Effect of vibration frequency and membrane shear rate on nanofiltration of diluted milk, using a vibratory filtration system

The effect of membrane shear rate during nanofiltration of skim milk diluted with two volumes of water representing dairy effluents, using a vibrating VSEP module with a Desal 5 DK membrane, was investigated. This shear rate (γmax) was varied by performing tests at 60.75, 60.2, 60.0 and 59.8 Hz. In long-term industrial use, it is necessary to limit the vibration amplitude d of the membrane rim to about 2–2.3 cm, against d = 3.0 cm at the maximum and resonant frequency of 60.75 Hz. At a pressure (TMP) of 4000 kPa, 45 °C and initial concentration, permeate fluxes decreased from 220 L h−1 m−2 at 60.75 Hz (with a shear rate of 1.35 × 105 s−1) to 95 L h−1 m−2 at 59.8 Hz (0.64 × 105 s−1, d = 1.5 cm). The permeate carbon oxygen demand (COD) due to lactose increased from 20 mgO2 L−1 at 60.75 Hz to 60 mgO2 L−1 at 59.8 Hz, remaining much smaller than in the feed (36,000 mgO2 L−1). Permeate conductivity increased from 500 μS cm−1 at 60.75 Hz to 800 μS cm−1 at 59.8 Hz, for a feed conductivity of 2000 μS cm−1. During concentration tests, performed at 4 frequencies, the permeate flux J at various volume reduction ratios of 1, 3 and 7 was given by a single equation J=3.0X10-6γmax1.56, showing that the flux is mainly controlled by shear rate whether this shear rate is lowered by reducing the frequency or increasing the concentration. Tests performed with a real dairy effluent gave a variation of permeate flux and conductivity with TMP similar to the model, up to 3000 kPa. Above 3000 kPa, the real effluent flux dropped due to higher membrane fouling.