A new look at seawater desalination

Earth is 70% covered by water but we cannot use it directly because of ‘salt’. This obvious problem might be seen as an opportunity.

Seawater is made fit for human use most efficiently by reverse osmosis (RO), and that is already a big deal, even though energy costs are high. In RO, water is pushed at pressure against a selective membrane, pure water goes through, and all dissolved salts stay behind. Here the ‘product’ is pure water and everything else is ‘waste’ and returned to the ocean. However, all of the chemicals are not ‘waste’ if they can be processed and sold, or further purified and sold.

The first thing to realize is that RO waste is highly enriched in the original ocean chemicals. There are many, and they have individual values, so let’s take a look, based on 1000 tons (cubic meters) of seawater. Global desalination is now about 80,000 times larger, per day.

First we consider the purified water as an economic product. One thousand tons of purified seawater has a current value in California of about $USD 760. Keep that number in mind.

The energy cost for this amount of RO (at 15 cents per kWh) is at least $450. We recognize that the RO plant is a large capital expense, but the economics don’t immediately look bad.

Have you heard about the vast quantity of gold dissolved in seawater? Globally, it is very large. Our thousand tons of seawater contains contain $0.38 of gold, $0.13 of silver, and similarly disappointing amounts of similar metals. They do not seem to be good targets.

So, what other value might be taken from the RO waste stream? The elephant in the room speaks. NaCl (plain old salt) has a value of $4530, and I am surprised to find that it so much exceeds purified water. Do RO to sell NaCl, with purified water as the byproduct? I wonder why no one seems to be saying this.

Magnesium is next, at $2670, and maybe you will be surprised (as I was) that it is the only metal now being commercially extracted from seawater. Those folks use electrolysis not coupled with RO. But if you have a car with ‘mag wheels’, more than half came from the ocean.

Potassium (as KCl) is $224; an agricultural fertilizer. If you grow crops in a ‘low- potassium’ setting you are screwed (it runs away every year), but if you sell the stuff, you have a captive market. Calcium (as CaCl2) =s $160. do not know where most of that goes.

Bromine ($200) and iodine ($5) present themselves because they are important for many chemical processes, and relatively easy to extract from this ‘waste stream’.

Seventy $ worth of sulfur (as S). do not know if this would be worth chasing or not. Maybe more valuable (or accessible) as sulfuric acid? That plays a huge role in industrial chemistry.

Lithium is $10.70. Maybe make some batteries? Strontium and barium are somewhat abundant, but I don’t believe the commodity prices I saw for them. Not examined.

I can’t imagine why you’d chase the $5 of silicon, as it is much easier to obtain from terrestrial sources. But it leads us to semiconductor chemistry. Those 1000 tons of seawater contain substantial amounts of Boron, Arsenic and Rubidium, along with smaller amounts of other elements very important for this field. The value of each depends entirely on how well they can be purified. No clever chemists seem to have addressed RO wastes for these elements yet.

What about other ‘rare earth elements’ for strong magnets and vehicle-exhaust catalysts? Not much in sea water. Total value 21 cents in this amount of seawater. Get them by terrestrial mining, RO wastes won’t help.

Here I examined potential values of some RO byproducts. Because people will need more water, this process is likely to expand, and it seems well to look at related financials. The dollar total from my list above (including the water, down to lithium, forgetting Ag and Au) is $8245. I made no effort to estimate extraction and purification costs of those other materials. But they are all many times more concentrated in the RO ‘waste’ stream than in seawater, and that is the first step.

In summary, the value of pure water from RO desalination of seawater is about 1/9 or the ‘potential’ value of chemicals that are now discharged back to the sea as waste. If you know any young chemists, ask if they would be interested in extracting things from RO waste. I'll do the same lots of chemists in this country.

 

Maybe. There was a TED talk on a very similar topic, using bacteria to bioconcentrate some of the chemicals. Would you have agreed with initial pessimistic assessments of powered flight, xerography, personal computers, etc. when they were first considered?

Maybe only if they were brought up in an online chat room eh?

 

Review of the subject published earlier this year

Feasibility of extracting valuable minerals from desalination concentrate: a comprehensive literature review
Arash Shahmansouri, Joon Min, Liyan Jin, Christopher Bellona
Journal of Cleaner Production 100 (2015) 4-16
Doi: 10.1016/j.jclepro.2015.03.031

Their Figure 1 has a lot in common with my analysis

For the moment it is to my advantage for most people to share your skepticism. What I need is interest from some chemists to write a grant proposal. Just last month I was close to these guys' shop in Potsdam NY. Shame to not yet have been bit by this bug.

John Mero has the idea in 1965 (wrote a book), so naming rights are out the window...

 

Ah, but which government? I am a bit of a man between countries now.

A fella could buy this report
Global Mining Metals Market Industry 2014
to see how things stand.

Shahmansouri also focused on the value of removing chemicals from RO waste prior to disposal. I guess this is not a big deal when you can dump at sea. However a lot of RO is done on inland brines and then you get super-duper brine. EPA no like.

 

A desal plant in Calif. is already selling calcium.

Israel as well as Saudi are heavily into RO, but it is actually global. Just need one desal plant, and tinker with their waste stream.

 

Marine biologicals hang on to (rare) phosphorous rather tightly, so, no. It seems that potassium for Ag is the killer app there.

Shahmansouri et al put uranium oxide 'above the line' at $300/kg. do not know its current price.

 

Most of these commodities have low price/ton, meaning that production near point of use saves transport costs.

The 'dream machine' takes the RO waste stream and turns it into money. Bisco Skep, supposing chemists to be purely rational beings. We'll see...

 

This stuff is way more fun for me than arguing 2015 temperatures. If by-product sales can make RO water more accessible in water-starved areas, I'm in.