Monday, April 26, 2010

Making fresh water from salt water

The water storage spheres I described in my last posting have many applications. Lack of fresh water is a rapidly growing problem around the globe, and the method described is an inexpensive, high strength, long lasting solution to this problem. Today I’ll be describing another application for this technology, one which is closely related to storing potable water.

There are two basic methods used for making fresh water from salt water. Both of these methods are gaining much wider use globally, as the need for fresh water increases dramatically while supplies are reduced. Fresh water supplies are expected to become even scarcer, as glaciers melt and this source of fresh water disappears.

The two basic methods for obtaining potable water from salt water are distillation and reverse osmosis. Distillation uses heat to evaporate fresh water from salt water, the fresh water is then condensed and collected. Reverse osmosis forces salt water through a semi-permeable membrane under very high pressure, removing the salt component so that fresh water passes through the membrane.

Both of these methods require a great amount of energy. Distillation requires a lot of heat to evaporate the fresh water, and reverse osmosis requires a huge amount of energy to create the high pressure needed to force salt water through the semi-permeable membrane.

Reverse osmosis desalination techniques require an external pressure of around 1,000 psi be applied to force salt water through a semi-permeable membrane, removing salts. The energy used for this is typically provided through electrical pumps, electricity is typically generated by burning fossil fuels. The energy consumed for this high pressure makes reverse osmosis very expensive.
At a depth of 2,225 ft. below sea level, a pressure of 1,003.4 psi is provided by the head pressure of seawater. If a hollow sphere made of concrete block were sunk to this depth, it is possible to force seawater through a semi-permeable membrane, attached to a portal on the sphere, to perform reverse osmosis and provide fresh water to the inside of the sphere. Concrete block can be inexpensively produced with a compressive strength of around 8,000 psi. This provides a significant safety factor for assembled concrete spheres to collect fresh water at great depth.

Typically, reverse osmosis occurs in two stages. There are a number of ways to provide this two-stage treatment, using the high pressure of seawater at depth to provide “free” pressure. (I’m just not giving away all my secrets here).

Once desalination is complete, a simple inflatable bladder could then be used to raise the water-filled concrete sphere to the surface, removing the need to pump this water to the surface (pumping water up 2,225 vertical feet would also be very expensive). Once fresh water is harvested from the sphere (at the surface) the empty sphere could be re-sunk to the same depth and the process repeated. The high pressure to perform the reverse osmosis is essentially "free."

Spheres made from concrete block can be made much less expensively than pre-cast concrete spheres which are cast as one piece. By practically all measures, this is an appropriate technology which should find use globally.

This could be done on a large scale to harvest a high volume of fresh water, and gain the economy of scale which would further reduce the operating costs of this system.

Another promising aspect of this system is the recent (and ongoing) research into carbon-based “fullerene” structures which can serve as reverse osmosis filters. Research indicates that fullerene materials can work at a lower pressure than current state-of-the-art reverse osmosis filters.

There is a huge, growing market for an appropriate technology for desalination. 


  1. What do you need to help advance your project?

  2. I like the sound of this, but I see problems. Some ships have been made of concrete, so presumably overcoming the buoyancy of even a small hollow sphere would require extra ballast to increase average density. Dumping that ballast at the bottom of the ride would go some way to counteracting the cost of compressing air for the bladder otherwise you'd need a bigger bladder. Both of this things would require energy though.