Oil is a common pollutant in wastewaters from a wide range of industries. Some of
the compounds found in oily wastewater are toxic and hard to degrade, and thus
pose a significant environmental risk. Treatment of such streams is mostly
performed by physical and chemical methods; however, those methods are limited
in their abilities to remove small and well-dispersed oil droplets. Membrane
separation, which has become widely common in recent years, operates as a
complementary treatment for these processes. The membranes’ versatility, high
efficiency and low demand for chemical additives serves as incentive for their
implementation as treatment processes. However, despite their excellent
capabilities, wider application of membranes for oil/water emulsion separation is
limited due to severe fouling. The tendency of oil droplets to deform and coalesce
seems likely to play an important role in fouling; however, such micro-scale
dynamics of the oil-water-membrane system are poorly understood. The present
study uses confocal microscopy at unprecedented resolution for direct observation
of oil droplet deposition, deformation and detachment during separation and
cleaning, respectively. The 3D shape of the droplets was imaged as a function of the
permeation rate, J, droplet radius, R, membrane permeance, k, water viscosity, μ,
and the water/oil interfacial tension coefficient, σ. Using an interfacial balance of
viscous and surface tension forces, these parameters are combined to yield a
modified capillary number, , which accounts for the extra
viscous ‘suction’ at close proximity to the membrane surface. A clear correlation was
observed between the degree of droplet deformation and an increasing .

Furthermore, the reversibility of droplet deposition and membrane performance
were assessed through microscopic surface coverage and flux recovery analysis,
establishing the existence of a critical flux. In general, operation at a low flux (3.9
μm/s) yields spherical droplets that are easily removed by crossflow cleaning,
whereas a high flux (85 μm/s) leads to significant deformation and mostly
irreversible deposition. It is surmised that, for oil droplets, the critical flux defines a
transition from a non-wetting state, where a thin water film separates the oil from
the membrane surface, to wetting of the membrane by the oil, induced by

hydrodynamic drainage of the film and eventual destabilization and rupture of the
thin film. This transition is expected to be delayed if the membrane material is highly
oleophobic and hydrophilic. These results shed important new insight on the
influence of hydrodynamic conditions on fouling reversibility during membrane based emulsion separation, and may be used to guide better design of surface modified membranes.