Salt Levels in the Sea!

Does the water ultimately get back to the sea?

The brine water is typically mixed with sea water and returned to the sea....

http://www.abc.net.au/science/expert/realexpert/desalination/01.htm

If there is a side loop maintained with little or no salt in it, then the main body of water will have a (slightly) elevated salt level. Some arithmetic can tell how much.

1) Compared with the volume of the oceans, the high salt waste is miniscule. Therefore it will have negligible overall effect on the oceans salinity. Only in the vicinity of the discharge will there be any change.

2) Much of the desalinated water will eventually find it's way back into the ocean via evaporation-to-rain and sewage discharge, to redress the balance.

I cannot think of any situation where the pure water extracted never gets back into the atmosphere or returned to the oceans.

There have been studies that state the Red Sea may experience higher salinity levels as there are no rivers discharging into that body. Back in the 90's (IIRC) Mexico came to the same conclusion concerning the Gulf of California. As for the Red Sea, I see no reason why the brine could not be dumped into evaporation ponds in Saudi Arabia. It is mostly uninhabitable desert. Eventually they could make building blocks from the stuff.

Unless we send water into space, then it may never get back to earth.

The oceans are getting saltier all the time due to the process of evaporation, rain, and runoff. So if you short circuit this process and remove the water for use without the runoff, you may actually be reducing the rate of salt build up.

The process of evaporation, rain and runoff are exactly why the oceans are not getting saltier all the time. It called the hydrologic cycle.

My understanding was that the minerals (primarily salts) in the oceans come from the rain dissolving them from rocks and carrying them to the ocean via rivers.

I can see there might be some fluctuations from dilution due to glacier melting, but it seems that there should be an overall trend to an increase in salinity. The salt stays behind when the water evaporates from the ocean.

if it was all dumped into a lagoon with poor mixing it would certainly have an effect. if discharged where it could combine correctly it's net effect wouldn't be traceable

It makes no difference, as others have said. The desalinated water eventually gets back into the sea.

I've got Bill Bryson's A Short History of Nearly Everything in front of me. This says "Of the 3% of Earth's water that is fresh, most exists as ice sheets. Only the tiniest amount - 0.036% is found in lakes, rivers and reservoirs, and an even smaller amount - just 0.001% exists in clouds or as vapour". On that basis, even if the (presumably used) desalinated water were stored somehow, or put in lakes, the effect would be quite negligible.

But Bryson also says something interesting - deep-sea vents pump out huge volumes of water, only discovered in 1977, which he implies without actually saying so is fresh water, and this explains why the oceans don't get saltier over time, due to salt being continually leached from the land and carried to the sea by rivers. This vent water doesn't of course come from a store, it's drawn in from the ocean and the salt is removed (he says) by some process. I don't know what natural process that would be, so I'm a bit sceptical. Also if the system is in equilibrium, an amount of salt equal to the input from rivers must be being stored somewhere.

As I understand it the salt in the sea is due to leaching from land over billions of years, the process presumably slowing over time as there is less left in the land (unless plate tectonics provides new sources). If salinity is still rising, my guess is the increase over the last few hundred years when measurements might have been taken is negligible. Also salinity is not the same in every part of the oceans. As evaporation is higher at low latitudes salinity is quite a bit higher there than nearer the poles, so very careful measurements would be needed to determine if it is changing.

Anybody got any thoughts?

Thank you, I read the same book, and recalled the passage about the vents, but couldn't remember where. Unfortunately, I can find no other references confirming the statement that all vents discharge pure water. (the ones in question could just be fed by clean springs, just like Silver Springs or Clearwater in FL. It would be interesting to discover how the vents remove all the dissolved salts as well as the other contaminents.

Yes, I thought afterwards about springs and underground rivers. There's a river Fleet under London, and according to Stephen Fry on QI, a UK TV series, there's one under the Amazon with higher flow than the Amazon (and that's a lot!). They must be fed from rainfall like normal rivers, and come out into the oceans somewhere, maybe via Bryson's vents, but already fresh. In that case they'd be no different from surface rivers on the salinity balance.

Also as Earth is about 4.6*10⁹ years old, if ocean salinity started out low say 2*10⁹ years ago (don't know what the experts say, but unlikely to be less than that), and has risen steadily since, in the last 200 years it has changed by 1 part in 10⁷, and there's no way that could be measured.

