Membrane and Desalination Technologies - TCE Moodle Website

646 P. Kajitvichyanukul et al.
follows (27). At low crossflow velocities, the movement of oil droplets is mainly dominated
by Brownian diffusion and electrostatic effects. Droplet coalescence may occur in localized
places at the surface of the membrane (depending on the local roughness) as well as inside of
the membrane pores. The coalescence of droplets at the membrane surface results in the
formation of a thin oil layer that would reduce the total number of available pores for
permeate flow, resulting in lower permeate flux. When steady state is reached, the thin oil
layer slowly permeates through the pores via surface flow due to the hydrophobic nature of
both oil droplets and membrane pores. However, during operation at higher crossflow
velocities, coalescence at the membrane surface may be reduced. The effect of crossflow
would be to keep the largest droplets away from the surface by shear-induced diffusion or
lateral migration. The droplets reaching the surface would be the smallest in the distribution
and would be able to penetrate into the pores without substantial film formation. In this
scenario, oil film forming at the surface of the membrane is diminished and there is less
reduction of the total number of available pores, resulting in higher permeate fluxes. At high
crossflow velocities, the amount of flux decline is also smaller. This is because the interfacial
oil film is not as pronounced during operation as it is at small crossflow velocities, and steady
state permeate flow is reached without significant flux decline (27). From Hong’s work, the
membrane coalescence under crossflow and intermittent permeate operation is found to be the
promising method to be a potentially novel process for the treatment of oily wastewater (27).
3. MEMBRANE TECHNOLOGY FOR OIL WATER SEPARATION
Oily wastewater treatment can be classified into two categories; primary and secondary
treatment systems. The primary treatment is employed to separate floatable oils from water
and emulsified oil. Free oil is readily removed in this step by normal mechanical separation
processes, such as gravity flotation or skimming. Secondary treatment system is aimed to
treat or break emulsified oil and, then, remove oil from water. An unstable oil–water emulsion
can be destabilized in this step by acid cracking and the oil is subsequently separated by
chemical flocculation or coagulation, followed by air flotation, centrifugation, or filtration.
Oil emulsions have, however, become more and more stable, and a number of them are now
almost impossible to crack by chemical means (37). For the time being, regulations imposed
on oily–water pollution have become tighter and the conventional treatment methods cannot
cope with the problem anymore. The membrane separation process has been one of the
candidates for oil–water separation (38–43). The obvious advantages of membrane process
would be lower capital cost, the absence of chemical addition, and the subsequent generation
of oily sludge. Concentrated oil from oily wastewater could be reused and handled with
accumulated straight oils and, subsequently, disposed of into existing incinerators.
Membrane processes are widely used in oil–water separation. In general, membrane is
classified into two groups; pressure-driven membrane and electrical membrane, known as
electrodialysis. The most applicable process for oily wastewater removal is the former type.
The pressure-driven membrane applications include microfiltration, ultrafiltration, nanofiltration,
and reverse osmosis. All of them are categorized by the molecular weight or particle size
cut-off of the membrane as shown in Table 15.5. Possible guidelines for membrane-process