Читать книгу Introduction to Desalination - Louis Theodore - Страница 18

Electrodialysis was the membrane separation process employed for desalination a century ago. Here, the ions forming the salt are removed from the saltwater by electric forces and concentrated in separate compartments. However, the higher the salinity of the raw water, the more electric power is needed for this process. Hence, this process was applied primarily in the treatment of moderately brackish waters containing 1,000 to 2,000 mg/L of dissolved salts. In recent years, reverse osmosis (RO) (Theodore and Ricci 2010) has displaced electrodialysis as the primary membrane separation desalination process, leaving the latter as the choice for medical kidney applications.

An RO system consists of an intake, a pre-treatment component, a high-pressure pump, a membrane apparatus, remineralization, and pH adjustment components, as well as a disinfection step. Generally, a pressure of about 1.7 to 6.9 MPa is required to overcome the osmotic pressure of saltwater.

This advanced separation technique may be used whenever low molecular weight solutes such as inorganic salts or small organic molecules (e.g. glucose) are to be separated from a solvent (usually water). In normal (as opposed to reverse) osmosis, water flows from a less concentrated salt solution to a more concentrated salt solution as a result of an innate concentration driving force (thermodynamically referred to as the chemical potential). As a result of the migration of water through the membrane, an “osmotic pressure” is created on the side of the membrane to which the water flows. In RO, the membrane is permeable to the solvent or water and relatively impermeable to the solute or salt. In order to make water pass through an RO membrane in the desired direction (i.e. away from a concentrated salt solution), a pressure must be applied that is higher than the osmotic pressure. The membranes used for RO processes are characterized by a high degree of semi-permeability, high water fluxes, good mechanical strength, chemical stability, and relatively low operating and capital costs. Early RO membranes were composed of cellulose acetate, but restrictions on process stream pressure, temperature, and organic solute rejection spurred the development of noncellulosic and composite material membranes (“sandwiches”). These membranes may be configured into a variety of geometries for system operation, including: plate and frame, tubular, spiral wound (composite), and hollow fiber.