If Wikipedia (and the back of my envelope) are to be believed, the total mass of seawater of Earth is about 1.3 x 10¹⁸ tonnes, and as the mass is about 3.5% salt, the total mass of salt in the oceans is about 4.5 x 10¹⁶ tonnes.

Whether or not the desalinated water, or indeed the extracted salt, is returned to the oceans, it would take a colossal amount of desalination activity to make any appreciable impact on the overall salinity.

Just wondering: do desalination plants utilize the discharged brine as a concentrated source for the recovery of the sea salts (MgCl2 and CaCl2 as well as NaCl)? If not, why not?

Apparently, it's not economical. Mining is usually cheaper.

http://www.abc.net.au/science/expert/realexpert/desalination/01.htm

Global Seawater Technologies is in the business of mining the oceans for (1)magnesium, (2) lithium, and (3) other useful salt minerals. They also sell the fresh water produces from the Gulf of California (Baja) to the local resort community, and also sell ultra-pure water to a local chip fab.

It has also been considered in the past to attempt to collect precious metals out of seawater by using ion exchange (or a combination of separation technologies) to arrive at an enriched brine, or trapped precious metals on ion exchange resin for further refining. I am not sure that anything was actually perfected for this, as they are literally trying to catch needles by winnowing a haystack, and with the salt content of seawater being what it is, it reduces ion selectivity in some cases.

many thanks for the input everyone!

Dear Mr.brich,

The return of the rejects in to the sea from de-salination plant - theoritically you are right.

But if you work out the Mass Balance - the pure water taken out is a very tiny fraction and salt content - added per day is also tiny portion when compared to the salt quantity in the sea water. and needs several billions of years

You have mentioned "De-sal plants sustainable in the long term? "---- the long long run will need several billions of years where as the scientists have estimated that the present sun in our Solar System has balance life of of 4.5 billions of year - just half life remaining and half already over. There after, asper scietists, the solar system will be extinct

DHAYANANDHAN.S

GA.. I'd suspected that the problem could be the size of the straits..... I debated this point rather than, as my pals decided, to take into consideration the WHOLE of the world's oceans!

I was not aware of the algae problem..... something else to consider!

Interesting figures, but OP didn't refer to discharge to a restricted body of water.

by virtue of the fact that the Majority of the Desal plants are on the East coast of the Arabian sub continent.... from the Straits of Hormuz going North.... its restricted!

That's where local knowledge comes into play

JH: There are emerging desalination technologies that may easily surmount the salinity issues in the Arabian Gulf, and any other area where the water is in a shallow inlet, and does not circulate or mix well with the rest of the ocean.

1) shock the water! This relies upon a channel containing a special media, ion exchange membrane at or near the electrodes, and axial flow. The electric shock produces an osmotic shock wave in the flow that results in a saline sub-channel, and a lower ionic strength sub-channel that are easily separated. No osmotic pressure limitation since it is not RO. This is a completely new research report from a couple months back, not ready for market.

2) Forward Osmosis - this has been around for years in recovery of mine leachate water, and other applications. Forward Osmosis where the draw solution is a thermally labile salt (ammonium bicarbonate for example) is a hybrid membrane-thermal process, and requires temperatures to break up the draw salt of 45-60 C. This is a typically lower quality heat input than distillation. The draw salt as ammonia and carbon dioxide gases are fed back into the draw solution (cool side), and fresh water is recovered (may opt to then RO or otherwise treat this fresh stream). FO is not nearly as subject to membrane fouling as is RO, but can easily suffer from salt gradient polarization at the membrane surface if the membranes are non-optimal, and water circulation rates on the front and back sides of the membrane are not optimal.

3) Boundary layer exclusion. It turns out that boundary layer at Nafion membrane (and other such membranes) extends much farther out into the bulk water than was earlier believed (Gerald Pollack's group discovered this). With the boundary layer up to 500 micrometers, it is a simple matter to flow the water in a Nafion tube (in sunlight) with a concentric tube taking off the still saline part of the solution, and fresh water recovered from the annulus. Apparently, a small amount of time is required to establish the exclusion zone, but this can be overcome easily by setting a large tube array.

The list keeps going.

"Would you like some fries with that salt...."?

'my questions are....would large scale De-sal plants and the dumping of waste (salty) water back to the ocean affect the salinity, would it have a far reaching effect....'

