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There are several ways in which it is possible for man to
convert water to his own use. As Cyprus is a small island, it is obvious
that one is never far from an almost infinite supply of water, the sea. The
problem is to treat the sea water in such a way that it becomes potable.
This is never easy and always costly.
Sea water contains about 35,000 milligrams of dissolved
solids per litre of water. These solids are mostly ionic salts, the main one
being sodium chloride or common salt. It is generally considered that the
maximum salt content for water to be
potable is 800 milligrams per litre. Even this may be considered too
high for persons suffering from certain chronic medical conditions. A value
of 500 milligrams per litre would be better as a target figure. The basic
problem is that to separate the salt from the water requires energy to break
the ionic bonds. The question remains as to how to do this in a reasonable
and economical manner. The two main contenders for this are
reverse osmosis and multi-stage
flash distillation. Each is capable of producing potable water from sea
water at a cost which is not too prohibitive. However, it must be thoroughly
realised that desalinated sea water will never be as cheap as water from
natural resources.
Reverse osmosis consists basically of pumping sea
water up to a very high pressure and allowing it to percolate through
semi-permeable membranes. This would seem to be similar to filtration
but, in fact, it is not the case. Without going into the technical
details, reverse osmosis depends on the use of a totally different
physical principle, differential surface tension. The advantage of
reverse osmosis is that it does not require an enormous space to
implement and that it can be applied for purifying volumes from about 1
tonne per day up to hundreds of thousands of tonnes per day, depending
on local requirements. The astonishing fact about reverse osmosis is
that the cost does not vary very much from throughputs of a few hundred
tons per day right up to the largest installations. The process is very
energy-intensive, particularly if a very high purity water is required.
The cost of installation is high, as well as the running costs. It
always requires automatic monitoring to ensure that the water quality is
sufficient.
The question must be asked whether the energy requirements of RO
desalination will upset the island's commitments to the Kyoto Protocol
and the EU's plans for greenhouse gas reductions. If, because we need
more water, we have to buy the right to emit more carbon, this water is
going to become even more expensive.
Large scale reverse osmosis is already well-known on
the island, from the installation at Dhekelia and south of Larnaka
airport and elsewhere. Such installations are very expensive to
implement and to run but can produce a continuous supply of potable
quality water at a cost of between €0.50 and €1.00 per tonne. The
Dhekelia plant has a throughput of about 40,000 tonnes per day, with a
planned upgrade to 50,000 t/day, while the Larnaka one is also 50,000
t/day. Of course, there is almost no theoretical limit to the amount of
water that can be treated if enough large-scale desalination plants are
installed, but the consumption of electricity would become prohibitive
for the island's generating resources. To circumvent this, they
frequently have internal generators, running off fuel oil, but this
electricity is much more costly than that produced from the power
stations, due to a lower overall efficiency. Internal generators may be
either gas turbine or diesel, the latter being more common. Either is a
severe source of air pollution unless the exhaust gases are carefully
treated: even so, additional carbon dioxide, the main "greenhouse gas",
is emitted. The motor-generators are also noisy and this causes
considerable resistance to the installation of such plants by local
residents. It is recommended that any new plants of this nature be
installed only in industrial zones, away from any habitation. Combined
with the high cost of production, the practical limit would not be
greater than about 100,000 tonnes per day on a full time basis. If, in
combination with restored natural resources, the total output from such
desalination plants exceeded the demand at any time in the future, the
excess water produced could be pumped into existing reservoirs. More
especially, they could be operated at night when there is an excess
supply of electricity available. When calculating the capacity of such
large plants, the down time for repairs and maintenance should be
considered at between 5 and 10 per cent because of membrane failures.
There is no evidence that the return of high-concentration salt water to
the sea will cause any severe environmental damage except within a
radius of a few tens of metres from the outlet pipe, which is totally
negligible. Notwithstanding, it is recommended that this be placed at
least 1 kilometre offshore in deep water to minimise any potential risk.
Plans to build a couple of new RO plants for Limassol and Paphos,
which were shelved by the Papadopoulos government, have been
resuscitated by the Christofias government and it is thought that
construction of them will be completed by 2010.
