Desalination is a process that removes salt from seawater so it can be used as drinking water. Some coastal cities consider desalination when rivers and reservoirs cannot meet demand. Many plants use reverse osmosis, pushing water through a membrane that blocks salt. This can create fresh water, but it also uses a lot of energy. The passage explains that desalination produces a salty waste called brine. If brine is released without care, it can harm ocean life near the outlet, so plants must dilute or manage it. Across the paragraphs, the central idea is refined: desalination can increase water supply, but its energy use and brine waste mean it must be planned responsibly.