a model filter. A method of calculation of the process of clogging of the fibrous air filter by monodisperse solid particles is described. The method accounts for the deposit permeability and for changes in the flowfield in the vicinity of fibres.

The fibre collection efficiency was determined by computing the limiting trajectory of a particle as a function of the porous shell radius and the porosity of the deposit. The results of the theoretical study on the kinetics of clogging in the depth of filters at low and intermediate Stokes numbers are presented. The results obtained are in agreement with available experimental data. Keywords: Aerosol filtration; dust loaded fibres; kinetics of clogging.

M. Rainer and W. Hoeflinger Experimental and CFD Analysis of Particle Retention on Different Filter Media in Solid-Liquid Filtration (pages 114-119)

bullet Abstract

bullet A number of filtration tasks require the deposition of a cake layer above the supporting filter media that itself acts as the actual filter medium. This study illustrates the cake layer formation on two different filter media (a dutch weave and a slotted metal sheet that have the same maximum pore size) and the resulting particle retention properties. In the first part of the paper the flow behaviour on the unloaded supporting filter media is calculated using the commercial computational fluid dynamic (CFD) software Fluent 5.4®. The differences in local fluid velocities and the particle tracks of single particles approaching the two different filter media are also calculated.

The second part of the paper describes an experimental study using a laboratory filter. The filtration of a diatomite suspension was observed under the microscope to show differences in the structure of the cake layer formed at different stages of the filtration process on a slotted metal sheet and on a twilled dutch weave. Higher particle retention of the cake layers built up on the metal sheet are caused by differences in the horizontal fluid velocities near the top surface of the supporting filter medium. Characteristic pressure drops over time during the formation of the first cake layer are discussed for the weave and the metal sheet in terms of the results from the CFD analysis.

Keywords: Solid liquid filtration; CFD analysis; filter media; cake layer; cake formation mechanism.

Published papers: Volume 2, Issue 3

Author(s) Title, page numbers and abstract

bullet Abstract

bullet Aspects of the safety of centrifuges used in the process industries are discussed taking the widely used batch centrifuge as an example. Three aspects of safety are reviewed, firstly the recent BS standard and CE certification, secondly control systems and finally basket design and inspection. The purpose of the review is to illustrate both good practice and some common pitfalls rather than give a definitive overview. Whilst the discussion refers to batch centrifuges many of the comments applies to other centrifuge types.


bullet Abstract

bullet The implementation of membranes in industrial processes offers access to a variety of applications. Their tailor-made process design with economic viability is decisive for the implementation of this technique. With the development of new membrane materials, both polymeric and ceramic, a wide range of industrial applications can be covered and some of these are illustrated with examples in this paper.


bullet Abstract

bullet It is almost universally accepted by those involved in the separation processing industry that the separation of liquid-solid suspensions is a highly complex subject. This, in part, is due to the almost infinite and diverse nature of materials available for processing, coupled with the vast array of separating equipment currently available on the market. Many knowledgeable and learned individuals have, over the years, gone to great lengths to examine and study the field of liquid-solid separation, producing detailed technical papers, formulae and theories. Whilst not attempting to undermine or vilify their valuable contributions in any way whatsoever there is also, just as importantly, a highly practical aspect to this field.

It is therefore the intention of this paper to cover the practical issues surrounding the vertical batch basket filtering type centrifuge, although most will apply to similar equipment, that must be addressed if a safe and efficient separation process is to be realised.


bullet Abstract

bullet Although bubble point testing has been used to calibrate fine filters and meshes, the technique begins to lose accuracy above pore sizes of about 30 microns. Test dust permeation has had some success in assessing the performance of filters with pore sizes above about 10 microns but the technique is more qualitative than quantitative because of the wide particle size distribution of the dusts. Furthermore, there is no traceability back to an International unit of length such as NIST (The National Institute of Science and Technology, USA). This paper describes the preparation, certification and use of a new range of 20 narrow size distribution glass microspheres for calibrating filters and meshes from 10 to 500 microns. Because the size distributions are so narrow the results are extremely accurate and reproducible.

