Membrane and Desalination Technologies - TCE Moodle Website

528 J.P. Chen et al.
Feed intake
Chemicals
Intake system and
mechanical screening
Pretreatment
Desalination processes
Post treatment
Product water:
storage and distribution
Disposal of waste
Fig. 12.2. General process for production of fresh water by desalination processes.
Membrane processes of RO and ED (EDR), on the contrary, are more favorable for treating
brackish waters (under most conditions) or highly saline wastes for which energy costs are
high or the flow rates are low.
In addition to the thermal and membrane processes, ion exchange (IX), freezing, and
membrane distillation can be used for the removal of salt in water. IX is presented in another
chapter of this handbook series. In the freezing process, saline water is frozen to form ice
crystals under controlled conditions. Before the water is completely frozen, the mixture is
usually rinsed to remove the salts in the remaining water or those adhering to the ice crystals.
The ice is then melted to produce fresh water. Lower energy requirement, lower potential for
corrosion, and lesser scaling or precipitation problems are the advantages; however, the
disadvantage is the difficulty in operations (e.g. handling huge ice crystals). Membrane
distillation process combines the use of both distillation and membranes (4, 5). Saline
water is warmed to enhance vapor production, and this vapor is exposed to a membrane
that can pass water vapor but not liquid water. After the vapor passes through the membrane,
it is condensed on a cooler surface to produce freshwater. In the liquid form, the freshwater
cannot pass back through the membrane; hence it is trapped and collected as the output of the
plant. The advantages lie in its simplicity and the need for only small temperature differentials
for its operation. However, when the process is run with low temperature differentials,
large amounts of water must be used, which affects its overall energy efficiency. A recent
development is adopting solar-powered membrane distillation technology for the production
of pure water from brackish water (5). Solar energy is collected for the operation of
membrane distillation system.
Before the raw water enters the aforementioned treatment systems, it is normally pretreated
by various technologies, such as filtration, chemical coagulation, softening, and pH