A hybrid desalination system that combines reverse electrodialysis
(RED) and reverse osmosis (RO) processes is another
example of a physical–chemical hybrid system. In this hybrid
process the RED unit harvests the energy in the form of electricity
from the salinity gradient between a highly concentrated solution
(e.g., seawater or concentrated brine) and a low salinity solution
(e.g., biologically treated secondary effluent or impaired water)
[44]. The RED-treated high salinity solution has a lower salt concentration
and serves as the feed solution for the RO unit to reduce
the pump work. The concentrated RO brine provides the RED unit
a better high salinity source for the energy recovery compared to
seawater. In addition, the concentration of the discharged brine
can be controlled by the RED unit for improving the water
recovery and minimizing the impact on the environment. Such
hybrid system is able to gain energy from the RED unit and
meanwhile save on the energy consumption on the RO system.
Fig. 3c shows a schematic diagram of an RED stack connected with
an external electric load. The cations and anions are driven
through the cation- and anion-exchange membranes (CEM and
AEM), respectively by the salinity gradient. The schematic diagram
of the RED-RO hybrid system is shown in Fig. 3d.