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What is done with the salt after making fresh water?

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An explanation of the distillation of seawater is shown in this video of the QM2

 

At "more than a thousand tonnes" of fresh water used on QM2 daily, and about 35g/kg salinity (Wiki), that's 35,000kg/77.000 pounds of salt/day.

 

One thing I don't understand is that by the looks of it, the way they remove the resulting salt wavers it can't be that much.

 

The other is: what do they do with it? Dumping anything in the sea seems forbidden, but returning salt a few hours later shouldn't be a problem? If it is kept on board, could I buy some?

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An explanation of the distillation of seawater is shown in this video of the QM2

 

At "more than a thousand tonnes" of fresh water used on QM2 daily, and about 35g/kg salinity (Wiki), that's 35,000kg/77.000 pounds of salt/day.

 

One thing I don't understand is that by the looks of it, the way they remove the resulting salt wavers it can't be that much.

 

The other is: what do they do with it? Dumping anything in the sea seems forbidden, but returning salt a few hours later shouldn't be a problem? If it is kept on board, could I buy some?

 

I think it is 3.5g/kg of water . Remember that the salt isn't pure salt . It has other impurities that you can't use . Since the water comes from the sea there will all sort junk in it.

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I think it is 3.5g/kg of water . Remember that the salt isn't pure salt . It has other impurities that you can't use . Since the water comes from the sea there will all sort junk in it.

 

I got the 35g/kg from Wikipedia, but even at 3.5g/kg there's still a huge amount of salt left.

 

There are more uses than eating it. You could create seawater with the exact right ingredients for use in aquariums, another use is to make my own little Dead Sea experience in a tub. And if it's not too contaminated, it could actually be sold for eating. Fleur de sel comes directly from the sea as well. Caribbean Ocean Salt would do nice next to Himalaya, Celtic and even smoked Norwegian salts I can find in shops.

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An explanation of the distillation of seawater is shown in this video of the QM2

 

At "more than a thousand tonnes" of fresh water used on QM2 daily, and about 35g/kg salinity (Wiki), that's 35,000kg/77.000 pounds of salt/day.

 

One thing I don't understand is that by the looks of it, the way they remove the resulting salt wavers it can't be that much.

 

The other is: what do they do with it? Dumping anything in the sea seems forbidden, but returning salt a few hours later shouldn't be a problem? If it is kept on board, could I buy some?

Now you know why the food tastes so salty!:eek:

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They use some of it for just-salt therapy. The rest they make pickles with.

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LOL...dumping salt in seawater is not prohibited. It has a negligible impact on the salinity of the ocean.

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After all, what happensto the salt when the H2O evaporation from the sea? Someone can't just pick up all that salt from the ocean when liquid becomes a gas right?

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Although I do not know the answer, I can not see what harm putting the salt back into the ocean from whence it came can do any harm to anything. It is not as if you are putting something into the ocean that was not already there.

 

DON

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I think Viking puts it in the soup they serve! We were on the Embla in April the soups were so salty they were awful!

 

 

Sent from my iPhone using Forums

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Looking forward to a definitive answer. All I could find from searching the Internet pertained to on-short desalination, such as this:

Q: What happens to the salt once it is removed from the seawater?

A: The concentrated saltwater is mixed with the circulating cooling water of the AES Generating Station, significantly diluting the concentrated seawater, and released back into the ocean, where it will be further diluted and dissipated to meet all local, state and federal standards including California’s Ocean Plan standards.

 

http://hbfreshwater.com/desalination-101

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I think Viking puts it in the soup they serve! We were on the Embla in April the soups were so salty they were awful!

 

 

Sent from my iPhone using Forums

That might be funny but you were cruising on rivers which are fresh water. Might want to learn science and geography between rants.

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That might be funny but you were cruising on rivers which are fresh water. Might want to learn science and geography between rants.

 

 

 

Your right, the truth is the food on the Viking Embla a least on our 21 day trip was Awful, it's so nice to have a nasty jerk like yourself, give me the opportunity to express that again!