In answer to this question posed, it would have effect to the local coast line by changing salinity in a local area, marine life would be affected. A high salinity brine would affect temperatures as it is denser, and this brine would also contain chemicals from the plant which might not be that good for marine life.

In dumping the brine, which contains chlorine, it would mix in the sea currents and simply be deposited elsewhere along the coast line, affecting a larger area. The more de-salination plants, the more damage to coast lines. Every action has a reaction which has a consequence, and mans actions take years to rectify.

http://www.paua.de/Impacts.htm

http://science1.nasa.gov/earth-science/oceanography/physical-ocean/salinity/

https://www.youtube.com/watch?v=5xQP_B18vMw

http://www.nasa.gov/press-release/international-spacecraft-carrying-nasa-s-aquarius-instrument-ends-operations

Salt concentrations in seawater are in dynamic equilibrium with both deposition and dissolution of mineral salts happening concurrently. Any added salt would result in increased deposition and any removal would increase the dissolution. Overall the seas are balanced but not equal at all points.

Can you explain this a bit more in detail?

It looks like the Saudis are planning ahead...

"SAN DIEGO (AP) -- Saudi Arabia's largest dairy company will soon be unable to farm alfalfa in its own parched country to feed its 170,000 cows. So it's turning to an unlikely place to grow the water-chugging crop - the drought-stricken American Southwest.

Almarai Co. bought land in January that roughly doubled its holdings in California's Palo Verde Valley, an area that enjoys first dibs on water from the Colorado River. The company also acquired a large tract near Vicksburg, Arizona, becoming a powerful economic force in a region that has fewer well-pumping restrictions than other parts of the state.

The purchases totaling about 14,000 acres enable the Saudis to take advantage of farm-friendly U.S. water laws. The acquisitions have also rekindled debate over whether a patchwork of regulations and court rulings in the West favors farmers too heavily, especially those who grow thirsty, low-profit crops such as alfalfa at a time when cities are urging people to take shorter showers, skip car washes and tear out grass lawns."...

http://news.yahoo.com/saudi-land-purchases-fuel-debate-over-us-water-061703957--finance.html

re pH value see the link below:

http://chemistry.stackexchange.com/questions/36904/does-dissolving-salt-in-water-change-the-ph

So there maybe is a change in salinity as talked about in the above threads, but the pH value should not see a change from feeding more salty water back into the sea.

This question has gone from the original question of, "affecting the oceans salinity", to obtaining fresh water. Began as an interesting question.

The fact remains, one will still need to be rid of the brine and salts and chemicals produced in the process so where would one dump the waste and what could be the final use of the waste, besides dumping in back in the seas?

Bonneville salt flats is so far a good idea but not sustainable in the long term.

Have you never heard of zero liquid discharge? There ways to boost fresh water recovery (HERO, etc.) to the point that other lower throughput technologies can take over at the high salinity limit of RO. I was in fact, completely on point with my answers, and I can't help it if some in the audience cannot keep up.

As to effects on ocean salinity, most people here are not seeing the full picture. The Earth is a vastly large open system (in spite of some of you thinking in a strict one dimensional or two dimensional sense with respect to salt balance in the oceans). Have none of you ever heard of subduction at tectonic plate boundaries? Although the salt input to the ocean has been looked at and looked at in this discussion (more of a glance really), no one has mentioned until now the ultimate outputs of (1) saline sediments being drawn down out of "circulation" at geologically active zones, and (2) geological processes that raise up shallow seas, isolate them, and thus entrap large deposits of salt. Mankind's activities diminish with respect to these processes.

Looking at de sal pants around Au, only a portion of fresh water is recovered from the sea. Much is returned to the sea as a brine as recovering the salts is not economical. However, in returning to the question posed, if the brine is not mixed properly with the sea at the discharge point, normally far enough off shore, or dumped by a vessel, the brine is devoid of oxygen and is heavier than the surrounding sea water, the effect can cause desolation of the area.

The brine generated as a wastewater during desalination is heavier than seawater, so if incorrectly discharged to the ocean, it would simply sink to the sea floor. In addition, the brine is devoid of dissolved oxygen as a result of the desalination process.

If it is released into calm water it can sink to the bottom as a cloud of salty water that can kill organisms on the sea bed, due to the lack of oxygen in the water.

