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Genesys application guide GENESYS INTERNATIONAL To ?nd out more and download our published papers visit our website at www genesysro com Application Guide Application Guide BWRO Antiscalants Scaling During the Reverse Osmosis process the solubility of sca

GENESYS INTERNATIONAL To ?nd out more and download our published papers visit our website at www genesysro com Application Guide Application Guide BWRO Antiscalants Scaling During the Reverse Osmosis process the solubility of scaling species may become ??supersaturated ? eventually leading to deposition on the membranes This process can be divided in main stages Stage ion clustering Stage nucleation Stage crystal formation Stage crystal growth IONS PROTONUCLEI NUCLEI CLUSTERING ORDERING CRYSTALS STAGE STAGE STAGE STAGE Insoluble deposits start forming on the membrane surface causing ux and salt rejection to decrease and resulting in higher long-term operational costs through increased pumping pressure cleaning frequency and membrane replacement The most commonly detected scales are calcium carbonate and calcium sulphate silica and other less common scales can also be problematic Some membrane deposits can be di ?cult to remove and irreversible membrane damagemay result Preventing salt precipitation is therefore paramount Scale Prevention Various di ?erent techniques are available which have been traditionally used to control scaling potential in RO feed waters They include the removal of calcium and magnesium ions from water by base exchange resin softening This method requires capital equipment and salt to regenerate the resin Acid injection lowers the scaling potential of calcium carbonate and calcium phosphate by reducing the feed water pH It has no e ?ect on calcium sulphate and a detrimental e ?ect on silica solubility A signi ?cant drawback of acid dosing is the handling problems and quantity required to have a small reduction of pH and e ?ect on scale inhibition Antiscalants o ?er some advantages compared to other methods for example capital and operational costs are lower than water softening and they can be designed to control speci ?c problematic species However not all antiscalants are equal di ?erent molecules mean varying modes of action and uneven levels of e ?cacy Application Guide BWRO Antiscalants Phosphonates o ?er the best combination of scale inhibiting e ?ciency metal ion sequestering and stability allowing reverse osmosis plants to operate at higher recovery rates thus reducing pumping costs General Purpose Antiscalants Antiscalants can disrupt the scaling process through a variety of mechanisms Threshold inhibition the antiscalant acts at stage of the crystal formation process inhibiting the ordering of protonuclei and thus preventing nucleation Active compounds polyphosphates polymetaphosphates phosphonates Crystal distorsion the antiscalant acts at stage disrupting crystal formation and thus preventing crystallization Active compounds organic polymers mainly polyacrylic polymaleic or polycarboxylic acid Dispersion dispersants use electrical repulsion to maintain crystals in suspension preventing them from adhering to the membrane surface and to each other Complexation ability to attract ions to create soluble and stable molecules Active compounds chelating agents such as EDTA The majority of modern antiscalant formulations are based on polymers or phosphonates Polymer-based antiscalants prevent scale formation mainly through crystal distortion and also have mild dispersant and threshold properties Although they are relatively cheap Polymers have B B technical limitations when compared to phosphonate chemistry they do B B not sequester metal ions polyacrylates can