The use of ultrafiltration and nanofiltration membranes in the treatment of metal-working fluids

Membrane filtration is used for separating oil from water and treating waste metal-working fluids. This work presents two different filtration systems and investigates the possibility of reusing their permeates. The first system consists of an ultrafiltration membrane (UF) followed by a nanofiltration membrane (NF). In this case, two types of PVDF ultrafiltration membranes, of 200 kD and 100 kD molecular weight cut-off, are used. The second system consists of direct filtration of the metal-working fluids by a nanofiltration membrane. Filtration experiments are carried out in a large-scale, semi-batch process of volume 200 l. The performance of each filtration system is discussed and compared. The permeate resulting from each system is used to prepare fresh metal-working fluid, to simulate the reuse of water in the manufacturing process. Characterisation tests, such as foaming and emulsion stability, are compared for the same metal-working emulsion when made with tap water in order to assess the feasibility of such water reuse.

This paper shows that the concentration polarisation layer plays a major role during the filtration of MWFs in limiting the permeate flux in both ultrafiltration and nanofiltration; high feed concentrations are required for the latter. Limiting the formation of this layer is essential for the achievement of good filtration performance and this can be realised by adjusting the filtration conditions. Increasing feed velocity is an effective way to avoid the effect of concentration polarisation. It has been shown that the nanofiltration membrane is susceptible to the concentration polarisation layer only when the feed concentration is increased. Therefore, using ultrafiltration as a pre-treatment improves the global filtration performance of a nanofiltration polishing step, the latter removing residual turbidity and lowering the COD. This paper shows that a non-turbid permeate, handled carefully, can be used to replenish MWFs recycled in the workshop. Furthermore, the reuse of permeate in such mixing saves water and reduces the waste from used MWFs. The impact on remixed MWF of bacteria developing in a clear permeate must be investigated in more depth. In addition, a poor permeate with high turbidity can lead to some problems because of phase separation and excess foaming.