Post on 22-Apr-2023
American University of SharjahCollege of Engineering
Department of Chemical Engineering
Spring 2015Introduction to Engineering and Computing (NGN 110)
CHE Lab Desalination Fundamentals – Reverse Osmosis
Group Members:
Arwa AbouGhareeb
ID:57898
Ala'a Khalaf ID:60149
Sara Saleh ID:59635
FatmaAlFalasi
ID:52985
Abstract
During our very first visit to the Chemical Engineering Lab,
the instructor, Mr. Muhammad, introduced us to the various fields
in which a chemical engineer may be involved. Not only are
chemical engineers in demand in the fields of pharmaceuticals,
cosmetics, petroleum and food industries, but most importantly,
in seawater desalination plants. As the world population and
industries grow, and natural sources of fresh water become ever
more polluted and scarce- constituting 2.5% of all water and
dwindling (Freshwater Crisis, 2015)-, it has become absolutely
necessary to resort to the huge body of ocean and seawater that
covers more than 70% of the surface of our planet. In this lab
session, we discussed the two most prominent techniques of
desalination: Thermal and Membrane processes, the tests and
standards available to qualify water for human consumption, and a
small-scale demonstration of the most prominent desalination
technique currently in use: Reverse Osmosis (RO). The RO machine
components and working were introduced; the device was operated
and almost instantaneously produced clear water from a tank of
highly turbid, saline water.
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Table of Contents
Abstract...................................................2
Table of Contents..........................................3
List of Figures............................................3
Introduction...............................................4
The tests for water quality...............................4
Water Desalination Techniques.............................6
Thermal Processes.......................................6
Membrane Processes......................................7
Experimental Set-Up: RO Machine Components.................9
Procedure:................................................11
Safety considerations:....................................11
Results...................................................12
Discussion.............................................12
The Industrial Procedure, Advantages and Disadvantages:. .12
Advantages of RO technique...............................14
Disadvantages:...........................................14
Possible sources of error, assumptions and shortcomings of
the small-scale demonstration................................14
Conclusion:...............................................15
References................................................16
4
List of Figures
Figure 1 MSF...............................................7
Figure 2 MED...............................................8
Figure 3 Reverse Osmosis...................................9
Figure 4 Forward Osmosis...................................9
Figure 5 Feed Water Tank..................................10
Figure 6 High Pressure Pump...............................10
Figure 7 Pressure Gauges (In and Out).....................11
Figure 8 Output Tank and Flow meters......................11
Figure 9 Conductivity Meter...............................12
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Introduction
Just because a cup of water looks clear to the naked eye does
not mean that it is safe to drink. There are several tests
available that are used to gauge the safety and purity of water.
The tests for water quality
Biological Oxygen Demand (BOD):
Is a measure of the level of organic pollutants in a
sample of water. Aerobic microorganisms in natural waters
oxidize the organic matter to respire, thus decreasing the
amount of dissolved oxygen. The test typically measures the
change in the amount of dissolved oxygen after a five-day
incubation period at a constant temperature of 20 ºC. The
resulting data is converted into a water quality index.
Chemical Oxygen Demand (COD):
Measures the quantity of Potassium Dichromate required
to fully oxidize all organic matter in the water sample.
Unlike BOD, COD encompasses organic matter that is
indigestible by some microorganisms.
Dissolved Oxygen (DO)
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The concentration of dissolved oxygen is vital for the
sustainability of aquatic life. Levels of DO that are too
low may indicate that the sample was drawn from a water body
suffering eutrophic conditions.
Conductivity (Specific Conductance)
Strictly “pure” water, that is, deionized water, is an
excellent insulator and should not conduct electricity.
However, even distilled and deionized water can contain some
ions, for water is a very good solvent. In fact, the human
body needs to replenish the electrolytes it loses every day,
so a moderate amount of salts in drinking water is healthy.
The presence of dissolved salts in water enables it to
conduct electricity. Thus, higher specific conductance means
a more saline sample. Conductivity is directly dependent
upon Total Dissolved Salts (TDS) of the water.
Turbidity
This is a measure of the clarity of a liquid. That is,
how well sunlight is able to pass through the water body.
