Desalinization Plant
Desalination,
desalinization, or desalinisation refers to any
of several processes that remove excess salt and
other minerals from water. Desalination may also
refer to the removal of salts and minerals more
generally, as in soil desalination, but the focus
of this article is on water desalination.
Water
is desalinated in order to be converted to fresh
water suitable for animal consumption or irrigation,
or, if almost all of the salt is removed, for
human consumption. Sometimes the process produces
table salt as a by-product. It is used on many
ships and submarines. Most of the modern interest
in desalination is focused on developing cost-effective
ways of providing fresh water for human use in
regions where the availability of water is limited.
Large-scale
desalination typically requires large amounts
of energy as well as specialized, expensive infrastructure,
making it very costly compared to the use of fresh
water from rivers or groundwater. The large energy
reserves of many Middle Eastern countries, along
with their relative water scarcity, have led to
extensive construction of desalination in this
region. Saudi Arabia's desalination plants account
for about 24% of total world capacity. The world's
largest desalination plant is the Jebel Ali Desalination
Plant (Phase 2) in the United Arab Emirates. It
is a dual-purpose facility that uses multi-stage
flash distillation and is capable of producing
300 million cubic meters of water per year.[4]
The largest desalination plant in the United States
is the one at Tampa Bay, Florida, which began
desalinizing 25 million gallons of water per day
in December 2007.
Methods
1. Distillation
1. Multi-stage flash distillation (MSF)
2. Multiple-effect evaporator (MED|ME)
3. Vapor-compression evaporation (VC)
4. Evaporation/condensation
2. Ion exchange[5][6]
3. Membrane processes
1. Electrodialysis reversal (EDR)
2. Reverse osmosis (RO)
3. Nanofiltration (NF)
4. Forward osmosis (FO)
5. Membrane distillation (MD)
4. Freezing
5. Geothermal desalination
6. Solar humidification (HDH, MEH)
7. Methane hydrate crystallisation
8. High grade water recycling
As
of July 2004, the two leading methods were reverse
osmosis (47.2% of installed capacity world-wide)
and multi-stage flash (36.5%).
The
traditional process used in these operations is
vacuum distillation—essentially the boiling
of water at less than atmospheric pressure and
thus a much lower temperature than normal. This
is due to the fact that the boiling of a liquid
occurs when the vapour pressure equals the ambient
pressure and vapour pressure increases with temperature.
Thus, because of the reduced temperature, energy
is saved.
In
the last decade, membrane processes have grown
very fast, and most new facilities use reverse
osmosis technology.[citation needed] Membrane
processes use semi-permeable membranes and pressure
to separate salts from water. Membrane systems
typically use less energy than thermal distillation,
which has led to a reduction in overall desalination
costs over the past decade. Desalination remains
energy intensive, however, and future costs will
continue to depend on the price of both energy
and desalination technology.
Forward
osmosis employs a passive membrane filter that
is hydrophilic and slowly permeable to water,
and blocks a portion of the solutes. Water is
driven across the membrane by osmotic pressure
created by food grade concentrate on the clean
side of the membrane. Forward osmosis systems
are passive in that they require no energy input.[citation
needed] They are used for emergency desalination
purposes in seawater and floodwater settings.
Considerations and criticism
Co-generation
Cogeneration
is the process of using excess heat from power
production to accomplish another task. In the
sense of desalination, cogeneration is the production
of potable water from seawater or brackish groundwater
in an integrated, or "dual-purpose",
facility in which a power plant is used as the
source of energy for the desalination process.
The facility’s energy production may be
dedicated entirely to the production of potable
water (a stand-alone facility), or excess energy
may be produced and incorporated into the energy
grid (a true cogeneration facility). There are
various forms of cogeneration, and theoretically
any form of energy production could be used. However,
the majority of current and planned cogeneration
desalination plants use either fossil fuels or
nuclear power as their source of energy. Most
plants are located in the Middle East or North
Africa, due to their petroleum resources and subsidies.
The advantage of dual-purpose facilities is that
they can be more efficient in energy consumption,
thus making desalination a more viable option
for drinking water in areas of scarce water resources.
Additionally,
the current trend in dual-purpose facilities is
hybrid configurations, in which the permeate from
an RO desalination component is mixed with distillate
from thermal desalination. Basically, two or more
desalination processes are combined along with
power production. The advantage to hybrid configurations
is two qualities, such facilities have already
been implemented in Saudi Arabia at Jeddah and
Yambu-Medina.
