The oceans cover nearly three-fourths of the earth’s surface and yet there is a global shortage of water. This is because the water in the seas and oceans is not usable for drinking or agriculture. Only 2.5 percent of the total water available in the world is potable and already some one-sixth of the world population has inadequate supply of clean drinking water. Water borne bacteria cause the largest set of preventable diseases like diarrhea. The United Nations has made the right to clean water as one of its Millennium Goals.
Seawater desalination plants are being built in coastal cities around the world to provide drinking water. The largest such plant is the Ashkelon desal plant in Israel with a capacity to produce 330,000 liters per day, meeting 5-6 percent of Israel’s total water needs. Gas turbines of 80 MW capacity provide the power needed. The total project cost is $212 million and the cost of water produced is 5.2 cents per liter, acceptable for an arid country like Israel.
While such large projects will take time and money to get built, there are several innovative small capacity desalination approaches that have been conceptualized or developed for various applications. Perhaps the UN or other government agencies could fund the deployment of these low cost systems in the water distressed poorer areas of the world.
1. Solar Cucumber desalination plant
The Solar Cucumber is a concept low cost solar desalination plant by the UK based designer Phil Pauley. The design uses solar power and reverse osmosis to replicate nature’s water evaporation and condensation cycle. A series of floating plastic tanks called mini bus units are anchored together on the seacoast. Seawater enters the lower compartment of the unit, with the plastic wall serving as the primary filtration unit. The salt water in the lower compartment is pushed up against the reverse osmosis (RO) membrane that separates the upper compartment of the mini sub unit by the pressure of the outside seawater. The top compartment is made of curved transparent plastic that helps to direct sunlight on the water surface causing evaporation and lowering of pressure. This pressure difference helps the RO process to convert salt water to potable water.
The capacity of each mini sub unit in terms of liters of water per day has not been disclosed by the designer. The cost of the system appears low and the use of solar energy avoids additional costs. The use of plastic materials should result in a self cleaning device. The designer also says stringing these mini bus units along the coast will act as a natural reef which would encourage marine organism growth.
2. Haitian Desal-A-Nation
This concept by designer Joe Kasper is a simple distillation jar. The lower jar is filled with seawater or other brackish water and kept in the sun. The solar energy causes the water to evaporate and the water vapor condenses in the upper jar as distilled water. The lower jar is made of black material to increase solar absorption.
The designer has named this device Haiti, after the island country that is surrounded by the sea but has severe drinking water shortage. This simple distillation device could also be used in urban slum areas to generate clean water from polluted ground or river water.
3. C – Water
The C-Water is another solar distillation device , conceptualized by the Chinese designer Caho Gao. The device is a very simple molded plastic component with an open bottom and a sloping top. When the C-Water is placed on any wet surface including swamps, the sunlight causes the water to evaporate. The water vapor formed condenses on the inside the plastic component and the slope causes the droplets to run off into the container at one end.
The designer estimates that it would take about 47 hours to fill a cup of potable water.
4. MIT solar powered desalination system
A team from MIT has collaborated with a team from the King Fahd University of Saudi Arabia to build a portable seawater desalination plant that can be quickly deployed to produce drinking water in natural disaster areas. The photovoltaic panels produce the electricity needed to drive the motors that pump seawater at high pressure through RO membranes. The membrane filters all contaminants and salt from the seawater, making it potable. A prototype to produce 80 gallons a day has been developed for testing. One major advance being attempted is a control algorithm that can vary pump loads and regulate valve positions to make the plant operational even in cloudy conditions, when the solar electrical output is low. This avoids the use of storage batteries that are heavy and expensive. The unit is designed to be usable by non-skilled people.
The MIT team estimates that a 1000 gallon per day portable unit can be built for $8,000 cost. The unit would be transportable in a cargo plane to provide quick relief in natural disaster areas where the power and water supply lines are disrupted.
5. Solar Cube
The Solar Cube is a self-contained portable desalination unit that can produce 1000 gallons or about 3500 liters a day. The Solar Cube was designed for marine applications to generate potable water on board ships and barges. That system is now being packaged for land based applications. One unit was deployed to supply drinking water at an earthquake relief camp in Pakistan in 2005. The Solar Cube has been developed by Spectra of California working with Trunz of Switzerland.
The Solar Cube uses the proprietary ‘Clark’ pump that has two opposing pistons on a single rod to pump seawater through the RO membranes. The Clark pump uses only 13 watt-hours of energy per gallon which is about a fifth of the power used by other pumps. The pump motor is designed to work on 24 V dc so that power from a solar photovoltaic panel or a wind turbine can be used without an Inverter. The Solar Cube costs around $ 16,000.