Click the image above to view a scaling case study!

Click the image above to view a scaling case study!

RO Scales are formed when dissolved substances in the feed water reach their maximum solubility limit somewhere in the RO unit. When further concentrated, they precipitate out of the solution and onto a surface. The surface may be a suspended particle in the feed/concentrate stream, the surface of the membrane, the feed water spacer, or any other surface.

The most common RO scales are:

• Calcium Carbonate (CaCO₃)

• Calcium Sulfate (CaSO₄)

• Barium Sulfate (BaSO₄)

• Strontium Sulfate (SrSO₄)

• Silica (SiO₂)

Other materials can cause scaling, such as metal hydroxides, calcium fluoride, and magnesium salts, but the above five are the most common. If these substances reach their maximum solubility in an RO unit, they precipitate onto a surface as a relatively hard, crystalline layer.

The following are the common symptoms of scaling:

1. Decreased salt rejection, especially last stage
2. Increased pressure drop, especially in the last stage
3. Decreased NPF (Normalized Permeate Flow)

To clean the scale and remove it, the crystalline precipitate needs to be redissolved. The following cleaning features are needed to facillitate the redissolving:

1. Low pH – Treating the scale with an acid will help to dissolve scale build-up. Common strong acids are sulfuric acid, hydrochloric acid, and nitric acid. *Note – Silica scales should be removed with a higher pH and need not apply.
2. Warm temperature – Chemical reactions occur quicker at higher temperatures. Removing scales will be better at 80°-90°F than 50°-60° F
3. Time – Chemical reactions are dependent on concentrations and time. The amount of cleaning chemical and the cleaning time are important for success.