The glass microspheres are certified using precision Electroformed sieves calibrated microscopically using a NIST Graticule and a calibration graph of percent passing against aperture size is constructed. The microspheres are then passed through the unknown filter or mesh and, from the percentage passing, the mean aperture size can be read off the calibration graph.

The use of sonic energy in the transportation of the beads through the filters means that the testing is very fast because it is performed dry. Sifting times of 1 minute are common for most of the range with accuracies and repeatabilities down to 1 micron. The absolute upper end of the pore size can be determined by analysing the particle size distribution of the microspheres passing the filter.


bullet Abstract

bullet The Chemicals and Pharmaceuticals Industry encouraged in the past the development of peeler centrifuges because of their configuration and mechanical arrangement. Fine chemicals and particularly pharmaceuticals needed a machine, the concept of which allowed separation of the processing area from the rest of the machine.


bullet Abstract

bullet Since 1893 Westfalia Separator has produced machines for mechanical separation; initially these were only separators and for the last 50 years also decanters. These two technologies are defined as sedimentation centrifuges from the process engineering viewpoint, which demarcates them from other centrifuges such as filtering, peeler and strainer centrifuges, etc. Prerequisites for mechanical separation with centrifuges are a difference in density as well as non-miscible substances. Thus, solid-liquid, liquid-liquid or liquid-liquid-solid separation is possible.

S. Cameron and C.J. Williams An Investigation Into The Fouling of Fine Bubble Aeration Diffusers in Activated Sludge (pages 26-30)

bullet Abstract

bullet It is estimated that 50-90% of the energy requirement of wastewater treatment is dedicated to the aeration of activated sludge. Accordingly, any factor that influences aeration efficiency will have a major effect on the overall operational costs of sewage treatment. One of the most efficient and therefore popular means of aerating activated sludge (AS) is the fine bubble diffuser. These come in a variety of designs but all rely on forcing air through a porous media located on the bottom of the AS tank. The bubbles produced at the diffuser/liquid interface disperse throughout the mixed liquor and oxygen diffuses from the bubbles into the activated sludge. This dissolved oxygen supplies the respiratory requirement of the aerobic micro-organisms that carry out biological treatment of the wastewater.

B. Madsen DECANTER PROCESS PERFORMANCE STUDIES: Flow Patterns, Sedimentation and Solids Handling (pages 30-32)

bullet Abstract

bullet In the design of a decanter centrifuge one can choose between a number of different principles for the flow path inside the machine. Over the years each manufacturer has developed strong arguments for their own favourite principle and for many years the most important choice was considered to be the choice between a co-current or a countercurrent design. As many thousands of each design type are operating successfully in the field one cannot truly claim that the one design is superior to the other.


bullet Abstract

bullet The influence of an electrical field on the kinetics of the accumulation and washing of a pressed filter cake was examined using laboratory filtration equipment (with a double-sided filtration cell) at the University of Karlsruhe in Germany. Under electrofiltration, the experiments conducted using quartz sand (average particle size d50% = 2 mm) suspended in Karlsruhe tap water (conductivity l @ 700 mS cm-1) showed that the electrokinetic effects (electrophoresis and electroosmosis) accelerate the kinetics of the pressure filtration.

Permeation experiments carried out using the untreated (non-polluted) filter cakes, pressed without an electrical field, were used as the control experiment to investigate the effect of washing. The kinetics of permeation was strongly influenced by the polarity of the electrical field. The permeation was accelerated when electroosmosis acted in the permeating direction (positive E-field) and decelerated, if electroosmosis acts against the permeating direction (negative E-field).

Examination of filter cake washing showed a kinetic effect that was only slightly accelerated by using the electric field. The conductivity due to the impurity of the cake was higher when the cake was washed in the presence of an electrical field than without it. As such cake washing without an electrical field for quartz sand SF800 seemed to be more effective. Therefore, an additional electrical field for the washing of quartz sand is not worthwhile.

