Session B9
Paper 49
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REVERSE OSMOSIS DESALINATION: THE SOLUTION TO WATER
SCARCITY
Mara Wrzesniewski, [email protected], Sanchez, 5:00, Elise Harrison, [email protected], Mena, 3:00
Abstract - Water scarcity proves to be one of the biggest
challenges the world faces today. Utilizing technologies such
as reverse osmosis (RO) desalination is extremely important
in combating water scarcity. The process of water
desalination creates fresh water by separating salt out of
seawater. Reverse osmosis desalination specifically
transforms seawater to drinkable water by forcing it through
permeable membranes. Of the many divisions of desalination,
reverse osmosis and multi-stage flash distillation (MSF) are
the most prominent due to their lower cost and simplicity.
However, RO is the favorable choice over MSF desalination
due to increased economic and environmental sustainability .
There are two leading, sustainable plants that use
the most modern version of this process: one named Sorek
located in Tel Aviv, Israel and the other, The Claude "Bud"
Lewis Carlsbad Desalination Plant (Carlsbad) in Carlsbad,
California. Sorek and Carlsbad each produce millions of
gallons of water daily, making them the top RO desalination
plants in the world.
The International Water Management Institute
estimated that 1.2 billion people today still have insufficient
water supplies. Of all of Earth’s water supply, only 0.5
percent is accessible fresh water, while 2.5 percent is
inaccessible in glaciers and 97 percent exists as seawater. To
fully take advantage of the abundance of seawater naturally
available on Earth, sustainable technologies such as RO
desalination are required to transform water in the oceans
into usable drinking water.
estimated in 2015 that 1.2 billion people still have insufficient
water supplies [2].
This is not just a problem that affects underdeveloped
nations. There are locations even in advanced countries, such
as the United States, where water scarcity is apparent. In
recent years, California has had an extreme and enduring
drought with a continuous lack of precipitation. There has
been a cutoff of water to many farmers as well as restrictions
to water in urban areas, all of which has been causing a
considerable negative effect throughout the state. Recent
studies have shown that this drought is causing great
economic losses, especially in the agriculture industry [3].
Efforts are being taken to reduce the magnitude of water
scarcity, but it is important to find a solution that is both
economically and environmentally sustainable. In order to
meet the economic sustainability requirement, the solution to
water scarcity must provide widespread drinking water while
be feasible for any nation to fund and uphold. Additionally, to
be environmentally sustainable, the solution must have
limited or negligible negative environmental impacts, and be
renewable by using inexhaustible resources as inputs.
Desalination, the process of transforming saltwater to fresh
drinking water could be that solution. Although Earth is made
up of over 70 percent water, only 3 percent is considered
freshwater, or water humans can bathe in, drink, and farm
with. Additionally, two thirds of that freshwater is frozen in
glaciers or otherwise inaccessible [4]. Of all of Earth’s water
supply, only this minimal amount of freshwater is readily
available while 97 exists as seawater [5]. To fully take
advantage of the abundance of seawater naturally available on
Earth, technologies such as desalination are required to
transform water in the oceans into usable drinking water.
There are several countries, Israel being a primary
example, that already depend on desalination for a majority
of their water supply [3]. John Lienhard, Director, of the
Center for Clean Water and Clean Energy at MIT and a
mechanical engineer, said, “We are already at the limit of
renewable water resources...on top of that we have global
warming, with hotter and drier conditions in many areas,
which will potentially further reduce the amount of renewable
water available. As coastal cities grow, the value of seawater
desalination is going to increase rapidly, and it’s likely we
will see widespread adoption.” [3]. The use of this technology
Key Words – Carlsbad, Desalination, Reverse osmosis, Sorek,
Water scarcity.
THE CHALLENGE: WATER SCARCITY
Even today, with all of the advanced technologies that
are being created every day, the issue of water scarcity is still
extremely relevant and extensive. The Millennium
Development Goals, part of the United Nations Department
of Economic and Social affairs, “aimed to halve the
proportion of people without sustainable access to safe
drinking water and basic sanitation between 1990 and 2015”
[1]. However, the International Water Management Institute
University of Pittsburgh, Swanson School of Engineering
Submission Date: 10.02.2017
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Mara Wrzesniewski
Elise Harrison
will continue to spread as more improvements are made to
make it more practical on a small scale.
