Desalination of Brackish Water by Reverse Osmosis

1
Desalination of Brackish Water by Reverse
Osmosis Jennifer Kanchukova, Natalie Tran, Jeff
Kunkle Advisors Jan Talbot, Ph.D. Richard
Herz, Ph.D. Chemical Engineering
Program University of California, San Diego
Objectives S To construct a reverse osmosis
setup to desalinate brackish water S To compare
the purity of permeate by varying the inlet
pressure of the pump and concentration
Membrane Specifications

• What Is Reverse Osmosis?
• S Reverse osmosis, also known as hyperfiltration,
  is the finest filtration known. This process will
  allow the removal of particles as small as ions
  from a solution. Reverse osmosis is used to
  purify water and remove salts and other
  impurities in order to improve the color, taste
  or properties of the fluid. It can be used to
  purify fluids such as ethanol and glycol, which
  will pass through the reverse osmosis membrane,
  while rejecting other ions and contaminants from
  passing. The most common use for reverse osmosis
  is in purifying water. It is used to produce
  water that meets the most demanding
  specifications that are currently in place.
• S Reverse osmosis uses a membrane that is
  semi-permeable, allowing the fluid that is being
  purified to pass through it, while rejecting the
  contaminants that remain. Most reverse osmosis
  technology uses a process known as crossflow to
  allow the membrane to continually clean itself.
  As some of the fluid passes through the membrane
  the rest continues downstream, sweeping the
  rejected species away from the membrane. The
  process of reverse osmosis requires a driving
  force to push the fluid through the membrane, and
  the most common force is pressure from a pump.
  The higher the pressure, the larger the driving
  force. As the concentration of the fluid being
  rejected increases, the driving force required to
  continue concentrating the fluid increases.
• S Reverse osmosis is capable of rejecting
  bacteria, salts, sugars, proteins, particles,
  dyes, and other constituents that have a
  molecular weight of greater than 150-250 daltons.
  The separation of ions with reverse osmosis is
  aided by charged particles. This means that
  dissolved ions that carry a charge, such as
  salts, are more likely to be rejected by the
  membrane than those that are not charged, such as
  organics. The larger the charge and the larger
  the particle, the more likely it will be rejected.

