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There are many different parameters effecting permeate flow rate of Ultrafiltration (UF) membrane in production of Whey Protein Concentratiun (WPC).In this study,we studied the impact of both Volume Concentration Ratio (VCR) and temperature on a Poly-Ether-Sulfone (PES) membrane with porosity of 10 kD (KMS HFKTM-131 (4333)) which has been placed in a housing containing of a series of two Spiral-Wound modules.Inlet and outlet pressures were 3.2 and 1 bar,respectively,and the flow rate of inlet whey was 7800 l/hr.While VCR was reaching 1.14 at T=10 ℃,a sudden drop of permeate flow-rate (from 3.54 l/min to 1.704l/min) was observed.It is obvious that it was caused by Concentration Polarization phenomenon [1].This sudden drop followed by a gradual declining of permeate flow rate (from 1.704 l/min to 1.44 l/min) that is attributed to membrane fouling [1].These statistically demonstrate that,in this pilot unit,concentration polarization is the main resistance in comparison with membrane fouling.It is also notable that during this experiment,concentration of protein increased from 15.2%/dry matter to 24.3%/dry matter while VCR reached 8 by a feed & bleed operation unit.Even though low temperature processes are more favorable,we have found that raising temperature from 10 ℃ to 50 ℃ would increase whey permeate flow rate of UF membrane from 3.508 l/min to 8.571 l/min,the fact that would be instrumental in economically modifying.