For whey products intended for human or animal consumption, demineralization enhances the nutritional value of the product. In industrial processes, whey is concentrated by evaporation (EV) and subsequently demineralized by electrodialysis (ED) and/or ion-exchange. Nanofiltration (NF) is an alternative for partial demineralization of whey. NF-membranes which are suitable for dairy applications have a high permeability for (monovalent) salts (NaCl, KCI) and a very low permeability for organic compounds (lactose, proteins, urea). The use of NF instead of EV followed by ED has the advantage of simultaneous concentration and demineralization of whey. This eventually reduces processing costs.Models were developed, based on the extended Nernst-Planck equation, which describe the salt rejection as a function of the flux for binary and ternary salt solutions. Effects of concentration polarization, composition of feed and concentration are incorporated in the model. In laboratory-scale experiments, rejection-flux curves of four different commercial membranes were established for three different model solutions (NaCl, CaCl2 and (NaCl + CaCl2)) and for ultrafiltration (UF) whey-permeate (pH 4.6, 5.8 and 6.6). The results indicated that the salt transport through all the NF membranes investigated depends on the flux. At low flux, when the contribution of diffusive transport is the most important, permeation of (especially monovalent) cations is high. At high flux, when transport by convection is the most important, rejection reaches a maximum (constant) value. From this it follows that the salt transport can be controlled by the flux.For binary salt solutions (NaCl or CaCl2), rejection data could be described by the (two-parameter) model for binary systems. For ternary systems (NaCl and CaCl2) the model was simplified from a model with four transport parameters to a model with three transport parameters. Rejection data for a ternary system could also be described adequately. Decoupling of transport parameters allowed that the model for the ternary system could be reduced from a four-parameter model to a three-parameter model without losing accuracy. For ultrafiltration (UF)-whey-permeate, a multicomponent mixture, it is shown that an approach in which monovalent cations, divalent cations and anions were grouped separately and lumped into one concentration can be used to describe the rejection-flux data adequately. The experimental data for the (cumulative) anion equivalent charges were predicted accurately only at pH 4.6 and 5.8. At pH 6.6, the rejection calculated for the anions based on equivalent charges was somewhat lower than the rejection actually measured. About halve of the difference could be ascribed to lactate and carbonate, which were not determined separately. As a result there was also a non-matching charge balance.The transport parameters derived from the results with UF-whey-permeate can be used to predict the salt rejection for similar multi-component systems like whey and UF-permeate in industrial systems.

Laboratory scale and pilot plant nanofiltration (NF) and reverse osmosis (RO) experiments with fermentation broths were performed with the following aims: (i) to quantify lactic acid rejection and to determine whether a theoretical model developed in a previous paper could be used to predict lactic acid rejection; and (ii ) to quantify fouling of NF membranes and to determine the major fouling mechanism.It was found that the rejection model developed, based on the extended Nernst-Planck equation, can be used to quantify lactic acid rejection of RO and NF membranes. Especially at high fluxes the prediction of lactic acid rejection using parameters determined with lactic acid/water mixtures was quite good. At low fluxes the predicted rejection of lactic acid was usually lower. Fouling of the membrane could be quantified in terms of three resistances: a membrane resistance, an initial fouling resistance, and a time-dependent fouling resistance. Empirical equations for the initial fouling resistances were developed and time-dependent fouling could be described either by a colloidal fouling model (ultrafiltered fermentation broth) or a gel layer model (fermentation broth). Evaluation of the three resistances by simulation of continuous and batch concentration experiments showed that during NF of an ultrafiltered fermentation broth the initial fouling resistance, resulting from concentration polarization effects. was the predominant resistance. For a fermentation broth the time-dependent fouling becomes more important than the initial fouling resistance. Protein fouling is the main cause of the time-dependent fouling. Therefore, it is recommended to remove proteins by ultrafiltration before NF.

Model studies were performed with the aim of improving lactic acid separation from fermentation broths by reverse osmosis and nanofiltration. A novel model, based on the extended Nernst-Planck equation, for the description of mass transfer of lactic acid through these membranes was developed. The model can be used to predict mass transfer of lactic acid under various pH and pressure conditions of the feed. The generalized model allows a simple calculation of the separation efficiency not only of lactic acid but of other acids as well.

Preadsorption of hydrophilic polymers on hydrophobic ultrafiltration membranes can reduce the susceptibility of the modified membranes to protein fouling. The mechanisms of this anti-fouling action were investigated. Polysulfone and nuclear track-etched membranes with different average pore diameters were hydrophilized by preadsorption of two water-soluble polymers. The fouling of the unmodified and modified membranes due to filtration of a whey protein solution or to adsorption of the whey protein at the membrane surface was characterized by flux measurements and by electron microscopy. Adsorption of protein at the pore walls of ultrafiltration membranes, resulting in the narrowing of pores, is prevented by partly sealing off the pore entrances by polymer molecules presorbed at the external membrane surface. The observed blockage of pores of microfiltration membranes cannot be averted by the preadsorption technique.

During ultrafiltration (UF) of whey, membrane/solute interactions appear to have a considerable effect on the separation characteristics of the membrane. This is primarily caused by precipitation of poorly soluble salts and/or adsorption of whey proteins, taking place inside the membranes and affecting the pore size distribution. Evidence for location of precipitation and adsorption inside the membrane is obtained by measuring the amounts of salts precipitated (using radiolabelling procedures) and protein adsorbed, and by comparing the protein adsorption with the adsorbed amounts on model surfaces having membrane-like characteristics. To quantify the influence of fouling in terms of pore narrowing a characteristic pore size for UF membranes is defined, based on the retention of low-molecular-mass saccharides. Membrane morphology and surface properties appear to have a considerable influence on the amount and type of protein adsorbed, suggesting a potential for controlling membrane fouling by modification of membrane properties.

Much research into the fundamentals of membrane formation and separation has been performed in order to improve the efficiency of the manufacture of ultrafiltration membranes. Determination of the membrane characteristics is a key problem in these investigations. In this paper, we report on a study of membrane morphology by fractional rejection measurements, using low molecular weight saccharides as the test solute, and by electron microscopy. Using a simple model for solute/solvent transport through cylindrical pores, a “characteristic pore size” was derived from saccharide rejection data. This pore size of a hypothetical isoporous membrane, interpreting the measured separation characteristics, provides a promising means of describing differences between membranes with respect to pore size and pore size changes caused by solute adsorption. From high resolution electron micrographs, information was obtained on the skin layer morphologies and, for some membranes the sizes of the larger pores could be estimated.