Desalination: is it worth it?

DESALINATION: IS IT WORTH IT? ABE 4905: Individual Study

Johnathan Woodruff Department of Agricultural and Biological Engineering, University of Florida

[email protected]

DESALINAT’N-WORTH IT?

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Abstract

The ongoing fresh water crisis that is apparent in many regions of the planet has posed

the challenge of finding sustainable sources of fresh water to engineers and environmental

scientists. One answer to this challenge is desalination. However, due to high costs and

negative environmental impacts, many critics believe that desalination is not a sustainable

answer to Earth’s fresh water shortages. The following report analyzes the pros and cons of

desalination and strives to make clear whether desalination is a suitable replacement to

traditional sources of fresh water. With the current desalination technologies, it was found that

the sustainability and benefits of the process were highly dependent on the area of the world in

which the plant was located. It was also made clear that as renewable energy technologies are

improved upon and made more affordable, desalination plants will continue to become a more

feasible choice for fresh water. The ability to convert seawater and other waste water into

clean drinking water will allow humans to lessen the stress put on natural water sources such as

springs and rivers, which in turn will make for a healthier environment.

Introduction

The desalination of water is the process of removing salts, minerals and other

contaminating substances from the water. This process is used to make sources of water that

would not ordinarily be used as fresh water sources into potable drinking water. These sources

include ocean water, brackish water and waste water from treatment facilities. Of the water

that exists on Earth, 97.5% of it is salt water. That leaves a measly 2.5% to be used in municipal

water systems, farming, and drinking water. Today 700 million people lack access to fresh

drinking water. It is projected that by 2025 that number will increase to 1.8 billion people [11].

As global population increases, fresh water sources are being drastically depleted to meet the

demands of ever growing municipal water systems. This is especially apparent in regions of the

world that do not receive adequate rainfall or do not have a steady supply of fresh water. These

regions include the Western United States (California in particular), the Middle East, the

Mediterranean region, and parts of Australia. As a result, these regions, especially the Middle

East and Mediterranean, have turned to desalination as one of their major sources of fresh

water.

At first glance, desalination seems like the “cure all” for humans’ constant need for a

sustainable fresh water source. However, along with the many benefits that desalination

provides to its users, there are also many drawbacks. These drawbacks include increased fossil

fuel consumption, increased CO2 production, environmental impacts from discharge streams,

and increased cost of fresh water.

Types of Desalination

To understand the benefits and costs of desalination, different types and classes of

desalination must first be addressed. There are two basic classes of desalination. The first class


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is the thermal distillation process. This process heats saltwater to boiling, collects and

condenses the steam, thus producing purified water. The other major class of desalination is

membrane desalination, which has been growing quickly in popularity. The membrane process

forces saltwater across a semipermeable membrane that separates salts from the water leaving

a saline solution or brine on one side and a “de-saline” solution (drinkable water) on the other

[1]. Both of these technologies have been around for several decades and are constantly being

improved on to increase efficiency.

Within these two major classes, there are several types of desalination. Multi-Stage

Flash Distillation (MSF) is a type of thermal desalination where salt water is vaporized in a series

of chambers under extreme pressure. The first chamber uses a lower pressure than the

incoming saltwater which vaporizes a portion of it. The vaporized water is collected and the

remaining salt water is passed into several more chambers each of which have a lower pressure

Figure 1. This figure shows the Multi-Stage Flash Distillation process and its components [12]. The Multiple Effect Distillation (MED) process is very similar to this except that it recycles its heat and is therefore more efficient.

than the previous. Once the water has passed through all of the chambers, the vaporized water

that was collected is re-condensed as distilled water and the remaining salt water that was not

vaporized leaves the facility with a higher saline content than when it entered. This waste water

is properly discarded and the distilled water is put into the municipal water supply as drinking

water [1]. The prominence in success of MSF plants is predominately due to their simple layouts

and reliability. Multiple Effect Distillation (MED) is another type of thermal distillation in which

salt water is heated under pressure and forced through a chamber. A portion of the entering

saltwater is evaporated leaving behind water with higher saline content. The remaining salt

water is passed to the next chamber which is at a lower pressure where the vapor produced

from the first chamber is used to heat the subsequent chamber. This process ends when the


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salt water has passed through all of the available chambers. The drinking water produced by

MED is distilled water. Since the heat produced in MED is recycled, it is typically a more efficient

form of thermal desalination. Vapor Compression (VC) is another type of thermal desalination.

