(GP/GT) for Additional Water Supply in the Lower Rio Grande

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At.: ELECTRODIALYSIS REVERSAL PROCESS
The Electrodialysis Reversal Process (EDR) is today the most technically advanced of all the membranerelated
processes for the purification of brackish waters. An outgrowth of the previously developed
Reverse Osmosis Process (described below), the EDR Process contributes several significant advantages
to its practitioners, chiefly:
Reduced operating costs via more efficient use of current densities
Employment of periodic current reversal (hence the name) for the prevention of fouling of the
membrane stacks
Greater sensitivity to anodic and cathodic radicals in salt separation, such that the chlorides and
the sulfates are more completely removed
Greater recycling efficiency of the concentrated brine stream, for improved system performance.
Basically, the EDR Process employs a series of semipermeable membranes (stacks) to progressively
remove the salts from the feed water via electrolytic action. Salts, when dissolved in water, are present
in the form of negatively and positively charged ions. When an electric current is applied, positively
charged ions in the brackish water, such as sodium, are forced through the cation-permeable membrane
toward the cathode. Negatively charged ions such as chloride are forced through the anion-permeable
membrane toward the anode. The water in the compartment between membranes is thus depleted of salt
while the water in the adjacent compartments increases in mineral content.
The membrane stack, or the EDR process unit itself, consists of several sets of anion- and cationpermeable
membranes. The quantity of salts removed by passage through one stack may range from 30
to 65 percent of the entering minerals, depending on the stack design and the characteristics of the
membranes themselves. Additional stacks are added in series to increase salt removal towards the desired
level of purity. Each added stack is known as a stage. Total volume of water processed is achieved by
arranging additional stacks in parallel.
The EDR Process operating costs are significantly influenced by the cost of energy at the plant site, as
is indeed the case with the Reverse Osmosis and the Multistage Flash Evaporation Processes as well.
Whether the energy needed is in the form of electrical power or steam of a certain quality, this category
of costs rules the fmal outcome of the economics of these processes.
Recent improvements in the design and spatial arrangement of EDR plants has led to the availability of
"packaged units" that are very compact compared to earlier versions, possessing a "small footprint" as
one would say in the computer world. This feature enables the effective use of these installations, in
skid-mounted fashion, in remote locations where power can be brought in or indeed produced at the site
by various means. This overall portability is a key feature in the success of several EDR installations
wherever space is at a premium.
Ruggedness of design and ease of maintenance are also additional features that have propelled the EDR
Process to the top of the list of processes selected for desalination of brackish waters, it being clearly
understood throughout that this process works best when raw-water salt contents do Dot exceed 5,000 to
6,000 ppm IDS.