The resistance-in-series model was used to analyse flux behaviour, which involved the resistances of membrane itself, the fouling and solute concentration polarisation. Response surface methodology was used to establish the relationships between operating parameters and ultrafiltration (UF) efficiency and thus to determine optimal conditions. Experiments were performed according to Box–Behnken design by changing the levels of three parameters, namely transmembrane pressure, feed flow rate and temperature. The fitted mathematical models allowed us to plot isoresponse curves. It was shown that the resistance due to solute concentration polarisation (Rcp) dominated the flux decline (40–74%). The fouling resistance (Rf) varied from 12 to 46%. To optimise simultaneously the three responses studied (Rf, Rcp and permeate limit flux), we applied the desirability function approach which allowed us to determine the best acceptable compromise. The selected UF conditions of the compromise were as follows: three bars, 0.95 L min−1 and 30 °C. Optimal values of Rf, Rcp and permeate limit flux were equal to 18%, 72% and 19 L h−1 m−2, respectively.