A study of Osmotic Distillation.

Richard Mark Pashley


Tertiary treated sewage water has a much lower osmotic pressure than seawater and so an osmotic distillation process could be used to simultaneously concentrate sewage waste water and dilute seawater, without direct fluid contact. Although this process has many potential advantages, it also has some practical difficulties which were examined in this study. In osmotic distillation, liquid is transferred through the vapour phase, from a solution with a low concentration of non-volatile solutes, to a solution with a higher concentration, in the process concentrating the dilute solution, and diluting the concentrated solution.  Partially or tertiary treated sewage water has a much lower osmotic pressure than seawater and so an osmotic distillation process can simultaneously concentrate waste sewage water and dilute seawater.  This process may have commercial advantages because the cost of desalinating water is directly linked to its salt concentration.  Also, concentrating treated sewage can reduce its disposal costs.  The use of a hydrophobic membrane allows only vapour transfer between the two solutions, possibly improving the public acceptance of ‘toilet to tap’ recycled drinking water, since there is no direct physical contact between the treated sewage and the product drinking water.  A model system was designed, using NaCl solution, pure water, and a commercially available hydrophobic hollow-fibre membrane.  The membrane flux that could be expected from a process such as this was measured, and compared with predictions based on theoretical calculations, and experimental data.  Although a low flux was observed with the laboratory scale system used in this study, improvements in design and scaling effects may significantly improve these results.


Membrane, distillation; water vapour; osmotic pressure

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