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

m-4
There are some built-in limitations in the EDR Process that must be very carefully handled, if a serious
loss of efficiency and onstream time is not to be encountered. Concentrations of Iron and Manganese
in the incoming feedwater have a major deleterious effect upon the operability of the membranes. Of
somewhat lesser importance is the concentration of Calcium in the feedwater. Tolerable limits for each
of these ions is as follows:
Calcium: Less than 400 ppm in the brine discharge stream
Iron and Manganese: Less than 0.3 ppm together. This imposes a great strain on the precision
and accuracy of the analyses to be made of the feedwater prior to its introduction into the EDR
unit. Potassium Permanganate is the preferred chemical employed for the removal of Iron and
Manganese and, thankfully, the amounts needed represent a very small fraction of the total
operating costs. Capital costs of the Iron/Manganese removal equipment are, however, not
insignificant. In a specific case, this equipment cost nearly an additional $2,000,000 on an
installed basis, for feedwater concentrations of 0.6 ppm (total for both the Iron and the
Manganese).
Yet another possible problem with the EDR Process is that any Silica in the feed water may tend
to concentrate upon (rather than be removed by) the membranes. Desilicifiers may be necessary
to sustain economic operation.
Please see FIG. 3 for a flow diagram of the EDR Process.