Richard J. Wakeman

Additional techniques to improve microfiltration

Abstract Concentration polarisation, cake formation and pore fouling lead to a performance loss of microfiltration membranes in terms of reduced permeate flux. Causes of these phenomena are varied and depend largely on components and properties of the feed suspension, the most common causes are summarised in this paper. Techniques to limit this reduction or retard fouling are available and are reviewed, the range of methods covered include feed pre-treatment, choice of membrane material, flow manipulation, the use of high shear, gas sparging and additional force fields.

Ion exchange modules formed from polyhipe foam precursors

Abstract Homogeneous, highly porous, low density, open cellular polyhipe polymer (PHP) has been produced by polymerisation of a high internal phase emulsion. Its surface properties were modified by sulphonation. By incorporating Na + ions onto the sulphonated PHP surfaces a modular form of ion exchange resin capable of effective exchange with metal ions in solution was produced. Isotherms were measured for samples of PHP of differing degrees of sulphonation, and compared with data obtained from commercially available ion exchange resins. The capacity of PHP increased with the degree of sulphonation. In single-pass dynamic adsorption tests the PHP showed better column utilization than the commercial resins.

Membrane fouling prevention in crossflow microfiltration by the use of electric fields

Abstract From an analysis of fluid velocity and electric field profiles the trajectory of particles through crossflow microfiltration units has been calculated. A tubular geometry filter leads to the most effective use of electrical power, when it is used as an aid to prevent membrane fouling. Results for plate, tubular and mutitube filters are given. Some experimental data from a tubular filter are presented, which demonstrate typical effects of the important parameters.

Factors affecting the formation and properties of wet compacts

Abstract New data are reported on the formation of wet compacts from slurries. The compaction data are analysed through two consecutive mechanisms, filtration and consolidation. Process design parameters for each mechanism are obtained from the models which might allow calculations to be made from small-scale experiments. The magnitude and dependence of the constitutive parameters on pressure, pH and surface charge, particle size and shape, and the nature of the particle—particle interactions are shown through the experimental results.

Colloidal fouling of microfiltration membranes during the treatment of aqueous feed streams

Abstract Experimental data has been obtained from a computer controlled crossflow microfiltration apparatus. Polymer membranes of differing properties have been used to filter suspensions carrying colloidal and fine particles of known shape, size, surface charge and chemical composition. The process parameters (feed concentration, crossflow velocity and pressure difference) have been changed systematically, and flux decline curves measured for the matrix of particle, suspension, and process properties. This paper presents results from the experimental programme, and in the light of these results explains some of the apparent anomalies which have been presented in previous publications.

Filtration characterisation and specific surface area measurement of activated sludge by Rhodamine B adsorption

Abstract Rhodamine B adsorption was used to measure the specific surface area of activated sludge. The surface area occupied by the Rhodamine B molecule was calibrated by measuring its adsorption onto polyvinyl chloride (PVC) granules. The area occupied was found to be dependent on pH and conductivity of the solution. To verify the measurements other uniform materials were used. The surface area of wood flour and an activated carbon were measured with satisfactory results. Micropores (

The influence of pH and temperature on the rheology and stability of aqueous titanium dioxide dispersions

Abstract The effects of pH and temperature on the rheology and stability of aqueous dispersions of rutile (titanium dioxide) were investigated systematically. The dispersions exhibit shear-thinning behaviour, conforming to the Herschel-Bulkley or the Casson-type models in the shear rate range investigated. Extreme sensitivity with pH was observed, whereby the dispersion viscosity can be changed by as much as an order of magnitude with pH variation at constant volume fraction. The temperature effect study indicates that the relative contribution of Brownian motion and shear flow to the viscosity is dependent on the flocculation state of the dispersion. The rheological behaviour of the dispersions in this study has been explained in terms of hydrodynamic interactions between particles as well as non-hydrodynamic interactions. These observations have important implications in crossflow microfiltration of these dispersions.

A finite volume model for the hydrodynamics of combined free and porous flow in sub-surface regions

Abstract In this paper, a finite volume model for the 3-D analysis of sub-surface water flow is described. The model is applied to simulate groundwater hydrodynamics in domains representing combined free flow and porous sections. The free flow section is isotropic, while the porous section is assumed to be a saturated and anisotropic medium with a constant porosity. The Navier–Stokes equations are used to simulate the free flow regime, while the porous flow is modelled by the Darcy equation. The governing equations of motion for each sub-domain are linked through the imposition of suitable matching conditions at the free/porous flow interface. At the exit from the domain, ‘no boundary condition’ is imposed to avoid forcing of any artificial condition on the flow system. This is guided by previously published work that recommends this approach for obtaining realistic simulations for complex flow systems. Results presented in this paper show that depending on pressure distribution, underground flow circulation inside the porous domain may occur. It is also shown that the direction of the porous flow may reverse at the interface, with the flow front reversal and the circulation centre moving away from their initial positions with time.

Microfiltration enhancement by electrical and ultrasonic force fields

Abstract ‘Assisted’ filtration techniques are emerging as technical alternatives to conventional separations. Experimental data presented in this paper show how electric and ultrasonic fields can assist microfiltration by reducing the flux decline caused by membrane fouling. Effects of the fields acting individually and in combination are illustrated, together with the influence of other filtration parameters.

Stability of latex crossflow filtration: cake properties and critical conditions of deposition

The critical permeation flux is the flux at which cake deposition starts to be detectable. The variations of the critical flux (determined simultaneously by a mass balance and successive variations of transmembrane pressures) during sub-micron latex particles filtration were studied under various operating conditions (membrane pore diameter, shear stress at the membrane surface, latex concentration, surfactant content). These variations were explained taking into account the properties of the suspension and of the fouling deposit (reversibility, specific resistance, thickness, porosity) formed beyond the critical threshold. It was shown that the critical parameter, Jcrit/τw, which defines the conditions required for stable filtration performance was more appropriate than ΔPcrit/τw previously suggested in the literature since Jcrit was actually independent of the clean hydraulic resistance of the membrane, and consequently of the membrane pore size. This indicates that there is no need to work with the largest pore size membrane: larger pores will not induce higher critical flux and will not improve the area of the stability zone of the filtration. This work also points out the major impact of surfactants on fouling phenomena and Jcrit, observations rarely reported in the literature: the higher the surfactant content, the higher the deposited mass and the lower Jcrit.