THE SOLUTION: REVERSE OSMOSIS
DESALINATION
The reverse osmosis (RO) process begins with treatment
of the water. This entails taking water either directly from the
sea or from underground sources [6]. The first step of
treatment is to guide the water through trash racks and
traveling screens to filter out the larger debris. Next, a
multimedia gravity filter removes other solids [5]. According
to Akili D. Khawaji in an article presented at the conference
on Desalination and the Environment sponsored by the
European Desalination Society and Center for Research and
Technology Hellas, “A typical pretreatment includes
chlorination, coagulation, acid addition, multi-media
filtration, micron cartridge filtration, and de-chlorination.
Various chemicals added to the seawater are sodium
hypochlorite for the prevention of microorganism growth,
ferric chloride as a flocculent, sulfuric acid for the adjustment
of pH and the control of hydrolysis and scale formation, and
sodium bisulfite to dechlorinate.” [5].
Next, the treated water must be at a specific pressure for
the RO membranes so that, when the water is pushed through,
the salts will be left behind. An appropriate pressure for the
membrane is reached by using stainless steel high pressure
pumps to raise the water pressure [5]. The pressure depends
on the concentration of salt in the feed water and is typically
between 250 and 400 psi for brackish waters, such as estuaries
and fossil aquifers, and 800 to 1000 psi for seawater [6]. There
are two configurations of membranes that are used most often
in commercial RO desalination. These are spiral wound and
hollow fine fiber (HFF) membranes, shown below [5].
FIGURE 2 [8]
Spiral Wound Membrane
These membranes are composed of cellulose acetate,
aromatic polyamides, or thin polymer composites, which are
used most often today [6]. Hollow fine fiber (HFF) membrane
is a fiber bundle composed in a u-shape made of cellulose
triacetate and polyamide [5]. HFF membranes use the same
principle as tubular and capillary filtration, however, smaller
diameter tubes are used to allow the bundle flexibility during
the desalination process [9]. Spiral wound membrane
contains membranes, feed spacers, permeate spacers, and a
permeate tube [10]. Both brackish water and seawater can be
fed through either type of membrane but the differing
pressures for the different concentrations of feed water have
an effect on the construction of the pressure vessel [6].
Some of the advantages to HFF membrane include its
very high packing density and a very small strand
diameter. This increases cost sustainability of RO since
higher packing density lessens the amount of membranes
necessary for purchase. The strands and fibers are more
flexible than other membranes so they can be used in a variety
of configurations. However, this flexibility is also a
disadvantage. It causes the fibers to be more vulnerable to
breaking when put under strain from the high pressure,
especially in comparison to spiral wound membranes [9]. The
advantages to using spiral wound membranes include the
variety of options of spacers, and types, lengths, and
diameters of membranes. These membranes are also easy to
clean and have a high packing density, but HFF membranes
are still superior in packing density [10].
The next step in the RO process is membrane separation.
In this step, a high external pressure is applied to the seawater
to overcome its osmotic pressure. This reverses the direction
of the water so that it travels across the membrane opposite of
its natural flow [5]. According to the Source Book of
Alternative Technologies for Freshwater Augmentation by
the United Nations Environment Program, the permeate,
which is the seawater, “is encouraged to flow through the
membrane by the pressure differential created between the
pressurized feed water and the product water, which is at nearatmospheric pressure.” [6]. A small, insignificant amount of
salt passes through the membrane but the majority of the
dissolved salts are left behind as brine that is later disposed of
[5].
FIGURE 1 [7]
Hollow Fine Fiber Membrane
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Mara Wrzesniewski
Elise Harrison
The final step of RO desalination is post treatment.
According to Khawaji from the Desalination and the
Environment conference, this process includes, “pH
adjustment, addition of lime, removal of dissolved gases such
as H2S (if any) and CO2, and disinfection” [5]. Specifically,
the water is passed through an aeration column to elevate the
slightly acidic product, with a pH of approximately 5, to a
neutral pH of 7, which is the pH of distilled water [9]. The pH
adjustment and other modifications must be implemented
before the product water is transferred to the distribution
system to be exported as drinking water. Often, this water that
has completed the desalination process is stored in a cistern to
be exported and used later [6].
