Water & Wastewater Asia November/December 2021

VIEWPOINT
Furthermore, the adoption of energy recovery
devices (ERDs) in RO systems have made
the economics of RO even more compelling
against its thermal counterpart. ERDs, in the
form of isobaric pressure exchangers, have
played a critical role in the shift from thermal
desalination processes to SWRO by reducing
the SWRO energy requirement by up to 60%.
Similar energy savings are also available
with the isobaric pressure exchanger in the
UHPRO process.
Figure 1
DEPLOYING ERDS TO AUGMENT THE
ECONOMIC BENEFIT OF RO
Basic RO systems employ a high-pressure
feed pump to pressurise the feed stream
to the required RO membrane pressure for
dissolved solid separation to occur.
The concentrate stream being discharged
from the system retains most of the pressure
from the feed stream as the pressure drop
across the RO membrane is only about
15-30 psi (1-2 bar). The hydraulic energy
in the concentrate stream needs to be
dissipated through a pressure regulating
valve to safely dispose the concentrate.
(Figure 1).
Figure 2
ERDs such as isobaric pressure exchangers
are designed to harness this wasted hydraulic
energy from the concentrate stream, thus
reducing the RO energy consumption.
possible to treat high-salinity wastewater
by operating RO systems at an ultra-high
pressure of more than 1,200 psi (83 bar)
– nearly 50% higher pressure than SWRO.
UHPRO can be deployed prior to the
thermal stage to further concentrate
and reduce the waste stream volume.
As a result, the required size and energy
consumption of the final thermal stage is
significantly reduced.
Figure 2 shows how the isobaric pressure
exchanger is integrated into a RO system
and how energy from the concentrate is
transferred to the partial RO feed stream,
which has a flow rate equivalent to that
48 WATER & WASTEWATER ASIA | NOVEMBER/DECEMBER 2021