Contrary the seawater or high salinity water before it

to the hybrid systems discussed so far, the FO-RO (osmotic dilution) hybrid
system is an indirect desalination process. In this system, a FO osmotic
dilution concept, which utilizes the salinity difference between two solutions
to drive the water towards the concentrated DS without the recovery of the DS,
is introduced to carry out a pre-dilution of the seawater or high salinity
water before it enters the downstream RO process 171. By coupling the FO
osmotic dilution process in this way, seawater RO desalination with lower
energy demand and/or water augmentation can be achieved. In this system, FO
working as a pre-treatment can serve two main purposes for RO: 1) the organic
matters and scaling precursors in the feed water are separated by the FO
process; and 2) FO works as a first barrier to partially remove trace
contaminants and boron that are generally poorly separated by the RO process
17,281,29. Moreover, no particular requirement on draw solutes or recovery
methods is necessary using this system. Fig. 10
shows a two-stage FO dilution process integrated with a SWRO system for
seawater desalination 18139. In the first FO unit, seawater/pretreated
seawater is osmotically diluted by an impaired water stream. Although seawater
as a DS does not cause fouling to the FO process, pre-treatment may still be
needed to reduce the fouling potential in the RO system 171. Prior to
discharge, the concentrated SWRO brine is diluted by the concentrated impaired water
from the first unit. As such, the FO units not only serve to osmotically
dilute the seawater/SWRO brine, they also help to reduce the volume of impaired
water and lessen the environmental impacts by lowering the electricity
requirement of the process as well as avoiding discharging brines of lower
salinity to the aquatic system 17,182,1831,40,41. However, the economic
sustainability of such hybrid FO-RO remains questionable as additional
investment cost for integrating the FO units is incurred, and there is no clear
advantage as compared to performing the same task using two distinct and
simpler established water treatment processes, i.e., water reuse and
desalination 18442. Ali
et al., 2017 . suggested that an improvement in the water permeation flux,
typically around 24 LMH for classical water recoveries (e.g., impaired feed water
against seawater DS), was a prerequisite to lower the investment cost to an
economically acceptable level as show in Table .