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R.O water treatment system

Abstract

Scaling means the deposition of particles on a membrane, causing it to plug. Without some means of scale inhibition, reverse osmosis (RO) membranes and flow passages within membrane elements will scale due to precipitation of sparingly soluble gas, such as calcium carbonate, calcium sulfate, barium sulfate and strontium sulfate. Most natural waters contain relatively high concentrations of calcium, sulfate and bicarbonate ions.

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Scaling means the deposition of particles on a membrane, causing it to plug. Without some means of scale inhibition, reverse osmosis (RO) membranes and flow passages within membrane elements will scale due to precipitation of sparingly soluble gas, such as calcium carbonate, calcium sulfate, barium sulfate and strontium sulfate. Most natural waters contain relatively high concentrations of calcium, sulfate and bicarbonate ions.

In membrane desalination operations at high recovery ratios, the solubility limits of gypsum and calcite exceed saturation levels leading to crystallization on membrane surfaces. The surface blockage of the scale results in permeate flux decline, reducing the efficiency of the process and increasing of operation costs. 
The effects of scale on the permeation rate of RO systems is illustrated in the following figure. Following an induction period, plant flow decrease rapidly. The length of this period varies with the type of scale and the degree of super saturation of the sparingly soluble salt.

As it is evident from the graph, the induction period for calcium carbonate is much shorter than that for sulfate scales, such as calcium sulfate. It is economically preferable to prevent scaling formation, even if there are effective cleaners for scale. Scale often plugs RO element feed passages, making cleaning difficult and very time consuming. There is also the risk that scaling will damage membrane surface.

There are three methods of scale control commonly employed:

  • acidification
  • ion exchange softening
  • antiscalant addiction.

Acidification: acid addiction destroys carbonate ions, removing one of the reactants necessary for calcium carbonate precipitation. This is very effective in preventing the precipitation of calcium carbonate, but ineffective in preventing other types of scale. Additional disadvantages include the corrosivity of the acid, the cost of tanks and monitoring equipment and the fact that acid lowers the pH of the RO permeate.

Ion exchange softening: this method utilizes the sodium which is exchanged for magnesium and calcium ions that are concentrated in the RO feed water, following the chemical equations:

Ca2+ + 2NaZ => 2Na+ + CaZ2
Mg2+ + 2NaZ => 2Na+ + MgZ2

(NaZ represents the sodium exchange resin).
When all the sodium ions have been replaces by calcium and 
magnesium, the resin must be regenerated with a brine solution. Ion exchange softening eliminates the need for continuous feed of either acid or antiscalant.

Antiscalants: they are surface active materials that interfere with precipitation reactions in three primary ways:

  • Threshold inhibition: it is the ability of an antisclant to keep supersaturated solutions of springly soluble salts.
  • Crystal modification: it is the property of an antiscalants to distort crystal shapes, resulting in soft non adherent scale. As a crystal begin to form at the submicroscopic level, negative groups located on the antiscalant molecule attack the positive charges on scale nuclei interrupting the electronic balance necessary to propagate the crystal growth. When treated with crystal modifiers, scale crystals appear distorted, generally more oval in shape, and less compact.
  • Dispersion: dispersancy is the ability of some antiscalants to adsorb on crystals or colloidal particles and impart a high anionic charge, which tends to keep the crystals separated. The high anionic charge also separates particles from fixed anionic charges present on the membrane surface.

During the past two decades new generations of antiscalants have emerged commercially, in which the active ingredients are mostly proprietary mixtures of various molecular weight polycarboxylates and polyacrylates.

Calculation procedures exist for predicting the likelihood of scale formation. Use of these predictors depends upon an up-to-date water analysis and a knowledge of system design parameters. The ions contained in the feed water concentrate though the RO system, the point of maximum scale potential is the concentrate stream. Antiscalant type and dosage is therefore based upon the mineral analysis at this point. 
It is important to find the optimization of antiscalant treatment with respect to type and dosage, identifying the proper antiscalant to use and the dosage-induction type relationship for the extended level of super saturation.

Parsian Farab is a manufacturer and supplier of broad spectrum antiscaling agents to prevent fouling of salts and disperse salts in suspension. For more information, please contact us.

Dosage Rate Description Product
generally, 2-5 ppm Antiscalant Hypotreat 440
 generally, 2-5 ppm Antiscalant Hypotreat 450
generally, 2-5 ppm Antiscalant Hypotreat 460
generally, 2-5 ppm Antiscalant Hypotreat 470

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