Incipient membrane scaling in the presence of polysaccharides during reverse osmosis desalination in spacer-filled channels

This study aims to shed light into the complicated phenomena occurring during the early stages of RO-membrane scaling when organic-foulants are present. Experiments were performed, using spacer-filled channel in once-through flow-mode, closely simulating conditions prevailing at tail elements of pressure-vessels. The effect was studied of a typical foulant (polysaccharide, sodium-alginate, SA), at small concentrations (~2 mg/L and ~10 mg/L), on incipient crystallization/deposition of calcium sulfate (CaSO₄·2H₂O, gypsum) during RO-desalination of synthetic brackish-water at small super-saturation ratio S (1.0 to 1.5). Scanning Electron Microscopy and membrane-scale/foulants dissolution techniques were employed. At small SA concentration (~2 mg/L) incipient-scaling inhibition is observed (likely due to crystallite-growth retardation by adsorbed organic molecules); however, at higher SA concentration (~10 mg/L), significant enhancement of incipient scaling occurs, attributed to formation of coherent organic-gel fouling-layer, favoring concentration increase of inorganic-scalants at the membrane-surface and consequent increase of supersaturation. Under such conditions, although the fouling-layer thickness remains roughly constant (due to constant deposited organic-matter), the specific fouling resistance α tends to significantly increase with increasing supersaturation, likely due to physico-chemical structural changes within the alginate gel-layer. The techniques employed allow determination and correlation of key fouling/scaling parameter-values (i.e. scale-mass deposition-rate and resistance α), that are necessary for developing reliable process modeling/simulation tools.