Reverse Osmosis for Desalination

Reverse Osmosis (RO) is a physical process that uses the osmosis phenomenon, i.e., the osmotic pressure difference between the saltwater and the pure water to remove salts from water (Figure1). In this process, a pressure greater than the osmotic pressure is applied on saltwater (feedwater) to reverse the flow, which results in pure water (freshwater) passing through the synthetic membrane pores separated from the salt. A concentrated salt solution is retained for disposal. The RO process is effective for removing total dissolved solids (TDS) concentrations of up to 45,000 mg/L, which can be applied to desalinate both brackish water and seawater.

Reverse osmosis needs energy to operate the pumps that raise the pressure applied to feedwater.

The amount of pressure required directly relates to the TDS concentration of the feedwater. For brackish water, the pump pressure requirement is between 140 and 400 psi. For seawater, pumps may need to generate up to 1200 psi. Therefore, the TDS concentration of the feedwater has a substantial effect on the energy use and the cost of the product water.

Two common types of membranes used in RO process for desalination include Cellulose Acetate (CA) membranes and Non-CA membranes. Cellulose Acetate membranes were developed in the 1960s and various modified and improved blends of CA membranes are currently used in the desalination process. The CA membrane has a relatively smooth surface that is resistant to fouling. It is theorized that if the membrane surface is rather smooth, the material that may cause fouling cannot deposit in the membrane crevices.

Non-CA membranes, typically called “thin-film composite membranes” include aromatic polyamide (PA) membranes and composite membranes using common organic materials such as polysulfone. These membranes have a higher flux rate (volume of freshwater per membrane surface area) and, compared to CA membranes, allow passage of lower salt concentration. Non-CA membranes are more stable over a broader pH range than the CA membranes, but are susceptible to degradation by chlorine.

Pre-treatment of feedwater is essential in order to protect the RO membrane, reduce energy costs, and increase salt retention. It should be free of large particles, organic matter, bacteria, oil and grease. Typical pre-treatment involves multimedia, cartridge, and sand filtration to remove larger particles, organic matter and other materials; and adding chemicals to prevent the formation of precipitates and scaling of the membrane. Often, pH adjustment is also needed.

Figure 1. Reverse Osmosis vs. Osmosis