Therefore, if there is a low current in the waters, the result is a problem for a coast line so it really means that siting a de-sal plant is optimum and the more we have along a coast line the greater the problem for a sustainable coast line. A coast line with a high energy swell is a requirement.

So the answer is two fold, it does affect the immediate coast line by adding brine to an area of the sea/ocean if a low current is present that prevents the brine mixing properly, and if a high current is present it helps mix the brine with the local waters, and if nature does not comply with fast currents, that brine can affect salinity by producing pockets of high saline water, as dumping the brine produces dead zones for marine life.

http://yaleglobal.yale.edu/content/dirty-pricey-obsolete-why-desalination-not-worth-its-salt

In the Middle East, where desalination is most heavily used, coastal areas around the facilities have become dead zones with incredibly high concentrations of salt and toxins.

In essence, the more run off one has, the more salts are being added and the more de-sal plants, the more salts we dump into the seas, so realistically we are increasing the salinity in the local area of the coast line.

As for cost to run a de-sal plant, the latest information is that de-sal plants are not economical due the the high costs of building one, around US$500-$900m, and the running costs.

http://www.bloomberg.com/news/articles/2013-05-01/energy-makes-up-half-of-desalination-plant-costs-study

Desalination plants on average use about 15,000 kilowatt-hours of power for every million gallons of fresh water that's produced, the Pacific Institute said today in a report. In comparison, wastewater reuse draws as much as 8,300 kilowatt-hours of power for the same volume and importing a similar amount of water into Southern California requires as much as 14,000 kilowatt-hours of electricity

And I think that helps answer the original question.

Where do you get the idea there is no oxygen in the brine? True that oxygen is at least as permeable through the membranes as water is, but it will not permeate "uphill" to the extent that the oxygen chemical potential in the permeate is higher than the oxygen chemical potential is in the brine. Any lost oxygen level is minimal, and that is easily recovered by (1) making sure the water is agitated with an air head space, and (2) ensuring the brine is at least as cold as the ocean source where it will be released.

Releasing the brine through a pipeline is admittedly the cheapest way to dispose, but this probably is not allowing good mixing, unless there is an eductor to draw in surrounding seawater, a static mixer, then the outlet. This does require slight elevation of discharge pressure at the point of entry to the pipeline, but this does not need to be more than a 10 psig increase in line pressure to drive the eductor and the mixer.

If the real problem is salinity, then brine discharge should stop. Find ways to further economically treat the water (that is consistent with environmental laws prevailing, and with nature conservation that we all depend upon for food). Once the brine is at or near saturation, it can be disposed of in deep wells further inland, or evaporated in brine ponds, or evaporate with salt recovery. Zero liquid discharge is no longer to be considered an expensive, exorbitant luxury, but necessary to protect the delicate balance of nature.

Try here as it is closer to your home for brine plumes and their effect.

http://www.ce.utexas.edu/prof/hodges/site2006/project_pages/project_desalination_brine.htm

Try here too, at page 20

http://ses.library.usyd.edu.au/bitstream/2123/1897/1/Desalination%20Plants.pdf

No way to insert the hyperlink, so the manual method is necessary.

OK, I reviewed the documents you provided link quotes to. It is not the paucity of oxygen in the brine that is a problem, it is the lack of mixing, and stratification of the shallow bays such that bottom dwelling life forms (and in that adjacent column of water) will die off due to apoxia by consumption. That is specifically why I suggested a better discharge method, further out if need be to catch currents, but more importantly, there needs to be a high ratio of mixing at the point of discharge, or the saline heavy solution will sink (even if warmer than the seawater present). Add to this changes in oxygen solubility due to the slight temperature increase, greater alkalinity (maybe), changes in ion activities, and pH (from pretreatment upstream of the desalinization steps), sure it is possible to kill off aerobic life within that zone.

When water becomes a critical need value, then complete recovery of the water is not only possible, it is the only answer. Zero liquid discharge is that answer. Technologies are emerging to deal with these highly saline streams by preparing them for electrodialysis followed by evaporative crystallizers that recover the last bit of water. Salt can then be hauled inland and buried.

