A comprehensive description of the threshold flux during oil/water emulsion filtration to identify sustainable flux regimes for tannic acid (TA) dip-coated poly(vinylidene fluoride) (PVDF) membranes

In the separation of oil/water emulsions using membrane separation methods, fouling remains a problem. To improve the fouling resistance effectively, a hydrophilic tannic acid (TA) coating was constructed on the poly(vinylidene fluoride) (PVDF) membrane via dip-coating method. Both the wettability and surface free energy were dramatically enhanced after the introduction of the TA coating. TA deposition with extended dip-coating time did not result in a corresponding increase in the pure water permeate quality, suggesting that there is a trade-off between the enhanced surface hydrophilicity and diminished pore size. Furthermore, the threshold fluxes were determined using the flux-stepping method, and explored to provide guidance on membrane fouling rates for oil/water emulsion filtration. The hydrophilic TA dip-coated PVDF membrane prepared at a dip-coating time of 9 h exhibited the highest threshold flux compared to the other membranes. The experimental evidences of the threshold fluxes can be used to predict the sustainable flux regimes for the TA dip-coated membranes. Constant flux verifiable experiments for oil/water emulsion were performed at fluxes near and below the threshold fluxes. By selecting a constant flux strategy below the threshold flux, membrane over-fouling can be avoided efficiently and the operating period can be prolonged favourably. More generally, operation below the threshold flux recognizes a modest fouling level, arising from the sufficient flux, which would result in the infrequent need for cleaning and is a compromise between capital expenditure and operating costs.