Membrane Technologies for Desalination

Membrane technologies are current and future technologies applied to the desalination of brackish and seawater to produce freshwater for supplementing drinking water supplies.

There are three basic categories of water purification technologies that are used for desalination: membrane technologies, distillation processes (thermal technologies), and chemical approaches. Some water purification plants use a combination of these technologies. Membrane technologies are the most common technology of desalination in the United States, while thermal technologies are not widely used in the United States.

A membrane is a thin film of porous material that allows water molecules to pass through it, but simultaneously prevents the passage of larger and undesirable molecules such as viruses, bacteria, metals, and salts. Membranes are made from a wide variety of materials such as polymeric materials that include cellulose, acetate, and nylon, and nonpolymeric materials such as ceramics, metals and composites. Synthetic membranes are the most widely used membranes in the desalination process and their use is growing at a rate of 5-10% annually.

In general, membrane treatment processes use either pressure-driven or electrical-driven technologies. Pressure-driven membrane technologies include reverse osmosis (RO), nanofiltration (NF), ultrafiltration, and microfiltration.
Reverse osmosis, and to some extent nanofiltration processes, are considered effective in salt removal. Electrical-driven membrane technologies that are effective with salt removal include electrodialysis (ED) and electrodialysis reversal (EDR). In 2003, the U.S. EPA issued the Membrane Filtration Guidance Manual. Membrane configuration refers to the arrangement of individual elements (cartridges) in a membrane treatment process.

Membrane technologies applicable to desalination are briefly described below.