Keywords: Electrofiltration; electrokinetic effects; electroosmosis; electrophoresis; permeate flux; pressure filtration.


bullet Abstract

bullet This research focuses on a Computational Fluid Dynamics (CFD) based approach for the design of dynamic membrane filters. In the investigation both geometry and process parameters are studied in order to achieve the required turbulence on the membrane surface. The hydrodynamics of these membrane filters are expected to be affected by variations in operating conditions, such as, flow rate, suspension density and RPM and also by geometric parameters such as disc or turbine radius and the distances between the membrane and the turbine. In this study two basically different industrially used membrane separation devices are investigated using a commercial computational fluid dynamics package. The filter is equipped with either a rotating membrane disc or with a turbine. The fluid is assumed to be Newtonian, incompressible and isothermal. The k-ε model is used to describe turbulent flow.

Pumping the solution through the separation units is the main cause of the operating expenses in a membrane separation plant. In addition, too high a pressure drop may rupture the membrane or its support. In this research the effect of the location of feed and the exit regions is investigated. Comparison of the different fluid entrance and exit alternatives using the CFD simulation helps to determine the minimum or satisfactory pressure drop. In addition, the effect of the rotational velocity of a rotary element and the feed velocity are considered.

The research also shows that the flow dynamic conditions at the membrane walls in a filter with rotating disc, change considerably with a relatively small change in feed velocity.

Keywords: CFD; cross flow filtration; dynamic filtration; dynamic membrane filters; fouling.


bullet Abstract

bullet Fouling and changes in the charge on the membrane during the filtration of protein solutions were characterised by simultaneous measurements of the flux and streaming potential through the pores of the membrane. Myoglobin was used as the model substance and both hydrophilic and hydrophobic ultrafiltration membranes and one microfiltration membrane were tested. The charge on the hydrophobic track-etched polycarbonate PC0.1 and the polysulfone GR51 membranes changed towards that of the protein during fouling. The charge on the GR51 membrane changed the most, while flux measurements indicated that the PC0.1 membrane fouled more than the GR51 membrane. The hydrophilic membranes that were made of regenerated cellulose did not become permanently fouled and their charge remained unchanged.

Although the flux increased when the flow velocity was increased in the laminar region the streaming potentials remained the same. A higher velocity prevented the formation of a concentration polarisation layer but not the permanent adsorptive fouling that occurred at the beginning of the experiment.

Keywords: Fouling; microfiltration; myoglobin; streaming potential; ultrafiltration.


bullet Abstract

bullet This research deals with the performance of very fine micro bubbles required for dissolved CO2 gas flotation of waste activated sludge. CO2 gas micro-bubbles are produced by dissolving CO2 gas in waste activated sludge under pressure and then injecting or spouting the sludge through a small nozzle to atmospheric pressure – the method is known as “supersaturate and release”. Discussion of the hydrodynamic performance of the CO2 gas micro bubbles is presented. The behaviour of micro bubbles is one of the most important factors in the flotation process and several researchers have previously investigated the behaviour of micro air bubbles. However, for the case of CO2 gas flotation, little is known, because the method is very new. In addition, CO2 has about 30 times higher water-solubility than air. Since the size control of CO2 micro bubbles is very important in new CO2 flotation its impact on flotation has been studied.

The best conditions for CO2 gas flotation and the case of the usual “supersaturate and release” method were investigated. As an extension of this research, another method for flotation using CO2 gas was examined where a weak vacuum was used instead of a release to atmospheric pressure. A comparison between the results of the two methods is presented.

Experiments were carried out to obtain useful data for the relation between CO2 micro bubbles size and flotation controlling factors. The principal factors are sludge temperature, the initial height of sludge, the bubbling pressure, nozzle diameter and the spouting pressure. Furthermore, some experiments were executed to check the effect of not spouting on bubble formation and flotation. The results clarify the relationships between the characteristics of CO2 micro bubbles and the principal flotation factors.

