Bittern (pl. bitterns), or nigari, is the salt solution formed when halite (table salt) precipitates from seawater or brines. Bitterns contain magnesium, calcium, and potassium ions as well as chloride, sulfate, iodide, and other ions.
Bittern is commonly formed in salt ponds where the evaporation of water prompts the precipitation of halite. These salt ponds can be part of a salt-producing industrial facility, or they can be used as a waste storage location for brines produced in desalination processes.
Bittern is a source of many useful salts. It is used as a natural source of Mg, and it can be used as a coagulant both in the production of tofu and in the treatment of industrial wastewater.