Membrane scaling in osmotically driven membrane processes
Date of Issue2016
School of Civil and Environmental Engineering
Singapore Membrane Technology Centre
Water shortage is a serious problem that has impelled many industries and researchers to investigate new access to clean and safe drinking water. Osmotically driven membrane processes, such as forward osmosis (FO) and pressure retarded osmosis (PRO) as emerging technologies for freshwater production, have attracted more research interests and applications. However, scaling is still a significant challenge in both FO and PRO application and has not yet been fully addressed in the literature. The unique phenomena in osmotically driven membrane processes (e.g., internal concentration polarization (ICP) and reverse solute diffusion (RSD)) and their effects on FO and PRO scaling require further investigation in order to provide a mechanistic understanding of FO and PRO scaling. Therefore, the work aims to systematically study the effect of water chemistry and operational conditions on FO and PRO scaling. In the study of feed solution chemistry on FO scaling, the effects of saturation index (SI), ionic strength, membrane orientation, and antiscalant (AS) addition on calcium sulfate dihydrate (gypsum) scaling during FO desalination were investigated. 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 with higher SI values accompanying with more sever flux reduction. At fixed Ca2+ and SO42- concentrations, reducing the ionic strength of the feed solution 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 the 0.15 M feed solution due to its ion activities and thus higher SI. The active layer facing draw solution orientation (AL-DS) 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 the contrary, the active layer facing feed solution orientation (AL-FS) had much more stable flux behavior. The current study also demonstrated the effectiveness of antiscalant addition and rinsing under cross flow conditions for FO scaling control. In the study of effect of draw solution chemistry on FO scaling, for the first time, the effects of reverse diffusion of draw solution on FO scaling in a bench-scale FO system using calcium phosphate as a model scalant were systematically investigated. Both scaling precursors such as Ca2+ and anti-scaling precursors here by ethylenediaminetetraacetic acid (EDTA) were selected to elucidate the relationship between reverse solute diffusion and membrane fouling. The results of the current study suggest that the presence of scaling precursors was not necessarily accelerating scaling process since the contribution of the reversed scaling precursors from draw solution should be considered as well. If the contribution is negligible, the scaling precursors containing salts can also be selected as candidate draw solutes. In addition, adding EDTA into the draw solution, the membrane scaling was reduced. The scaling of calcium phosphate also can be significantly affected by pH of the DS. If the draw solution pH is sufficiently low, even containing scaling precursors the scaling process may be improved. There is a competing mechanism between the scaling precursors and anti-scaling precursors. A conceptual model, capturing the relationship between scaling and scaling precursors and anti-scaling precursors in draw solution was presented. The findings in the current study have significant implications on the selection of draw solutes and membrane scaling in FO process. The scaling phenomena in PRO were also systematically studied. PRO is an osmotically-driven membrane process that can be used to harvest salinity-gradient power. The PRO performance (both water flux and power density) can be severely limited by membrane fouling. The current study, for the first time, investigates PRO scaling in a bench-scale pressurized system using gypsum as a model scalant. In addition to the bulk feed solution (FS) saturation index (SIbulk), gypsum scaling was found to be strongly affected by the draw solution (DS) type and concentration, the applied hydraulic pressure, and the membrane orientation. The commonly recommended AL-DS orientation was highly prone to internal scaling. In this orientation, severe ICP of scaling precursors induced gypsum clogging in membrane support layer even when the FS was undersaturated (e.g., SIbulk = 0.8). At higher SIbulk values, external gypsum crystal deposition occurred in addition to internal scaling. More severe scaling was observed when the DS contained scaling precursors such as Ca2+ or SO42-, suggesting that the reverse diffusion of these precursors into the FS can significantly enhance gypsum scaling. Increasing applied hydraulic pressure could enhance reverse solute diffusion and thus result in more severe gypsum scaling when the DS contained scaling precursors. A conceptual model, capturing the two important PRO scaling mechanisms (ICP of scaling precursors from FS and reverse diffusion of scaling precursors from the DS), is presented to rationalize the experimental results. Our results provide significant implications for PRO scaling control.