Microfiltration 147 membranes of the appropriate pore size in relation to the characteristics of the particles upended in the feed. Unfortunately it is very difficult to predict the optimal pore size from first principles, and experimentation is required. However, a recent study of Tarleton and Wakeman(1993)has attempted to understand the relationship between pore size and particle size in flux decline. Clearly the particle size distribution is an important factor specially when even a small proportion of fines are present in the feed. Many anomalies remain, and further systematic studies are required A large amount of effort has been put into devising means of avoiding or counter- acting the effects of fouling in MF, and several promising techniques have evolved. The first requirement is to maintain high shear in the retentate and thus reduce the thickness of the boundary layer at the membrane surface and remove deposits from the membrane It is desirable that turbulent flow be maintained on the retentate side of the membrane Murkes(1989)has described a new high shear cross-flow method for Mf, and claimed an order of magnitude increase in flux rates over conventional CMF Forcing a quantity of permeate back through the membrane (i.e. backflushing )can ive deposited particles away from the membrane surface, as well as breaking up con centration polarisation and or gel layers in the bulk flow. This can lead to regeneration of permeate flux, but it is necessary to ensure that a higher volume of permeate is not used for the backflushing than is gained from the resulting increased permeate flux (Milisic and Bersillon, 1986). However, assuming the correct sequencing and volume of pulsations are used, the technique can be most beneficial(Fig. 5.4). An alternative method of backwashing is to use cleaning sequences consisting of blowing air through the mem branes-i.e. gas backflushing(Dietrich et al., 1988; Peters, 1989). This may prove beneficial with problem feed streams. Another approach to maintaining flux is to use the uniform transmembrane pressure' mode of operation. This requires simultaneous operation of a retentate pumping loop and a permeate pumping loop, adjusted so that the pressure drop across the membrane is small, and is uniform along the length of the mbrane. This system has allowed very high fluxes(500-7001 m-2h-)to be mair ained during processing of skimmed milk using ceramic membranes in the Bactocatcl system(see Section 5.3.1) The application of an electric field has been shown to reduce fouling due to colloids and particles during CMF (Visvanathan and Ben Aim, 1989)and thus prevent flux decline(Tarleton and Wakeman, 1988), although the mechanism of action is not full understood. Bowen et al. (1989)reported that electrical pulses were effective in both improving flux rates and restoring flux rates in fouled systems, and could be used as an alternative to back washing and conventional cleaning techniques Other approaches to reducing fouling during Mf have included the use of abrasives to break down the fouling layers, and the application of pulsations to the feed stream (Milisic and Bersillon, 1986). Chemical pretreatments of the feed streams can also sed to reduce the problems of fouling(Bedwell et aL., 1988), although this is frequently not a suitable option. Pretreatment of the membrane may reduce the flux decline during harvesting of yeast cells by conditioning the MF membranes with respect to pl x during fouling. Taddei and Howell (1989)reported a 70% improvement of flt The initial challenge to the membrane has been found to affect the subsequent flux during MF. Care must be taken during start-up procedures to prevent too-rapid fluxMicrofiltration 147 membranes of the appropriate pore size in relation to the characteristics of the particles suspended in the feed. Unfortunately it is very difficult to predict the optimal pore size from first principles, and experimentation is required. However, a recent study of Tarleton and Wakeman (1993) has attempted to understand the relationship between pore size and particle size in flux decline. Clearly the particle size distribution is an important factor, especially when even a small proportion of fines are present in the feed. Many anomalies remain, and further systematic studies are required. A large amount of effort has been put into devising means of avoiding or counter￾acting the effects of fouling in MF, and several promising techniques have evolved. The first requirement is to maintain high shear in the retentate and thus reduce the thickness of the boundary layer at the membrane surface and remove deposits from the membrane. It is desirable that turbulent flow be maintained on the retentate side of the membrane. Murkes (1989) has described a new high shear cross-flow method for MF, and claimed an order of magnitude increase in flux rates over conventional CMF. Forcing a quantity of permeate back through the membrane (i.e. backflushing) can drive deposited particles away from the membrane surface, as well as breaking up con￾centration polarisation and/or gel layers in the bulk flow. This can lead to regeneration of permeate flux, but it is necessary to ensure that a higher volume of permeate is not used for the backflushing than is gained from the resulting increased permeate flux (Milisic and Bersillon, 1986). However, assuming the correct sequencing and volume of pulsations are used, the technique can be most beneficial (Fig. 5.4). An alternative method of backwashing is to use cleaning sequences consisting of blowing air through the mem￾branes - i.e. gas backflushing (Dietrich et al., 1988; Peters, 1989). This may prove beneficial with problem feed streams. Another approach to maintaining flux is to use the ‘uniform transmembrane pressure’ mode of operation. This requires simultaneous operation of a retentate pumping loop and a permeate pumping loop, adjusted so that the pressure drop across the membrane is small, and is uniform along the length of the membrane. This system has allowed very high fluxes (500-700 1 m-* h-’) to be main￾tained during processing of skimmed milk using ceramic membranes in the ‘Bactocatch’ system (see Section 5.3.1). The application of an electric field has been shown to reduce fouling due to colloids and particles during CMF (Visvanathan and Ben Aim, 1989) and thus prevent flux decline (Tarleton and Wakeman, 1988), although the mechanism of action is not fully understood. Bowen et al. (1989) reported that electrical pulses were effective in both improving flux rates and restoring flux rates in fouled systems, and could be used as an alternative to backwashing and conventional cleaning techniques. Other approaches to reducing fouling during MF have included the use of abrasives to break down the fouling layers, and the application of pulsations to the feed stream (Milisic and Bersillon, 1986). Chemical pretreatments of the feed streams can also be used to reduce the problems of fouling (Bedwell et al., 1988), although this is frequently not a suitable option. Pretreatment of the membrane may reduce the flux decline during fouling. Taddei and Howell (1989) reported a 70% improvement of flux during harvesting of yeast cells by conditioning the MF membranes with respect to pH. The initial challenge to the membrane has been found to affect the subsequent flux during MF. Care must be taken during start-up procedures to prevent too-rapid flux