A membrane desalination process uses a physical barrier - the membrane -  and a driving force. The driving force can be an electrical potential, which is used in electrodialysis or electrodialysis reversal, or a pressure gradient, which is used in reverse osmosis. Membrane technologies often require that the water undergo chemical and physical pretreatment to limit blockage by debris and scale formation on the membrane surfaces. The three major membrane processes are electrodialysis, electrodialysis reversal, and reverse osmosis.

• Electrodialysis and electrodialysis reversal: The membranes used in electrodialysis and electrodialysis reversal are built to allow passage of either positively or negatively charged ions, but not both. Ions are atoms or molecules that have a net positive or net negative charge. Four common ionic molecules in saline water are sodium, chloride, calcium, and carbonate. Electrodialysis and electrodialysis reversal use the driving force of an electrical potential to attract and move different cations (positively charged ions) or anions (negatively charged ions) through a permeable membrane, producing fresh water on the other side.
  The cations are attracted to the negative electrode, and the anions are attracted to the positive electrode.When the membranes are placed so thatsome allow only cations to pass and others allow only anions to pass, the process can effectively remove the constituents from the feed water that make it a saline solution.
  The electrodialysis reversal process functions as does the electrodialysis process; the only difference is that in the reverse process, the polarity, or charge, of the electrodes is switched periodically. This reversal in flow of ions helps remove scaling and other debris from the membranes, which extends the system’s operating life.
• Reverse osmosis (RO) uses a pressure gradient as the driving force to move high-pressure saline feed water through a membrane that prevents the salt ions from passing. There are several membrane treatment processes, including reverse osmosis, nanofiltration, ultrafiltration, and microfiltration. The pore sizes of the membranes differ according to the type of
  process. Because the RO membrane has such small pores, the feed water must be pretreated adequately before being passed through it. The water can be pretreated chemically, to prevent biological growth and scaling, and physically, to remove any suspended solids.

(Mechell & Lesikar 2010)