This process uses compression to heat and evaporate water rather than using direct heating.

The VC process has seen very little implementation compared to the other forms of

desalination. This in part may be because of lack of scalability. VC processes are most

competitive cost-wise in facilities with production capacities less than 5000 m3/d. Due to this,

the expansion of VC has been very limited with only 200 facilities in operation in the year 1994.

Most of these facilities operate at capacities around 500 m3/d [10]. At this scale, the process is

not extremely efficient. To improve overall efficiency, this process is often used with other

desalination processes such as MED [2].

Reverse Osmosis (RO) is a type of membrane desalination. Incoming saltwater is forced

through a semipermeable membrane under high pressures that produces relatively pure water

on the downstream side but leaves a saline-rich water on the intake side. In the RO process,

membrane cleanliness is an extremely important factor. To aid in the efficiency of the RO

process, filters are used as an initial treatment for the intake water to remove particulate

matter and anything else that may have been sucked in from the salt water source. The initial

filtration helps protect the costly semipermeable membranes from getting clogged with

undesired particulate matter which would otherwise reduce the working life of the membrane.

The energy cost of running the plant would also increase without the initial filtration because as

the semipermeable membrane gets clogged, the pumps have to work harder to push water

through them, which requires more energy. In addition, after the water passes through the

membrane, it is treated to kill any microbes that may be present and to adjust the pH of the

water to the desired level [1].

Figure 2. This figure shows a cross section of the filters used in the reverse osmosis desalination process. As this technology becomes more main stream, it also becomes cheaper to produce and use [13].


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Distribution of Desalination Plants

There are around 15,000 major desalination plants worldwide that produce 71.7 million

m3 per day (26.2 billion m3 per year), which contributes around 0.6% of the global water supply.

Of the 15,000 desalination plants, 60% percent draw their water from saltwater sources and

two thirds of them are thermal processes. The dominant form of desalination is RO, which

accounts for 60% of the current global desalination capacity. MSF accounts for 26.8% of this

capacity with the remaining 13.2% being comprised the various other types of desalination [2].

The majority of desalination plants can be found surrounding the Arabian Gulf. The sum

of these plant’s capacities is approximately 11 million m3/day. Most of the plants located in this

area of the world are thermal plants because of relatively low fuel costs. The majority of the

plants outside this region are membrane desalination [7]. The main producers in the Gulf region

are Saudi Arabia, which produces 23% of the worldwide desalination capacity (9% from the

Arabian Gulf and 13% from the Red Sea) and the United Arab Emirates, which produces 26% of

the worldwide capacity.

Figure 3. This figure shows the distribution of desalination plants around the Arabian Gulf. Plants that are under constructions and fully online are show with their respective capacities. All plants with capacities >1000 m3/day are shown and plants with capacities >100,000 m3/day are specifically identified [7].


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The Mediterranean region produces around 4.2 million m3/day. Of the countries that

use the Mediterranean Sea as their desalination water source, Spain is the largest producer

with 7% of the worldwide capacity. Contrary to the plants that occupy the Arabian Gulf region

which use mostly thermal distillation, 95% of Spain’s desalination plants are RO. The third

largest desalination producing region in the world is the Red Sea, which has a combined

capacity of 3.4 million m3/day. Although desalination is well established in this region as the

dominant source of fresh water, the rest of the world has only just begun digging into this

resource. Plans have already been started to increase the production capacity of California to 2

million m3/day with the addition of 20 new desalination projects expected to be in operation by

2030. There are currently two >200,000 m3/day plants located in Carlsbad and Huntington

Beach, which began operation in 2009. Australia and China also have big plans to dramatically

increase their daily desalination capacities within the next decade [7].

Figure 4. This figure shows the distribution of desalination plants around the Mediterranean Sea. All plants online and under construction were included as well as their respective capacities [7].


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List of Benefits of Desalination

Allows people to have access to water that was previously not safe to drink.

Not only coastal cities can benefit; inland cities with soiled or brackish municipal water

supplies can also benefit.

More available fresh, drinkable water.

The larger the scale of the desalination plant the better, since cost and efficiency increase with increased size [1].

