Environmental Engineering Reference
In-Depth Information
compared with UF treatment (Chua et al., 2003). A summary on the SDI values using
different pre-treatments is given in Table 12.15.
The advantages of membranes over the sand filters are the use of lesser
chemicals and more consistent permeate quality (Chua et al., 2003). Membranes
pretreatment offers a higher confidence in delivering the designed quality despite
fluctuations in feed water quality, reduced requirement on chemicals, ability to prolong
the RO membrane lifespan, avoidance of downtime on processing, and lower space
requirement due to a more compact membrane system. MF and UF membranes only
serve to enhance the removal of colloidal particles and suspended particles. Even though
MF and UF membranes are able to reduce requirement on chemicals dosing,
chlorination-dechlorination and most importantly anti-scalant dosing are still required as
part of the pre-treatment when MF and UF membranes are used. In most cases, fouling
of RO membranes due to scaling still occur even with the use of MF and UF membrane
as pretreatment. In addition, MF and UF membranes are not designed for TDS rejection,
which lead to RO as the final stage to reject the high amount of TDS in the desalination
unit. Therefore, the power consumption remained high due to the energy consumed by
the RO booster pumps.
Table 12.15 SDI values of different pretreatment in seawater desalination using RO
membrane.
Type of Pretreatment
SDI Value
References
Dual Membrane Filtration
3.5-4.3
Alawadhi (1997)
Sand Filtration
3.0-6.3
Chua et al. (2003)
Dual Membrane Filtration with
Cartridge Filtration (5
3.3-4.0
Alawadhi (1997)
μ
M)
Sand Filtration with Cartridge
Filtration (10-5-1 μM)
2.8-3.8
Chua et al. (2003)
2.5-4.0
Taniguchi (1997)
MF Membrane (0.1
μ
M Pore Size)
UF Membrane (0.01
μ
M)
< 3.0
Taniguchi (1997); Chua et al.
(2003); Pearce et al. (2004)
The above observation has led to the use of nanomembranes as part of the
pretreatment in RO desalination plants due to its high rejection for divalent ions and also
enables certain level of TDS rejection prior to the RO membrane. More than 95% SO 4 2-
rejection has been reported using various types of NF membranes, while monovalent
ions have rejections of less than 60% (Rautenbach et al., 1997). A pilot-scale testing of a
combined NF-SWRO using Gulf seawater demonstrated that NF membrane was able to
remove TDS by 37.3% from 44,046 mg/L to 27,619 mg/L and remove almost all
turbidity and microorganisms (Hassan et al., 1997). High rejections of divalent ions of
 
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