Treatment of dairy process waters modelled by diluted milk using dynamic nanofiltration with a rotating disk module

We have investigated the recovery by nanofiltration of lactose and milk proteins as well as carbon oxygen demand (COD) and ionic concentration reduction in dairy process waters simulated by UHT skim milk diluted 1:2 with an initial COD of 36,000 mgO2 L−1. The filtration system consisted in a 14.5 cm diameter metal disk (smooth or equipped with vanes) rotating near a circular Desal 5 DK membrane. At initial concentration, the permeate fluxes at a transmembrane pressure (TMP) of 4000 kPa and 45 °C ranged from 130 L h−1 m−2 for a smooth disk at 1000 rpm to 230 L h−1 m−2 using a disk with vanes at 2000 rpm. Permeate COD was minimum for this last case which gave the highest shear rates and decreased with increasing TMP from 60 mgO2 L−1 at 1400 kPa to 22 at 4000 kPa. In concentration tests at a TMP of 4000 kPa, at 2000 rpm with vanes, the permeate flux decayed with increasing volume reduction ratio (VRR) but was still 100 L h−1 m−2 at VRR = 7.5. The maximum VRR, reached using a disk with 6 mm vanes rotating at 2000 rpm, was 14.3, corresponding to 38% of dry matter. Permeate COD remained quasi independent of shear rate (rotation speed and disk type) until VRR = 5 but increased more rapidly at lower shear rates for higher VRR, to reach 350 mgO2 L−1 at 2000 rpm with vanes. Comparison with recent data using same fluid, and same membrane, but installed in a vibrating system (VSEP), showed that the rotating disk yields better performance than the VSEP, due to its higher membrane shear rate.