List of Immediate Drawbacks

Desalination requires a lot of energy. A typical MSF plant consumes around 23.5

kilowatthours (kWh) of heat energy plus 2.5-3.5 kWh of electricity per m3 of water

produced [16].

Figure 3. This figure shows the distribution of desalination plants around the Red Sea. All plants, online and under construction, are included with their respective capacities.


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Large scale RO requires around 3.5-5.0 kWh of electricity per m3.

Current desalination worldwide uses 75.2 TWh per year to produce 0.6% of the world’s

water (65.2 million m3 per day), which is around 0.4% of the global electricity

consumption [2]. To put this into perspective, the average American household

consumes around 10,908 kWh annually. This means that roughly 6,894,020 homes could

be powered annually by the electricity that desalination plants consume.

The cost of desalination is extremely dependent on energy cost. If for some reason oil or

coal prices climb dramatically, the cost of desalination increases dramatically [2].

Costs Associated with Desalination

The cost to build and maintain desalination plants vary significantly with the area that

the plant is being built, the type of plant being built, the contract type, and the desired capacity

(m3/day) of the plant. In 2011, the city of Adelaide, Australia built a RO 2-pass desalination

plant with a contract to design, build, operate, and maintain the facility. This plant has a daily

capacity of 273,000 m3 per day and cost $1.79 billion [5]. This would be considered a very large

plant, hence the large price tag.

The price at which the desalinated water can be produced varies with several factors,

such as the type of desalination facility (RO, MSF, MED), the energy sources that power the

plants, the capacity of the plant, as well as by the Total Dissolved Solids (TDS) in the saline

water source. For instance, a RO facility that intakes brackish water and has a capacity of

60,000 m3/day will produce fresh water at a cost ranging from $0.26-$0.54 per m3. If this water

was produced in a similar RO facility that used a saltwater source, the cost would range from

$0.50-$1.00 per m3. This is mostly because saltwater has more TDS and therefore will sully

intake filters, RO membranes and mechanical equipment faster than brackish water [4].

The cost of desalination is also largely due to the energy consumption of the plants.

Even though the MSF and MED plants are for the most part more reliable, RO plants consume

much less energy. The typical energy consumption of a large scale MSF plant is 18 kWh/m3, 15

kWh/m3 for an MED facility, and 5 kWh/m3 for an RO plant. However, the reliability and low

cost of fuel makes MSF and MED plants highly competitive against RO facilities. Recent studies

have shown that for similarly scaled plants, production costs for each of the three technologies

are very similar. In addition to this, there are many MSF plants that are 20-30 years old and still

functioning. Many of these facilities have been refurbished to extend their production lives for

an additional 10-20 years. MSF reliability and long life would further drop production cost as

well as increase the plants capital, which typically accounts for 30-40% of the production cost

[10]. The main price contributor to the RO process is the cost of the new technologies. The RO

membranes used are very expensive and wear out relatively quickly. As RO technology

improves and its cost becomes more and more manageable, these prices will inevitably drop.


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Environmental Impacts of Desalination

The environmental impacts of desalination are numerous. Along with large energy

consumption comes large amounts of CO2 entering the atmosphere. However, this is not the

only negative impact of desalination. In actuality, there are many negative pressures that

increasing use of desalination puts on the environment. Other risk factors associated with

desalination include increased temperature of discharge, increased salinity of discharge and

remnants of pre/post water treatment chemicals. The discharge mentioned is referred to as the

“brine” which consists of high salinity water, chemicals, and anything else that was not desired

in the initial water intake. All of these risk factors have different degrees to which they impact

the ambient environment and each type of desalination technology has its own set of risk

factors.

Table 1. CO2 emission of the various types of desalination technologies [6].

MSF plants discharge a brine solution that is 7-15○C hotter than the feed water

temperature. This brine solution can also have a 15-20% higher salinity than the feed water.

MED plants are similar to MSF plants in that MED plants require both electrical and thermal

energy. However, MED requires less electrical energy than MSF and as a result, MED has

started to gain prominence as a desalination process. The output brine of MED is very similar to

MSF. In terms of CO2 emissions, MED has a lower rank than MSF because the electrical energy

consumption is less. However, both MED and MSF have a higher CO2 output than RO [6]. The

different CO2 emissions can be seen in Table 1.