 

 

Sent from my iPhone using Forums

Edited by goldenrod

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LOL...dumping salt in seawater is not prohibited. It has a negligible impact on the salinity of the ocean.

 

It is in some jurisdictions.

 

Really.

 

I had for a client years ago a salt company. When at their facility in MD, I noticed the entire paved area had a 6+ inch high curb. SO I asked why?

 

The reason was, the State of Maryland required them to capture all the run off from rain on there paved area and test the salinity before releasing it. INTO THE CHESAPEAKE BAY, which is estuarian (partially salty, less than the ocean).

 

But the salinity of the bay varies from month to month and year to year.

 

STUPID.

 

Never underestimate the stupidity of government agencies.

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It isn't dry salt, the flash evaporators have two outputs, fresh water from one spot and brine from the other, which is nothing but water with a lot more salt than regular sea water. It just gets pumped overboard when out to sea.

 

FWIW, if they use the same process we used in the Navy, the water is boiled at about 5 inches of vacuum which reduces the boiling temp to about 145F. Therefore they won't "make" water when near shore, since the water is usually much dirtier, and often polluted with sewage and other such nasties. Therefore they also chlorinate the water to ensure the nasties are dead.

 

Hope that helps.

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Thanks for the laugh. 77 lbs per day is perfectly safe to dump back into the ocean.

 

Except it's 77,000 lbs, not 77 lbs.

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It isn't dry salt, the flash evaporators have two outputs, fresh water from one spot and brine from the other, which is nothing but water with a lot more salt than regular sea water. It just gets pumped overboard when out to sea.

 

Unfortunately, the original video I posted was removed. I remember that it showed dry salt which was put in bags.

Maybe most salt is removed as brine, but at least some of it is "harvested" as dry salt before being thrown overboard.

 

I have bought sea salt in France before, which was quite expensive. So, if a ship arrives in Rotterdam and happens to have stored a few tonnes of salt, I'd certainly be interested.

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It isn't dry salt, the flash evaporators have two outputs, fresh water from one spot and brine from the other, which is nothing but water with a lot more salt than regular sea water. It just gets pumped overboard when out to sea.

 

Found the video again.

just after 31 minutes they shows the salt in the form of wafers. Still doesn't look as much as what you'd get from 1000 tonnes of water each day, but it sure is a lot.

 

It actually got a bit more interesting today. I spoke with someone from a company that literally buys seawater, delivered at 60 Euro/metric tonne, to grow seaweed on land. The water can only be used for a certain time before all essential minerals and whatever are depleted. And, the water is "harvested" directly from shore, not 12nm away with probably less pollution and bacteria. The latter wouldn't even be a problem at all as the process would kill them anyway.

 

These evaporators seem to produce exactly everything that seawater contains, except for water itself. 1000 liters is suddenly a 35 kg bag that could simply be sent by mail at a fraction of the cost.

 

So I'm still very interested whether Cunard and other lines found a buyer already, or if they simply throw the precious material overboard.

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Okay, I'm over here.

 

My internet on the ship is too bad to stream video, so not sure what you are viewing. If it is of watermaking on a ship, I can tell you categorically that they do not produce dry sea salt. Both evaporators and RO units will pump about 3 times the amount of sea water into the unit as the amount of fresh water that is produced. So, the two parts of sea water not turned to fresh water exits the ship with a concentration of salt about 50% higher than it entered the ship with.

 

Unlike what wraithe posted, most evaporators today operate at 28-30" of vacuum (near perfect vacuum), and boil water at about 120*F (48*C). RO's, of course, produce fresh water at whatever the sea temperature is.

 

Not sure what "process" you are referring to with the "harvested" water, but I can guarantee that water closer to shore has more pollutants and bacteria than open ocean, which is why ships don't produce water near shore.