After the completion of the RO desalination process, the
brine must be discharged. This highly concentrated salt
solution also contains chemicals that are used in other steps
of the process and incites questions about the environmental
sustainability of the technology. The majority of desalination
plants dispose of the brine in oceans and estuaries. The salt
concentration of the brine is twice of that of the ocean and is
more dense, but the long-term impacts of this disposal are
unknown [11]. Some of the better solutions that are expected
to have lesser environmental impacts include multi-port
diffusers and diluting with effluent or cooling water. The
multi-port diffusers are placed on the pipe that discharges the
brine to mix it and the effluent or cooling water can be mixed
with the brine to dilute it and weaken the concentration so it
is not as harmful to the environment [11]. However, RO
technology has been developed too recently to conclude the
brine disposal as a serious non sustainable environmental
impact. This possible downside of the RO desalination
process has been lessened by these solutions but is still being
investigated as the use of this technology becomes more
widespread.
disposal, space requirements, operation and maintenance
aspects” according to a journal presented at the conference on
Desalination and the Environment [5]. However, RO
desalination is superior to MSF desalination in regards to
efficiency, power required, construction and maintenance
cost, and size. According to Jonathan Zactruba, science
journalist for an online database for engineers, RO
desalination provides triple the yield compared to MSF
desalination. Essentially, for the same input of seawater, RO
desalination methods produce three times the output
compared to the alternative process. All desalination
processes have a high expense, but the efficiency of RO
desalination outweighs the cost. One critical example of RO
desalination efficiency payoff is the reduced cost pumping
seawater to the plant and a decreased amount of brine to be
disposed of [12].
In addition to the favorable efficiency of RO methods,
the physical energy required for RO desalination is also
favorable compared to MSF. MSF processes require 12 more
kilowatt hours per cubic meter of power, or 43.2 million more
pascals, than RO processes for the same input amount of
seawater [12]. Since MSF plants are historically larger than
RO plants, the cost of construction and maintenance are
subsequently higher than that of RO plants. In comparison,
improved versions of both spiral wound and HFF membranes
are being advanced daily, reducing maintenance costs and
increasing reliability of RO desalination processes. Problems
with MSF arise concerning the corrosion of materials due to
rapidly changing temperature conditions throughout the
desalination process. RO desalination plants use metal alloys
and polymeric materials to resist warping and reduce
replacement costs. Technological advances in the RO system
have reduced the unit cost of water making it easier to choose
between the two systems [5].
RO desalination is effective on a large scale, so it is
currently a good option for water supply companies that
provide for substantial populations. One criticism is that it
may not be as effective for small populations. However,
small-scale RO plants have been built in several rural areas
that don’t have other options for their water supply [6]. For
example, in the RO plant in the British Virgin Islands, the
government monitors the quality of the water produced,
distributes the water, and assists in the plant’s operation,
making it extremely practical and the best option out of all
types of water purification for that smaller territory.
Additionally, in the past 15 years, the operating technology
has been greatly improved for RO plants. There are
essentially no long-term operational problems as long as daily
monitoring and preventative maintenance are completed at
the plants [6].
All of this proves that RO is the best form of
desalination, but the question still stands if other forms of
water purification outside of desalination are more practical
in economic terms. Overall, the production costs of
desalinated water have decreased from the large amount of
research and development going into the technologies [5].
BEST FORM OF DESALINATION:
REVERSE OSMOSIS
Reverse osmosis is one of the most commercially
successful forms of desalination alongside multi-stage flash
distillation (MSF). The fundamental concept of this process is
flash evaporation, where the pressure is lowered to evaporate
the seawater. Heat input, heat recovery, and heat rejection are
the three sections of an MSF plant. At the start of the process,
saltwater is introduced into a flash chamber where it boils
rapidly and evaporates. Some of the heat produced in each
flash chamber is used to heat the water in the next chamber.
The water vapor produced by flashing is then cooled and
condensed back to the liquid state by colder salt water that
flows in tubes through the condenser [5].
For many, the choice between the trusted MSF
desalination and the newer RO desalination can be difficult,
and dependent on many factors including “seawater
characteristics, product water quality, source of energy and
consumption, plant size, plant reliability, concentrate
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Mara Wrzesniewski
Elise Harrison
The amount of initial treatment required has increased, as well
as the costs of this treatment, due to more rigorous and strict
quality standards. However, this additional cost is the same
across all distillation methods. Some other factors that affect
the cost include the location of the plant and the materials for
construction, but the cost of energy is the largest component.
