Abstract
The pilot-scale fertiliser driven forward osmosis (FDFO) and nanofiltration (NF) system was operated in the field for about six months for the desalination of saline groundwater from the coal mining activities. Long-term operation of the FDFO-NF system indicates that simple hydraulic cleaning could effectively restore the water flux with minimal chemical cleaning frequency. No fouling/scaling issues were encountered with the NF post-treatment process. The study indicates that, FDFO-NF desalination system can produce water quality that meets fertigation standard. This study also however shows that, the diffusion of solutes (both feed and draw) through the cellulose triacetate (CTA) FO membrane could be one of the major issues. The FO feed brine failed to meet the effluent discharge standard for NH4+ and SO42+ (reverse diffusion) and their concentrations are expected to further increase at higher feed recovery rates. Low rejection of feed salts (Na+, Cl−) by FO membrane may result in their gradual build-up in the fertiliser draw solution (DS) in a closed FDFO-NF system eventually affecting the final water quality unless it is balanced by adequate bleeding from the system through NF and re-reverse diffusion towards the FO feed brine. Therefore, FO membrane with higher reverse flux selectivity than the CTA-FO membrane used in this study is necessary for the application of the FDFO desalination process.
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•Pilot-scale FDFO-NF desalination of saline coalmine water was conducted.•Physical cleaning was effective to recover water flux without chemical cleaning.•NF post-treatment works efficiently due to high water quality from FDFO process.•Water quallity from FDFO-NF system meets standard for fertigation.•High selectivity FO membrane is essential to lower salt diffusion on both sides.