Economics
A
number of factors determine the capital and operating
costs for desalination: capacity and type of facility,
location, feed water, labor, energy, financing
and concentrate disposal. Desalination stills
now control pressure, temperature and brine concentrations
to optimize the water extraction efficiency. Nuclear-powered
desalination might be economical on a large scale,
and there is a pilot plant in the former USSR.
Critics
point to the high costs of desalination technologies,
especially for poor third world countries, the
impracticability and cost of transporting or piping
massive amounts of desalinated seawater throughout
the interiors of large countries, and the byproduct
of concentrated seawater, which some environmentalists
have claimed "is a major cause of marine
pollution when dumped back into the oceans at
high temperatures"
It
should be noted that typically the reverse osmosis
technology that is used to desalinate water does
not produce this "hot water" as a byproduct.
Additionally, depending on the prevailing currents
of receiving waters, the seawater concentrate
byproduct can be diluted and dispersed to background
levels within relatively short distances of the
ocean outlet.
While
noting that costs are falling, and generally positive
about the technology for affluent areas that are
proximate to oceans, one study argues that "Desalinated
water may be a solution for some water-stress
regions, but not for places that are poor, deep
in the interior of a continent, or at high elevation.
Unfortunately, that includes some of the places
with biggest water problems." and "Indeed,
one needs to lift the water by 2000 m, or transport
it over more than 1600 km to get transport costs
equal to the desalination costs. Thus, desalinated
water is only expensive in places far from the
sea, like New Delhi, or in high places, like Mexico
City. Desalinated water is also expensive in places
that are both somewhat far from the sea and somewhat
high, such as Riyadh and Harare. In other places,
the dominant cost is desalination, not transport.
This leads to relatively low costs in places like
Beijing, Bangkok, Zaragoza, Phoenix, and, of course,
coastal cities like Tripoli." For cities
on the coast, desalination is being increasingly
viewed as an untapped and unlimited water storage.
Israel
is now desalinizing water at a cost of 53 cents
per cubic meter. Singapore is desalinizing water
for 49 cents per cubic meter. Many large coastal
cities in developed countries are considering
the feasibility of seawater desalination, due
to its cost effectiveness compared with other
water supply options, which can include mandatory
installation of rainwater tanks or stormwater
harvesting infrastructure. Studies have shown
that desalination is among the most cost-effective
options for boosting water supply in major Australian
state capitals. The city of Perth has been successfullyoperating
a reverse osmosis seawater desalination plant
since 2006, and the West Australian government
has announced that a second plant will be built
to service the city's needs. A desalination plant
is to be built in Australia's largest city, Sydney,
and Wonthaggi, Victoria in the near future.
The
Perth desalination plant is powered partially
by renewable energy from the Emu Downs Wind Farm.
The Sydney plant will be powered entirely from
renewable sources, thereby eliminating harmful
greenhouse gas emissions to the environment, a
common argument used against seawater desalination
due to the energy requirements of the technology.
The purchase or production of renewable energy
to power desalination plants naturally adds to
the capital and/or operating costs of desalination.
However, recent experience in Perth and Sydney
indicates that the additional cost is acceptable
to communities, as a city may then augment its
water supply without doing environmental harm
to the atmosphere. The Gold Coast desalination
plant will be powered entirely from fossil fuels
and at a time when the coal fired power stations
have significantly reduced capacity due to the
drought. At a rate of over 4 kWh per cubic meter
to produce this will be the most expensive source
of water in Australia.
In
December of 2007 it was announced that the city
of Port Stanvac, Australia would fund a new desalination
plant by raising water rates. An online, unscientific
poll showed that nearly 60% of votes cast were
in favor of raising water rates to pay for desalination.
Environmental
One
of the main environmental considerations of ocean
water desalination plants is the impact of the
open ocean water intakes, especially when co-located
with power plants. Many proposed ocean desalination
plants initial plans relied on these intakes despite
perpetuating ongoing huge impacts on marine life.
In the United States, due to a recent court ruling
under the Clean Water Act these intakes are no
longer viable without reducing mortality by ninety
percent of the lifeforce of the ocean; the plankton,
fish eggs and fish larvae. There are alternatives
including beach wells that eliminate this concern,
but require more energy and higher costs while
limiting output. Other environmental concerns
include air pollution and greenhouse gas emissions
from the power plants that provide electricity
and/or thermal energy to the desalination plants.
