Desalination
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Description
Desalination is the removal of salts from water, typically seawater. It can be used to provide water of a suitable quality for drinking, industrial use or for irrigation, depending on the equipment. Because of the tight ionic bonds between the water and salt cations and anions, desalination is a process which requires much energy to break the bonds and thereby separate the pure water from the salt.
Desalination is used mainly in countries with a littoral but with insufficient natural rainfall or ground water. These include many countries along the Mediterranean coast and in the Middle East etc.
Detailed description
The four most common ways of desalination are distillation, solar flash distillation, reverse osmosis and ion exchange.
Distillation
Distillation involves heating the seawater to boiling point and condensing the steam. Because water has a high latent heat of vaporisation, the energy required is very high. This process may be used therefore only where there is abundant low-cost energy. Because the boiling seawater allows the steam to escape by bubbling, the bursting bubbles throw up some raw seawater into the steam. A certain amount of salt is therefore carried through to the distillate. To produce relatively pure water of potable quality, the distillation has to be repeated three times or a reflux still used. The water is sterile.
Because of the fuel costs in most places, this process is rarely used today for large scale desalination, although it was popular over a century ago.
Solar flash distillation
This is a lower-cost distillation process where the sun is used to provide most of the energy. It was much promoted fifty years ago, but is less popular today because of very high maintenance costs. The seawater is heated to about 80 °C in solar heating panels and is then fed, by a fine spray, into a vacuum chamber, where about three-quarters of the water is instantaneously flashed off into vapour, the remainder falling to the bottom, with the dissolved solids. The vapour is condensed and the process repeated three times. The throughput of water is a fine balance between the water temperature and the pressure in the vacuum chamber, to maintain the equilibrium of vapour and water in the right proportions.
There are two problems with this process: the area of land required for the solar panels for large scale plants (e.g., supplying drinking water to a town of, say, 50,000 inhabitants) and the cost of maintenance because even stainless steel corrodes easily with hot concentrated seawater. Titanium could be used for the metalwork, but would be very costly.
Reverse osmosis
Osmosis is the process of salts and water passing through a semi-permeable membrane, such as a plant taking in the right proportion of water and nutrients through its roots. It works on the principle that the surface tension of water is lower when ionic salts are present. Reverse osmosis (RO), as the name implies, means forcing high-surface tension pure water through a membrane, thus leaving the salts behind. It is not a filtration in the accepted sense of the term, where particulate matter is captured in a porous medium. The process therefore consists of pumping the seawater at very high pressures into a membrane assembly. One point that surprises many is the ratio of dissolved solids before and after RO is nearly a constant with a given set of conditions, irrespective of the concentration. Ocean water has about 35,000 ppm total dissolved solids (TDS). A single stage RO assembly may have a purification ratio of 20:1, so that the purified water will have dissolved solids of about 1750 ppm. A second, similar, stage will produce 87.5 ppm water and a third stage 4.4 ppm. Ideally, drinking water should have a sodium ion concentration of less than 20 ppm. This level is problematic with 2-stage RO, while 3-stage RO is too good. If the RO water is used to complement low-sodium natural water, it is usually sufficient to mix the two waters. Alternatively, a small third stage may be used to dilute the second-stage water.
One of the big advantages of RO is that it can be scaled to suit any need. Commercial RO units are available to desalinate from 1-5 tonnes of potable water/day up to large plants that can supply cities with millions of inhabitants. The strange thing is that the cost of RO desalination does not change much from small to large plants, i.e., there is little economy of scale. RO does require some maintenance to prevent fouling of the membranes and to periodically replace them but the biggest cost is undoubtedly the energy required to drive the pumps at pressures of about 5-6 MPa. It should be noted that RO water is not necessarily sterile and either chlorination or UV irradiation is required to kill microorganisms before it can be considered potable.
Ion exchange
By itself, ion exchange would be too costly for desalination but it may be used to "polish" the water from any of the above processes to provide very high quality water for industrial processes. The principle of ion exchange is to capture a cation and replace it with a hydrogen ion and to replace anions with hydroxyl ions. The hydrogen and hydroxyl ions combine to form pure water. In practice, the medium for the exchange are small plastic beads which are packed into columns through which the water passes. There are two types of bead, cationic and anionic. These may be packed into separate columns, which are easily regenerated in situ or they may be mixed into a single bed which cannot be regenerated in situ. The latter is capable of giving the absolutely purest water with a conductivity of 0.056 µScm at 20 °C.
Energy
We have seen that water purification does require very high energy levels. Desalination is therefore usually a very polluting process with important greenhouse gas emissions, unless it can be done with nuclear power. Renewable energy is generally insufficient for large scale desalination (desalination would not even be justified if hydroelectricity were available!).
Water quality
Purified sterile water may be perfectly potable, but the flavour is often disagreeable because it is too pure; it lacks mineral salts. It is usual, if it is not mixed with naturally mineralised water, to add a minimum of essential minerals, notably calcium or magnesium bicarbonate to render it acceptable to the palate.
