Asalt evaporation pond is a shallow artificialsalt pan designed to extractsalts fromsea water or otherbrines. 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 onGreat Inagua inthe Bahamas, or the ponds in Jasiira, a few kilometres south ofMogadishu, where seawater is trapped and left to evaporate in the sun.
During the process ofsalt winning, seawater or brine is fed into artificially created ponds from which water is drawn out byevaporation, allowing the salt to be subsequently harvested.[1]: 517 [2]
The ponds also provide a productive resting and feeding ground for many species ofwaterbirds, which may includeendangered species.[3] However, Ghanaianfisheries scientistRoseEmma Mamaa Entsua-Mensah also noted that salt winning can destroymangrove forests andmudflats, altering the environment and making it unproductive for other development or fish growth.[4] The ponds are commonly separated bylevees. Salt evaporation ponds may also be calledsalterns,salt works orsalt pans.
Minimum least work of evaporation from saline water to atmosphere, as a function ofspecific humidity (ω), temperature (T), and salinity (C, mass fraction salt)[5].
There is an associated loss of available energy when evaporating into dry air. ThisGibbs 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.[5]
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, (), which is athermal efficiency that compares incoming light energy to the enthalpy of vaporization. This is the same as the gained output ratio (GOR) in desalination.[5]
Due to variablealgal 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 thesalinity of the pond increases. In low- to mid-salinity ponds, green algae such asDunaliella salina are predominant, although these algae can also take on an orange hue.Halobacteria, a type of halophilicArchaea (also known asHaloarchaea), are responsible for changing the color of middle to high-salinity ponds to shades of pink, red, and orange. Otherbacteria such asStichococcus also contributetints.[citation needed]
The ancient salt pans inMarsala andTrapani, Sicily. Salt has been farmed here since thePhoenician period, with archaeological evidence still present in nearbyMotya.
UntilWorld War II, salt was extracted from sea water in a unique way in Egypt nearAlexandria.[12] 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.
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.
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