98 M.J. Lewis 4.2 PROCESSING CHARACTERISTICS This section will deal with some of the important processing parameters encountered in ultrafiltration. There are various factors which will influence the outcome of the process, such as the concentration factor and rejection. See Section 3.3 The extent of the concentration is defined by the concentration factor (), defined as VE/V(see eq.(3.5). Usually the permeate is the biggest fraction by volume. Milk for heese making is concentrated by UF fivefold, whereas cheese whey is concentrated twentyfold for the production of protein concentrates. Sometimes the resulting permeates are further concentrated by reverse osmosis 4.2.1 Rejection or retention factors The rejection or retention factor(R)of any component is defined as where ce is the concentration of component in the feed and cp is the concentration in the permeate. The rejection is determined experimentally for each component in the feed, by sampling the feed and permeate at the same time and analysing that component. It is very important and will influence the extent(quality)of the separation achievable Rejection values normally range between 0 and 1; sometimes they are expressed as percentages(0 to 100%) when Cp=0; R=l; all the component is retained in the feed when cp = c R=0; the component is freely permeating In ultrafiltration experiments, some workers have measured negative rejection,i.e Cp>CE, particularly for minerals. It is not immediately obvious why this should have occurred. Possible explanations for this are higher concentrations at the membrane Irface than in the bulk, due to concentration polarisation. However, this is unlikely to be the case for freely permeating species. Another explanation is the basis on which concen- tration is measured (Glover, 1985). This may arise when there is substantial fat in the eed which is rejected by the material. It is suggested that concentrations be expressed in the aqueous portion. A third explanation lies in the Donnan effect; Donnan predicted and later demonstrated that concentration of electrolyte in the solutions on either side of a alysis membrane were unequal when the colloid on one side was electrically charged (see later). For example, at low pH values, where proteins are likely to be positively charged, this could lead to higher concentrations of cations in the permeate membrane for a particular application. Rejection values may also be influenced by operating conditions An idealultrafiltration membrane would have a rejection value of 1.0 for high molecular weight components and zero for low molecular weight components. However, typical values observed for real membranes are between 0.9 and 1.0 for high molecular weights and between 0 and 0. 1 for low molecular weight components. values for98 M.J.Lewis 4.2 PROCESSING CHARACTERISTICS This section will deal with some of the important processing parameters encountered in ultrafiltration. There are various factors which will influence the outcome of the process, such as the concentration factor and rejection. See Section 3.3. The extent of the concentration is defined by the concentration factor cf), defined as VF/Vc (see eq. (3.5)). Usually the permeate is the biggest fraction by volume. Milk for cheese making is concentrated by UF fivefold, whereas cheese whey is concentrated twentyfold for the production of protein concentrates. Sometimes the resulting permeates are further concentrated by reverse osmosis. 4.2.1 Rejection or retention factors The rejection or retention factor (R) of any component is defined as R = (CF - cp )/cF (4.1) where cF is the concentration of component in the feed and cp is the concentration in the permeate. The rejection is determined experimentally for each component in the feed, by sampling the feed and permeate at the same time and analysing that component. It is very important and will influence the extent (quality) of the separation achievable. Rejection values normally range between 0 and 1; sometimes they are expressed as percentages (0 to 100%). when cp =O; when cp = cF R = 1; all the component is retained in the feed R = 0; the component is freely permeating. In ultrafiltration experiments, some workers have measured negative rejection, Le. cp > cF, particularly for minerals. It is not immediately obvious why this should have occurred. Possible explanations for this are higher concentrations at the membrane surface than in the bulk, due to concentration polarisation. However, this is unlikely to be the case for freely permeating species. Another explanation is the basis on which concen￾tration is measured (Glover, 1985). This may arise when there is substantial fat in the feed which is rejected by the material. It is suggested that concentrations be expressed in the aqueous portion. A third explanation lies in the Donnan effect; Donnan predicted and later demonstrated that concentration of electrolyte in the solutions on either side of a dialysis membrane were unequal when the colloid on one side was electrically charged (see later). For example, at low pH values, where proteins are likely to be positively charged, this could lead to higher concentrations of cations in the permeate. Rejection characteristics can readily be determined for different substances using different membranes. This is one practical way of selecting the most appropriate membrane for a particular application. Rejection values may also be influenced by operating conditions. An ‘ideal’ ultrafiltration membrane would have a rejection value of 1.0 for high molecular weight components and zero for low molecular weight components. However, typical values observed for real membranes are between 0.9 and 1.0 for high molecular weights and between 0 and 0.1 for low molecular weight components. Values for