Membrane and Desalination Technologies - TCE Moodle Website

596 J.P. Chen et al.
major concepts: (1) pressure resistance increment and (2) control of the concentration
polarization. RO module with increased pressure resistance (from 7 to 8.2 MPa) was incorporated
in the system. The system was tested for two different situations: (1) for standard
seawater with TDS of 35,000 mg/L and (2) for Middle East seawater with TDS of 43,000 mg/L.
The tests were carried out in Japan in a facility with the capacity of 50,000 m 3 /day for
standard seawater and in Saudi Arabian RO plant with the capacity of 14,000 m 3 /day for
Middle East seawater. It was found that the recovery of standard seawater can be increased
from 40 to 60% for standard seawater and for the Middle East seawater, the recovery was
increased from 35 to 50% successfully (80).
Fouling control is another concern of improvement. Biofouling is a major problem in
seawater RO desalination. Intermittent chlorine injection was found to be one of the effective
biofouling control strategies. However, it is required to use high chlorine resistant membranes.
Cellulose triacetate membranes are recommended for use with direct chlorine sterilization,
with intermittent injection of chlorine due to the membrane’s high chlorine resistance.
The process was optimized (81, 82). In another study, membrane surface was modified with
nanosilver to mitigate biofouling (83). Effective pretreatment is important in fouling control.
Conventional pretreatment and UF/MF membrane filtration pretreatment were discussed
earlier in this chapter. Although UF and MF are cost effective compared to double filtration
steps, it is costlier compared to single filtration. On the contrary, it is argued that the
utilization of UF/MF only shifts the fouling from RO to upstream UF/MF membranes.
It was found that dissolved air flotation (Aqua DAF) prior to UF membrane filtration
improved the UF flux by 60% and the cost was optimized. It is said that this arrangement has
improved the flux of RO and reduced the fouling (84). Flotation, a well-known option in
wastewater treatment, can be used in seawater RO desalination as a pretreatment step. This
technology will effectively remove oil, grease, and algae (85).
Because of the low ionization product of boric acid at pH lower than 10, seawater
desalination by RO cannot effectively remove boron from the feed water. Electrodialytic
boron removal is proposed recently to avoid the abovementioned problem. Two different feed
water samples were used: (1) assuming 40% water recovery and (2) a higher rejection. TDS
and boron concentration of the first feed water were 604 and 2.25 mg/L, respectively. For the
second feed water, TDS and boron concentration were 400 and 1.3 mg/L, respectively.
Electrodialytic unit with AMX and CMX Neosepta (Tokuyama Co.) membranes was used
for the boron removal experiments. Membrane to membrane distance was 0.4 mm. Boron
concentration was reduced to 0.4 mg/L using the above system, while the energy consumption
of the first feed water was 0.237 kWh/m 3 and that of the second feed water was
0.186 kWh/m 3 (86).
Apart from the technological concerns over seawater desalination, environmental concerns
are arising. During the process, concentrate and chemicals are released to the marine
environment. On the contrary, energy demand is very high and air pollutants are emitted.
Therefore, environmental impact assessment should be conducted for each desalination
project (71). In Israel, the issue of seawater desalination related environmental pollution is
addressed seriously. Guidelines for pretreatment, discharge composition, marine monitoring,
etc., are adopted (87).