Experimental study on the effect of polysaccharides on incipient membrane scaling during desalination

Experimental results are reported on the effect of polysaccharides in the early stage of membrane scaling. Sodium alginate at small concentration (2 and 10 mg/L) was employed as typical polysaccharide. Synthetic solutions, characterized by small bulk supersaturation ratio S in calcium sulfate (S ~ 1.4), were filtered through a desalination membrane in dead-end cells under agitation. Three types of tests were performed; i.e. membrane scaling in absence of alginates (as reference), combined fouling-scaling (i.e. scaling in presence of alginates) and scaling tests after a fouling layer was formed on the membrane. Examination of various deposits by Scanning Electron Microscopy (SEM) and determination of calcium on membrane deposits and in filtered solids (from retentate and feed-fluid samples) provide useful insights including clear evidence that alginates tend to inhibit CaSO4 crystallization, likely due to calcium binding with carboxylates of alginate macromolecules and formation of gel deposits. The inhibition effect, manifested in the reduced deposit-mass density on membranes and crystallization-rate, as well as in crystal-shape modification, is stronger at the higher alginate concentration. Similarly, incipient membrane scaling, pretreated with alginate foulant solution, appears to be significantly inhibited compared to clean membranes. Under the conditions tested, membrane salt rejection was unaffected.