In order to evaluate the performance of modified PVDF membranes,
the PVDF and PVDF–PVP membranes were used to treat
the oily waste water by ultrafiltration. Fig. 11 indicates the changing
trend of permeation flux with the extension of time. As seen in
Fig. 11, the permeation flux decreased sharply and then kept constant
along with the process of ultrafiltration. Flux decline was due
to membrane fouling by soluble oil. Moreover, the average permeation
water flux through the PVDF–PVP membranes was more
than that through the PVDF membranes. Fig. 12 indicates the
changing trend of permeation flux with the different oil concentration.
It was found that the permeation flux through the PVDF and
PVDF–PVP membranes both decreased slightly with different oil
concentration while that seemed to be nearly independent of oil
concentration. In theory, substantial increase of permeation flux
often accompanies the decline of separation factor. Fig. 13 represents
the changing trend of rejection ratio of oil waste water with
time. It was seen that that all the membranes had achieved excellent
rejection ratio of oil waste water.