Tuesday, July 13, 2010

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.


  1. The ballast required to sink an air-filled sphere to that depth would be huge. The difference in density between sea and fresh water is quite small which means a full sphere will have lost most of it's buoyancy. Therefore a lot of air will be required to refloat the ballast.

    Compressing air to the required pressure will be very energy intensive. Assuming the air compressors or water pumps have equal efficiency and that you have to pump (or compress) an equal volume without friction it would take the same amount of energy.

    As I type this I realise you could sink a solid to produce gas via chemical reaction when required. Whether or not you can make this economical depends on how much chemical reactant you would need and the cost(there's not much profit margin in the water game).

    In terms of sustainability the solid reactant will need to be produced/sourced which will have an energy associated with it (most likely fossil-fuel derived).

    1. While it is true that significant work must be done to sink a sphere, if ballast is used then it doesn't matter how deep the sphere is sunk; with enough ballast it will simply keep sinking.

      A large sphere filled with water will have near neutral buoyancy, so it does not require a large force to make it buoyant. Just a slight buoyancy will bring the water to the surface.

      See my other entry for some calculations (flawed and incomplete as they are):

    2. Yeah, my post sounds kinda negative, it wasn't meant to be. I just meant that unless you have a ballast that you can keep dumping in the ocean, you'll have to haul it back up - that will be your energy input. I'll read your newer post now. I've just found your blog and am starting from the very beginning