In RO plants, the brine is left after the pure water permeates through the membrane.

The brine discharge may have a salt concentration ranging from 20-70% higher than that of the

feed water. This range depends highly upon the original salt content of the feed water. RO

systems often use energy recovery in the form of a pressure differential between the pure

water pressure (low) and the brine pressure (high). This is done with the aid of pressure

exchangers. Implementing this energy recovery system has largely contributed to the decrease

in cost of RO desalination. Since RO plants use less specific energy than MED or MSF plants,

their greenhouse gas emissions are often less. Water entering the RO system is pretreated with

Configuration (Fuel) MSF MED RO

kg/CO2/m3 kg/CO2/m3 kg/CO2/m3

Non-cogeneration, Natural gas (NG)

20.4 - 25 11.8-17.6 2.79

Cogeneration, steam cycle, NG 13.9 – 15.6 8.2-8.9 2.13

Cogeneration, combined cycle, NG

9.41 7.01 1.75

Cogeneration, hybrid RO, steam cycle, NG

9.45 7.33 --

Cogeneration, hybrid RO, combined cycle, NG

5.56 4.38 --


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chemicals that are left behind in the brine. These chemicals add to the toxicity of the brine

discharge. Therefore, not only is the discharge extremely high in salinity, it is also high in toxic

chemicals. RO desalination process makes use of high pressure pumping systems. These

systems tend to make a lot of noise which results in noise pollution. On the other hand, RO

discharge is at feed water temperature since there is no use of thermal energy and therefore,

there is no need to worry about thermal pollution as in MED or MSF processes [6]. The degree

of environmental impact of the risk factors is highly dependent on its temperature and density.

High temperature, low density brine discharge has a lesser impact than low temperature, high

density discharge. This is because hotter, less dense brine concentrate will stay on top of the

water and float away from the discharge source, whereas colder, denser brine concentrate will

sink to the bottom and cause harm to sea life in the immediate area of the desalination

discharge source.

Preventing Environmental Impacts

The breakdown of water sources that desalination plants draw from is as follows: 5%

draw from treated wastewater, 22% from brackish water, 58% draw from sea water, and 15%

from other non-conventional water sources. Therefore most of the desalination technologies

have been geared toward desalinating seawater. Before further implementation, regional

regulation and management plans need to be made so that desalination plants do not enable

extreme coastal development while causing significant damage to the outside environment. As

stated before, the two major problems with desalination are (1) emissions of pollutants due to

considerable energy consumption and (2) concentrate and chemical discharge. However, there

are many more problems to consider when proposing the building of a desalination plant.

When desalination plants intake water directly from “open seawater,” marine

organisms can be pulled by the intake current and collide with the screen (impingement) or can

be sucked in with the water becoming trapped (entrainment) [7]. Most desalination plants use

some sort of pretreatment. In thermal plants, pretreatment includes treatment against

biofouling, scaling, foaming, and corrosion. In membrane plants, this process typically involves

treatment against biofouling, suspended solids and scale deposits. As a result, much of the

chemical residues and heavy metals used in these treatment processes are washed out into the

sea along with the concentrate produced by desalination. The effects of this discharge are

dependent on location. Shallow, enclosed environments with a large population of marine life

are considered more sensitive than a high energy, open sea location, which can dilute and

move the discharge more effectively. Therefore, to prevent environmental impacts of the brine

discharge, desalination prospectors should be required to release their plant’s discharge in high

energy, open sea locations.


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Table 2. The typical effluent properties of RO and MSF seawater desalination plants [7].

RO MSF

Physical Properties

Salinity Temperature Plume Density

Up to 65,000-85,000 mg/L Ambient seawater temperature Negatively Buoyant

About 50,000 mg/L +5 to 15○C above ambient. Positively, neutrally or negatively buoyant depending on the process, mixing with cooling water from co-located power plants and ambient density stratification.

Dissolved Oxygen (DO)

If well intakes used: typically below ambient seawater DO because of the low DO content of the source water. If open intakes used: approximately the same as the ambient seawater DO concentration.

Could be below ambient seawater salinity because of physical de-aeration and use of oxygen scavengers.

Biofouling control additives and by Products

Chlorine If chlorine or other oxidants are used to control biofouling, these are typically neutralized before water enters the membranes to prevent membrane damage.

