Morton Bahamas’ Inagua solar salt field:
Contribution to environmental sustainability

Great Inagua is the third largest and southern-most island in the Bahamas with 380,800 acres of land. The island’s location and geography mean that it is ideally suited to solar salt production – the process of transforming sea water to commercial salt.

 

Great Inagua is relatively flat and arid and enjoys constant easterly trade winds year round. It is located in the major shipping lanes for cargo transport between the Americas. The Morton Bahamas solar salt field is located at Man O’ War Bay on the northwest sector of Great Inagua Island. The entire salt field uses 30,000 acres which is approximately 8 per cent of the island.

 

Through careful management, the salt field has been able to support and protect naturally occurring plants and animals and has assisted in the recovery of the population of the protected Caribbean Flamingo.

 

Location of Great Inagua Island

How is salt harvested at Inagua?

Sea water, the world’s largest natural source of brine (which consists of water and dissolved salts) is pumped from the ocean into a system of reservoirs. Sun and wind evaporate the water, concentrating the brine.

The majority of the dissolved salt is sodium chloride but the brine also contains other forms of salt including magnesium chloride, magnesium sulphate, calcium chloride, calcium sulphate, potassium chloride, magnesium bromide and calcium carbonate (Baseggio 1973). The brine gravity flows or is pumped through successive reservoirs until it is fully concentrated. Concentrated brine is moved into crystallizing ponds where a salt cake begins to form on the floor of the pond. Salt is collected in these crystallizing ponds until the cake reaches a depth of three to ten inches. Depending on the season and weather conditions this can take six to twelve months to occur on Great Inagua.

Once the desired depth of salt cake has been achieved, the brine is drained from the ponds and solar salt is harvested, washed, screened and stockpiled.

Environmental benefits

Solar salt manufacturing takes only a fraction of the man-made energy required for solution mining. Heat from the sun is used to evaporate 43.7kg of water for each 1 kg of salt crystallized. Solar salt production is one of the most efficient uses of solar energy, next to agriculture and forestry (Sedivy 2008).

The Morton Bahamas solar salt field on Great Inagua provides an integrated ecosystem that supports wildlife on the island. The reservoir system supports the growth of an benthic cyanobacterial (algal) mat which in turn provides a home and food for a number of species of birds, fish and crustaceans, including the protected Caribbean Flamingo (Phoenicopterus ruber), the national bird of the Bahamas.

The algal mat that develops prevents the seepage of brine from the ponds and the contamination of ground water sources.

The system relies on the continuous flow of fresh sea water into the initial reservoirs. Nutrients and biological material in the sea water get consumed in the reservoirs by a succession of species (Davis 2003). As the brine becomes more concentrated the species that can live in it change. Two examples of organisms that thrive are Dunaliella Salina, a type of green micro-algae, and the Artemia Salina, also known as brine shrimp. Brine shrimp are the primary food source for many species of birds including the Caribbean Flamingo.

Prior to the development of the salt field, the ecosystem fluctuated during droughts or when brine flow was disrupted. The salt field stabilised the ecosystem by providing continuous flow of brine which contributed to bringing the population of Caribbean Flamingo back from near extinction.

According to researchers Korovessis and Lekkas (2009), working in synergy with the ecosystem provides three direct benefits to the salt field as outlined below.

  1. Algae mats/organic matter reduces the permeability of the ponds reducing loss of brine through seepage;
  2. Halobacteria (Archaea) colours the brine red which maximises the solar absorption and increases the evaporation rate; and
  3. Brine shrimp balance the conditions in the evaporation ponds and crystallizers by controlling algal bloom for optimum production of high quality salt (excludes highly dissolved organic materials to prevent early precipitation of gypsum).

Another environmental benefit of the salt field is that the reservoir system also helps keep the land cool. When water evaporates, the process removes heat from the surroundings. Ponds provide evaporative cooling especially where temperatures are high and humidity is low. This effect helps animals in extreme arid environments stay in their optimal temperature zones (Norris and Kunz 2012).

