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

Lab Instructor: Mr. Muhammad Qassim

Experiment Date: 4/3/2015

Submission Date: 11/3/2015

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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

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5

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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