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

REPORT ON
TASK m: ALTERNATE-DESIGN SYSTEMS EVALUATION
A. PROCESS TYPES REVIEW
INTRODUCTION
Dr. Alan D.K. Laird, Professor Emeritus of the Seawater Conversion Laboratory of The
University of California at Berkeley and a Co-Editor of the definitive work: "Principles of
Desalination" with Dr. K.S. Spiegler (Academic Press) remarked during a speech, that: "The
gradual spread of desalination throughout the world will continue as potential users become aware
of its benefits. Its costs .., will come down as economies of scale assert themselves and where
people's priorities shift increasingly towards this technology, in the face of dwindling
groundwater supplies and decreasing water qUality. But whatever its rate of growth, those who
wrest fresh water from our planet's finite supply of saline and soiled water - as a chemical of life
- wiII rely increasingly on desalination as an essential tool. •
Three major processes exist today for the commercial-scale conversion of brackish groundwaters
and/or seawater to potable quality water, most often delivered at a guaranteed maximum
concentration of 500 ppm of Total Dissolved solids (TDS). They are:
The Reverse Osmosis Process (RO)
The Electrodialysis Reversal Process (EDR)
The Multistage Flash Evaporation Process (MSF)
As a general rule (since in reality the specific process selected is almost totally a function of the
salinity of the raw water as a principal determinant), the RO Process is useful for the desalination
of waters containing up to 35,000 ppm TDS, with the addition of ·seawater" membranes for TDS
levels above 10,000 ppm; the EDR fmds its best applications at concentrations not exceeding
6,000 ppm TDS; the MSF Process is useful over the entire range of salinities, up to a high of
50,000 ppm TDS.
Selection of the appropriate process is also a site-specific function, inasmuch as it is dictated by
location, length of raw-water lines to the plant, proximity to disposal receptacles, etc.
The RO and the EDR Processes are basically molecular-diffusion processes wherein a semipermeable
membrane stack is utilized to separate the salts from the water, in varying process
configurations. The MSF Process, in any of its variations, is basically a distillation-based
process, with the differences in its versions based on methods of energy and efficiency
improvement, thus influencing total operating costs.
Some pro-forma economics for the RO and the EDR Processes, using purchased grid electricity
as the power source, are presented herein, the MSF pro-formas to follow later on.
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