Further developments are being made in this area through
research focused on using lower cost alternative energy
sources and the reduction of energy consumption by
desalination plants [5].
According to a journal presented at a conference
sponsored by the European Desalination Society and Center
for Research and Technology Hellas, “Over the last two
decades, a great deal of progress has been made in seawater
desalination processes, which have resulted in the significant
reduction of water production costs. This has led to a higher
acceptance and growth of the industry worldwide, particularly
in arid regions of the world. However, because desalination
costs still remain high, many countries are unable to afford
these technologies as a freshwater resource.” [5]. More
technology improvements need to be made, and are currently
being investigated, that will considerably lower the
production costs, making desalination more easily attainable
for underdeveloped countries that have less resources. The
goal of the industry is to lower the costs to a point where
desalinated water can be produced for only 50 US cents per
cubic meter and only require power that costs 2 US cents per
kilowatt hour. Currently, the cost of materials and
construction of RO plants is high and could make the process
less suitable for underdeveloped countries. However, existing
plants have been successful in producing water that can be
distributed at a low cost, proving that in the long term, reverse
osmosis is a viable and sustainable process. The research and
development endeavors to lower costs are primarily focused
on improving the RO membranes, the environmental impacts
of brine discharge, efficient power requirements, renewable
energy usage, and lower cost materials [5].
TWO LEADING PLANTS
There are two leading, sustainable plants that use the
most modern version of the RO desalination process: one
named Sorek located in Tel Aviv, Israel and the other, The
Claude "Bud" Lewis Carlsbad Desalination Plant (Carlsbad)
located in Carlsbad, California. Both plants have been
recently constructed, and have begun producing fresh water
in the past 5 years. The Sorek and Carlsbad desalination plants
lead the world in modern and efficient RO desalination and
foreshadow
the
widespread
use
of
RO
desalination technology to combat water scarcity.
Sorek
The largest modern reverse osmosis desalination plant
in the world is Sorek, located on a beach approximately 10
miles south of Tel Aviv, Israel [14]. In 2000, the Water
Desalination Administration launched a desalination plan for
the country of Israel that included the construction of Sorek
[15]. It was built by Israel Desalination Enterprises and cost
about $500 million to build for the Israeli government.
Though the cost of building the plant was high, this single
plant is responsible for producing and providing 20 percent of
all of the water consumed throughout the nation by
households. This is a substantial portion of the country’s
water supply. Also, Sorek is producing water that can be sold
at a lower price than most desalination plants today and it will
still make a profit. The Israeli water authority purchases the
Sorek plant’s water at the cost of 58 US cents per cubic meter,
which is approximately the amount of water one person in
Israel uses weekly. This plant also has one of the lowest
amounts of energy consumption out of all of the world’s
large-scale desalination plants [14].
Construction of the plant was completed late in 2013,
however the production of water has just started reaching its
full capacity even more recently than that. The Sorek plant is
expected to generate 627,000 cubic meters, over 165 million
gallons, of potable water daily. This proves the practicality
and efficiency of large scale desalination plants and is a great
improvement in Israel’s water supply. In 2004, the majority
of the water used in the country came from groundwater or
rain. Today, Sorek is the largest of the four desalination plants
currently running in the country. This technology is being
implemented on such a large scale that around 40 percent of
Israel’s water supply can be accounted for by their four plants.
It was expected that by 2016, that already large number would
increase to 50 percent with the addition of a few more
desalination plants [14].
The specific pretreatment for seawater or brackish water
depends on the plant. At Sorek, pretreatment begins with
chemical dosing from two pumps, a flocculation basin that
removes solids that are suspended in the water, and dual
media gravity filtration to remove the leftover contamination.
FIGURE 3 [13]
Salt Water Reverse Osmosis Cost Trend
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Mara Wrzesniewski
Elise Harrison
Then, this filtered water is pumped to the section of the plant
that performs the RO desalination [15].
