TM 5-813-8
solution, provided that the pressure differential exceeds
d. Process specifications.
When an electro-
the osmotic pressure. A typical reverse osmosis flow
dialysis-reversal process has been identified as most
sheet is shown in figure 7-3.
economical, the design will be limited to the single
b. Energy recovery. Reverse osmosis produces a
process. The process design for any electrodialysis-
concentrated, high-pressure brine.
With reverse
reversal process will include a minimum/maximum
osmosis, the energy lost in depressurizing the brine can
allowed product water conductivity. The design must
be returned efficiently to the feed water by mechanical
show the required product conductivity that must be
methods.
In small systems, consider a flow-work
obtainable at the required product flow, based on the
exchanger; in large systems, consider an energy
worst conductivity raw water. A 10-micron cartridge
recovery turbine.
filter to be placed before the membranes must be
(1) Flow-work exchanger.
A flow-work
included in the design. When a particular metallurgy or
material is required for strategic, corrosion design, or
exchanger, figure 7-4, is a simple piston driven by
process economic reasons, this metallurgy or material
pressurized brine to compress the saline feed. When
will be designated for all applicable parts and spare
the piston has traveled a full stroke, the valving is
parts and equipment. All required instrumentation,
changed, and the saline feed is used to expel
depressurized brine. Flow-work exchangers have been
electrodialysis-reversal stack must be designed. The
under development since 1980.
system design must be based on equipment with a
(2) Energy recovery turbines. Several large
history of water treatment system experience. The
reverse osmosis systems have been built with energy
required experience history should include a minimum
recovery turbines. These turbines can be installed to
of 2 years of operating experience meeting water quality
assist directly in pumping or to drive synchronous
and system design goals, current operating capacity,
motors and generate electricity. While reverse osmosis
maximum allowable repair frequency and duration, and
is an energy efficient desalination process for highly
maximum allowable ratio of experienced capital cost to
saline waters, energy recovery can reduce the amount
repair cost. The requirement for successful experience
of energy used by as much as one-third.
will limit the amount of untested innovation used at a
c. Mechanical strength and packing of membranes.
facility.
For containment of high pressures with thin membranes
in reverse osmosis, three alternative arrangements have
been developed.
Reverse osmosis. Diffusion through materials
7-3.
(1) Porous tubes. Porous tubes lined with
is influenced by the nature of the diffusing material. A
semipermeable membrane material have been
number of materials allow water to pass through with
developed for concentration of valuable products in
relative ease. Some of these materials allow only a
industry. Such systems are no longer used for water
minute passage of ionized material compared to the
desalination. A packing density of less than 110 square
passage of water through them. These semipermeable
feet of surface area per cubic foot of volume makes this
materials are used for desalination. If a thin barrier or
configuration too expensive for water production. See
membrane is used, water can be forced through the
figure 7-5.
membrane while ions are stopped by the membrane. In
(2) Spiral-wound membranes.
By using
general, nonionized materials, such as some gases and
many organics, will not be removed by these
spacers it is possible to roll a membrane envelope onto
membranes. Some larger organic molecules may not
a slotted product water tube as shown in figure 7-6. This
pass through the membranes.
reverse osmosis membrane configuration is known as
a. Osmotic pressure. When a semipermeable
the spiral-wound configuration. This arrangement allows
for surface densities of greater than 250 square feet of
membrane that will pass solvent is placed between two
surface area per cubic foot of volume. With the
solutions of different concentrations containing the same
development of this spiral-wound configuration, water
solvent at identical temperatures, the solvent must pass
production from brackish water sources by reverse
from the less concentrated to the more concentrated
osmosis became economical in many applications. See
solution as shown in figure 7-2. This flow of solvent
figure 7-6.
produces a pressure head difference. The equilibrium
liquid pressure head difference is called the osmotic
pressure difference of the solutions (see App A for the
calculation). If these pressures are reversed, pure water
will be forced from the more concentrated solution
through the membrane into the less concentrated
7-3
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