4. The World

4.1 The USA

     Only 19 states (including Florida) have desalination plants in the US.  Florida ranks first with about 120 plants, then California with 17 plants in 1997, followed by Texas with 11.  The Virgin Islands has 14 SWRO plants.  More than half the desalination plants are found in Florida. The plant with the largest capacity is found in Yuma, AZ, a BRO plant which produces almost 20% of the desalted water of the US (74 MGD) by itself (Bureau of Reclamation 1997) in order to provide drinking water for export to Mexico.  This Arizona plant discharges its concentrate to the freshwater canal from which the feedwater is pumped.  Florida plants produce more than 60% of the Membrane water of the country (70%, if the Yuma plant is excluded).
 

Table 4-1 Methods used for Concentrate Disposal in the USA in 1998 by the number of plants (AWWA-MTRC, 1998) and by volume of concentrate disposed
By plant number Number of plants By volume Volume of concentrate (MGD)
Surface Water Discharge 43% 86 53.5% 45.5
Discharge to Sewer  29% 57 17.5% 14.9
Land Application  12% 24 5% 4.3
Deep Well  10% 21 23.5% 19.5
Evaporation Pond 6% 12 0.5% 0.4

     Surface discharge is the most common means of concentrate disposal (Table 4-1).  The use of POTW is common for small capacity plants. One fourth of the concentrate generated by the 200 plants is disposed via Deep Well Injection.  The disposal of concentrate in Florida reflects disposal practices around the US.
 

4.2  The World

4.2.1 Middle East
     All the plants found in the Middle East are SWRO and discharge their concentrate in the ocean.  Most of the research done in this region is concerned with process upgrades and ways to lower the costs of maintenance and operation of the desalination facilities.  Concentrate disposal does not seem to be a major environmental concern.  Only one reference concerning problems with concentrate has been found.  This paper discussed sediment contamination near a concentrate outfall (Sadiq, 1995).
     Drinking water sources have been the cause of enormous political tension all over the Middle East for centuries. Membrane technology has been a mean to provide water in the desert.  No other alternative could make this possible.  Therefore, one has to understand that the concerns for concentrate pollution are lowered to the extreme in such conditions.  Moreover, it seems to be a general opinion that seawater concentrate presents a very low level of toxicity and impact on the environment.
4.2.2 Caribbean Islands
     The concerns in the Caribbean Islands about membrane technology and concentrate disposal are very similar to the Middle East.  On islands that depend on tourism for ninety percent of their income, drinking water is extremely valuable.  Without membrane technology to produce potable water, supplies would be severely limited, affecting tourism and agriculture.  Desalco Inc., which is responsible for the SWRO plants in the Cayman Islands, in the Bahamas, and in the Bermuda, uses deep well injection for disposal of concentrate (http://www.desalco.bm/).
 

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