17.4. Reverse Osmosis (RO)

Reverse osmosis (RO) is a process commonly used to purify and desalinate water (Reisch, 2007). RO is also an integral part of industrial water management plans to reach zero-liquid discharge goals (Kucera, 2010). The liquid water is forced under pressure through a non-porous membrane in the opposite direction to osmosis (osmosis is defined shortly). Most salts and uncharged molecules are retained by the membrane. Thus, permeate is much purer water and retentate becomes significantly more concentrated. The commonly used membranes shown in Figure 17-3 are: 1) a blend of cellulose acetate and cellulose triacetate, 2) aromatic polyamides (aramids), and 3) cross-linked aromatic polyamides (Eykamp, 1997). Currently, the best membranes are thin-film interfacial composite membranes (Baker, 2004). Both hollow-fiber and spiral-wound modules are used for RO (Figure 17-1), but about 85% of applications use spiral-wound membranes (Baker, 2004). Spiral-wound membranes are used because techniques for making hollow-fiber composite membranes have not been developed. In addition, spiral-wound membranes do not clog as easily as hollow fibers; thus, less pretreatment is needed for spiral-wound membranes.

The feed to an RO system usually requires pretreatment to remove any particulates that would clog the membrane. If there are ions or solutes in solution that have limited solubility, the design must include a solubility calculation to determine if they will precipitate onto the membrane when retentate is concentrated. If precipitation is likely, these ions or solutes must either be removed or made more soluble to prevent them from precipitating. A simple schematic of a simple RO system including the most important auxiliary equipment is shown in Figure 17-8. In practice, large-scale systems may have hundreds of membrane modules arranged both in series and in parallel (Figure 17-2C) in what is often called a “Christmas-tree” pattern (Baker, 2004). More details on equipment are available in Baker et al. (1990), Eykamp (1997), Ho and Sirkar (1992), and Noble and Stern (1995). Kucera (2010) discusses details of operation and maintenance of RO systems.