Treatment of dairy process waters using a vibrating filtration system and NF and RO membranes

Abstract

The permeate flux and chemical oxygen demand (COD) reduction were investigated in dairy process waters using a vibratory shear-enhanced filtration system (VSEP) and various nanofiltration (NF) and reverse osmosis (RO) membranes. Dairy process waters were simulated by UHT skim milk diluted 1:3 to obtain an initial COD of 36 000 mg O 2 l −1. In NF the highest permeate flux (270 l h −1 m −2 at 4 MPa, 45 °C and initial concentration) was obtained with a Filmtec membrane which yielded also the highest permeate COD (94 mg O 2 l −1). The best compromise was obtained with a Desal 5 DK membrane which yielded a COD of 36 mg O 2 l −1 and a flux of 240 l h −1 m −2 under same conditions. In concentration tests, the permeate flux decreased with increasing volume reduction ratio (VRR) to reach 25 l h −1 m −2 for the 5 DK membrane at VRR=7 while permeate COD soared to 1050 mg O 2 l −1. A comparison with published data collected using a spiral module and same test fluid at 25 °C and 1.9 MPa showed a definite advantage for the VSEP equipped with the same 5 DL membrane and operated at same pressure and temperature. The VSEP yielded a permeate COD of 24 mg O 2 l −1 versus 128 mg O 2 l −1 for the spiral module together with a higher flux 71 l h −1 m −2 versus 24 l h −1 m −2. The better performance of the VSEP can be attributed to its higher membrane shear rate which reduces lactose concentration at membrane and its transmission. As expected, permeate COD was further reduced to less than 22 mg O 2 l −1 at a VRR of 5.6 by using an RO membrane.