Turbidity is directly influenced by the Total Suspended
Solids (TSS) in the water. A high amount of TSS causes the
scattering of light off the mixture, which makes the water
cloudy. It is not the unpleasant appearance that is the only
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concern to the consumer; since turbidity shields pathogenic
bacteria from disinfectants, which can lead to water-borne
disease outbreaks.
pH
Is a measure of acidity or alkalinity of water. Acid
rain is often the main cause of acidic water bodies, while
waste chemicals from industrial areas leaching into rivers
and ground water relate to high pH for water. Safe water
should have a pH of about 7.0
Colony Counter
This is an important test to discover the types and
abundance of bacteria that inhabit the sampled water body.
Most of the bacteria that are found in lakes, streams and
rivers are harmless to humans. However, the presence of
fecal coliform bacteria and E. coli are warning signs of
contamination with human and/or warm-blooded animals’ waste.
Sedimentation:
Is the measurement of the concentration of eroded soil
and debris from the surrounding landscape as well as mud
from the bottom of rivers that has been picked up and
suspended.
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The above information on the variety of water tests has been
provided courtesy of the US Geological Survey website (USGS.org).
Water Desalination Techniques
According to water.org, 1 in 9 people do not have access to
safe water. Therefore, it is detrimental to not only provide safe
water solutions in the most energy-efficient method, but also
solutions that can be easily and reliably implemented in the
underprivileged communities.
As per the latest review on water desalination methods, the
Foundation for Water Research (2011) categorizes current methods
into two main branches:
Thermal Processes
These rely basically on the evaporation of sea water,
followed by collection of the condensed vapors to produce fresh
water. What distinguishes one thermal technique from the other is
energy-efficiency, achieved by decreasing chamber air pressure,
thus decreasing the boiling point of water, recycling latent heat
of fusion, and accelerating evaporation by increasing the exposed
surface area of the heated water. Extensive use is made of the
fact that the more concentrated the brine, the lesser energy
required to boil off whatever water is remaining in it.
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Multi- Stage Flash (MSF) Desalination
Seawater is initially heated under high pressure then allowed
to flow into a chamber containing air pressure that is
significantly lower than atmospheric pressure. The heated water
flashes, that is, it boils immediately, and condenses on pipes at
the top of the chamber carrying relatively cold seawater. The
brine is then led into the following chamber which is slightly at
a higher air pressure, where water boils off the brine once more.
This process is repeated anywhere from 4-40 times until
atmospheric pressure is reached. See Error: Reference source not
found.
Multiple Effect Desalination (MED)
In principle, MED is
very similar to MSF, with
a few
exceptions.
In the
first
chamber
(the first
effect),
seawater
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Figure 1 MSF- Source: The Foundation for WaterResearch
Figure 2 MED- Source: The Foundation for WaterResearch
led through pipes at the top of the chamber is sprayed on the
surfaces of pipes at the bottom carrying very hot steam. The
rising water vapors are then used to heat - while the brine is
used to cool- the second effect. Through multiple effects, the
boiling temperature gradually decreases. See Figure 2
Membrane Processes
Reverse Osmosis
In this process, a semi permeable membrane only allows water
molecules to pass through its pores and removes larger particles
from drinking water. External pressure is applied to overcome
osmotic pressure, as a result water molecules are forced through
the semi-permeable membrane and dissolved salts are left behind
in the stream. The amount of pressure applied depends on the
concentration of salt in water. Reverse Osmosis is capable of
removing many substances such as dissolved salts and organic
substances, in addition to microorganisms like bacteria and
viruses. See Figure 3.
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Forward Osmosis
In this process, unlike in reverse osmosis where mechanical
pressure is applied, forward osmosis makes use of osmotic
pressure to draw pure water from seawater onto the other side of
a semi-permeable membrane, to dilute a very strong solution of
ammonium carbonate. The dilute solution is then gently heated to
drive off the ammonium carbonate compound as ammonia and carbon
dioxide gases, leaving behind pure water. The technique is known
to soldiers who, in natural disasters, can use sugar instead of
ammonium carbonate to make fresh water. See Figure 4. The main
disadvantage of this method is the prolonged time needed to
produce freshwater.
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Figure 3 Reverse Osmosis -Source: The Foundation for Water Research
In the Chemical engineering lab, we
were introduced to the water treatment
process by Reverse Osmosis. Chemical
engineers undertook the design of such a
technique to purify water to a grade high
enough for drinking without a phase
change.
