(367b) Operating Strategy for the Energy-Efficient Reverse Osmosis (EERO) Desalination Process

Operating Strategy for the Energy-Efficient Reverse Osmosis
(EERO) Desalination Process

William B. Krantz^a,c,, Tzyy Haur Chong^b,c, Siew-Leng Loo^b,c

^aDept. of Chemical & Biological
Engineering, University of Colorado, Boulder CO 80309-0424

^bSchool of Civil & Environmental Engineering,
Nanyang Technological University, Singapore 639798

^cSingapore Membrane Technology Center, Nanyang
Technological University, Singapore 637141

The
energy-efficient reverse osmosis (EERO) process, patented in 2014 by the Singapore
Membrane Technology Center, reduces the osmotic pressure differential (OPD) and
the cost for potable water production from a saline water feed by combining  Single-Stage Reverse Osmosis (SSRO) with a countercurrent
membrane cascade with recycle (CMCR). The retentate from the SSRO stage is the
feed to the CMCR and its permeate is blended with that
from the CMCR to produce the potable water product. Key features of the EERO
process are: (1) optimum injection of the SSRO retentate into the CMCR; (2)
operating all stages at the same OPD; (3) countercurrent retentate and permeate
flow; (4) permeate recycle to the retentate side in the CMCR; and (5) retentate
recycle to the permeate side by employing one or more NF stages in the CMCR. These
features reduce the concentration difference across the membrane in each stage,
thereby reducing the OPD and net specific energy consumption (SEC_net).

Since the permeate
from an NF stage has a reduced divalent salt concentration and is the feed to
the terminal CMCR stage, the latter can be operated at a higher recovery. Operating
the EERO stages at the same OPD requires that the recoveries of adjacent CMCR stages
add to one. Hence, fouling in the NF stage whose feed has a high divalent salt
concentration can be mitigated by operating it at a lower recovery (higher
safety factor).

Optimal operation of the EERO process can
require using NF membranes with very low salt rejections in one or more of the
CMCR stages. If a membrane with a higher than optimal rejection is used, near optimum
EERO performance can be achieved at the cost of a small increase in the SEC_net
by boosting the pressure in the NF stage.

EERO process performance for different
operating strategies is compared to that for SSRO and two SSROs in series based
on the OPD and SEC_net. The annualized total cost of water production
for EERO operation at 75% recovery is shown to be less than that for SSRO
operation at 50% recovery.

At 75% recovery EERO results in a
two-fold increase in the retentate concentration that not only reduces the
brine disposal, but also translates to a four-fold increase in the energy
density possible via a hybrid EERO-PRO (pressure-retarded osmosis) process for
recovering the osmotic potential energy in the brine. As such, this hybrid
process would cross the 10 W/m² threshold for economic PRO operation.

Acknowledgment

This research was supported by the
Singapore National Research Foundation under its Environmental & Water
Technologies Strategic Research Program and administered by the Environment
& Water Industry Program office (EWI) of the PUB.  The Singapore
Membrane Technology Center, Nanyang Environment and Water Research Institute. Nanyang
Technological University is supported by the Economic Development Board of
Singapore.