Membrane and Desalination Technologies - TCE Moodle Website

Desalination of Seawater by Reverse Osmosis 585
filtrate varied from 2.8 to 3.8 and spikes as high as 6.3 were frequently observed. Membrane
pretreatment produced filtrate of a better quality; SDI of the filtrate produced was consistently
less than 3, a prerequisite for proper operation of an RO desalination plant.
The competitiveness of UF pretreatment over conventional pretreatment was assessed
by evaluating the impact on RO hydraulic performances (52). The study showed that UF
provided permeate water with high and constant quality, resulting in a higher reliability of the
RO process than with a conventional pretreatment. The SDI of surface seawater was reduced
from 13–25 to less than 1, whereas the conventional pretreatment failed to reduce it to less
than 2.5. The combination of UF with precoagulation at low dose helped in controlling UF
membrane fouling and providing filtered water in steady state conditions. The performance of
RO membranes, downstream UF, exceeded the usual operating conditions encountered in
seawater desalination. The combined effect of higher recovery and higher flux rate significantly
reduced the RO plant costs.
A study was performed to investigate MF and UF pretreatment performances in treating a
broad range of water for RO desalination (53). The results of 2-year testing showed that MF
gave the best solution for the pretreatment of raw water to produce infiltration water. The
effective mean filtrate production of the pilot plant is higher than that for the other systems
that were tested, and the amount of chemicals needed in the process is much lower owing to
the effectiveness of the air backwash system. The quality of the filtrate produced with UF
membranes was slightly better compared with that of the filtrate produced with MF membranes.
A demonstration experiment using 35 pilot-scale MF and UF plants showed no
significant difference in membrane filtrate quality and energy consumption between MF
and UF membranes. No coliforms were found in the filtrates (44).
MF and UF processes are capable of reducing biofouling tendency in RO membrane as
they pose a physical barrier to these microorganisms. MF is capable of removing protozoa
(approximately 10 mm), coliforms (approximately 1 mm), and cysts (approximately 0.1 mm).
The pore size of UF membranes is smaller and thus can further remove viruses (approximately
0.01–0.1 mm) (54). It has been shown that the indicator organisms Escherichia coli
and spores of sulfite-reducing Clostridia, present in conventionally treated water, could be
reduced by UF to values below the detection limit (55). The application of membranes thus
reduces the dosage of aggressive chemicals such as chlorine and ozone, thereby minimizing
or avoiding the production of undesirable disinfection byproducts. The interest in using
membranes as part of the disinfection process has intensified with the emergence of chlorine-resistant
pathogens. Chlorine-resistant Cryptosporidium parvum has been reported to
cause outbreak of diarrhea epidemic in USA and UK. Water supply authorities are looking to
UF and MF application to act as an absolute barrier to Cryptosporidium oocysts, which range
from 4 to 6 mm in size (56).
The use of MF membrane as pretreatment also allows the use of TFC RO membrane
instead of CA RO membrane. TFC membranes are anisotropic membranes with a porous
sublayer supporting a very thin top layer ranging from 0.1 to 0.5 mm in thickness. Such
membranes have higher flux compared to thicker isotropic membranes, as flux is inversely
proportional to the thickness of the membrane (57). TFC membranes have higher solute
rejection capability and can be operated at lower pressure (58).