The feed water against seawater DS), was a prerequisite

The ‘FO-RO’ (osmotic dilution) hybrid system is
an indirect desalination process. In this system, a FO osmotic dilution
concept, which based on 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