Concentration polarization is intrinsically associated with the selective character of membranes and often means flux decline and which causes a subsequent decrease of ultrafiltration and nanofiltration performance. More important is the fact that it acts as a precursor of membrane fouling and creates severe fouling problems in the longer times range. The quantification of its dependence on the operating parameters of cross-flow velocities and transmembrane pressures makes recourse to the film theory to introduce mass-transfer coefficients that generally are calculated by dimensionless correlations of the Sherwood number as a function of the Reynolds and Schmidt numbers. In the present work, the mass-transfer coefficients are obtained through the fitting of experimental results by the pressure variation method. The ultrafiltration/nanofiltration of the winery wastewaters from the racking operation is carried out with the membranes ETNA 01PP (Alfa Laval) and NF 270 (Dow Filmtec) under a wide range of cross-flow velocities and transmembrane pressures up to 15 bar.

A wide range of nanofiltration membranes are investigated for the fractionation of a winery effluent and recovery of the polysaccharides of low molecular weight and of the polyphenols. Permeation experiments were carried out with three laboratory-made cellulose acetate membranes and two commercial membranes: NF270 membrane supplied by Filmtec Corp., Minneapolis, MN (USA) and ETNA01PP membrane supplied by Alfa Laval, Nakskov, Denmark. The five membranes were characterized by the rejection coefficients to a set of reference solutes and evaluated in terms of the rejection coefficients to polysaccharides, polyphenols, conductivity and total organic carbon. The rejection coefficients to polyphenols were overall lower than the ones to polysaccharides, meaning that the polyphenols permeate preferentially through all the membranes. Among these membranes, the NF270 membrane displayed the highest rejection coefficients for all parameters evaluated, with rejections in the order of 93.8% and 99% for polyphenols and polysaccharides, respectively. On the other hand, the ETNA01PP membrane presented the lowest rejection coefficients: around 27% to polyphenols and 72% to polysaccharides.