Salt evaporation pond

A salt evaporation pond is a shallow artificial salt pan designed to extract salts from sea water or other brines. The salt pans are shallow and expansive, allowing sunlight to penetrate and reach the seawater. Natural salt pans are formed through geologic processes, where evaporating water leaves behind salt deposits. Some salt evaporation ponds are only slightly modified from their natural version, such as the ponds on Great Inagua in the Bahamas, or the ponds in Jasiira, a few kilometres south of Mogadishu, where seawater is trapped and left to evaporate in the sun. During the process of salt winning, seawater or brine is fed into artificially created ponds from which water is drawn out by evaporation, allowing the salt to be subsequently harvested. The ponds also provide a productive resting and feeding ground for many species of waterbirds, which may include endangered species. However, Ghanaian fisheries scientist RoseEmma Mamaa Entsua-Mensah also noted that salt winning can destroy mangrove forests and mudflats, altering the environment and making it unproductive for other development or fish growth. The ponds are commonly separated by levees. Salt evaporation ponds may also be called salterns, salt works or salt pans.

Metrics and energetics

There is an associated loss of available energy when evaporating into dry air. This Gibbs Free Energy becomes positive when the salinity is high enough (or air humid enough) for the salt solution to cause water to condense into it. That is how liquid desiccants work. Evaporation systems are also often evaluated by the water evaporation rate per unit area. When the energy is largely provided by sunlight, these are often evaluated with a solar efficiency, ( η s o l {\displaystyle \eta _{\rm {sol}}} ), which is a thermal efficiency that compares incoming light energy to the enthalpy of vaporization. This is the same as the gained output ratio (GOR) in desalination.

Algae and color

Due to variable algal concentrations, vivid colors (from pale green to bright red) are created in the evaporation ponds. The color indicates the salinity of the ponds. Microorganisms change their hues as the salinity of the pond increases. In low- to mid-salinity ponds, green algae such as Dunaliella salina are predominant, although these algae can also take on an orange hue. Halobacteria, a type of halophilic Archaea (also known as Haloarchaea), are responsible for changing the color of middle to high-salinity ponds to shades of pink, red, and orange. Other bacteria such as Stichococcus also contribute tints.

Examples Notable salt ponds include:

The Salterns of Guérande, in Loire-Atlantique, France. The salt produced in the salterns are a protected geographical indication in Europe. The Cáhuil salt ponds, in the O'Higgins Region, Chile. The Manaure salt ponds, in La Guajira Department, Colombia. The Salineras de Maras, Peru, in the Cusco Region. The saltworks of Alcácer do Sal, Comporta, and Castro Marim in Portugal The El Caracol solar evaporator, on the outskirts of Mexico City, Mexico. The Sečovlje and Strunjan salt ponds on the northern edge of the Adriatic Sea in Slovenia. The San Francisco Bay salt ponds in the United States, formerly operated by Cargill, including Charleston Slough. Cargill has since ended salt production in the area, and most of the ponds are being restored to a more natural state. South Bay Salt Works in San Diego, California. The Dead Sea salt ponds in the West Bank, Israel and Jordan. The salt ponds in Salina, Malta. The name of the village is the Maltese word for salt pan. The Port Hedland, Dampier, Lake McLeod, Useless Loop and Onslow salt ponds in Western Australia. Yellow Walls, Malahide, Ireland; active from 1770 to 1837. Lake Grassmere in New Zealand The ancient salt pans in Marsala and Trapani, Sicily. Salt has been farmed here since the Phoenician period, with archaeological evidence still present in nearby Motya. The Nature reserve of Margherita di Savoia, in Apulia, Italy. Known since antiquity, it's one of the largest in Europe. Flamingos often nest in the area. The salt works on the island of Great Inagua owned by Morton Salt. The salt harvesting by the Tsonga women of Baleni on the Small Letaba River, Limpopo, South Africa. Until World War II, salt was extracted from sea water in a unique way in Egypt near Alexandria. Posts were set out on the salt pans and covered with several feet of sea water. In time the sea water evaporated, leaving the salt behind on the post, where it was easier to harvest.

Production

Salt pans are shallow and open, and metal pans are often used to evaporate brine. They are usually found close to the source of the salt. For example, pans used in the solar evaporation of salt from seawater are usually found on the coast, while those used to extract salt from solution-mined brine will be found near the brine shaft. In this case, extra heat is often provided by lighting fires underneath.

History

Archaeological evidence Archaeological evidence for early salt production is fragmentary. Coastal installations have been lost to rising sea levels, furnaces were often dismantled after use, and the ceramic containers used to hold and heat brine were broken up, so few direct traces survive.

Europe From the Neolithic to the Iron Age, salt making in Europe relied mainly on inland brine sources and on evaporation over a fire, using the ceramic vessels and supports known as briquetage. Methods varied with local conditions, and large coastal evaporation ponds were best suited to warmer and drier regions. Marine salt making in western Europe may go back to the 5th millennium BC, when seasonal lagoons behind low sandy shores could have served as natural basins in which seawater was concentrated before being heated. Organised production is attested archaeologically in west-central France from the 3rd millennium BC, where distinctive ceramic forms indicate installations for heating brine. This does not rule out earlier concentration by solar evaporation in coastal lagoons, such as those near Locmariaquer and Carnac in Morbihan, or at Varna in Bulgaria.

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