DESALINATION

     Desalination is the process of purifying sea water to drinkable water.  Although you may not think this process is very important, ask the majority of people living along the shores of Florida and California where they get their drinking water.  The answer is from the ocean!  Many island nations lack sufficient ground water and also utilize desalination.  Desalination must reduce typical seawater at about 34,000 part per million (ppm or mg/L) to an acceptable drinking water standard of below 500 ppm total dissolved solids (tds).  So how is this done?  The most common methods are reverse osmosis and distillation.

Desalting by Reverse Osmosis

     Osmosis refers to the movement of water from an area of high concentation to an area of lower concentration.  Therefore, reverse osmosis is the forcible movement of water from an area of low concentration to an area of higher concentration, or against the water's natural concentration gradient.  In desalination, a semi-permeable membrane (a membrane that allows some material to pass through, but not other materials) is used to separate water from salt and other dissolved solids.

     The most common desalination membran used is cellulose acetate (a chemically modified plant cellulose).  Below is a typical block flow diagram for a reverse osmosis desalination plant:

     The brine or salt solution is usually disposed of by land spreading, sewer systems, deep well injection, piping to ocean, or solar evaporation ponds.  The disposal method chosen is dependent on geographical location, brine concentration, and waste disposal costs.

Desalting by Distillation

     Desalination can also be performed by distillation.  Distillation of saltwater is less common than reverse osmosis due to the large energy costs involved to achieve the necessary purities for municipal use.  Generally, reverse osmosis setups have lower operating costs than those using distillation.

Concerns of the Engineer

     1.  What is cheaper?

              a.  Producing purer water and brine solution of higher concentration

              b.  Producing less pure water and brine solution of lower concentration

              c.  Issue:  Operating Costs vs. Waste Disposal Costs

     2.  What type of unit is better for each situation, reverse osmosis or distillation

              a.  How much water is to be purified?

              b.  How much money is available for the capital investment?

              c.  Does the unit need to be portable?

              d.  Other legal implications.  (noise levels, land available, location)

     3.  Public relations between company and public

              a.  Emphasize "good" points

              b.  Point out why the process is necessary for meeting water needs

Environmental Aspects of Desalination

     Write to:

                             National Technical Information Service
                                       Operating Division
                                  5285 Port Royal Road
                                  Springfield, VA 22161

     Request free of charge:

                            Water Treatment Technology Report No. 13
              "Desalting as an Environmentally Friendly Water Treatment Process"

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