Man-made Resources
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Man-made Resources

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.

Desalination

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 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

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.

Large-scale

Large scale reverse osmosis is already well-known on the island, from the installation at Dhekelia and the proposed installations 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 $1 and $1.50 per tonne. The Dhekelia plant has a throughput of about 40,000 tonnes per 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.

Small-scale

A wide choice of commercially available 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 $1 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. 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

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.

Air Conditioning

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 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 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.

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