Approx. 10-25% of source water feed dosage, if not neutralized.

Halogenated organics Typically low content below harmful levels. Varying composition and concentrations, typically trihalomethanes

Removal of suspended solids

Coagulants (e.g. iron-III-chloride)

May be present if source water is conditioned and the filter backwash water is not treated. May cause effluent coloration if not equalized prior to discharge.

Not present (treatment not required)

Coagulant aids (e.g. polyacrylamide)

May be present if source water is conditioned and the filter backwash water is not treated.

Not present (treatment not required)

Scale control additives

Antiscalants Acid (H2SO4)

Typically low content below toxic levels. Not present (reacts with seawater to cause harmless compounds, i.e. water and sulfates; the acidity is consumed by the naturally alkaline seawater, so that the discharge pH is typically similar or slightly lower than that of ambient seawater).

Typically low content below toxic levels. Not present (reacts with seawater to cause harmless compounds, i.e. water and sulfates; the acidity is consumed by the naturally alkaline seawater, so that the discharge pH is typically similar or slightly lower than that of ambient seawater).

Foam control additives

Antifoaming agents (e.g. polyglycol)

Not present (treatment not required). Typically low content below harmful levels

Contaminants due to corrosion

Heavy metals May contain elevated levels of iron, chromium, nickel, molybdenum if low-quality stainless steel is used.

May contain elevated copper and nickel concentrations if inappropriate materials are used for the heat exchangers.


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Recent Developments in Sustainable Desalination

One of the main drawbacks of thermal desalination is the use of carbon fuel sources to

produce thermal energy. Burning these carbon fuels increases CO2 output and increases the

cost of desalinating water. To absolve this problem, solar collectors may be input into

desalination plants to convert solar energy into thermal energy, thereby reducing the need and

dependence on carbon fuel sources.

One of the most basic ways of collecting solar energy and turning it into thermal energy

is by running a fluid (water or synthetic oil) through an absorber pipe. This fluid is heated by

solar radiation and can either be stored for later use in an insulated tank or can be used

immediately in the thermal desalination process. These absorber pipes can either be fixed or

can track the sun as it moves across the sky. The latter method will produce more thermal

energy but is also more expensive and costly to run. Regardless, either of these methods would

decrease dependence on carbon based fuels [8].

Another way to collect solar energy to use in the thermal desalination process is with

salinity-gradient solar ponds. These shallow ponds use a vertical salt water gradient. This means

that the denser, saltier water stays at the bottom of the pond while the fresh water stays on

top and does not mix with the bottom layer. The salinity gradient ponds absorb both direct and

diffuse sunlight. As the light strikes the pond, the water heats up. Since the bottom layer has

such a high salt content, it is too dense to rise to the surface; therefore, convective heat

exchange cannot occur. The bottom layer water continues to heat up to very high temperatures

(70-85○C). The middle gradient layer in the pond acts as an insulator keeping the bottom layer

hot long after the sun has gone down. A transfer fluid piped through the bottom layer carries

the thermal energy away from the pond to be used in the desired application. Since the

temperatures in salinity-gradient solar ponds do not exceed 100○C, they are typically best

suited for supplying direct heat for thermal desalination. Due to the high salt content in the

brine discharge of desalination plants, salinity gradient solar ponds provide an environmentally

friendly disposal site for this discharge [8].

Cleaning Chemicals

Cleaning chemicals Alkaline (pH 11-12 or acidic (pH 2-3) solutions with additives such as: detergents (e.g. dodecylsulfate), complexing agents (e.g. EDTA), oxidants (e.g. sodium perborate), biocides (e.g. formaldehyde).

Acidic (pH 2) solution containing corrosion inhibitors such as benzotriazole derivatives.


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Evacuated tube collectors (ETCs) make use of a vacuum sealed cover which minimizes

heat loss due to infrared radiation. The cover is made of glass and is cylindrical in shape. As light

filters into the evacuated cover of the tube, it is trapped between the fluid-filled absorber tube

and the glass cover. The absorber tubes can either be metallic or glass and are typically colored

black to absorb more solar radiation. These tubes are usually assembled together in series for

large collectors. At a cost of $300-$550/m2 this technology is not cheap, especially for a large

scale desalination production facility. However, as fuel costs rise and ETC technology becomes

less novel and cheaper to produce, this will become a more viable alternative to carbon based

fuels. With temperature production in upwards of 200○C, ETCs can be used as a source of

thermal energy in a high temperature thermal desalination plant [8].

