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

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A2: THE REVERSE OSMOSIS PROCESS
The oldest of the membrane-related processes is the Reverse Osmosis (RO) Process, the earliest
installations of which date back to before 1950, no matter that these early units were strictly laboratorysized
and often were more trouble than they were worth. Shipboard-sized units, capable of desalting up
to 15,000 gallons per day of seawater, appeared on vessels in the early Sixties. It was not until 1966 that
commercial-sized units, capable of desalting up to 100,000 GPO, were installed in municipal use, one
of these early types being installed in the City of Plains, Texas, in 1967.
The RO Process relies on a natural phenomenon: Osmosis, involving fluid flow across a membrane called
"semipermeable". The term arises from the fact that certain components of a solution, usually the
solvent, can pass through such a membrane while others, usually dissolved solids, cannot. The direction
of solvent flow is determined by its chemical potential which is a function of pressure, temperature and
the concentration of the dissolved solids.
Thus, if pure water is on both sides of a semipermeable membrane at equal pressure and temperature,
no net flow can be realized inasmuch as the chemical potential is equal on both sides. If a soluble salt
is now added to one side of the membrane, the potential on that side is reduced, causing flow to occur
from the pure water side to the salt water side, thus diluting the concentration of salt in the water on that
side. If a reversal of this flow is desired, the pressure on the salt water side is increased and now the
flow occurs from the salt side to the pure side. This reversal mechanism gives the process its name and
accomplishes the desalting of the feedwater without a change of phase (i.e. the water is not required to
be converted into steam prior to its desalination).
As can now be perceived, the design of the membrane, its chemical composition and its physical
characteristics (e.g. pore size, etc.) must be carefully balanced for the intended job. Most desalinationplant
RO membranes are designed for the flow of water across them. It must be realized that since this
is a specific design balance predicated on the passage of water, dissolved compounds which are
chemically similar to water will also pass readily through the membrane, since they will interact with the
membrane in a similar manner. If the composition of the feedwater is such that these compounds are
present in excessive quantities, then pre-treattnent of the feedwater becomes a must. It is for this reason
that detailed analyses of the feedwater are required, including tests for biological compounds that would
affect the TOC, BOD, COD, etc., as well as tests for colloidal matter that may entrain such compounds.
As to the rest of the features of an RO Process unit, they are quite similar to the EDR unit, in that
feedwater needs to be pre-treated, brine discharge is recycled for greater efficiency, temperature and
pressure are carefully controlled (albeit pressures in an RO Process are altogether higher than those in
an EDR Process unit) and the ease of operation is sensitive to the salinity of the feedwater.
Today's RO units can handle salinities up to those found in seawater (35,000 ppm IDS), although their
best performance is usually realized when salinities are no greater than some 15,000 ppm IDS. Part of
this constraint arises from the fact that, after a period of continuous operation, the membranes suffer from
"concentration polarization", a phenomenon in which the salt concentration on the face of the membrane
exposed to the feedwater side is greater than that in the feedwater itself. This then requires periodic
flushing and dilution procedures that can raise overall operating costs significantly, especially in those
cases where membranes are required to maintain high water flows per unit area. Recent advances in