Seawater Desalination

Written by Dr. Klaus L.E. Kaiser

A new process, called “Electrochemically Mediated Seawater Desalination” has recently been described. The inventors claim that it could potentially lead to large-scale 99% desalination of seawater at low cost. Some people have dubbed the invention the “water chip.”

The EPA (Environmental Protection Agency) is listed as a “partner” on the company’s websiteDesalination water chipFig. The prototype plastic “water chip” contains a micro-channel that branches in two, and utilizes a process known as electrochemically mediated seawater desalination. Image source: Okeanos Technologies.

Laboratory vs. Nature

What may work well with trace quantities of high-purity materials in the laboratory does not necessarily work in the real environment. Most water, especially ocean water contains a multitude of constituents, from dissolved (mostly inorganic) salts to a large variety of organic compounds. Even quite pristine freshwater, like that of the Laurentian upper Great Lakes develops visible foams of natural compounds around its perimeter upon wave action due to the presence of natural surfactants.

Ocean water is full of such natural surfactants and a host of other “chemicals.” Therefore, any desalination process has to deal not only with the salt content but also with the other natural constituents of ocean water. That is why a simple, large scale desalination method has been a long sought after quest.

Current Desalination Methods

Currently, the most common desalination method is “reverse osmosis.” The process uses physical pressure to push seawater through membranes with pores so small that even the “atomic” salt ions (sodium and chloride ions) cannot pass through. As you can imagine, that process requires both a lot of energy and frequent cleaning or replacement of the membranes.

There are also other methods, such as freezing or distillation of the water, but they all require mega amounts of energy.

New Desalination Method

So far, the researchers reporting the new methodology have achieved a 25% desalination of 40 nano-liter (4×10^-9 L) of saline water per minute. To get an amount of, say 4 L per minute, the process would have to be scaled up by a factor of 10^9, or 1,000,000,000 [factor 1]. With 1,440 min/day, this would lead to 4×1,440=5,760 L/day. For a city the size of the one I am living in (half a million residents plus various industries), the consumption (combined industrial and personal use) of potable water is in the order of 300,000,000 L/day. To get that amount of water, the process will have to be scaled up another 300,000,000/5,760 = 50,000 [factor 2].

However that would still only be at a 25% salt reduction, reducing the salt content from 5% to 4%. Therefore additional similar salt-reduction steps would be required to lead to anything even semi-comparable to potable water (less than 0.05% salt to be consumable) in terms of salt content. At a 25% salt reduction in each step, a total of 16 [factor 3] such additional steps would be necessary.

Now, let’s put the factors altogether: 1,000,000,000 x 50,000 x 16 and you arrive at approximately 10^15 = 1,000,000,000,000,000 of such “water chips” required for this city – for the hardware alone. Of course, the energy required to produce the necessary electric field and current are extra.

I think you get the drift.

Problems of Scale

Many processes work fine on a small scale but are difficult or impossible to scale up to a commercially viable size. There are often numerous reasons for such problems. Moreover, any desalination process step may work well in the initial step (producing a 25% salt reduction from 5 to 4%) with normal ocean water containing approximately 5% salt, but may be much less efficient or fail to work entirely at the subsequent steps which would start with sequentially lower salt concentrations. Therefore, the new process could still fail to produce any potable water for human consumption.

In short, the road from this “water chip” to a viable large-scale system for producing potable water from the oceans is very long, perhaps with no end in sight – ever.

 

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Comments (1)

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    ewiljan

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    What is the total amount of earthling desalinization via reverse-osmosis and other means v.s the Sun’s evaporative seawater distilation that costs nothing, and provides 63% of the freswater production on this planet?

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