title: Multi-criteria evaluation of an alternative diluted seawater RO desalination process incorporating wastewater recycling
Creators: name: Pazouki, Pirooz
Publisher: name: Griffith University
Year: 2022
Subjects: subject: desalination
subject: dilution desalination process (DDP)
subject: wastewater recycling

description: Desalinated seawater is often the primary source of drinking water on coastal islands. While seawater reverse-osmosis (SWRO) desalination technology has significantly reduced the energy intensity of the desalination process over the past decade, it is still much higher than that of conventional water treatment technologies. There are also concerns about the potential environmental impacts of SWRO desalination plants because of direct disposal of concentrated brine into the ocean and the high energy demand of the process being predominately met by fossil fuels in off-grid island settings. This study evaluated the capacity for the dilution desalination process (DDP) to lower energy use and reduce brine disposal in the desalination of seawater. In the DDP, lower salinity water recovered from treated wastewater effluent is used to dilute seawater before it enters the reverse-osmosis (RO) membrane. As such, less energy is required to desalt the (diluted) feedwater. In my study, two DDP processes are assessed: osmotic dilution desalination (ODD) and mixing dilution desalination (MDD). The ODD process involves a hybrid forward osmosis (FO)–RO configuration and the MDD process an ultrafiltration (UF)–RO configuration. In this thesis, these dilution processes will be referred to as simply MDD (UF-RO) and ODD (FO–RO). Treated wastewater, or secondary effluent (SE), is used as a supplemental water source for reducing salinity in seawater desalination, and pre-treated seawater from a seawater treatment plant is used as the feedwater in the dilution process. In the first configuration, the FO−RO hybrid (applying the ODD process), pre-treated SE from the wastewater stream is used to dilute seawater before RO. In the second configuration, UF is integrated with an RO section. The same impaired water from the sewage plant diffuses through a UF membrane for pre-treatment before moving through the RO membrane. As such, diluted seawater with lower salinity is prepared for SWRO desalination. In both configurations, less energy is required for desalination and safe water is delivered for direct potable reuse (DPR). Recent reviews on the development of the FO process have highlighted mass transfer limitations, internal concentration polarisation (ICP), external concentration polarisation, fouling and solute rejection flux as key challenges leading to low water flux in FO membranes. UF achieves higher water flux, as the driving force of permeate water is hydraulic pressure, in contrast to the FO membrane process, which relies on natural osmotic pressure. Moreover, the fouling and ICP problems are resolved by the UF process due to the bigger mesh size of the membrane. However, UF uses a hydraulic energy-dependent membrane and requires more power than is required for FO membranes, which could be considered an environmental drawback. Using two successive dense membrane processes in hybrid ODD (FO–RO) configuration could increase consumer confidence in wastewater use in seawater desalination. This study undertook a comprehensive assessment of wastewater reuse combined with seawater desalination in two DDPs. Prior to undertaking this assessment, the benefits, advantages and challenges of alternative dilution methods (e.g., UF) and knowledge gaps were discussed. The key factors considered in the multi-criteria evaluation were technical performance, economic feasibility, environmental impacts, and health and quality.