Of the 13 Saudi Arabian desalination plants I have worked on 4 were powered by redirected flare gas, 2 were powered by waste heat from power stations and the rest were oil fired. In Saudi where the state is producing both the oil and the water (the water industry is pseudo privatized with some members of the royal family among water company shareholders) and where "fresh" water is 8-10 times more valuable per liter than oil, your economic figures are complete nonsense. Stand alone desalination plants may not be economically viable in the US or Australia, so don't build them as stand alone plants. Combine them with large quantities of waste heat that has no other more viable uses. Or use newer technologies that can be run from wave power. If you have enough waves to power the plant you also have enough waves to mix and displace brine waste effectively. You only need fresh water where people live. All of Saudi is desert, most of Saudi is uninhabitable desert, so they are stuck with plants on Gulf with a couple on the Red Sea to feed Jeddah and Mecca. Water for Riyadh is piped 467km from the Gulf and is typically only available to the general populace one day in three. For Jeddah fresh water that becomes one day in nine.(Figures based on 2009) When you travel to the Kingdom you may not notice this because all the hotels catering for westerners have large storage tanks and receive priority supplies.

My roll in desalination plants was to consult on the commissioning of chlorine dosing equipment and more importantly to instruct locals how to commission and maintain the plant on their own. I traveled to Saudi every couple of months over a two year period and stayed for five to six days per trip. I was never involved in the desalination process only the treatment of the resulting "fresh" water. It is a fascinating country with as many good points as bad and where I still have lots of friends. I would bet that most CR4 users don't know that Saudi Arabia has been bombing Yemen for the last two years using arms supplied by the US and UK.

Like I said, new tech will solve this disposal issue. Saudis deserve a much improved water supply, since now most of the coastal fresh water wells they had are spent.

They should be able to take advantage of solar power, solar thermal, reverse osmosis, forward osmosis (using lower grade heat), electrodialysis, distillation/crystallization, and some of the emerging technologies that do not foul, and do not have to overcome osmotic pressure to drive water (or draw water) through a membrane.

15 000 kwh for 1 000 000 gallon = 66.67 gallons/kwh.

i.e.: 1/66.67 kwh/gallon, i.e.: 0.015 kwh/gallon water production.

15w/gallon !

That's about 4w/liter.

And it did not specify what type of plant this is, i.e.: if it's R.O or vacuum type.

Vacuum distillation with solar or waste heat should be far lower.

Is it really expensive to produce water this way then ?

According to what has been researched on plants word wide and not specific to a country that has wave energy, wind farms, solar farms, and all the mod cons, it seems so.

Adelaide's Lonsdale plant is running at a cost of $40m p.a. and running at minimum capacity. I also know the salt and brine, (when I lived there), was a major hassle to dispose of and all sea dumping was stopped and the brine sent to a marsh land fill for a while. More costs were incurred in transporting the waste and that was not included in running costs. It is now dumped by ship off shore and via the pipe line 1000m off shore. The sea there is freezing cold, very rough all the time so the mix of the brine is pretty good.

So in generalising, it all depends where it is placed, what resources are available to drive the plant and how it is mixed in the sea off shore as to the final impact on a coast line. It is a relatively short fix in countries that suffer regular long term droughts, i.e Kuwait, Doha, Saudi, S Africa, Ethiopia, Iraq, Iran, most parts of Australia, etc. South Australia is the driest state in the driest inhabited continent on Earth. But long term, it has its downfalls.

This link is worth reading.

https://www.sawater.com.au/community-and-environment/our-water-and-sewerage-systems/water-sources/desalination/adelaide-desalination-plant-adp

Interesting.

A re-purposed V/ULCC tanker might do the trick.

Head out to sea, make full load of fresh water, from where seawater is cleanest, away from 'meddling' human, and get back to port/discharge point when tanks are full.

Plenty of mix and match ways to do this too, vis-a-vis, to maximize land based solar energy catchment facility.

(Takes a lot of heat to heat up water from ambient condition to, say, 80C.)

Expensive is not an absolute term. It is a relative term. Expensive relative to what? Not drinking? I don't think so.

Expensive compared to having a pristine river nearby to draw water from? Probably yes.

Expensive compared to a cool, clear, pure water well to pull from? Probably yes.

Utility.

i.e.: it's bloody cheap !!

Potable water, and regular wash water, including body-part hygiene maintenance

The rest is poet talk, which I'm not quite qualified

At least you are succinct and spot on with comments.

No need for 80 C water when 45-60 C is plenty hot for main driving force of thermolytic FO.