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Water Desalination

A general comparison of methods for the deionization of sea water and brackish water to produce drinking water

 

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1. Introduction

 

Water seems to be a superabundant natural resource on the planet earth. However, only 0.3 per cent of the world's total amount of water can be used as clean drinking water. Man requires huge amounts of drinking water every day and extracts it from nature for innumerable purposes. As natural fresh water resources are limited, sea water plays an important part as a source for drinking water as well. In order to use this water, it has to be desalinated. So, sea water desalination is a real challenge for western civilisation.

  • The base method for sea water desalination is distillation. Approximately 620 kWh of evaporation enthalpy is necessary to obtain 1 t of drinking water.


    •

  • In technical installations, a multiple stage flash (MSF) evaporator is used requiring approximately 100 kWH/t (hot steam at 2 bar) and 3.5 kWh/t of electric energy for pumps.

    •

  • In contrast, the energy demand of reverse osmosis (RO) can be brought down to 7 kWh (large plants with pump pressure recycling and low water recovery rate) of electric energy for pump operation to produce 1 t of drinking water with a salt content of below 500 ppm from sea water (total salt content of 3.7 percent). In small plants, however, pump pressure recycling is not applicable. In this case, energy requirements rise to 30-40 kWh / t of water.

    •

  • In electrodialysis by means of ion exchanges mebranes, however, the driving force is the electric power removing the ions from the water which is to be desalinated. As the amount of electric power needed is proportional to the salt content of the water, it is impossible to give a specific amount of electric energy required. Therefore, this method is much more favourable in case of low salt contents, such as brackish water or water extracted from briny sources (heavily exploited fresh water sources near the sea) with an energy consumption of 3-8 kWh/t, than in case of high salt concentrations (energy required up to 15-25 kWh/t).

    •

     How does it work?

    In case of relatively low salt contents, electrodialysis is fully competitive with regard to RO. To opt for one method or the other, the differences between both as dicussed below have to be considered.

     
    2. Drinking water

    Water demineralization processes differ not only in the driving force but also in the water quality obtained. The energy consumption of the different methods depends on the quality of the water produced and the feed water composition. Water deionization generally intends to produce drinking water, whose composition is given in the following table.

    The WHO standards for drinking water are [1]:
    Substance Desired maximum concentration in mg/l Permitted maximum concentration in mg/l Isotonic solution in mg/l ,[2]
    total dissolved solids

    500

    1500

    9000
    Mg

    30

    150

    -
    Ca

    75

    200

    -
    Chloride

    20

    60

    3550-3800
    Sulfate

    200

    400

    -
    Sodium

    -

    -

    3050-3400
    Potassium

    -

    -

    150-210
    total content in mmol/l

    approx. 10

    approx. 30

    approx. 150

       

    3. Comparison of reverse osmosis (RO) and electrodialysis (ED)

     

    Since 1990, RO has been improved continuously with regard to its performance, i.e. investment cost, performance per membrane area and power demand.
    In contrast, there has been no considerable progress concerning the (well-known) problems of ED. However, the advantages of ED remain:

  • It is the only method which offers "living" water, i.e., the desalinated water is close to the original water. The salt is extracted from the water, but the water as such remains untouched. It is not forced through a membrane as in the case of RO. Therefore, the water retains all other substances which might be in it.
    •
  • With ED, the salt is concentrated in a real brine. It can be used for salt production whereas RO does not produce any highly concentrated solutions.
    •
  • ED requires less than 1.5 m3 H2O/t, whereas RO requires 2-3 t. Depending on the specific application, the question of what to do with the highly concentrated salt acquires a certain importance. - This will not pose any problem when talking about sea water, but what about briny sources?
    •
  • In case of water containing nitrate, nitrate concentration can be reduced selectively by means of ED.
  • This calculation example shows how to get a rough idea of the membrane area you might require for your application.
    •

    As a conclusion, it can be stated that RO has already reached its optimum, whereas ED still has a large potential for further development.

     
    4. Potential for future developments of ED

     

    The most important areas of optimization are still:

  • concentration polarisation,
  • membrane resistance,
  • ion-conduction spacers.

    • PCA GmbH ist working on these problems. Do you have any questions? Would you like to participate in research? Are you looking for a subject of a thesis?
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    5. Literature

     

    [1] N.N. Greenwood, A. Earnshaw , Chemie der Elemente, VCH, 1988

    [2] R. Ludewig KH. Lohs, Akute Vergiftungen, VEB Gustav Fischer Verlag Jena, 1981

     
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