RO MEMBRANE
flowing across the surface of the membrane
Effects of Feedwater Pressure S In Figure 4 the
variable inlet salt concentration is plotted
against permeate flowrate at a constant pressure
of 70 psi. The relationship seen is inversely
proportional. Since osmotic pressure is a
function of concentration of salts in the
feedwater, an increase in the inlet salt
concentration increases the osmotic pressure.
Therefore the pressure needed to overcome the
osmotic pressure must also increase. If feed
pressure remains constant, higher salt
concentration results in lower membrane water
flux. The increasing osmotic pressure offsets the
feedwater driving pressure. S As the water flux
declines, the purity of the permeate is
decreased. Figures 5 and Figure 6 show the
relationships of concentration of the permeate as
a function of inlet salt concentration and salt
rejection percentage as a function of inlet
concentration, respectively. A high concentration
of salt results in high molecular forces within
the solution. Molecular forces must be overcome
by applied pressure before water molecules are
able to separate and pass thorough the membrane.
Since the applied pressure remains constant, it
is difficult to overcome the forces resulting
from a high feed concentration of salt solution.
Salt becomes coupled with the water and passes to
the permeate side. Therefore, the salt rejection
increases initially and reaches a maximum value
of 89.2 at 27 mmol NaCl per liter of inlet
water. After the maximum value has been reached,
the percentage of salt rejection declines.
under pressure and high velocity allowing the
water phase to pass through the membrane as
(permeate) free of contaminants
40
waste water feed is introduced into the membrane
module
4
HIGH PRESSURE PUMP
PERMEATE
RETENTATE
INLET FEED
contaminants are rejected by the membrane and are
collected as (concentrate)
Experimental Methods S Before running the
experiments, a calibration curve for the
conductivity meter was prepared by measuring and
plotting the conductivities of solutions against
known concentrations. S To test the performance
of the reverse osmosis membrane, the first set of
experiments was ran at constant concentration of
31mmol NaCl per liter of water and different
pressures.  S Second set of experiments involved
varying the inlet concentrations of salt in the
feed at a constant pressure of 70 psi. S Solid
NaCl was weighted and dissolved in a container
with tap water at 76F. S The feed tanks
capacity is 19.5 L, in which the solution was
thoroughly mixed. S Once the system was turned
on, the pressure of the pump was adjusted by
opening or closing the valve on the retentate
side. S During each run 5-7 samples of the
permeate were collected at 1.5 minute intervals,
analyzed for purity with the conductivity meter,
and recorded.
Conclusion S Reverse osmosis is a high-pressure,
energy-efficient technique for purifying process
streams. Pressure applied across the RO membrane
must be greater than the osmotic pressure of the
solution. The salt ions and other contaminants
are rejected by the RO membrane, while the
purified water is forced through the membrane by
pressure. The permeate contains a very low
concentration of dissolved solids. There is a
positive correlation between the pressure applied
to the membrane and the flowrate of the permeate,
and an inverse proportional relationship between
the inlet feed concentration and permeate
flowrate. The optimal pressure of the membrane
was found to be 62 psi at a specific solution
concentration of 31mmol NaCl per liter, which
gives a permeate concentration of 3.2 mmol NaCl
per liter, and salt rejection of 89.6. At a
constant pressure of 70 psi, the salt rejection
increases initially and reaches a maximum value
of 89.2 at 27 mmol NaCl per liter of inlet water.
Effects of Feedwater Pressure
Effects of Feedwater Concentration
References S Alamo Water, Tips for Calibrating
Testing Instruments, http//waternet.com/article.
asp?IndexID5210209.  Last updated February 4,
1998. Accessed March 13, 2004. S Applegate,
L.E., Membrane Separation Processes, Chem.
Eng., 91(12), 64 (1984). S Bates, W.T. and R.
Cuozzo, Integrated Membrane Systems,
Hydranautics, 38, 67 (2002). S Dababneh, A.J.,
M.A. Al-Nimr, A Reverse Osmosis Desalination
Unit, Desalination, 153, 265 (2002). S Filmtec
Membranes, Factors Affecting RO Membrane
Performance, http//www.pacificro.com/DeFilmFa.pd
f.  Last updated August 28, 2001.  Accessed
January 10, 2004. S GLI International,
Electrolytic (Contacting) Conductivity
Measurement,  http//www.gliint.com/library/tb-c1
.pdf. Last updated March 3, 2004. Accessed    
March 14, 2004. S Jonsson, A. and G. Tragardh,
Fundamental Principles of Ultrafiltration,
Chemical Engineering Process, 27, 67 (1990). S
Seader, J.D. and E.J. Henley, Separation Process
Principles, W. Anderson and T. VenGraitis, eds.,
John Wiley Sons, Inc., New York, NY (1998). S
Sheiknoleslami, R. and J. Bright, Silica and
Metals Removal By Pretreatment To Prevent Fouling
of Reverse Osmosis Membranes, Desalination, 143,
255 (2002). S Teng, C.K., M.N.A. Hawlader, and A.
Malek, An Experiment With Different Pretreatment
Methods, Desalination, 156, 51 (2003). S Vial,
D., G. Doussau, and R. Galindo, Comparison of
Three Pilot Studies Using Microza Membranes for
Mediterranean Seawater Pre-treatment,
Desalination, 156, 43 (2003).
Effects of Feedwater Concentration S A linear
relationship is found between the pressure and
permeate flowrate, as seen in Figure 1. As the
pressure, above the osmotic pressure of the
solution, is applied to the feed, the flux across
the membrane varies in direct and linear
relationship with the amount of pressure applied.
As the pressure increases, more water is pushed
through the membrane, resulting in greater flux.
Every membrane has a maximum operating pressure,
above which the membrane will blow out. Until the
maximum pressure is reached the flux will
continue to increase in proportion to the amount
of force applied. S Conductivity readings were
converted to concentrations of NaCl and plotted
as a function of pressure in Figure 2. Pressure
was also plotted against salt rejection
percentage, as seen in Figure 3. Since the RO
membranes are imperfect barriers to dissolved
salts, there is always be some salt passage
through the membrane. As feed pressure is
increased, the salt passage is overcome as water
is pushed through the membrane at a faster rate
than salt can be transported. However, there is
an upper limit to the amount of salt that can be
rejected via increasing feedwater pressure. Above
that limit, some salt flow remains coupled with
water flowing through the membrane, which
decreases the permeate purity. As can be seen on
the graphs, the membrane used has an optimal
pressure of about 62 psi for the specific
solution concentration of 31mmol NaCl per liter,
and an average conductivity of 22.4 ?S. The
permeate concentration at this pressure is 3.2
mmol NaCl per liter, and salt rejection is 89.6.
Above an applied pressure of 62 psi,
concentration of the permeate increases and salt
rejection decreases.

• Acknowledgements
• Media Separation Systems
  Hydranautics
• Lyndon Cacho Mike Watson