Other subjects that might have an effect on dissolved CO2 gas flotation of waste activated sludge were investigated experimentally including bubble formation, bubble size and bubble-particle interactions.

Keywords: Flotation; carbon dioxide; micro bubbles; sludge; waste activated sludge.

S. Obermair, J. Woisetschläger and G. Staudinger The Flow Pattern in the Dust Outlet Geometry of a Gas Cyclone and its Effects on Separation Efficiency (pages 80-85)

bullet Abstract

bullet Since the end of the 19th century many researchers have attempted to clarify the influence of all the different operating and geometric parameters on the separation efficiency of gas cyclones. However, the dust outlet geometry was rarely investigated. Tests performed by the authors prove that by changing the dust outlet geometry, the resulting change in flow pattern can lead to a significant improvement in the separation efficiency. For a cyclone with a bin, Laser Doppler Anemometry (LDA) measurements show a vortex moving downwards along the wall. A second vortex transports the gas back from the bottom of the bin into the cyclone. The maximal axial upward velocity is of the same magnitude as the inlet velocity into the cyclone. By using a cone insert the tangential and axial velocities in the bin are reduced, but the two vortices are forced into the gap between the cone insert and the wall causing high turbulence.

The flow pattern of a cyclone with a downcomer tube is similar to that of a cyclone that incorporates a bin. However, as a consequence of the high tangential velocities and lower turbulence in the lower part of the downcomer tube and in the bin, there is an additional separation. Once the particles reach the downcomer tube, they will circulate in the bin and downcomer tube. As a consequence those particles are unlikely to be re-entrained back into the cyclone chamber.

The findings of this study show that cyclones that incorporate a cone insert or a bin should be replaced by cyclones with a downcomer tube. The downcomer tube works as a concentrator for the particles that are carried in the boundary layer at the lower end of the cyclone.

Keywords: Gas cyclone; dust outlet; LDA; particle separation; particle deposition.

Published papers: Volume 2, Issue 2

Author(s) Title, page numbers and abstract
A. Willis Air Filtration Using Glass Filter Media (pages 13-15)

bullet Abstract

bullet Hollingsworth & Vose (H&V) Company Ltd. manufacture wet-laid, bound glass microfibre media that end users convert into forms suitable for removing particles from air or liquid. In this paper, an explanation of the key properties that air filter manufacturers look for in our media are described, along with how H&V can help custom design media to meet exact final filter requirements.

A. Gibson Use of Tetratex ePTFE Membrane laminates in fine particle filtration (pages 15-17)

bullet Abstract

bullet Air pollution control has become a major concern worldwide. Emission control standards are becoming more and more stringent. Today, dust control is a necessary part of any process and is subject to strict emission regulations.

Under the current climate a filter has to operate as economically as possible and take into consideration process parameters such as gas stream chemistry and flow distribution, water and acid dew points and dust characteristics. The end result should allow the filter to operate with a good air flow distribution, low pressure differential and to meet the demands of both the end user and regulatory authorities.


bullet Abstract

bullet This paper focuses upon the development of electrostatically charged filter media. The topics covered include an introduction to Hollingsworth & Vose, the history and development of Technostat media, its manufacture and applications, a review of composite media and finally the use of appropriate test equipment.

S. Smith Activated Carbon Media and its Application in Compressed Air (pages 22-26)

bullet Abstract

bullet When specifying filtration for a compressed air system, the presence and likely effects of an oil vapour are often overlooked. Activated carbon is a versatile medium, available in a wide variety of formats, with a large capacity for the removal of these vapours and is the primary selection for the filtration design engineer.

This paper describes the source of these vapours, gives a brief overview of the range of activated carbon filter materials that are generally available and describes a novel test method, to help determine operating life for a specific filter design under conditions that more closely simulate operation.