Experimental Set-Up: RO Machine Components
1. Feed tank: is the initial tank in which the saline
water is fed, as in Figure 5
2. High-
pressure
pump supplies
the needed
pressure
to overcome
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Figure 4 Forward Osmosis- Source: The Foundationfor Water Research
Figure 5 Feed Water Tank
osmotic
pressure and
force the
water to
penetrate the
membrane. See
Figure 6.
3. Flow-meters and pressure gauges are placed on the inlet
and outlet streams across the membrane to monitor the water
pressure levels (Figure 7) and volume rates of flow.
4. Output tank: Collects water after
it has
passed
through the
membrane,
leaving
behind
particulates
and salts.
See Figure
8.
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Figure 6 High Pressure Pump
Figure 7 Pressure Gauges (In and Out)
Figure 8 OutputTank and Flow
meters
5. Brine collector: Collects the concentrate that is left
behind on the saline side of the membrane. In this lab
experiment, the brine is simply flushed out through the
sewage system. However, the industrial process will usually
inject it into the seawater after adjusting its
concentration to mitigate its harms on aquatic life.
Procedure:
1. Firstly, ensure that the water level in the feed
tank is adequately higher than the minimum level of the
suction tube in order to avoid air bubbles entering the
system and damaging the membranes.
2. Using a 500-ml beaker, take a sample of the
saline, turbid water and use the conductivity probe (Figure
9) to directly read off the initial
TDS value of this sample.
3. Turn on the main power switch
and the High Pressure pump switch
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Figure 9 ConductivityMeter
4. Once more, use another 500-ml beaker to collect a
sample of the clear water produced in the output tank.
5. Now measure the conductivity of this sample as
before. You will notice that the water conductivity (more
specifically, TDS) has significantly dropped.
Safety considerations:
The RO machine is relatively safe to operate and there should
be no major risk during operation, given that water spills near
main power switches and supplies are avoided, and wet hands are
dried well before using the machine’s controls in order not to
expose the operator to electrocution.
Results
The conductivity of the output sample was measured to be 600
mg/L.
The output sample was visibly clearer (much less turbid) compared
to the feed sample, and its odour was remarkably more agreeable.
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Discussion
The Industrial Procedure, Advantages and Disadvantages:
The large-scale industrial procedure involves several
processes as explained below:
Clarification:
Coagulants are added to the water so that suspended solids
agglomerate and can thus be removed easily by settling to the
bottom or filtered directly.
Sedimentation:
The water will be transferred from the screeners into the
sedimentation tank. Then, in the sedimentation tank the smaller
suspended particles will settle down at the bottom of the tank
while the water will flow on the top in order to enter the pipes.
Filtration:
In this step, the remaining particles, any floating solids
that are still in the water will be removed by the sand filter.
This filter is the most common because of its efficiency and low
cost.
Activated Carbon:
The clear water will pass into a tank which is filled with
activate carbon which removes chlorine and other chemicals from
the water flow, as well as absorb the bad odor that is caused by
some chemicals like ammonia.
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Softeners:
In many parts of the world, natural waters may contain
relatively high concentrations of calcium carbonate, calcium
sulfate or some other calcium ions. This is termed “hard water”.
In order to “soften” water, one way, called “Zeolite softening”,
is to add chemical compounds that replace the calcium ions with
sodium ions.
High pressure pump (H.P.P) and RO membrane:-
In order to let the water enter the reverse osmosis membrane
(RO membrane) we need high pressure applied on the water, which
is higher than the system’s osmotic pressure, by means of a High
Pressure Pump. Now the water passes through the semi-permeable
reverse osmosis membrane (RO membrane), which contains small
channels, each has the same size of a water molecule. Through the
RO membrane, pure water passes while the unwanted salts will not,
as the salts have much larger molecular sizes.
As the membranes are very delicate and prone to damage, the
water pressure on either side of it should be monitored with
care. Two pressure gauges will read the inlet and outlet water
pressure respectively. For the RO process to be functional, the
inlet water pressure (measured in bars) must be greater than that
of the outlet water. If the two pressure readings are too close,
or have too large a difference, this may signal a ruptured or a
clogged membrane, respectively.
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In addition, the inlet and outlet volume flow rate must be
equal. Otherwise, this could indicate a leakage.
Adding Minerals Back:
During the filtration process in the osmosis system, almost
all of the mineral ions are stripped out, and we got deionized
water which, although strictly “clean”, is deficient of the
mineral ions that the human body needs. Hence, a measured amount
of minerals is added back to the water to make it more palatable.
