The effect of milk pH at four levels (6.67, 6.43, 6.25 and 5.97) on dynamic behaviour of permeate flux(JP), total hydraulic resistance(RT) and milk solutes rejection (protein, RP; fat, RF; lactose, RL; minerals, RM and total solids, RTS) were studied. Experiments were carried out using the pilot plant UF membrane system equipped with a spiral wound module and a polysulfoneamide membrane.A three-stage strategy based on a resistance-in-series model (boundary layer-adsorption) was used to determine the different hydraulic resistances. The experimental results showed that JP at all levels of pH decreases with increase in operating time(t), either of flux decline rates for pH levels of 6.67 and 6.43 were obtained higher than these for 6.25 and 5.97 levels. Decreasing pH within 6.67-6.43 range had no effect on psuedo-steady state flux, but it considerably declined with further decrease in pH. RT increased during process at all levels of pH, although there was a slight difference observed between pHs 6.67 and 6.43. In total decrease in pH led to increase in both reversible and irreversible hydraulic resistances, but irreversible was greater than reversible fouling at all levels of pH. The results of solute rejection showed that RP and RF at any value of pH don't change with t, whereas RL, RM and RTS increase considerably with t at the corresponding pH. Meanwhile increasing pH did not have much effect on RP and RF, but the rejection of other solutes strongly decreased.
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