NAMS 2002 Workshop - ICOM 2008

Desalination II – 4
Thursday July 17, 4:00 PM-4:30 PM, Honolulu/Kahuku
A New Niche for Electrodialysis: Improving Recovery from RO Desalination
D. Lawler (Speaker), The University of Texas at Austin, Austin, Texas, USA -
dlawler@mail.utexas.edu
Y. Kim, The University of Texas at Austin, Austin, Texas, USA
W. Walker, The University of Texas at Austin, Austin ,Texas USA
Desalination continues to grow in importance because freshwater supplies for
drinking water dwindle while demand grows with increasing population. Dramatic
improvements in membrane technology have made reverse osmosis (RO)
systems the industry standard for desalination. Nevertheless, RO is not the
universal panacea; in particular, when RO is used on inland brackish waters, the
recovery (the fraction of the influent water that becomes product) is rarely higher
that 80%. The other 20%, the concentrate, becomes a waste stream that is
expensive and environmentally troublesome to dispose.
One possible means to improve recovery of RO desalination systems is to use
electrodialysis (ED) as an interstage or post-treatment; that is, to treat the
existing concentrate of an RO system using ED and thereby increase the overall
recovery. The objectives of this paper are to present a simple mathematical
model of ED that helps define the niche for ED in this application and to reinforce
that model with laboratory experimental results.
In ED, alternating cation- and anion-exchange membranes create alternating
clean (diluate) and concentrate streams. For an ion to be transported from the
bulk solution in the feed stream to the bulk solution of the concentrate, it must
move through five separate regions: (i) the bulk solution on the diluate side of an
ion exchange membrane, (ii) a diffusion boundary layer on the diluate side of the
membrane, (iii) the membrane itself, (iv) a boundary layer on the concentrate
side, and (v) the bulk solution on the concentrate side. In each region,
electroneutrality must be maintained, so cations and anions do not act
independently of one another; slower moving ions tend to control the overall
transport. The ion concentrations and potential drop adjust in various parts of the
system to achieve the requirements of electroneutrality and equal electrical
current (ion flux) being carried in each step at steady state. The behavior in each
region is defined by the Nernst-Planck equation. Operating ED systems have a
critical or limiting current, and the actual current (and consequent potential drop)
must be held below this value.
In both bulk solutions and both membranes in a single-salt system, the
concentrations are uniform (so diffusion is zero) and the current and potential