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

ill-7
It is not surprising, therefore, that the development of the EDR process followed logically as an answer
to these RO constraints, noting that current reversal in the EDR process on a set frequency is an elegant
and efficient way to avoid (or at least greatly reduce) the advent of "concentration polarization".
The one clear difference between the operational requirements of the RO Process as compared to the EDR
Process is the much higher pressures at which RO operations have to be optimized. Pressure ranges from
400 to 1,000 psig are not uncommon, and these occasion higher fixed costs for electricity for pumping,
in addition to the electrical power costs for the rest of the RO Process. It has been estimated that
electrical costs for pumping alone can reach as high as 50 cents per thousand gallons. This is the reason
that part of these costs are sought to be recovered by the inclusion of a power recovery turbine in the
brine discharge line of RO plants. Approximately 20% of such losses are estimated to be reasonably
recoverable.
On balance, then, the overall operating costs of an RO plant as opposed to an EDR plant are higher for
otherwise equivalent conditions. Process selection, however has to be influenced by the fact that EDR
plants cannot cope with salinity conditions in excess of 6,000 ppm TDS, and this fact alone guarantees
that there will always be a significant share of total installations that will accrue to the RO Process.
Please see FIG. 4 for a flow diagram of the RO Process.