 

What I believe you are seeing as you mention salt in "wafers" is them cleaning the evaporator. If the evaporator uses the plate heat exchanger (some are shell and tube), then the minerals (not just salt) left when the water boils off can adhere to the plate surfaces and reduce efficiency (heat transfer). There are chemicals added to the sea water entering the evaporator to help prevent this scale formation, but it isn't always 100% effective, so routine maintenance is to open the plate stack (perhaps 100 or more titanium plates) and clean them. These mineral "wafers" are very common, I just cleaned our evap last month, and pulled 10 or 20 off the plates.

 

As to the harm in changing the salinity of the sea water, it can have a large impact on marine life, both from dumping concentrated brine solutions (more concentrated than most evaporators) or dumping fresh water pool water into the sea. Just like salmon, who spend most of their life in sea water, and if placed in fresh water before sexual maturity would die, once their body makes an incredible change they can no longer live in sea water and must travel up rivers to fresh water. Other marine life require precise salinity levels to live, and changing those levels can threaten the species. This is why countries like the US have laws about power plants or water treatment plants putting out too much effluent at a time, and places like Saudi Arabia, where they use distillers and RO's to make municipal water on a grand scale, have had problems with marine life die offs.

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Okay, I'm over here.

 

My internet on the ship is too bad to stream video, so not sure what you are viewing. If it is of watermaking on a ship, I can tell you categorically that they do not produce dry sea salt. Both evaporators and RO units will pump about 3 times the amount of sea water into the unit as the amount of fresh water that is produced. So, the two parts of sea water not turned to fresh water exits the ship with a concentration of salt about 50% higher than it entered the ship with.

 

Unlike what wraithe posted, most evaporators today operate at 28-30" of vacuum (near perfect vacuum), and boil water at about 120*F (48*C). RO's, of course, produce fresh water at whatever the sea temperature is.

 

Not sure what "process" you are referring to with the "harvested" water, but I can guarantee that water closer to shore has more pollutants and bacteria than open ocean, which is why ships don't produce water near shore.

 

What I believe you are seeing as you mention salt in "wafers" is them cleaning the evaporator. If the evaporator uses the plate heat exchanger (some are shell and tube), then the minerals (not just salt) left when the water boils off can adhere to the plate surfaces and reduce efficiency (heat transfer). There are chemicals added to the sea water entering the evaporator to help prevent this scale formation, but it isn't always 100% effective, so routine maintenance is to open the plate stack (perhaps 100 or more titanium plates) and clean them. These mineral "wafers" are very common, I just cleaned our evap last month, and pulled 10 or 20 off the plates.

 

As to the harm in changing the salinity of the sea water, it can have a large impact on marine life, both from dumping concentrated brine solutions (more concentrated than most evaporators) or dumping fresh water pool water into the sea. Just like salmon, who spend most of their life in sea water, and if placed in fresh water before sexual maturity would die, once their body makes an incredible change they can no longer live in sea water and must travel up rivers to fresh water. Other marine life require precise salinity levels to live, and changing those levels can threaten the species. This is why countries like the US have laws about power plants or water treatment plants putting out too much effluent at a time, and places like Saudi Arabia, where they use distillers and RO's to make municipal water on a grand scale, have had problems with marine life die offs.

 

Thanks, Chief!

 

I've got a still and found a way to post it:

 

wafers.png

 

Yes, this must be cleaning. In the first post I already guessed that is not a way they could handle 35,000kg/77.000 pounds of salt/day, but the narrator says they do.

 

Still, is 35,000kg/day a correct guess?

 

What kind of chemicals are added to prevent scale formation?

 

 

Not sure what "process" you are referring to with the "harvested" water, but I can guarantee that water closer to shore has more pollutants and bacteria than open ocean, which is why ships don't produce water near shore.

 

The "process" I meant is making dry salt which would kill the bacteria, and by "harvesting" I meant that the salt a ship would produce would indeed be less poluted than the water they use now.

 

The video says (I'm getting a bit skeptic about it now :)) that water is pumped into one of three low pressure evaporators, heated by waste heat from the engines, and the steam condensates.