Salt pans of Inagua

The Caribbean Flamingo

The Caribbean Flamingo population has risen from a low of 5,000 to current numbers of 50,000 nesting birds. The Bahamas National Trust (BNT) attributes the Morton Bahamas solar salt field on Great Inagua with being “largely responsible for the continued existence of this population in the Bahamas” (BNT 2015). The BNT’s paper explains that the salt field is beneficial to the flamingo population for a number of reasons including:

  • it enhances the production of naturally existing food sources,
  • the ponds are regularly stocked by Morton with brine shrimp that provide flamingos with a source of high quality protein,
  • the water level in the ponds are constant and are not subject to ephemeral cycles, and
  • employment of local people assists in limiting hunting of flamingos as a source of food (many of the historic flamingo populations in the Bahamas disappeared following emancipation when many local people were forced to exist on a subsistence diet). The stable and accessible population has led to a Bird Tour Guiding Initiative on Inagua further promoting conservation of this and other species.

The Morton Bahamas salt field shares its climatological data with flamingo researchers and provides scientists access to its ponds and reservoirs. The BNT goes on to point out that:

“The Morton operation is large, and abuts the western boundary of the Great Inagua National Park, where the birds nest. The location of the Morton property helps to limit easy access to the park and to nesting flamingos. In addition, the logistic support extended to the Bahamas National Trust, the local wardens, and visiting scientists by Morton is invaluable. The leadership role of Morton in the community helps BNT leverage conservation initiatives with donors and donor organisations and helps make sure that the day to day work of the wardens gets done.”

Carribean flamingos on Inagua salt field

Continued focus on improved sustainability

The by-product of salt production is referred to as bitterns. Bitterns is the solution that remains after evaporation and crystallization of salt from seawater. It contains in concentrated form the calcium and magnesium chlorides and sulfates, bromides, iodides, and other chemicals originally present in the brine.

Morton Bahamas has focused on improving its impact on the surrounding environment by reducing the quantity and concentration of bitterns discharged into the ocean.

Rather than discharging bitterns back to the sea after only one cycle through the salt field, the bitterns are now put through another round of salt production in order to extract as much of remaining salt content as is possible.

This recycling step resulted in a greater than 40% reduction in the amount of bitterns returned to the sea from the salt field in 2014 compared to 2013. This occurred despite an overall increase in brine use and salt production at the facility in 2014.

As part of the process of improving sustainability, Morton Bahamas is planning to keep the brine in the system longer. This will increase the amount of salt reclaimed from the water (potentially hundreds of thousands of extra tons per year) which can be used to create a commercially viable de-icing/ice-melt blend product along with higher value products.

The relatively low grade de-icing product made from the recycled bitterns will not need to be washed which will mean no additional demand for brine and pumping for washing the product – saving on brine and energy.

As part of its continuous improvement program the Morton Bahamas solar salt field is currently preparing to implement an environmental management system in accordance with the International Standard for such systems (ISO 14001). Purpose designed software will be used to manage all environmental aspects of the operation including audits, procedures, impact prevention and environmental training courses.

References

  • The Bahamas National Trust (BNT) (2015) Salt Industry and Flamingos on Great Inagua, The Bahamas National Trust Published Online: http://www.bahamas.com/vendor/morton-salt-factory
  • Baseggio G.(1973) The Composition of Sea Water and its Concentrates, Morton Salt Co. Wood Stock IL, Fourth Symposium on Salt, Vol 2, pg. 351 – 358
  • Davis, J.S.(2003) Importance of Microorganisms in Solar Salt Production, University of Florida, Gainesville, Florida, Fourth Symposium on Salt, Vol 2, pg. 369-372
  • Korovessis N. A. and Lekkas, T.D. (2009) Solar Saltworks Production Process Evolution – Wetland Function, Global NEST Journal, Vol 11, No 1, pp 49-57, Published Online http://conferences.gnest.org/Conferences/Saltworks_post/011-030.pdf
  • Norris. Amy L. and Kunz. Thomas H. (2012). Effects of Solar Radiation on Animal Thermoregulation, Solar Radiation, Prof. Elisha B. Babatunde (Ed.), Published Online: http://www.intechopen.com/books/solar-radiation/effects-of-solar-radiation-on-animal-thermoregulation
  • Sedivy, V.M.,2009, Environmental Balance of Salt Production Speaks in Favour of Solar Saltworks, Global Nest Journal, Vol 11, No1 ,pp 41-48 , 2009

This case study was prepared by EnviroWorks, Perth, Western Australia, www.enviroworks.com.au

 

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