The Desalination of Sea Water

Old Finnish well ~> 1950 Device, Gill mat and Wave power plant

The distilling of sea water is an old dream because salt-free water is in danger of running out. The
lack of water has become acute quite recently because the level of groundwater has gone down.
At the same time the population and the needs of the population have grown. Access to water is
being fought for. It may be that there will even be water wars. Where the author of this article lives,
far away in the north on latitudes 62 - 63oN, there is plenty of water for all needs, however. The
seas have endless quantities of water. Water becomes salt-free by distilling it, but it requires
a lot of energy. With the help of the heat from the sun desalination has become possible recently,
but still desalination is expensive, at least in the sense that it requires large investments.


Another idea which has been put forward and studied is desalination through reverse osmosis, i.e. compressing
sea water with strong pressure through microscopically small holes. This does not necessarily entail large
investments. The process of desalination needs less than one percent of the energy needed for boiling water,
and it can be produced through sun, wind or wave energy. The goal would be to have large desalination plants
on the sea shores. These plants would pump large quantities of sea water inland through large irrigation canals,
the water to be used as irrigation for farms.

Will the rivers of the future run inland from the sea transforming the present sand deserts into
fertile gardens by salt-free water for millions of people? Pumps and energy will be needed.

Osmosis in nature generally works so that the water in the cells tries to reach the more concentrated solution - i.e. from the outside of the cell to the inside. We say that disorder or entropy grows when the differences in concentration tend to diminish between the cell and its surroundings. This tendency causes pressure and the cell swells. The reason for this is the fact that large salt molecules don't get through the cell membrane, the small ones do, though. The pressure difference causes a balance between in-going and out-coming molecules. To decrease the entropy we need an outside energy source to separate the water from the salt.
Below you can see a diagram of a Desalinating device

The desalination device

The Idea

The idea is to pump the salt water with high pressure (> 50 bar) through a thin foil. Big chloride ions don't penetrate the foil, the smaller water molecules do. The pressure needed is as big as tens of atmospheres which requires strong structures for the plastic foil to hold out. In addition, the plastic must endure the eroding chemicals of sea water and it has to be full of holes of suitable size. The size of the holes should be such that water molecules get through but chloride ions don't, chloride being the main component of salt. Regular-sized holes can be made through ion bombing, perhaps with fluorine, caesium or chloride ions. Finding the right ion and plastic requires experiments. My own idea for a solution is shown in the diagram above.



A gill mat

A gill mat

The Realisation of the Idea

There are a kind of mats inside a pressure tank. The distillation takes place on the surfaces of these mats which I here call "gills". The gills are mats made of fibreglass and steel wire, enveloped by suitable plastic foils. The water seeps through the foil on the sides of the gills and is then stored. The foil must be carefully chosen so that sea water won't erode it. There can be hundreds of gills so their total area becomes large. A gill is a fibreglass mat with at least seven layers. The inner nets are stronger, woven steel nets. The water runs into the collecting pipe between the wires of the steel nets. Every gill has a pipe with an electric safety valve at the end. This valve will close the pipe immediately if the current grows too big, i.e. leaks out, or the outflowing water becomes too salty. The gills are mounted in their stands, they shouldn't rub on each other because they tear easily. Also unwanted substances may seep through the plastic, especially small molecules, so the water may not be everywhere drinkable but is suitable for irrigation, as washing water and for the industry. Drinkable the water will be after radiation of ultraviolet rays with lamp or sun.

In a typical case the area of the gills of a refinery would be from an are to a hectare.
The unit size could be closer to an are than a hectare because smaller units are
easier to maintain. Several smaller units can function side by side. This has the
advantage that during maintenance it is not necessary to wholly stop the flow of the
water, and the paid laborers can work continuosly. Furthermore, a standard-sized
unit is easier to manufacture, maintain and transport.

The recycling pump tries to put the sea water into a strong, swirling motion. This
prevents the formation of slime in the gills and especially the formation of a salt
layer which would clog up the gills. Salt is crystallised at 35% and on the surface
of the plastic foil that concentration can easily form. The strong circulation of the
water prevents that.

Before the actual desalination all rubbish must be separated from the sea water and
the water must be filtered with the help of sand layers in the normal way. The simplest
way to take the water is from a perforated pipe running in the sand of the sea shore.
The pipe takes the sea water seeping through the ground layers by the same method
which produces water called artificial ground water.

The pressure pump pushes water with considerable power inside the de-salting
apparatus. Part of the power can be retained if you put a turbine, which is re-
connected to the pump, in place of the choke, because choke spends energy.
The turbine can produce also electric energy continuously and according to need, if
concentrated soluted water is leaded into high (>500 m) located and big reservoir
before turbine. Most of the pumped-in water runs slightly more concentrated back
to the sea. The sea's wave, wind or sun energy can possibly be used directly to
achieve the necessary pressure. A wind power plant nearby is perhaps the best
source of power until now.

My draft is a wave power plant:

Wave power plant

The wave power plant is a concrete pontoon, which can be located by beach (See picture above).
The pontoon pumps high pressure water for gills (and to a turbine for energy) with four cylinders.
(By the way. I like wave power plants are good purely for energy production somewhere and right-sized.)

The gills must be washed at times, perhaps daily, with some alkaline detergent solution by pumping
it in. The water used here must not get into the pipe system because it's dirty. There is a break in
supplying the water at this stage. Even a continuing chemicalisation may be needed, e.g. because
of regulating the pH - value to prevent the formation of slime and corrosion of the pipe system.
Ozonisation is also possible but, because of corrosion, only after the desalination device. During
regular maintenance the worn-out plastic foils of the gills are changed.
The plastic foils will be need much, but plastic can be clean and smelt for many purposes, in other
word: recycle. If the energy needed for the desalination is discontinuous and/or the need of water is
periodic or randomly variable: Small lake or large water tank is needed as a water reservoir.

Tapani Hakonen Äänekoski EU

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