P. Hodgson Low Density Ceramic Filters for Hot Gas Filtration (pages 27-30)

bullet Abstract

bullet Low density ceramic filters were developed over 10 years ago. Based on ceramic fibre, mineral fibre or calcium silicate they are capable of handling gases at temperatures in excess of 1000°C. The ultimate temperature capability is dependent on the composition of the filter element and of course very much on the material of the element supports and filter structure. They have had a chequered history, with a mixture of some success and some outstanding failures. However, with greater understanding of optimum plant design and operating conditions for this type of filter the list of successful applications is growing rapidly.


bullet Abstract

bullet Modern time controlled high pressure diesel injection systems require extreme fine fuel filters and an efficient removal of emulsified water. The required border values of the filtration efficiency and water separation efficiency are described. Fuel filters with star-pleated elements and cellulose based filter media do not fulfill these new requirements. The necessity of a huge increase of filtration performance in terms of particle retention efficiency and particle holding capacity presupposes a new filtration concept.

The new multigrade media generation consists of two single filtration layers. The upstream layer is made of polymer meltblown fibres with very high particle holding capacity, the downstream layer combines the functions mechanical support and of the final filtration step with high particle retention efficiency. Compared to common filter elements either an increase of capacity by a factor of 3 or an increase of the initial particle retention efficiency by one order of magnitude is possible. Together with the excellent removal of emulsified water by coalescence, the multigrade media fulfill all new requirements of modern diesel injection systems. A further increase of depth filter media perfomance requires a CFD supported design of fibre composition and arrangement. The simulation of the flow field and particle trajectories is done with FLUENT. The geometrical model is a 2-dimensional array of circular cylinders. The calculated initial particle retention efficiency is shown to be strongly dependent on the arrangement of the fibres.


bullet Abstract

bullet Electrostatic discharge of accumulated charge generated in vehicle fuel systems can occur when system components are made of insulating materials. Since charge separation is a surface effect and fuel filters provide a large amount of surface area, the electrostatic charge and potential on filters can reach high levels, which are sufficient to cause electrical breakdown, resulting in pinholes in fuel filters.

This paper investigates the factors affecting charge generation during the filtration process in automotive fuel filters. The experimental investigation was concentrated primarily on a determination of electrical currents and potentials encountered in fuel filter elements made of different filter media and different resistivities of filter housings at specific flow rates.

T. Ciach BIODEGRADABLE FILTERS (pages 37-40)

bullet Abstract

bullet Disposable air and water filter cartridges, because of their high chemical resistance and popularity, have a significant environmental impact. Biodegradable polymers and melt blow technology are used in an attempt to reduce this problem. Biodegradable polymers available on the market were the raw material for melt blown fibrous filter production. The filters were tested in water and air environments. A biodegradable electret filter was also made.


bullet Abstract

bullet Over the last 25 years cleaning technologies have changed little apart from refining process technologies for improved performance and efficiency. However, the future could be different. Industry needs to standardise validation procedures whilst performing continuous monitoring to serve customer requirements. The biggest change the industry faces is one of environmental responsibility and performance criteria to meet international standards.


bullet Abstract

bullet Fibres have been taken from a commercially available aviation fuel filter-water-coalescer cartridge and their surface affinity for water examined in an Environmental Scanning Electron Microscope (ESEM). The fibres present a mixture of silica and epoxy resin surfaces. Water droplets were found to form only on the silica surfaces.

Fibres that had been soaked in jet fuel containing a naphthasulphonate (a model surfactant), equivalent to “disarming” the coalescer, had unusual wetting characteristics consistent with this additive lowering the interfacial tension of the fuel water interface. Water spontaneously spread across the entire fibrous matrix. Aqueous film structures have been observed within the fibrous matrix pointing to a novel mechanism to explain some specific field observations of coalescence failure.


bullet Abstract

bullet The effective wobble angle is a special parameter used in tumbler centrifuge design and operation; its relationship with the structural wobble angle can be calculated. A model to describe the motion of a tumbler centrifuge is summarised, enabling the importance of structural and operational parameters such as the effective wobble angle to be explained and related.