Adding Disinfection products:
The semi-permeable membrane should have prevented bacteria
and viruses from passing through. However, it remains necessary
to disinfect the water, at least in order to keep it safe to
drink even after it enters plumbing and storage tanks.
Disinfection chemicals, such as Iodine, are used.
Testing the water:
Finally, water is now tested to ensure that it conforms to
drinking water-grade standards by means of the water quality
tests outlined in the Introduction to this report.
The details of the above procedures have been adapted from
Kucera’s Properly Apply Reverse Osmosis (1997).
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Advantages of RO technique:
According to the Organization of American States- Department
of Sustainable Development (2005), Reverse osmosis, as evaluated
by health officers and sanitary engineers, carries the following
advantages and disadvantages:
It disposes of 95% to 99% of the unwanted
compounds and solids.
It is considered to be a greener method due to
reduced energy consumption and heat dissipation.
No harmful or dangerous chemicals produced during
this process.
Available in self-contained, mobile units that can
be installed in homes.
Output water is produced very shortly after
turning on the machine. Compare to Thermal Processes which
take a relatively longer time. They are perfect when
emergency water supply is needed.
Disadvantages:
Bacteria can get caught in the membrane, and
although they do not cross over, they result in odors and
tastes that can be found in the output water prior to post-
treatment.
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The system cannot operate continuously as it has
to be shut down periodically to replace the membrane tubes.
Possible sources of error, assumptions and shortcomings of the small-
scale demonstration
Note that the lab demonstration used feed water of
conductivity 2000 mg/L, which 20 times less saline that that of
seawater. The reason is to avoid having to replace the semi-
permeable membrane very often.
In addition, the industrial process of Reverse Osmosis
involves pre-treatment of feed water to remove the suspended
solids, and the post-treatment process involves disinfection, pH
adjustment and addition of some salts to make the water more
potable. Such processes, for demonstration purposes, were left
out.
Conductivity of an electrolyte (or specific conductance) is
measured in siemens per meter (S/m). Conversion from specific
conductance to Total Dissolved Solids (TDS) in milligrams per
Liter (mg/L) has been performed assuming that the salt is Sodium
Chloride.
Therefore, 1µS/cm is equivalent to 0.64 mg of NaCl/Kg of water.
Realistically, the feed water must have contained a variety of
salts than just NaCl.
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Measurements of Conductivity also depend on the temperature
of the sample, assumed to be 25 °C. The temperatures of the feedand output samples were not measured as it is assumed that the
temperature difference should be insignificant.
Conclusion
In conclusion, the purpose of this experiment is to
demonstrate the far-reaching influence and importance of chemical
engineering principles in the treatment of saline water in the
most efficient and reliable technique, using Reverse Osmosis as a
contextual example. We learnt that there are different
technologies used for desalination of seawater such as
conventional desalination technologies and membrane processes,
and we observed that the Reverse Osmosis is currently the best
method for desalination of seawater as it consumes relatively
much less energy than thermal processes, thus reduces the carbon
footprint and the unit cost of water to end consumers. RO is a
solution that can be easily implemented in homes, thus it
alleviates clean water scarcity problems in poor and suburban
communities. Certainly, chemical engineers have had major
contributions in providing the world with sustainable, greener
present and future.
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Bibliography
(2005). Retrieved from Organization of American States:
http://www.oas.org/DSD/publications/Unit/oea59e/ch20.htm
Foundation for Water Research. (11, May). A Review of
Current Knowledge: Desalination for Water Supply. Marlow,
UK.
Freshwater Crisis. (2015). Retrieved from National
Geographic:
http://environment.nationalgeographic.com/environment/freshw
ater/freshwater-crisis/
Kucera, J. (1997). Properly apply reverse
osmosis. Chemical Engineering Progress, 93, 54. Retrieved
from
http://ezproxy.aus.edu/login?url=http://search.proquest.com/
docview/221587150?accountid=16946
Millions lack safe water. (n.d.). Retrieved from Water.org:
http://water.org/water-crisis/water-facts/water/
Perlman, H. (2014, May 30). dissolved oxygen. Retrieved
from US Geological Survey:
water.usgs.gov/edu/dissolvedoxygen.html
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Water Quality. (n.d.). Retrieved from US Geological
Survey: http://water.usgs.gov/edu/waterquality.html
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