Preventing Scale Formation in Reverse Osmosis (RO) Systems

As pure water permeates the RO membrane, the salts are left behind in a concentrated stream. If the solubility of any salt is exceeded, the salt will form scale directly on the membrane surface or possibly within the flow channels through the membrane element.

?This scale formation will occur first in the tail end of the RO system, within the last elements through which the water flows before exiting the system in its concentrate stream.?The scale will cause the RO system to require increased pressure to achieve the same permeate flow rate, and the permeate conductivity may increase in the tail end of the system.?Restoring performance will require cleaning with an acidic solution, although membrane cleaning is only going to be effective with certain types of scale.?It is unlikely to be fully effective with silica scale, or with most of the sulfate salts. If cleaning cannot restore performance, membrane replacement becomes the only option for restoring the lost RO performance.

?For most water sources, preventing scale formation is not a necessarily difficult challenge. It can usually be accomplished by applying one or more of the following three options to the RO feed stream:

?1. Acid injection

2. Water softening

3. Injection of a scale inhibitor

?Acid injection is effective at preventing calcium carbonate scale, but it is only marginally effective at preventing sulfate or silica scale.?A major disadvantage of acid injection for many applications is it results in the formation of carbon dioxide that will readily permeate the RO membrane.?

Lime softening will reduce the water hardness, alkalinity, and dissolved silica to a point that an RO unit might be able to operate at a low permeate recovery with reduced potential for scale formation.?However, increased permeate recovery that takes full advantage of the RO feed pressure and minimizes water waste will require that the water hardness be reduced to a much greater extent. An advantage of this approach is it enables caustic (sodium hydroxide) to be injected upstream of the RO system to raise its feed water pH, which will result in increased removal of alkalinity, dissolved silica, and boron. Operation at elevated pH will also reduce the potential for the formation of silica scale.

?The most cost-effective method for preventing scale formation is injecting a scale inhibitor. The formation of scale crystals can be slowed sufficiently to allow those crystals to exit the RO system in its concentrate stream. In this manner, saturation points may be safely exceeded for calcium carbonate and sulfate salts if the RO system is operating.

Some of the scale inhibition products consisted of a single polymer that tended to lose its solubility when injected into certain water sources, such as ones that contained iron. This would result in heavy fouling of RO pre-filter cartridges, as well as of the membrane elements.

Other products are blended that contain both polymers and phosphonates, they work synergistically to improve their solubilities, while providing superior prevention against scale formation. In fact, they will also assist in keeping iron in suspension to reduce its potential for fouling the RO system.

Threshold inhibition: it is the ability of an antiscalant to keep supersaturated solutions of springily 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: 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.

With the correct application of the chosen scale prevention method, most RO systems should never have to experience scale formation.