Additional vibratory forces which are generated in a tumbler centrifuge and which affect cake dewatering are identified and analysed, together with the motion of particles in the centrifuge drum. An alternative drum design, the step drum, is analysed and shown to increase the solids residence times. Keywords: Centrifuge; filter; modelling; design; residence time; solids discharge; vibration.

A. Yelshin, J.A. Teixeira, W.R. Bowen, and M. Mota ROLE OF FRACTIONAL CAKE COMPOSITION IN CAKE RESISTANCE (pages 45-48)

bullet Abstract

bullet A model for binary mixtures was applied in order to provide an explanation of the discrepancy between the calculated and experimentally measured specific cake resistance in TiO2 cakes. The case under consideration shows clearly that, even where physicochemical factors influence the particle size distribution, for some bimodal dispersed systems where the ratio of larger diameter particles to those of smaller diameter is greater than 4, the specific cake resistance can be defined by using the particle mixture approach.

Consideration of tortuosity as a variable parameter dependent on porosity may improve the degree of fit between the cake resistance modelling and experimentally measured results. Keywords: Microfiltration; porosity; tortuosity; specific cake resistance.


bullet Abstract

bullet Flocculation is a common method to increase the efficiency of a solid/liquid separation process for fine grained material. Operators are faced with a wide range of possible solutions to their specific flocculation problem because of the wide range of flocculants on the market. It is not easy to identify the best flocculant and the ideal flocculation conditions (flocculant dosage and energy input during flocculation) without considerable effort. Laboratory methods to determine the best flocculant for a given application lag far behind the high level of development of different flocculants. This particularly applies to the sedimentation of flocculated particles in a centrifugal field. There is still no method to characterise the effect of flocculating agents on the three main parameters that play a role in a separation:

1) The sedimentation velocity of the flocs,

2) The compression kinetics and

3) The stability (transportability) of the sediment.

These properties can be determined, independently of each other, using a transparent disc centrifuge. In operation only the sedimentation and the compression kinetics can be detected. This paper presents the results of a preliminary study using a disc centrifuge.

Keywords: Centrifugation; compression; decanter centrifuge; flocculant testing; flocculation; sedimentation.

Published papers: Volume 2, Issue 1

Author(s) Title, page numbers and abstract

bullet Abstract

bullet An emerging technology is outlined that appears destined to play a major role in how liquids in the future will be purified in the major liquid processing industries. Instead of piecemeal tinkering with “case history” methodology accompanied by the proliferation of purification stages and a worsening of the environmental pollution problem, the possibility of a generic solution is now in the offing. The goal is the realisation of a liquid purification ROBOT.The paper outlines how this goal is to be achieved with reference to the chemical, pharmaceutical, beverage, food and water industries, whereby all waste stream pollution is to be eliminated at source. The R&D program to realise this goal is outlined with references to web-site detailed information including patents.


bullet Abstract

bullet This paper describes the principal factors that affect the selection of woven and needlefelt filter media. These factors are discussed in relation to the basic forms of filter media, the finishing treatments that can be applied and the methods of testing media.


bullet Abstract

bullet This paper considers some of the important design and manufacturing issues involved in the selection of needlefelts for use in hot gas and dust collection filtration applications.


bullet Abstract

bullet In many industrial filtration applications, the life of the filter media can be limited by abrasive damage. Such damage can be caused by impact of hard and sharp particulate matter in the slurry against the medium, as well as fretting damage between the filter medium itself and the filter press. Increases in filter media lifetime are desirable, and will result in improved process up time and hence greater process efficiency. This paper describes a range of new composite filter media that have been specifically developed to address abrasion issues in liquid filtration applications. The new range of media is called Tuf-tex™ and consists of a specially formulated abrasion resistant resin system, which can be applied to both woven and non-woven substrates. The resin is applied as a liquid system in such a way as to provide an abrasion resistant impregnation into the surface of the substrate.

The influence of the coating on the abrasion resistance, permeability, filtration efficiency and filtration throughput is described. Results from laboratory tests are employed to illustrate the process savings that can be expected from the use of these new media in industrial filtration processes.

