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This study focused on membrane filtration of neutralized pond water, which may be necessary when good quality water is required for hydrometallurgical processes. Neutralized mine water can still have fairly high metal and sulphate levels, which can hinder discharge and reuse possibilities. Both nanofiltration and reverse osmosis are effective in removing metals and sulphate, but scaling can be a severe problem. Microfiltration as a pre-treatment method, although meant for particle removal, seemed to decrease the amount of scalants, thus delayed scaling on the membrane surface and increasing water recovery for both nanofiltration and reverse osmosis. It is possible that the presence of particles in the feed water promoted crystal growth in the turbulent flow and caused the removal of dissolved constituents. Alternatively, supersaturation could have occurred, allowing microfiltration to remove the scalants as particles. The Liqum sensor indicated that redox values started to increase again just before scaling began due to precipitation in the supersaturated membrane concentrate solution. Thus, the sensor seemed to provide real time, in-situ, early-stage scaling warning.

The emerging thin film composite (TFC) forward osmosis (FO) and pressure retarded osmosis (PRO) membranes generally have better separation properties compared with their cellulose triacetate (CTA) counterparts. Nevertheless, their scaling performance has been rarely reported. In the current study, the phenomenon of membrane integrity loss as a result of scaling is reported for the first time for osmotically driven membrane processes (ODMPs). The results show that the TFC membrane suffered marked flux reduction during the scaling in the active-layer-facing-feed-solution (AL-FS) orientation, accompanied with the severe damage of the membrane active layer. The membrane integrity loss is attributed to the scale formation and growth in the confined space between the membrane and the feed spacer. Compared with the CTA membrane, the TFC was more prone to scaling and membrane damage due to its unfavorable physiochemical properties (presence of Ca2+ binding sites and ridge-and-valley roughness). Although antiscalant addition was shown to be effective for scaling control in AL-FS, it was ineffective in the active-layer-facing-draw-solution orientation. The current study reveals the critical need for scaling control in ODMP processes with respect to the membrane integrity and flux stability. The results also have far-reaching implications for FO and PRO membrane design and process operation. MOE (Min. of Education, S’pore) Accepted version