Gypsum scaling during forward osmosis process—a direct microscopic observation study

Abstract

A B S T R A C TThis study investigated the effect of supersaturation index (SI), ionic strength, membrane orientation, and antiscalant (AS) addition on gypsum scaling during forward osmosis (FO) desalination. Scaling tests were performed in a cross-flow FO system, and the development of gypsum scalants on FO membrane was directly observed using an optical microscope integrated with the FO filtration cell. Greater surface coverage by gypsum crystals and larger crystal sizes occurred on the scaled FO membranes for feed solutions (FS) with higher SI values accompanying with more severe flux reduction. At fixed Ca 2+ and SO 2À 4 concentrations, reducing the ionic strength of the FS from 0.55 to 0.15 M resulted in longer induction time. Nevertheless, more flux loss and surface coverage by scalants occurred at longer filtration duration for 0.15 M FS due to its greater ion activities and thus higher SI. The active layer facing (AL)-draw solution membrane orientation was found to be prone to internal scaling, which is likely as a result of unfavorable internal concentration polarization of scaling precursors inside the FO membrane support layer. On contrary, the AL-FS orientation had much more stable flux behavior. The current study also demonstrated the effectiveness of AS addition and rinsing under cross-flow conditions for FO scaling control.

Membrane fouling in osmotically driven membrane processes: A review

“…It is worthwhile to note that most of the existing studies on inorganic scaling in ODMPs focused on the use of gypsum as model scale that is independent of solution pH and has relatively high solubility [28,31,35,69,70,205,206]. Only few studies reported other types of scaling [80,186].…”

Insights into the Influence of Membrane Permeability and Structure on Osmotically-Driven Membrane Processes

“…In this study, FS containing Ca 2+ and SO 4 2− were used to investigate the impact of scaling on TFC and CTA-W(P). The membranes were tested in AL-FS mode with mitigated internal scaling [ 50 ]. Water flux of TFC and CTA-W(P) in the scaling tests are shown in Figure 5 .…”

“…The tests were performed in the AL-FS mode. This mode is preferred in FO applications with low scaling/fouling potential as it can eliminate the internal clogging of support layer and achieve more stable flux in long-term operation [ 50 ]. In the scaling tests, mixed solution containing Na 2 SO 4 (72 mM), CaCl 2 (26.1 mM), and NaCl (10 mM) was used as FS, of which the gypsum saturation index (SI) was 2.0 [ 51 ].…”

A Review of Fouling Mechanisms, Control Strategies and Real-Time Fouling Monitoring Techniques in Forward Osmosis

“…When the concentration of these salts in the feed exceeds their solubility at higher water recovery rates, precipitation may occur near or on the membrane surface, leading to scaling of the membrane surface and flux decline [44,45]. Amongst the various scaling compounds reported in the literature are silica, calcium carbonate, gypsum and calcium sulphate [45][46][47][48][49][50].…”