Membrane filtration with reversible adsorption: Hollow fiber membranes as collectors of colloidal particles

Abstract

Microfiltration (MF) and ultrafiltration (UF), in which a semipermeable porous membrane is used to clean a slurry from the suspended particles, macromolecules, fine solids or viruses, are often accompanied by the formation of a cake on the membrane surface [1][2][3]. The hydraulic resistance of the cake, which may dramatically reduce the process driving force, is considered to be the factor limiting or narrowing the industrial and municipal application of MF and UF, for example, to wastewater and surface water treatment. A large number of methods to prevent or reduce the cake formation was proposed: high shear-inducing velocity of a liquid over the membrane surface, turbulence promoters, air sparging, vibratory membranes, a charged membrane surface, and the like [4].These methods often result in a more complicated design of membrane plants and always lead to higher operating and/or capital costs. In spite of all these efforts, almost all MF and UF plants require periodic membrane surface cleaning by flushing or backflushing. As a rule, it is especially difficult to "oppose" the cake formation in membrane separation processes where the size of colloidal particles to be removed is a couple of dozens of nanometers, that is, where the UF and MF particle-size ranges are overlapped.Many experimental and theoretical studies, for example [2,[5][6][7], suggest that the surface interactions and aggregation of colloidal particles at a membrane surface is the main factor accounting for the cake formation in membrane processes. As a result, all the efforts were directed to building mathematical models taking into account the surface interaction forces of colloidal particles, which are a necessary tool in predicting the performance of membrane plants. At the same time, no research was carried out to study microfiltration and ultrafiltration as separation processes in which the separation can be achieved not only by the rejection of suspended particles (SP) at the membrane surface but also by their adsorption on the same surface. It should be noted that the second mechanism, which is widely used in conventional depth filtration, adsorption, and chromatography, has never been considered as an engine that could produce an additional amount of clarified water in parallel to the production of permeate, the liquid that passes through the membrane.The membrane device most suitable to implement the idea of combining the rejection and adsorption of suspended particles at the membrane surface in one apparatus in order to increase the yield of clarified water is a hollow fiber filter in which the feed stream would flow around the shells of hollow fibers perpendicular to their axes. The permeate would be withdrawn from the HF lumens while the other product stream, the filtrate, which would be produced by collection of suspended particles on adsorbing or particle-collecting shells of hollow fibers, similar to conventional depth filtration [8], would be discharged from the other outlet (Fig. 1). The two clarified streams, the permeate and f...

Membrane Filtration with Reversible Adsorption: The Effect of Transmembrane Pressure, Feed Flow Rate, and the Geometry of Hollow Fiber Filters on Their Performance

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Depth membrane filtration (DMF) with reversible adsorption is a novel pressure-driven membrane separation process in which the feed suspension is treated in a hollow fiber (HF) filter so that the clarified liquid produced by the semipermeable membranes is made up of a mixture of permeate and filtrate [1]. The permeate is a liquid that passed through the semipermeable membranes, which reject almost all suspended particles.

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“…The porous HF membranes completely reject the suspended particles. The electrokinetic properties of the membrane surface and suspended particles, along with other assumptions, are adopted from [1].The equation of mass conservation and the equation for the rate of reversible adsorption with appropriate initial and boundary conditions that govern DMF in a radial filter can be written as in [1]:Abstract -The effect of transmembrane pressure, feed flow rate, and the geometry of hollow-fiber filters on their performance is studied for depth membrane filtration with reversible adsorption. Mathematical models for rectangular and radial filters are used to find the maximal operation times with a filter retention of not less than 0.9 at constant product (permeate plus filtrate) flow rates for the treatment of a latex suspension in continuousflow and batch operations.…”

“…The porous HF membranes completely reject the suspended particles. The electrokinetic properties of the membrane surface and suspended particles, along with other assumptions, are adopted from [1].…”

“…In [1], we developed a mathematical model describing the process of depth membrane filtration with reversible adsorption in a rectangular hollow-fiber filter. The differential equations that account for the adsorption and peptization mass fluxes of particles in the boundary layer of surface interaction forces at the membrane surface and for the local and overall material and volume balance in the filter were solved using an approximate method based on the averaging of permeate flow rate.…”

“…As in [1], we assume that the suspension under treatment in a radial DMF filter (Fig. 1) is a dilute incompressible liquid with constant viscosity, which contains suspended quasi-lyophilic (quasi-stable) particles.…”