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Introduction to Desalination Plants
When I was a lad at sea as an engineer on ESSO Tankers many, many years ago, there were water evaporators in these old ships' engine rooms which produced distilled water from seawater for the boilers. Because of their age, these were a constant source of annoyance requiring patience, liberal injections of Dow and Foss chemicals, several good cussings per watch, then a lots of cold beers after watch, in that order.
These older types of steam tube vaps are still in use today, however newer more modern designs with the latest technology are coming on the market. (Too late to calm my shattered nerves, but see my article on ships water-makers.)
This is an article on water makers, and in particular desalination of seawater using modern technology. We begin then with an examination of various types of new technology desalination plants under research and development or seen as candidates for future use.
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Modern Desalination Techniques
There are several promising new technologies emerging in the desalination of seawater. These include
- Forward Osmosis,
- Carbon Nanotubes Membranes,
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Modern Reverse Osmosis Desalination (RO)
Reverse osmosis has been used for various purposes since the early 70s, seawater desalination using reverse osmosis being the main method of producing drinking and irrigation water.
The process is quite simple; basically seawater under high pressure flows through a polymer matrix semi-permeable membrane. This membrane prevents the passage of salt ions and other solids in the seawater.
The water on the high concentration side of the membrane is pumped to 250 psi for brackish water treatment, (used for process and irrigation) but increased to 1000 psi for seawater, in order to overcome the inherent natural osmotic pressure of seawater. (350 psi)
The most modern desalination plant using the reverse osmosis technique has been installed in Salt City, California, being used to desalinate seawater from the Pacific Ocean. The seawater is drawn up through wells sunk into the sand, so the water is actually sand filtered before entering the desalination plant.
The desalination plant uses the latest reverse osmosis technology to turn the seawater into drinking water, along a new innovation of energy recovery to drive the pressurizing seawater pumps. This technique developed by ERI of California uses seawater at different temperatures in a PX pressure exchanger to provide the energy to run the pumps, with a saving of 90% of the energy used by conventional pumps.
Reference: Scientific American - Coastal California City Turns to Desalination to Quench it's Thirst
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Forward Osmosis Desalination (FO)
Forward osmosis, like reverse osmosis, uses a semi-permeable polymer membrane to separate the dissolved solids from the liquid being treated. However, unlike reverse osmosis, forward osmosis does not need expensive energy consuming pumps.
Forward osmosis uses a draw solution; a concentrated solution having a high osmotic pressure such as an ammonia carbon dioxide solution. This solution is fed from an adjacent tank into the osmosis cell, which, where due to its high osmosis pressure, draws the seawater through the membrane, with the membrane preventing the passage of solids.
The draw solution is then pumped into another tank where it is heated to convert it back to ammonia and carbon dioxide, before being recirculated to the osmosis cell.
Meanwhile the residual fresh water left in this tank is pumped away for post processing and storage for future use.
This technique is in the research and development stage; however in lab tests its ability to produce large quantities of fresh water from seawater, coupled with its projected energy efficiency, almost guarantees it will not be too long before we see a working model.
Reference: APEC - Novel Forward Osmosis Desalination Plant
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We have examines the desalination of seawater using osmosis, but there are other methods such as desalination of saltwater using thermal methods. Currently osmosis and thermal desalination share equally about 80% of the desalination plants operating worldwide, the other 20% being made up of electrodialysis, multi-effect and vapour distillation plants. The thermal methods all apply heat to the seawater, causing it to evaporate, this process is followed by distillation to freshwater. The most popular thermal type is the multi-stage flash, which has been on the go since 1950s. The modern ones can have up to twenty stages, each stage being under a lower pressure than the previous one. The hot seawater passes through each stage and flashing of to steam, the steam being condensed to fresh water for use as drinking water or irrigation purposes. Thermal desalination plants are costly to operate, but many plants use waste heat to boil the seawater, and these plants are very efficient producing up to 15 million gallons of fresh water a day. Main drawbacks are costs and scaling of internal pipes and surfaces.
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Fast Flow through Carbon Nanotubes Desalination Technique (NF)
Desalination of seawater using carbon Nanotubes technology has been tested and still under research and development by Lawrence Livermore National Laboratory. They have shown the feasibility of desalinating seawater using their nanotubes, with significant savings in energy and costs.
Carbon Nanotubes are made from rolled sheets of carbon atoms. Their micro size means that their smooth inside diameter can accept only seven water molecules side by side, being 50,000 times thinner than a human hair.
One would think that due to their microscopic size, the flow of water through them would be restricted however; tests have shown that the flow of water through the ultra-smooth bore of the tubes is comparable to that of considerably larger bore tubes. This results in a reduction in water pressure through the nanotube membrane, resulting in smaller pumps and energy savings.
The membrane consists of a silicon wafer chip impregnated with a carbon catalyst to produce the carbon nanotubes that form vertically and closely packed.
The spaces between the tubes are packed with a ceramic material which gives the membrane strength and help the billions of nanotubes to adhere to the silicon chip,with the nanotubes acting as pores, allowing water to pass through them but preventing passage of solids.
Although this process is a new concept, it could replace conventional membranes in the desalination of seawater.
Reference: Technology Review - Nanotube Mebranes Offer Possibility of Cheaper Desalination
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Biomimetics and Desalination
Biomimetics is the name given for mimicking natures processing techniques; the transfer of ideas from biology to technology.
The majority of living cells contain minute water tubes that conduct water but exclude dissolved solids such as salt.
In water desalination technique, an example is to be seen in tropical coastal mangrove swamps. The reeds (species of Rhizophoraceae) grow in salty water, sucking up water through minute capilliaries in their root system in a process known as transpiration. The membranes in the pipes allow water through, excluding the passage of the salt, a process that made biomimetics suitable for further research.
Website: Ask Nature (Biomimicry Institute) - Mebranes Desalinate Water: Mangrove
Website: Sandia National Laboratories - Membrane Technologies
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Pre-Treatment and Post-Treatment Of Desalinated Water
- Seawater Filtration/Screening
This water is passed through several fine mesh screens, then very fine cartridge filters to prevent the membrane from becoming clogged with solids.
Anti-biofouling inhibitors are used to prevent fouling of the membrane surface.
Chlorine dosing in small measures is used to kill bacteria within the unit.
Anti-scaling inhibitors are used to prevent pipes and internal surfaces from becoming coated.
The fresh water now requires treatment to make it safe to use as drinking water, through disinfection by UV, Ozone, or Chemicals.
- Ultraviolet (UV)
A UV light beam is directed at the water as it flows through a specialized tube, destroying micro-organisms by interrupting cell division, rendering reproduction of micro-organisms impossible.
Ozone is an oxygen molecule which has had an extra atom added. The ozone is normally added to the water to destroy micro-organisms by oxidization – using that extra atom of oxygen. It acts much faster than chlorine dioxide, being a much stronger disinfectant, but more expensive.
The current most popular and cheapest means of disinfectant for treatment of drinking water is chlorine dioxide, which destroys micro-organisms by cutting off nutrient transport links across the cell walls. .
Chlorine dioxide also destroys the microbes which are responsible for Legionnaires Disease, should the water be used as process water in air condition units.
Website: Lenntech Water Treatment Solutions - Water Treatment and Purification
Website: Advanced Equipment & Service - Pre and Post Treatment (for reverse osmosis and nanofiltration membranes)