The ‘FO-RO’ (osmotic dilution) hybrid system isan indirect desalination process. In this system, a FO osmotic dilutionconcept, which based on the salinity difference between two solutions to drivethe water towards the concentrated DS without the recovery of the DS, isintroduced to carry out a pre-dilution of the seawater or high salinity waterbefore it enters the downstream ‘RO’ process 171. By coupling the ‘FOosmotic’ dilution process in this way, seawater ‘RO desalination’ with lowerenergy demand and/or water augmentation can be achieved. In this system, FOworking as a pre-treatment can serve two main purposes for RO: 1) the organicmatters and scaling precursors in the feed water are separated by the FOprocess; and 2) ‘FO’ works as a first barrier to partially remove tracecontaminants and boron that are generally poorly separated by the ‘RO’ process17,281,29. Moreover, no particular requirement on draw solutes or recoverymethods is necessary using this system. Fig. 10shows a two-stage ‘FO’ dilution process integrated with a SWRO system forseawater desalination 18139.
In the first ‘FO’ unit, seawater/pretreatedseawater is osmotically diluted by an impaired water stream. Although seawateras a DS does not cause fouling to the FO process, pre-treatment may still beneeded to reduce the fouling potential in the RO system 171. Prior todischarge, the concentrated SWRO brine is diluted by the concentrated impaired waterfrom the first unit. As such, the FO units not only serve to osmoticallydilute the seawater/SWRO brine, they also help to reduce the volume of impairedwater and lessen the environmental impacts by lowering the electricityrequirement of the process as well as avoiding discharging brines of lowersalinity to the aquatic system 17,182,1831,40,41. However, the economicsustainability of such hybrid FO-RO remains questionable as additionalinvestment cost for integrating the FO units is incurred, and there is no clearadvantage as compared to performing the same task using two distinct andsimpler established water treatment processes, i.
e., water reuse anddesalination 18442. Ali et al., 2017 *suggested that an improvement in the water permeation flux, typically around 24LMH for classical water recoveries (e.g., impaired feed water against seawaterDS), was a prerequisite to lower the investment cost to an economicallyacceptable level as show in Table