Regardless
of the method used, there is always a highly concentrated
waste product consisting of everything that was
removed from the created fresh water. This is
sometimes referred to as brine, which is also
a common term for the byproduct of recycled water
schemes that is often disposed of in the ocean.
These concentrates are classified by the United
States Environmental Protection Agency as industrial
wastes. With coastal facilities, it may be possible
to return it to the sea without harm if this concentrate
does not exceed the normal ocean salinity gradients
to which osmoregulators are accustomed. Reverse
osmosis, for instance, may require the disposal
of wastewater with a salinity twice that of normal
seawater. The benthic community cannot accommodate
such an extreme change in salinity and many filter-feeding
animals would be destroyed when the water is returned
to the ocean. This presents an increasing problem
further inland, where one needs to avoid ruining
existing fresh water supplies such as ponds, rivers
and aquifers. As such, proper disposal of concentrate
needs to be investigated during the design phases.
To
limit the environmental impact of returning the
brine to the ocean, it can be diluted with another
stream of water entering the ocean, such as the
outfall of a wastewater treatment plant or power
plant. While seawater power plant cooling water
outfalls are not freshwater like wastewater treatment
plant outfalls, the salinity of the brine will
still be reduced. If the power plant is medium
to large sized and the desalination plant is not
enormous, the flow of the power plant's cooling
water is likely to be at least several times larger
than that of the desalination plant. Another method
to reduce the increase in salinity is to spread
the brine over a very large area so that there
is only a slight increase in salinity. For example,
once the pipeline containing the brine reaches
the sea floor, it can split off into many branches,
each one releasing the brine gradually along its
length through small holes. This method can be
used in combination with the joining of the brine
with power plant or wastewater plant outfalls.
The
concentrated seawater has the potential to harm
ecosystems, especially marine environments in
regions with low turbidity and high evaporation
that already have elevated salinity. Examples
of such locations are the Persian Gulf, the Red
Sea and, in particular, coral lagoons of atolls
and other tropical islands around the world. Because
the brine is more dense than the surrounding sea
water due to the higher solute concentration,
discharge into water bodies means that the ecosystems
on the bed of the water body are most at risk
because the brine sinks and remains there long
enough to damage the ecosystems. Careful re-introduction
can minimize this problem. For example, for the
desalination plant and ocean outlet structures
to be built in Sydney from late 2007, the water
authority states that the ocean outlets will be
placed in locations at the seabed that will maximise
the dispersal of the concentrated seawater, such
that it will be indistinguishable from normal
seawater between 50 metres and 75 metres from
the outlet points. Sydney is fortunate to have
typical oceanographic conditions off the coast
that allow for such rapid dilution of the concentrated
byproduct, thereby minimising harm to the environment.
In
Perth, Australia, in 2007, a wind powered desalination
plant was opened. The water is sucked in from
the ocean at only 0.1 meter per second, which
is slow enough to let fish escape. The plant provides
nearly 40 million gallons of clean water per day.
Desalination compared to other water supply options
Increased
water conservation and water use efficiency remain
the most cost effective priority for supplying
water. While comparing ocean water desalination
to wastewater reclamation for drinking water shows
desalination as the first option, using reclamation
for irrigation and industrial use provides multiple
benefits.[22] Urban runoff and storm water capture
also provide multiple benefits in treating, restoring
and recharging groundwater.
Experimental techniques and other developments
In
the past many novel desalination techniques have
been researched with varying degrees of success.
Some are still on the drawing board now while
others have attracted research funding. For example,
to offset the energy requirements of desalination,
the U.S. Government is working to develop practical
solar desalination.
As
an example of newer theoretical approaches for
desalination, focusing specifically on maximizing
energy efficiency and cost effectiveness, we may
consider the Passarell Process.
Other
approaches involve the use of geothermal energy.
An example would be the work being done by SDSU
CITI International Consortium for Advanced Technologies
and Security. From an environmental and economic
point of view, in most locations geothermal desalination
can be preferable to using fossil groundwater
or surface water for human needs, as in many regions
the available surface and groundwater resources
already have long been under severe stress.
Recent
research in the US indicates that nanotube membranes
may prove to be extremely effective for water
filtration and may produce a viable water desalination
process that would require substantially less
energy than reverse osmosis.
See also
* Biofuel from algae (cogeneration).
* Soil salination
* Solar Powered Desalination Unit
Desalination
Plants
* Kwinana Desalination Plant
* Point Paterson Desalination Plant (Credit:
Wikipedia).
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