RO Membrane Scaling Explained
What is RO Scaling?
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 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 scales are:
- Calcium carbonate (CaCO)
- Calcium sulfate (CaSO)
- Barium sulfate (BaSO 4 )
- Strontium sulfate (SrSO)
- Silica (SiO)
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.
Scaling in Daily Life
Recall a time that you boiled a pan of water to dryness. A white material formed on the walls and base of the pan. It was not removed with detergent alone. It either took a chemical, like vinegar or an abrasive cleaning compound, like “Ajax”, “Comet”, etc. to remove the scale. This whitish scale is usually mostly calcium carbonate with calcium sulfate being the next most prevalent component. All the other salts that were in the water made up the rest of the scale.
A scale, then, is not a bunch of particles, like dirt, which we can wash off with flow rate and/or detergent. It is a crystalline material which must be redissolved. It will take a certain set of conditions to get this material, which has come OUT of a solution, to go back INTO a solution.
The following are the common symptoms of scaling:
- Decreased salt rejection, especially in the last stage
- Increased pressure drop, especially in the last stage
- Decreased NPF (Normalized Permeate Flow)
RO Scaling Case Study
Complete Water Solutions was called out to inspect our client’s 10-12 Gallons Per Minute RO System. It was found that the Industrial RO System had several problems:
- The RO System pressure differential on the pre-filter was very high
- The permeate flow was low
- The feed pressure was very high
Complete Water Solutions was able to determine that the prefilter needed to be changed more frequently. We were also able to find that the Antiscalant was not dosed properly. The overfeeding of antiscalant caused massive scale in the RO System which caused the limited flow, and high pressures. Complete Water Solutions was able to clean the membranes that were not damaged and was successful in returning the Industrial RO system back to normal operations.