Parabolic trough collectors are one of the most commonly used sources of large scale

solar energy production. They consist of many linear collectors with parabolic cross sections.

These collectors have a receiver tube that runs along the focal line of the collector. All of the

sunlight captured by the collector is concentrated on this receiver tube. Depending on the size

of the parabolic collector, concentration ratios can range from 10 to 100, producing

temperatures of 100-400○C. An advantage to parabolic trough collectors is that unlike many

other forms of solar collectors, only one axis of solar tracking is needed to produce sufficient

concentration ratios. To track the sun, most parabolic collector facilities have a mechanical

control which maintains the parabolic collector in the appropriate position to capture the most

solar energy throughout the day. Due to the wide range of fluid temperatures which parabolic

collectors can provide, they are used to produce both hot water and steam for commercial and

industrial facilities.

Figure 5. This figure displays the layer and heat convection currents typical of a salinity gradient solar pond [14].


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Along with solar ponds, parabolic troughs are among the most commonly used solar

technologies for desalination plants today. Parabolic troughs can produce such high

temperatures that they are typically used for electrical energy generation, which makes them

useful for most desalination methods, not just thermal distillation. However, thermal

distillation is the most suitable application for parabolic troughs since these plants can make

use of both the thermal and electrical energy produced. Whereas for example, RO desalination

would have little to no use for the thermal energy produced.

The cost of parabolic troughs is directly correlated to the temperatures they can yield.

The higher the temperature, the higher the cost. Due to this, other forms of solar energy

production, such as solar ponds, are preferred because of their low cost and ability to store

energy. However, where space is limited and electricity and high temperatures are needed,

parabolic troughs are typically the best method of solar energy production for desalination

plants [8].

Desalination Plants Currently Under Construction in the United States

The Carlsbad Desalination Project is currently under construction and will provide 50

million gallons of fresh, high quality water to San Diego County each day. Upon completion, it

will be the largest desalination plant in the United States with an intake capacity of 100 million

gallons per day (mgd). Half of the seawater processed by this facility will be converted into

fresh water, while the other half will be discharged back into the discharge channel before it is

diluted with additional seawater. This is done to ensure that environmental impacts due to

sharp increases in salinity at discharge sites will be kept at a minimal. The daily power

consumption of the Carlsbad Project will be enough to power 28,500 homes per day when it

Figure 4. This figure displays a large scale parabolic trough. As described the black absorber tube is positioned in the focal line of the mirror so that all sunlight caught by the mirrors is focused on the absorber [15].


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goes online in 2016 [11]. Even though this plant will be providing substantial amounts of fresh

water to the San Diego area, many critics believe that the project is wasteful and will only be

“putting a Band-Aid” on an ever increasing problem. Other solutions such as conservation

efforts and waste water treatment facilities would be both more cost effective and

environmentally friendly.

Conclusion

Desalination is an extremely costly process, both in monetary costs as well as

environmental and energy costs. Cities such as Santa Barbara, California built a $34 million

desalination plant in 1991 after severe drought had parched the landscape and was threatening

the local supply of fresh water. Five years after the completion of the plant, Santa Barbara

began to receive adequate rainfall again and the desalination plant was shut down. Therefore,

the $34 million used to build the plant was wasted. Australia built six desalination plants with

the price tag of $10 billion when they had a drought in the 1990’s and four of these plants are

now shut down due to an increase in rainfall. It can be said that without proper planning and

environmental analysis, desalination can simply not be worth the cost of building and running

the facilities. Even in areas where desalination has been effectively implemented for decades,

such as Saudi Arabia, the process is highly dependent on cheap thermal energy costs. In the

future if oil prices begin to climb due to scarcity, these plants will inevitably be too costly to run.

With that being said, investment of money, time, and energy should still be put forth to

develop more environmentally friendly and cost effective desalination methods. As the global

population grows and demands more fresh water, traditional sources will be exhausted and

desalination will need to take their place. In the meantime, alternatives such as water

conservation efforts and sustainable farming initiatives should be put in place to ensure that

future generations will have safe, reliable sources of fresh water.


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Desalination: is it worth it?