Overall it is both cost efficient and energy efficient in its
production of a considerable amount of fresh water to meet
the needs of Israel. Raphael Semiat, a desalination expert and
chemical engineer at the Israel Institute of Technology said,
“This is indeed the cheapest water from seawater desalination
produced in the world. We don’t have to fight over water, like
we did in the past.” [14].
one meter stainless-steel piping [3]. Then the water is pumped
through a specific sand and anthracite filtration system to
extract the suspended particles from the solution. Other
smaller salts and dissolved particles are then removed as the
water flows through 2000 spiral wound fiberglass reverse
osmosis membranes. Essential minerals are reintroduced to
the water before it is transported first to the Water Authority’s
aqueduct, then through a new 10 mile, $159 million dollar
pipeline to a second aqueduct, Water Authority's Twin Oaks
Valley Water Treatment Plant. Here the newly desalinated
water is mixed with existing drinking water supplies to be
distributed throughout the region [16].
Carlsbad
The state of California is prone to dry weather
conditions and spontaneous drought patterns. However, a
study in 2015 that analyzed blue oak tree rings in the state’s
Central Valley claimed the most recent drought, spanning
from 2011 until 2016, was the driest period in 500 years
according to senior author Valerie Trouet, an Associate
Professor at the University of Arizona [11]. In addition,
according to a study performed by the University of
California, Davis, “The drought inflicted $1.5 billion in
agricultural losses in 2014 alone” [3]. Years of drought
caused record-high temperatures from January through March
of 2015 combined with 1000 more documented wildfires than
the year prior, and widespread mandatory water restrictions
put the state in extreme distress. Fortunately, where other
water sources were not able to supply California with the
resources it desperately needed, reverse osmosis desalination
proved to be a suitable solution.
The Claude “Bud” Lewis Carlsbad Desalination plant
was officially opened on December 14th, 2015 to relieve
California’s water crisis. The San Diego County Water
Authority worked in tandem with Poseidon Water to swiftly
construct the Carlsbad plant in only three years after
negotiations began in 2010 [16]. The Carlsbad Desalination
plant is the largest reverse osmosis desalination plant in the
United States to date [3]. The plant produces approximately
50 million gallons of water per day that is intended for the
county of San Diego in order to reduce the impact of water
scarcity across the state. The Carlsbad plant serves over 24
local water agencies and supplies the entire region with 10
percent of its water demand, an equivalent of 3.3 million
people. Monthly costs average around $5 a unit per
household, which is considered low according to Water
Authority’s prediction in 2012. An estimated annual inflation
for the Carlsbad plant water is 2.5 percent per year, which is
a drastic improvement from the 9.9 percent inflation rate for
imported treated water from the Metropolitan Water District
of Southern California [16].
The Carlsbad plant covers six acres of utility zoned land
near the Encina Power Station towards Agua Hedionda
Lagoon in Carlsbad, California. The entire project cost
approximately $537 million to build according to San Diego
County Water Authority [16]. The basic premise of the plant
starts with water from the Pacific ocean traveling to the
Carlsbad plant located in close proximity to the coast through
IMPROVEMENTS TO THE TECHNOLOGY
Desalination in general began to appear as a viable water
source around 1960. The reverse osmosis process was first
developed in the 1970s, but didn’t emerge as a commercially
successful process until further technological developments
were made and operating costs were lowered. Because reverse
osmosis is a fairly new form of desalination, there are many
changes and improvements that have been put into effect over
the past several years and even more that are currently being
researched and developed. Two major advancements in RO
technology revolve around improvements of the membranes
and energy recovery devices [5].
Energy recovery devices are mechanisms that are
comprised of turbines or pumps and their purpose is to
convert pressure drops into energy [5]. A large difference in
pressure between the feed water and the product water is
necessary for the water to flow through the membranes, so
these devices were engineered to take advantage of the energy
that can be produced by this process [6]. The pressure of the
brine drops by between 14 and 60 psi when leaving the
applied pressure from the high pressure pump. To convert this
into rotational energy, the devices are connected to the stream
of concentrated brine that flows out of the pressurized
chambers. Without any energy recovery, the energy
consumption of a large RO plant is around 6 to 8 kilowatt
hours per cubic meter. Instituting an energy recovery device
reduces that number to around 4 to 5 kilowatt hours per cubic
meter [5]. This impressive reduction of energy cuts costs and
makes RO desalination a more sustainable process.
