(305d) Kinetics Of Ro Membrane Scaling In The Presence Of Antiscalants

Most inland brackish water sources contain precursor ions of sparingly soluble mineral salts (e.g. calcium, sulfate, bicarbonate, etc.) that can lead to membrane scaling in RO desalting operations and thus impair plant productivity (i.e. permeate flux), product water quality, and membrane operational life time. Currently, membrane scaling is typically mitigated using commercial polymeric formulations, commonly known as antiscalants, to retard the onset and growth of mineral salt crystals in the bulk and onto the membrane surface [1,2]. The use of antiscalants can add up to 15% of the total water production cost and thus optimization of antiscalant use can be beneficial to lowering the cost of RO desalting. Current understanding of antiscalants action is primarily inferred based on permeate flux decline studies and studies of mineral salts precipitation in bulk solutions [3]. Direct measurements of the kinetics of mineral salt scaling on RO membranes are needed in order to provide data needed for the development of predictive scale formation models, optimize RO operating conditions and antiscalant choice and dosage. While a number of studies have been recently published on direct observation of scale formation, there is lack of much needed information on the direct influence of antiscalants on mineral scale crystallization on membrane surfaces.

In the present study, the influence of antiscalants on retarding the onset and growth of mineral salt crystals on RO membranes was quantified and elucidated using a novel approach for direct (during high pressure RO process) optical imaging of surface mineral scale growth [4]. RO membrane scaling tests were conducted, in a novel transparent RO cell, using model solutions representative of field samples of brackish water from typical locations in California. The study focused on gypsum scale which is the most problematic scalant in inland water desalination in most areas in the U.S. A number of commercial antiscalants were first evaluated with respect to their effectiveness in suppressing gypsum bulk crystallization. The antiscalants were then tested in membrane RO experiments, at varying doses, to directly measure their impact on the time-evolution of crystal site density and the growth rate of individual surface crystals. An approach to quantify antiscalant effectiveness was employed based on determination of kinetic growth parameters using a simple surface scale formation model.

The results of the study have shown, for the first time, that different commercial membranes differ in their response to antiscalant scale suppression treatment,. Specifically, it was found that surface nucleation was affected by both the specific antiscalant and membrane used. In general, the ranking of antiscalant effectiveness based on bulk crystallization induction time correlated with membrane surface scale coverage and flux decline measurements in membrane RO studies. Membrane surface roughness and hydrophobicity (characterized by contact angle measurements) were shown correlate to some degree with scaling propensity of the membranes and with antiscalant effectiveness in suppressing scale. The present approach to evaluating antiscalant effectiveness provides a rationale approach to antiscalant and membrane selection for brackish water desalting where propensity for mineral scaling can be an impediment for successful process implementation.

[1] Rahardianto, A., W. Shih, R. Lee, Y. Cohen. Diagnostic characterization of gypsum scale formation and control in RO membrane desalination of brackish water. J. Mem. Sci. 279 (2006) 655.

[2] Wen-Yi S., A. Rahardianto, R. Lee, Y. Cohen. Morphometric characterization of calcium sulfate dihydrate (gypsum) scale on reverse osmosis membranes. J. Mem. Sci. 252 (2005) 253.

[3] Amjad, Z. Kinetics of crystal growth of calcium sulfate dihydrate. The influence of polymer composition, molecular weight, and solution pH. Can. J. of Chem. 66 (1988) 1529.

[4] Uchymiak, M., A. Rahardianto, E. Lyster, J. Glater, Y. Cohen. A novel RO ex situ scale observation detector (EXSOD) for mineral scale characterization and early detection. J. Mem. Sci. 291 (2007) 86.