Desalination: an Idea Validated

A few entries ago, I wrote about the possibility of using a concrete sphere for desalination: or removing salt from salt water to produce fresh, potable water. I had written that a pressure of around 1,000 psi is required for forcing salt water through a semi-permeable membrane to create freshwater; furthermore, that a concrete sphere with a compressive strength of around 8,000 psi seemed like an adequate safety factor for this application; and that concrete block can be consistently, reliably and inexpensively produced with this level of strength.


If a sphere made from block with a compressive strength of 8,000 psi were sunk to a depth of 2,225 feet, the head pressure, or water pressure, at that depth would be around 1,000 psi; enough to force salt water through a semi-permeable membrane, and enough to perform reverse osmosis necessary for desalination.

Currently, desalination through reverse osmosis is problematic because of the high energy requirements necessary to force salt water through a membrane. The practice of desalination through reverse osmosis is growing rapidly around the globe, as the need for potable water increases while its availability decreases. If we can provide fresh water from salt water while avoiding the high energy requirements, this would be a giant leap forward in dealing with the problem of providing fresh, potable water. Currently, the energy needed for desalination is typically provided by burning fossil fuels to generate electricity. This is inherently problematic and ultimately not sustainable.  It also creates salt-rich effluent, which is problematic in its disposal.  Below is a picture of a desalination plant.

The key to my approach is to sink the hollow concrete sphere to the proper depth, let the “free” pressure fill a sphere with fresh water, and then to allow an inflatable bladder to float the sphere (filled with fresh water) to the surface, so no energy is required to pump fresh water up 2,225 feet. At the surface, the fresh water would be harvested and the process repeated. This would be done a large scale (with hundreds or thousands of spheres) to gain the economy of design and produce a substantial supply of fresh water.

I did some additional research to investigate whether or not my idea was valid, or would hold water, if you’ll pardon the pun. I came across an old paper titled “Laminated Concrete For Deep Ocean Construction” (authors: M.H. Karsteter, Florida State U.; W.R. Karsteter, Environmental Concrete Design Inc.; and M.E. Roms, Consultant) published for Offshore Technology Conference, 2-5 May 1988, Houston, Texas.

Here is the Abstract from this paper: “The U. S. Naval Civil Engineering Laboratory has developed formulas to predict the collapse of hollow concrete spheres or cylinders and has shown that they can remain watertight under the pressure of chemically active deep ocean seawater. This paper tabulates wall thickness, volume of concrete, weight of displaced water, and gives a concrete cost factor for several interior dimensions of one-atmosphere habitats or valve chambers for oil and gas wells, and describes a low-cost method for building submersible concrete structures by shotcrete laminating in floating formwork.”

Here is Background from this paper: “In order to determine the long-term durability of concrete in the deep ocean, the U. S. Naval Civil Engineering Laboratory immersed 18 concrete spheres in the Pacific Ocean at depths ranging from 1800 to 5000 ft. Each sphere was 66 inches in diameter with 4-inch thick walls and was designed for a working depth of about 3000 feet at 1300 psi.

The design strength of the concrete was 8000 psi but after 5 years, tests showed a 15 percent increase. This remained the same after 10 years. No visible deterioration of the concrete was observed in any of the spheres and leakage varied from 0 to only 14 gallons after 10 years.

Spheres immersed beyond the designed depth collapsed and a formula was developed to predict the wall thickness needed for concrete spheres and cylinders of various outside diameters to survive at various depths (3). The authors contemplated that in the future, methods may be developed to build massive structures on the seafloor at which time it would be desirable to have designs for negative buoyancy and deeper depths.”

This paper was done largely for and by the deep oil drilling industry. There was no intent on using this research for desalination back in 1988. Nonetheless, the results are absolutely relevant to the problem I’ve been thinking about, and I feel as though my initial insight is completely valid and justified. The authors even speak of 8,000 psi concrete. They also note that strength of concrete spheres actually increases over the first five years.

This is a significant opportunity for a large market.  Fresh water provided sustainably.