Thursday, September 1, 2011
Coleridge's Ancient Mariner
Singapore is the third most densely populated country in the world. Water is a huge issue and constraint for them. With 7,000 people per square kilometer, its land mass is not large enough to supply its five million inhabits with water.
Seawater has always been seen as our collective solution to local water problems (when the oceans hold 97% of our surface water, getting salt out of saltwater is everyones Plan B). The issue has always been cost - - cost in the context of energy requirements for salt removal. German engineering may have a solution - - Siemens electrochemical desalination is currently being tested in Singapore (a full-scale pilot plant should be completed by 2013).
The following was in the September 3, 2011 issue of The Economist (Drops to drink):
"To make seawater fit for human consumption its salt content of approximately 3.5% must be cut to 0.5% or less. Existing desalination plants do this in one of two ways. Some employ distillation, which needs to have 10 kilowatt-hours (kWh) of energy per cubic meter of seawater processed. Brine is heated, and the resulting water vapor is condensed. Other plants employ reverse osmosis. This uses molecular sieves that pass water molecules while holding back ions, such as sodium and chloride, that make water salty. Generating the pressure needed to this sieving consumes about 4 KWh per cubic meter. The Siemens system, by contrast, consumes 1.8 kWh per cubic meter, and the firm hopes to get that down to 1.5 kWh.
It works using a process called electrodialysis, in which the seawater is pumped into a series of channels walled by membranes that have slightly different properties from those used in reverse osmosis. Instead of passing water molecules, these membranes pass ions. Moreover, the membranes employed in electrodialysis are of two types. One passes positively charged ions and the other passes negatively charged ones. The two types alternate, so that each channel has one wall of each type. Two electrodes flanking the system of channels then create a voltage that pulls positively charged ions such as sodium in one direction and negatively charged ions such as chloride in the other.
The result is that the ions concentrate in half the channels, creating a strong brine, while fresher water accumulates in the other half. As the brine emerges, it is thrown away. The fresher water is put through the same process twice more and eventually has its salt concentration reduced to 1%. That is not bad, but it is still double what is potable. There is therefore one further step. This is to employ an ion-exchange resin in addition to the membranes. Such resins increase the electrical conductivity of the system and allow one more passage, bringing the salt concentration below 0.5%."
Two key points. The first is the notion that all water constraints and issues are local or regional problems - - yet water technology is a perfect example of potential global solutions. The solution to your particular water supply problem may have a solution on the other side of the planet. The second point is that metanational firms, such as a Siemens or GE, are very good at taking technology and research from Point A and applying it at Point B. Those engineers that can think and capture knowledge globally, yet apply it locally, are going to be in high demand.
Samuel Taylor Coleridge's The Rime of the Ancient Mariner probably said it best - - water, water, everywhere, nor any drop to drink.