But technological advances in recent years have altered the equation. The most common form of desalination is reverse osmosis; it involves forcing water through cartridges that contain thin-film composite polyamide membranes, which trap salt and other impurities but allow the fresh water through.Some environmentalists have long opposed desalination because of the energy the process demands, as well as other considerations such as the impact of sucking in large quantities of seawater from the ocean.