A wide choice of commercially available containerised
reverse osmosis units exists, with throughputs of 1 tonne per day up to a
few thousand tonnes. The very small ones are not economically viable,
costing typically €5 to run a 1 tonne per day unit. From about 250 - 500
tonnes per day, the cost is typically between €0.50 and €1.50 per tonne,
similar to large-scale units. An installation of this size could be
installed in a basement room of about 20 m2 floor space, in a
lorry or in a small external prefabricated building, as well as being
supplied in a ready-to-use container. This scale of unit can also be
equipped to run off diesel fuel, with low-noise engines, rather than from a
mains electricity supply, at a slight increase in overall costs. It would
therefore be viable at places without an adequate high-power electricity
supply available and, above all, as portable units which could be trucked to
any place on the littoral where there is an emergency supply required. The
small units are capable of supplying potable water of the required purity,
including bacterial count, to meet any standards, depending on equipment
specifications.
Multi-stage solar flash distillation is a very economical
process for large-scale desalination, both in terms of capital investment
and, above all, running costs. The main disadvantage is that it requires
very large tracts of land with unsightly panels, close to the sea. This is
really incompatible with most of the littoral in Cyprus. The principle of
operation is very simple: sea water is heated in solar panels to a
temperature of about 60°C and then sprayed into a chamber at a reduced
pressure. It boils off instantaneously and the vapours are condensed. The
process is repeated, typically three times, until the quality of the water
is sufficiently good. Because most of the energy required to operate the
system comes from the sun, the consumption of electricity is usually less
than one-fifth of that required for reverse osmosis for an equal production.
It is therefore less polluting and less likely to over-burden existing
electricity supplies.
Maintenance costs of this process is high because of the corrosivity of
the brackish water in the intermediate stages.
Air conditioning can also supply large quantities of
high-quality distilled water. This is the condensate from the coolers within
the individual rooms, whether the system be a central one with a chiller or
with separate compressors for each room. Obviously, this source of water is
seasonal but, in summer, one litre of water per person in a room is lost
through perspiration and respiration every four hours. This can be collected
at virtually no cost and added to other water for any use. In reality, this
water would be too pure for use by itself, except possibly for washing
purposes. The taste would be unpleasant due to a lack of mineralisation,
although it should be safe to drink if the recipient is changed frequently.
If used consistently for watering plants, it would require some additives to
ensure the good health of the plants, because there is no nutritive value in
the water.
The major obstacle to collecting this valuable water is in
the small quantity produced per room. Nevertheless, it is foreseeable that
aggregate quantities in the order of hundreds of tonnes could be collected
every day over the hottest six months of the year, particularly from large
buildings.
Importing water
In 2008, as an in extremis emergency measure, it was decided
to import water from Greece. This turned out to be a Comedy of Errors,
as 50 kilotonnes of precious water sat in the Limassol roads for 3
weeks, unable to be offloaded from the tanker ship because of a
succession of stupid errors due to a lack of foresight and planning. At
the time of writing, the first tanker load has still not been offloaded
while a second tanker is now sitting there, figuratively twiddling its
thumbs, and a third one is due to arrive. Has anyone bothered to work
out how much it costs to have tankers sitting idle because a pipe was a
few metres too short?
Be that as it may, importing water by tanker is a horrendously
expensive exercise and has become necessary only because of the
incompetence of the authorities in previous years. Technically, there is
no reason why it should not be done, but it places the country at the
mercy of the supplying country. It is hoped that this temporary measure
will never become permanent because Greece has its own water problems
and may close the tap if water becomes scarce there.
It would be much more logical to have a pipeline from a country with
an assured supply. Turkey springs to mind, but for the political
aspects: no one would favour an essential supply from a country that
could use it as a stranglehold. Of the surrounding countries, only Egypt
has an assured supply of fresh water, from the Nile. A pipeline from the
delta region to, say, Dhekelia would be technically feasible, even over
the Cyprian Arc fault line. It would probably be cheaper than using
tankers over more than a few years but the political aspects of ensuring
a constant supply would have to be negotiated as a long-term agreement.
This would allow the reservoirs in Cyprus to be constantly topped up,
giving a good autonomy in the event of a catastrophic failure. The big
advantage is that the presence of a quasi-unlimited supply would allow
agriculture and forestry to improve, increasing transpiration and
improving the local microclimates. This is something that should be
seriously studied.
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