In the last two decades alone, the RO desalination
process has improved substantially, as reflected by
considerably lower capital and operation costs. This
breakthrough is primarily attributed to the advancement of the
membranes used in the RO process. The end goal for
scientists is to create a membrane that is able to withstand
high pressures without breakage or replacement, and that has
improved flux and more precise salt filtration. Membranes are
being engineered to operate under these conditions to cut
down on both the construction and operation costs [5]. There
are a few stand out groups emerging with cutting edge
membrane technology that shows great promise for the future
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Mara Wrzesniewski
Elise Harrison
of RO desalination. One group consists of mechanical
engineer Rohi Karnik and his team who are currently working
on a membrane that is only a single atom thick [3].
Today, water scarcity still plagues 1.2 billion people
from both developed and underdeveloped countries [2]. Some
of this water scarcity is impacting Americans on the West
Coast in counties such as San Diego, California. RO
desalination proves to be the most promising solution to the
water scarcity crisis. Several different desalination processes
have been successfully implemented over the past few
decades. With the emerging technologies of RO desalination,
water scarcity will impact fewer nations worldwide, therefore
making clean water more accessible to society. The high cost
of RO is currently a constraint for some countries, but the
favorable outcomes of all of the existing plants indicates the
potential of the technology in terms of efficiency and
practicality. Modern technology and engineering aim to make
the process less expensive and more energy efficient in order
for desalination to become viable for less developed countries
as well. Currently, several technologies including energy
recovery devices, improved membranes, safeguard systems,
and larger diameter tubes, are moving the RO desalination
process to become more economically and environmentally
sustainable for widespread, long term use. The future of RO
desalination shows incredible promise in solving the problem
of water scarcity.
Karnik and his group of researchers at the Massachusetts
Institute of Technology (MIT), are blasting graphene with
beams of ions and bathing the strips of graphene in chemicals
to create nanometer size holes in the material [3]. Ideally, the
water molecules will experience minimal resistance while
flowing through these adapted membranes, which would
decrease the amount of pressure needed for the RO
desalination process. Recall that the single largest cost in the
entire RO process is associated with the pressure required to
push water through the permeable membranes. Computer
models performed by Jeffrey Grossman’s materials science
and engineering group at MIT predict the newly developed
graphene membranes could cut the energy used in RO
desalination processes by 15 to 46 percent [3]. The high
permeability of Karnik’s membranes would decrease the
surface area necessary to filter out the salt ions from water,
doing the same job as polymer membranes with less material.
This could theoretically cut the RO desalination plant size in
half decreasing cost across all sectors of construction and
maintenance [3]. Although Karnik’s graphene membrane
shows incredible promise in the field of desalination, there is
still years of research and testing to done before the world will
see graphene membranes implemented in commercial plants.
Specific advancements at the Carlsbad plants include
several safeguard systems in order to guarantee long term
productivity. Carlsbad, one of the newest desalination plants
in the world, was designed with “Extra pumps, treatment
capacity, and membrane tubes” according to Jonathan
Loveland, Vice President at Poseidon Water, partner of the
plant. He went on to say, “Because it is a critical asset for the
region, there is a tremendous amount of redundancy to give
high reliability. If any piece fails, something else will pick up
the slack.” [3].
The Sorek desalination plant is also a good example of
recent advancements being implemented. The first major
innovation that has been put into practice at Sorek is a change
in the pressure tubes. With other large-scale desalination
plants, it is standard to use pressure tubes that are eight inches
in diameter, but Sorek is the first to use sixteen inch diameter
tubes. This requires a fourth of the amount of piping the
alternative tubes require [14]. Reducing the amount of
hardware required is a primary way of cutting costs for the
construction, maintenance, and running of desalination plants.
The developments at both of these plants are increasing
efficiency and lowering costs and can be applied at other RO
desalination plants as well.
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FINAL THOUGHTS ON REVERSE
OSMOSIS
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ACKNOWLEDGMENTS
We would like to thank the writing instructors for
providing input and excellent feedback that has greatly
improved our writing. Specifically, Julianna McAdoo gave us
very beneficial corrections and feedback during our writing
center appointment. We would also like to thank Michael
LaBella who runs a nonprofit organization called The Trinity
Help Foundation who provides clean water to families in
Haiti. His work has inspired us to research a topic that lessens
the impact of water scarcity on underdeveloped countries
such as those he works in.
7
Mara Wrzesniewski
Elise Harrison
8