Membrane and Desalination Technologies - TCE Moodle Website

648 P. Kajitvichyanukul et al.
l Energy costs are lower compared to thermal treatments.
l The treatment plant can be highly automated and does not require highly skilled operators.
The development of membrane technologies in the last 30 years has embodied applications
in the processing of emulsions. Both microfiltration and ultrafiltration have been used for
concentrating emulsions (43–45). Ideally, the continuous phase is driven through the membrane
by the application of a transmembrane pressure as the dispersed phase is retained at the
surface by a sieving mechanism (43). However, because the liquid droplets are deformable,
they can still be squeezed into smaller pores if the applied pressure exceeds the dispersed
phase capillary pressure (44). This would then result in the breakthrough of dispersed phase
as well as permeate contamination.
The separation efficiency (f ) for the separation of o/w emulsions is given by (43):
oil concentration in permeate
f ¼ 1
oil concentration in feed
Among the membrane processes, UF is an attractive alternative for the treatment of
emulsions, as no chemical additives are required, high removal efficiencies achieved, and
low capital and operating costs (44). Macrofiltration (MF) membranes have been used to
recover surfactants in the permeate. Microfiltration membranes give higher fluxes, but pose a
higher risk of oil breakthrough. If the salt content of oily wastewater is too high for direct
reuse of the permeate in the plant, it can be treated by reverse osmosis and nanofiltration
(7, 46). In addition, reverse osmosis can selectively reject solutes of the same size order as
water molecules.
3.1. Ultrafiltration
UF is a pressure driven membrane process which can concentrate and fractionate macromolecular
solutes and separate suspended species from water. UF provides a non-destructive
separation, which can be performed without any phase transition (47). Because the osmotic
pressure exerted by the high molecular weight solutes can be negligible, this process is
operated at relatively low pressure in range of 0.69–6.91 atm (70–700 kPa) (48, 49). The
concentrate may contain up to 50% of oil. The oily concentrate can be further separated by
centrifugation.
UF membranes, which have tight pores, have been selected in most applications to ensure
steady permeate quality. The flux frequently declines with time; this has been attributed to
surfactant adsorption on the pore walls, to the buildup of a polarized layer of concentrated
emulsion at the membrane surface and also pore plugging by oil droplets (44, 45). However,
this technique has two drawbacks; concentration polarization and membrane fouling (50). As
a filtration process, membranes used are truly porous and separation is a physical process
requiring elevated pressure to achieve passage of fluid through the filter. The water and other
low molecular weight solutes pass through the membrane pores. The larger molecules or
aggregates are rejected. The successful ultrafiltration separation performance is usually
obtained when discrete and stable emulsion particles of oil, larger than the membrane pore
size, are maintained. However, the mechanism of separation of oil and water in ultrafiltration