R. White and C. Murton STAR FLOW PROFILE CERAMIC MEMBRANES (pages 23-26)

bullet Abstract

bullet This paper describes ceramic membranes that have a star profile cross-section. Their use in solid/liquid and liquid/liquid separations is described and for the latter it is shown how contaminate concentrations below statutory limits can be achieved.

M.G. Frey, A. Jensen, K. Dam-Johansen and S. Rasmussen BAG HOUSE FILTER PERFORMANCE STUDIED BY NUMERICAL SIMULATIONS (pages 3-12)

bullet Abstract

bullet This paper addresses aerodynamic considerations and the effects of particle separation in industrial bag house filters. A commercial Computational Fluid Dynamics (CFD) tool was applied to analyse the airflow and particle transport in industrial bag filter vessels. The aim of the study was to obtain a better understanding of both processes and to optimise current filter design. The study has investigated the effect of two commonly applied filter inlet configurations, namely the tangential bottom and top inlet types, on the particle deposition pattern on the filter bags. A considerable proportion of the particles were found not to reach the filter bags. This is attributed to a pre-separation caused by inertia effects. The CFD calculations further indicated that fewer particles are captured on the filter bags in a bag house with a tangential top inlet compared with the same bag house using a bottom inlet. In addition, different particle deposition patterns were observed for the two configurations and are discussed.

Keywords: Bag filter design; computational fluid dynamics (CFD); gas-solids filtration; particle deposition.


bullet Abstract

bullet Sedimentation of a particle towards a vibrating filter medium is modelled to calculate the motion of the particle relative to the oscillating medium. The effects of particle properties (particle size and density), vibration characteristics (frequency and acceleration) and filtrate flow rate on the particle behaviour close to the filter medium are investigated. The simulation shows that the vibration of the filter medium directly affects the motion of particles in suspension. For a particle with particular properties, critical vibration conditions exist when the particle makes infrequent contact with the medium, leaving a liquid layer above the medium and hence a higher liquid flow rate through it. This is an unstable situation; under most circumstances the particle periodically contacts the medium and rebounds into the liquid above it, whence the particle settles again. The filtrate flow rate has a considerable dampening effect on the vibration induced motion of the particle; a high flow drags the particle onto the medium and can nullify any potential advantage that may otherwise be offered by the vibrations.

Keywords: Settling, vibration filtration, modelling.


bullet Abstract

bullet The promotion of convection within an aluminium oxide tubular membrane using gas-liquid two-phase flow was studied for the microfiltration of aqueous titanium dioxide dispersions. The influence of gas flow velocity and periodical gas flow is also presented. The results of these experiments show that a constant gas-liquid two-phase flow has a positive impact on flux. It may be concluded that two-phase flow seems to expand the particle cake as it increases both cake porosity and thickness, thus allowing higher fluxes. In the case of periodical gas flow the improvement in permeate flux is less. However, this phenomenon depends on the mode of periodical gas flow and on the concentration of the dispersion used. The most important consideration is that for all the dispersion concentrations tested, the injection of air was always associated with an enhancement of the permeate flux. There is no concentration for which air injection had no effect on the permeate flux.

Keywords: Microfiltration; gas-liquid two-phase flow; dispersion; particle cake.


bullet Abstract

bullet Electro-osmotic dewatering is typically operated under continuous direct current (dc) conditions, however it has recently been shown that intermittent power application using half-wave rectified alternating current (ac) improves the dewatering process. The conditions of electro-osmotic dewatering under intermittent power application are further investigated in this study. The effects of intermittent power application are experimentally investigated for a range of low ac frequencies and under pulsed dc for a range of on/off time ratios. The intermittent electric field was found to improve the performance of electro-osmotic dewatering. An optimal frequency of rectified ac in terms of the total amount of water removed was identified. The efficiency of electric power consumption was determined and an optimal on/off time ratio for pulsed dc was identified.

Keywords: Solid-liquid separation; electro-osmotic dewatering; intermittent power application; on/off time ratio; electrical contact resistance.

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