 

Both evaporators and RO units will pump about 3 times the amount of sea water into the unit as the amount of fresh water that is produced. So, the two parts of sea water not turned to fresh water exits the ship with a concentration of salt about 50% higher than it entered the ship with.

 

Here is another still of one evaporator:

 

evap.png

 

So if I'm correct, the output of this machine is one pipe that collects steam (33% of the water), and another that collects 66% of the same steam that also condensates into water, and returns it to sea, a bit saltier than before. From Wiki I understand that seawater is already at near saturation level, so it would even just push out dry salt?

 

How you can manage a near perfect vacuum while pumping in 1000 tonnes of water a day? That feels like very expensive pumping CO2 out of Coca Cola..

 

I also don't understand why you'd get three times as much water as needed to boil, only to pump out hot water. Wouldn't it be more efficient to get 100% of the water turned into steam, and 100% of the salt as dry material?

 

Google isn't helping much to help understand these mysterious machines. The one thing I do understand is that the ships won't be selling clean salt :rolleyes:

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Yes, that is a photo of cleaning the evaporator plates. No, you don't produce that much salt. For an evaporator that makes 500 tons of fresh water per day, you might get 3-4 55 gallon drums of scale every 6 months. The goal is to not have these "wafers" form at all. These wafers tell you that you are not doing something right, either running the temperature too high, or not dosing enough feed water treatment chemicals to prevent them.

 

The chemicals used to prevent scale formation are predominately polymers and anti-foaming agents. The polymers create a film around the scale particles to keep them from adhering to the plates, and the anti-foamers work to do just that, because the minerals and such in sea water, when boiling (ever done a crab or lobster boil?) will cause foam which will interfere with efficient heat transfer.

 

Just like boiling water in a pot on a stove, there is some water boiling off, while some water remains in the pot. When boiling sea water, the steam formed is pulled into another part of the evaporator, where the incoming sea water is used to cool and condense this water (shell and tube or plate type heat exchanger, so no contact), and the incoming sea water gets a pre-heat. The water remaining "in the pot" increases in salinity, and is pumped overboard. This is a constant flow process, so 3 ltrs of sea water are pumped in, 1 ltr evaporates, and 2 ltrs are pumped overboard. Doing it this way, along with the use of vacuum to reduce the temperature of boiling, reduces the amount of scale that builds up on the heat transfer surfaces. Some of these scale deposits need to be removed using an acid soak, so even after you remove the "wafers" from a plate type unit, you will take the plates and soak them in acid to remove the remaining hard scale. To reduce this downtime, and man-power intensive maintenance is why you pump twice the amount of fresh water made overboard as high salinity brine. Otherwise, you would be cleaning the unit daily.

 

So, you are not boiling the 2/3rds of the water that goes overboard, that is just the water "in the pot".

 

You create the vacuum by using some of the sea water that is pumped for the evaporator to drive an eductor. Here the water is passed through a nozzle, which increases the speed of the water, and creates the Bernoulli effect around the nozzle, or drawing a low pressure there. This does two things. It draws air out of the evaporator shell to make the vacuum, and it also draws the brine out of the extremely low pressure in the shell to send it overboard (its hard to pump from a near vacuum). An eductor like this uses a small percentage of the sea water, and can draw a large evaporator (600 tons/day) down to 30" of vacuum within 3-5 minutes.

 

Some units will also use the warm brine to heat the cold sea water in another pre-heat exchanger, to increase efficiency. Again, the heat is "waste heat" in that if the heat from the diesel engine cooling systems isn't used in something like the evaporator, it would just be transferred to sea water that is sent overboard, losing that heat energy. So, since it is already "waste heat", "wasting" some sending warm brine overboard isn't a problem, if it reduces the amount of maintenance needed. These types of evaporators can work 24/7 for 6 months to a year without any maintenance what so ever.

 

Here's a quick, down and dirty description of evaporator operation and maintenance:

 

http://marineengineeringonline.com/fresh-water-generator-or-evaporator-alfa-laval-type/

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I'm having flashbacks to my navy days and getting my ESWS pin with all of this evap talk. :o

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