Reverse osmosis (RO) systems behave similar to a boiler or a cooling tower in that they cause the dissolved salts in their make-up water to become more concentrated. As pure water permeates the RO membrane, the salts are left behind in a concentrated stream. If the solubility of any particular salt is exceeded, the potential exists that 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:
- Acid injection
- Water softening
- Injection of a scale inhibitor
The specific requirements for the water source can be predicted using U.S. Water's Scale Inhibitor Projection Program (SIPP), which takes into account the particular make-up of the water source, the water temperature and pH, and the salt concentration factor relative to the desired permeate recovery of the RO system. The right method will often depend on the priorities of the application.
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. It may then pose a removal burden for downstream ion exchange or deaeration equipment.
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. This then would require zeolite softening of the RO feed water, which uses a strong-acid cation exchange resin that is regenerated using sodium chloride. 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 (where applicable) is injecting a scale inhibitor. Using similar chemistry to the antiscalants used in boilers and cooling towers, 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 as long as the RO system is operating. When the RO unit shuts down, it is critical that the system be flushed of supersaturated salts using permeate water, or by allowing low pressure feed water to displace the water held up within the system.
Some of the scale inhibition products used in the early years of the RO industry 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.
Products such as U.S. Water's RO 503 and 504 are blend products that contain both polymers and phosphonates can be utilized in the RO systems. 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. The appropriate dosage can be determined using the SIPP program.
With the correct application of the chosen scale prevention method, most RO systems should never have to experience scale formation.
For more information on U.S. Water’s Reverse Osmosis Systems, chemical or service programs, contact us U.S. Water today!