Desalination
Quick Outline
Desalination is a practical way of making salty water drinkable and it
is used widely around the world now, especially in very dry countries
and on ships and small islands. The process is quite expensive and
it uses a lot of energy. It also produces a very concentrated
waste stream of brine which has to be disposed of responsibly. For
these reasons, it is generally a source of last resource, implemented
when all others have failed. The most common, modern methods of
desalination are thermal processes and reverse osmosis (RO) although
there is an increased trend to RO due to the advances in this technology
in the last 10 years. For practical reasons, the most attractive
desalination options are for water that does not have much salt in it to
start with, ie brackish water or recycled water.
Background
As Australia has been severely affected by both drought
conditions and changes in rainfall patterns, a great deal of attention
is now focused on new sources of water to supplement traditional
supplies. One of the most obvious alternative sources is
desalination and it has been advocated, and seriously considered, for
many applications and can provide drought proofing for water short
areas. This fact sheet has been prepared to help people without
expert knowledge in this area to understand how desalination works and
what its potential and drawbacks are. At the end, detailed sources
are listed for anyone who wishes to know more.
Taking Salt Out of Water
Distillation: Removing salt from water is a process
that has been used for a long time, in the form of distillation.
The natural process evaporation from the surface of the sea, to form
clouds, which then result in rain, is the most widespread distillation
process. Boiling salty water and condensing the steam, or even
just putting a dish of water in the sun and collecting the vapour on a
clear cover are both very simple methods of distillation.
Commercial desalination plants have been operating now for decades,
using the distillation process. When distilling large quantities
of water, say for a town supply, there are practical problems to be
dealt with: firstly, the energy needed to evaporate water is quite
considerable, so the process can be very expensive, unless a cheap
source of electricity or heat is available. For instance,
running a power station and a desalination plant together (commonly
called cogeneration) can be cost effective, since the waste heat from a
generator can be used, as well as cheap electricity.
Membrane processes: A more recent development, and
now more widely used, relies on what is called a semi-permeable membrane
to separate salt from water. Simply put, a synthetic membrane is
made, with pores so tiny that water molecules can pass through it, but
other molecules, especially salts, cannot. This separation doesnot
happen easily, though, and it requires very high pressures to force the
water through the membrane. A natural process, called osmosis,
operates in all living cells, to equalise the salt concentration on
either side of the membrane. Because the process for desalination
is the exact opposite, it?s called reverse osmosis, or just RO. A
pre-treatment step is required before RO to provide high quality water
and reduce membrane fouling. The most common pre-treatment steps include
coagulation and filtration or microfiltration.
Other processes: There are other ways of removing
salt from water, but they are not practical for community supplies, so
this fact sheet doesn?t discuss them any further.
Considerations
Energy: One of the first things to note about desalination is
that, whichever method is used, a lot of energy is needed; that is both
expensive and the use of energy can create undesirable greenhouse gas
emissions. It is very important to note that the amount of energy
for distillation is fixed for a given volume of water, but the energy
for RO depends on how salty the water is to start with. For this
reason, it is much more attractive to desalinate brackish (ie slightly
salty) water or treated sewage effluent than it is to deal with
seawater, which has roughly 35 grams of salt in every litre of
water. For RO, the energy also depends on the temperature of the
water, less energy being needed for warmer water. This means that
RO in Darwin would probably cost less than the same process in
Hobart.
Cost: A significant part of the cost equation for
desalination is owing to the high energy consumption, but desalination
plants are sophisticated pieces of equipment with high capital costs and
quite significant maintenance requirements too. Desalination
plants do not last as long as traditional water treatment plants, so the
capital cost has to be amortised over a relatively short life, which
also adds to the cost. The actual cost for a given plant is very
site specific and also dependent on the size of the plant. To give
a very rough idea, it is now possible to produce desalinated water from
seawater for slightly more than A$1 per kilolitre, if the plant is
large, say 100 megalitres per day (serving, say, a medium-sized
city). For smaller plants and less favourable conditions, the cost
could be $4 per kL or more.
Environmental Impacts: It is not possible to use
desalination as a water source and to assume that it has no
environmental impact. As mentioned earlier, the high energy
consumption leads to greenhouse gas production and the salt that has
been extracted has to go somewhere. Depending on the process, the
salt concentration in the waste stream could be anywhere from double
that of the source water, up to a solid salt product (although that
would seldom happen in practice). Managing that salt stream is not
a trivial problem and even in a coastal community, the environmental
management issues associated with the brine disposal have to be
carefully managed. In an inland community, of course, there is no
easy disposal route, so dedicated brine ponds may be needed. There
have been proposals to link gourmet salt manufacturing to desalination,
but that is not a common occurrence.
Maintenance and Operation: A desalination plant is complex and has
serious challenges in terms of corrosion and fouling, regardless of the
specific process being used. While it is possible to design a
small RO plant, say, for rugged conditions, it is not feasible to expect
a large desalination plant of any sort to operate reliably without
expert operational and maintenance support. This means that
implementing desalination in small, remote communities has to be dealt
with carefully, if significant breakdowns are to be avoided.
Treatment Plants
There has been an exponential growth in desalination plants worldwide
with the reduction in capital and operating costs and improvements in
energy efficiency of RO systems. The Middle East is still the largest
user of desalination and seawater desalination plants of capacity over
300 ML/d are being constructed there (e.g. Ashkelon plant in Israel).
There is increasing use in Europe in countries such as Spain and in
North America with plants of over 100ML/d capacity in the Caribbean.
Other Sources of Information
International Desalination Association (IDA) http://www.idadesal.org - Journal -
The International Desalination and Water Reuse Quarterly
Australasian Desalination Association (ADA) - http://www.ceic.unsw.edu.au/ada/
- publishes regular newsletters for members.
European Desalination Society (EDS) http://www.edsoc.com
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