Calcium carbonate scaling by reverse draw solute diffusion in a forward osmosis membrane for shale gas wastewater treatment

In this study, we investigated the potential of membrane scaling and its reversibility in the FO process for produced water treatment. Sodium chloride (NaCl) and ammonia-carbon dioxide (NH₃-CO₂) were used as draw solutions to examine the effect of draw solute on membrane scaling behavior when treating model produced water, with a high concentration of calcium ions. While a negligible water flux decline was observed for NaCl draw solution, a severe water flux decline occurred for NH₃-CO₂ draw solution due to calcium carbonate (CaCO₃) scaling. Bidirectional solute diffusion analysis and scanning electron microscopy results (SEM) revealed that the CaCO₃ scaling formed only on the active layer of the membrane at the feed side due to the reverse diffusion of carbonate species from the draw solution and the high level of removal of Ca²⁺ by the FO membrane. Scaling formation enhanced the reverse diffusion of draw solutes implying a significant loss of draw solutes in the FO process. Osmotic backwashing did not recover the initial water flux by CaCO₃ scaling. Although ethylenediaminetetraacetic acid (EDTA) cleaning is effective for scaling, the rapid flux decline indicates that a more fundamental strategy is required for long-term FO operation when NH₃-CO₂ draw solution is used for water treatment. A two-step FO process (first FO using a NaCl draw solution and second FO using an NH₃-CO₂ draw solution) was proposed for the pretreatment of feed solution and stable FO operation for shale gas wastewater treatment.