Scaling and Antiscalants

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.

Threshold Mechanism

Dispersancy

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.

Lenntech can help you in the selection of the best antiscalant for your particular application.

Economical analysis

Acid addition is not very cost effective because of the cost of acid, tanks and monitoring equipment. Unless removed by degasification, excess of carbon dioxide contained in the permeate of acid-fed systems increases the cost of ion exchange regeneration.
Antiscalants are relatively cheap products and have no additional costs.
When compared to either acid or antiscalant addition, the main disadvantage to softening is cost factoring in equipment costs. Through a present worth analysis there is no level of hardness in which softening competes economically with antiscalants addition.
The following table gives a cost comparison between softening and antiscalant treatment options for different levels of hardness, on a basis of a RO system designed to produce 75 gpm (17 m³/h) of permeate at 75% recovery.

Contact us for more information about antiscalants.