Home Forums Water Management Sea water desalination with neutral CO2

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    Francois Fayolle

    Research Project
    Sea Water Desalination using CO2

    Description / Background
    Although Mauritius has a resourceful aquifer and abundant fresh underground water, some experts aver that the island is inexorably moving into a water-scarce zone due to continued population growth, the growing number of hotels and high class residences under the RES and IRS schemes, not to mention the water management crisis at national level.
    According to recent studies, demand for water is increasing at the rate of 2.5% per year and supply is unable to keep pace. Accordingly, water availability could become a serious limiting factor to our socio-economic development.

    Local domestic water consumption is estimated to range between 180 to 200 liters per day per person, while in the tourist sector, the consumption can be considerably higher. For example, in 5 star resorts incorporating golf courses, the latter can exceed 1000 liters per day per person. Rightly enough, it is government’s policy to encourage high class tourism, which unfortunately entails higher consumption patterns.
    Some hotels on the coast have found it necessary to invest in costly desalination plants so as to generate part or whole of their potable water from sea water. In all cases, the technology used is the reversed osmosis (RO), an effective but rather expensive process, consuming much energy, and producing desalinated water at a cost of over four times the cost of potable water obtainable from the CWA (a cost per cubic meter of >Rs 20 compared to Rs 4.50 from the CWA). The process also generates an important quantity of concentrated brine which has the potential of endangering life species in the lagoon if not properly diluted before being rejected into the sea.

    According to a recent report from the World Wildlife Fund (WWF), RO desalination plants could have a negative impact on the environment, mainly because of the important amount of energy required for the process. This is why, WWF officials encourage research aimed at finding new ways of getting fresh water supplies having less environmental endangering impacts.

    In that respect, the Research and Development committee of Gaz Carbonique Ltee has set the objective to develop a low cost sea-water desalination process, using little energy, and which involves the use of neutral Carbon Dioxide (CO2) which otherwise would have been rejected into the atmosphere. This constitutes a net removal of carbon dioxide from the atmosphere, and it is in line with the country’s commitment towards the 2015 Paris agreement, to reduce CO2 emissions.

    About Gaz Carbonique Ltee
    Gaz Carbonique Ltee is a gas manufacturing and distribution company incorporated in Mauritius since 1959. It produces food grade CO2 and some CO2 derivatives such as liquid CO2 and dry ice, medical and industrial oxygen, acetylene, nitrogen, liquid nitrogen, and also imports such gases as helium , hydrogen, nitrous oxide, argon mixes etc.
    Since 2015, Gaz Carbonique Ltee has invested heavily into a factory to collect neutral CO2 emanating from the fermentation of sugar molasses at Omnicane Ethanol Production Ltd, and to process the said CO2 gas into green labelled food grade CO2 for use in the beverage and some other industries. This project has obtained the status of Clean Development Mechanism (CDM) from the United Nations Framework for Climate Change (UNFCC)

    The CO2 produced is said to be green labelled because, being generated from biomass sugar cane which had absorbed the CO2 from the atmosphere through photosynthesis while growing, its rejection back to the atmosphere constitutes no additionality.

    Potential Solutions / working principle
    The present research proposal relates to further develop and fine tune a low cost sea-water desalination process developed by Gaz Carbonique Ltee, for use in hotels situated on the coastal regions around the island. This innovative process, which uses little energy, provides the additional benefit of consuming green carbon dioxide (CO2), which would have otherwise been rejected to the atmosphere.

    Green CO2 of food grade quality, is fed into a reactor of proprietary design, wherein, sea water, after being mixed with ammonia liquor to produce NH4OH, (added as a catalyst to weaken the bond of the salt molecules), is made to interact with the NaCl within the reactor, through a special process requiring very little energy. Two heavy solids (Na2CO3 and NH4Cl) are precipitated and these settle in a clarifier below and are removed by a mechanical system. Desalinated sea water, over flows from the clarifier, and is available for uses such as cleaning, washing, gardening, laundering, toilets flushing, golf courses watering etc.

    A small reverse osmosis desalination plant can be used to further purify the required portion of the desalinated water into potable water which will be needed by the kitchens, showers and baths etc. Since the energy requirement for a RO system is proportional to the osmotic pressure, very little energy will be used for this small scale potable water conversion stage, from an already desalinated water. Additionally, the lifetime of the membranes will be greatly extended, contributing to lowering operational cost for the small RO process.

    By-products and risks to environment
    The by-product will be a mixture of Na2CO3 and NH4Cl. The research will need to establish if it would be an economically viable option, to collect the latter on a regular basis and further process it in order to separate the precipitated solids NH4CL from the NA2CO3:

    The solid NH4CL is to be recycled using a thermal treatment with calcium hydroxide, which is obtained as a byproduct free of charge from the acetylene manufacturing process at Gaz Carbonique Ltee CaC2 + 2 H2O → C2H2 + Ca(OH)2

    The thermal treatment of the mixture produces the following reaction
    Ca(OH)2 + 2NH4Cl → 2NH3 + CaCl2 + 2H2O

    Thus, free ammonia is generated, which is to be diluted to produce ammonia liquor, to be reused in the desalination process upstream and the final residue CaCI2, is a sellable product extensively used in the food and other industries:
    Since the other solid Na2CO3 is also a highly demanded sellable product, the sales of both products are expected to generate non-negligible returns which may help in reducing further the desalination process as a whole.

    A much smaller amount of concentrated brine per unit volume of water treated, could be produced from the RO system producing the potable water, and can thereby more easily disposed without harm to the environment.

    -Low cost desalination system
    -Low energy consumption and thereby low carbon footprint for operation.
    -Helps in removing CO2 from the atmosphere

    #707 Reply
    Sookur sooviraj

    Why so many expenses to recycle seawater desalination for co2 .
    If we afford the expenses how many people will use the water as portable for house used.
    We can use seawater for irrigation then let create natural flat soil for plantation and let the soil itself together with plants root will distillation saltwater into mineral water if needed a design am here to establish it

    #804 Reply
    Francois Fayolle

    Many plants cannot tolerate salt because it interferes with their cells’ normal function. Some plants can tolerate salt water because they have adapted by using reverse osmosis. Reverse osmosis allows a plant to use a selective membrane to keep the salt from interfering with normal processes needed for growth.

    #805 Reply
    Sookur sooviraj

    My objective is not to irrigation system with seawater
    Seawater can be used as underground irrigation ether drop by drop or hydroponics system where seawater traveling from a basin to planting area five layers as filter bed then collect the water in another basin as pure water

    #993 Reply

    Dear Sir,
    This is a very good idea. But if we consider the sustainability of the project on our country then we must analysis its efficiency. There should have both priductive efficiency which means that our current system is producing good water at a good price and alternatives already exists. Allocative efficiency where we could spend such a huge investment for other projects.

    The project is very good for hotels where they search for good water source and also filtered water. They can have a small desalination system for their consumption.

    But your idea is very good and you should continue with this project as it seems that it is a very good alternative for Rodrigues.

    It is also good water source.

    With regards.

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