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Thread: Question about high salinity brine released to sea!

  1. #1

    Question about high salinity brine released to sea!

    Hello everyone!

    New to the forum and the subject of marine biology in general.
    I'm trying to compare two desalination technologies on long periods of time (think 30 years from now).

    First technology produces a brine with salinity of 75,000 mg/L and the other technology produces a brine with salinity of 50,000 mg/L.

    Let us assume that I have the amount of brine produced from each plant over the next 30 years. How do I compare between the two salinity wise? Is it as simple as calculating the volume of 'salt' released?

    Thank you for your tips.

  2. #2
    Hi there! (this looks like one of my homework questions, hope you're not cheating lmao). The biggest thing to consider is, you're right, calculating the volume of salt, assuming they're releasing the same amount of brine. If this is just being dumped directly into the water then another thing to consider in terms of ecosystem effects is how it compares to the previous salinity of the area (for example, a 35g/kg cove would be a lot more chill receiving 35g/kg brine than a freshwater lake would). You could also look at how the change affects a particular species in the area (sharks and alligators can be pretty adept with salinity changes, but is their prey?) and how 'open' the water is (for example, one bucket of brine in the ocean won't do much, but a bucket of brine in a bathtub would completely change the water).

    I'm new to the forum too and still a student, but this stuff's always fun to talk about!
    Last edited by Ella Pearl; 07-25-2020 at 04:17 PM.

  3. #3
    Senior Member Pod of Cali Merman Storm's Avatar
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    One thing to consider: For a given amount of fresh water produced, you will get far more 50 g/L brine than 75 g/L brine. Starting with 35 g/L sea water with the goal of making 1 L of fresh water you can
    Start with 3.33 L of sea water, extract 1 L of fresh water, leaving 2.33 L of 50 g/L brine
    Start with 1.88 L of sea water, extract 1 L of fresh water, leaving 0.88 L of 75 g/L brine.

    In the second case, if you mixed the 0.88 L of brine with 1.45 L of sea water, the result would be 2.33 L of 50 g/L brine. In other words, it does not mater which desal method you use, you end up in the same place once mixing with sea water occurs.

    Usually, the brine discharge is done through multiple small holes to encourage rapid mixing, in order to get the salt concentration down to a level similar to the natural variability of sea water. In addition, the desalted water eventually makes its way back to the ocean, keeping its salinity level in long term balance.
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  4. #4
    Quote Originally Posted by Ella Pearl View Post
    Hi there! (this looks like one of my homework questions, hope you're not cheating lmao). The biggest thing to consider is, you're right, calculating the volume of salt, assuming they're releasing the same amount of brine. If this is just being dumped directly into the water then another thing to consider in terms of ecosystem effects is how it compares to the previous salinity of the area (for example, a 35g/kg cove would be a lot more chill receiving 35g/kg brine than a freshwater lake would). You could also look at how the change affects a particular species in the area (sharks and alligators can be pretty adept with salinity changes, but is their prey?) and how 'open' the water is (for example, one bucket of brine in the ocean won't do much, but a bucket of brine in a bathtub would completely change the water).

    I'm new to the forum too and still a student, but this stuff's always fun to talk about!
    Quote Originally Posted by Merman Storm View Post
    One thing to consider: For a given amount of fresh water produced, you will get far more 50 g/L brine than 75 g/L brine. Starting with 35 g/L sea water with the goal of making 1 L of fresh water you can
    Start with 3.33 L of sea water, extract 1 L of fresh water, leaving 2.33 L of 50 g/L brine
    Start with 1.88 L of sea water, extract 1 L of fresh water, leaving 0.88 L of 75 g/L brine.

    In the second case, if you mixed the 0.88 L of brine with 1.45 L of sea water, the result would be 2.33 L of 50 g/L brine. In other words, it does not mater which desal method you use, you end up in the same place once mixing with sea water occurs.

    Usually, the brine discharge is done through multiple small holes to encourage rapid mixing, in order to get the salt concentration down to a level similar to the natural variability of sea water. In addition, the desalted water eventually makes its way back to the ocean, keeping its salinity level in long term balance.

    Hello Ella and Merman!
    Thank you very much to both of you for taking the time to respond to me. Very useful tips and I was able to calculate the tonnage of salt released.
    I guess to get any understanding of how significant that number is, I would need to understand the mixing and the current salinity of the water where the plants would be.

    I did quick search and here is what I understood..
    There is an EPA upper limit for salinity in the region waters and its set at 42PPT. The current salinity in areas of existing plants is 40 ppt. Now I would imagine double the desalination capacity would drastically increase the water body salinity.

    I'm truly grateful for any additional tips on how to possibly relate the amount of concentrated brine released on the existing seawater salinity or the ppt in general.

    Originally I was simply going to compare two technologies and suggest that one with less salt tonnage is better. But if you believe that the tonnage alone is not enough i'll go ahead and get better resolution details.


    By the way, the two technologies will be producing billions of brine and not at equal rates. SO a 50g/L vs 75g/L is actually significant difference. But not producing equal amounts of brine makes the comparison not much straight forward.

    Thanks again for taking the time to respond to me and looking forward to hear from you.

  5. #5
    Senior Member Pod of Cali Merman Storm's Avatar
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    Where on earth is this, eastern mediterainean? That is a rather high salinity level.
    Another thing that can be done to lower the discharge salinity is to mix the brine with sewage outfall water. That also helps keep the sewage from pluming back up to the surface.
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  6. #6
    Quote Originally Posted by Merman Storm View Post
    Where on earth is this, eastern mediterainean? That is a rather high salinity level.
    Another thing that can be done to lower the discharge salinity is to mix the brine with sewage outfall water. That also helps keep the sewage from pluming back up to the surface.
    It is in the Persian Gulf where countries depend 100% on desalination as source of potable water!
    Perhaps some background information would help... I have constructed a techno-economic model with main focus on freshwater use at home. Typically in such models technologies would compete based on price of the technology itself, price of fuels, and emissions to air (because of carbon tax and climate change). I'm interested in including the salinity to help the technologies compete beyond the price of fuel and the carbon emissions. Sure there are so many ways to reduce the brine concentration... but for the sake of simplicity and since I'm not going to go deeper into environmental aspects in my study... I would like to generate a column in my table that would quantify the harmful aspect of the plant effluent.


    Now I have the brine volume.
    From the brine volume and salinity info I was able to calculate the salt resealed.
    I have the EPA seawater salinity levels in ppt (part per thousand?).
    And I know the current actual salinity in the area of the existing plants.

    Is there a way to estimate how that amount of salt released can affect the current sea ppt?
    Or any other suggetions for how to link the expected released brine with the current sea salinity?

    Thanks!

  7. #7
    did everyone give up? </3

  8. #8
    Senior Member Pod of Cali Merman Storm's Avatar
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    PPT is parts per thousand.
    One thing to remember: Seawater desalination does not add salt to the ocean. The mass discharged salt is about equal to the amount taken in by the water intake. Desalting water from the Persian gulf does not increase the amount of salt in the gulf.
    What it does is remove water, which will gradually increase the salinity of the remaining sea water in the gulf. Again, this is irrelevant of the method used to desal the water, it only depends on the amount of fresh water recovered.

    While you are doing this new, lower salinity water is mixing in through the strait of Hormuz. I found this paper discussing this:
    http://www.rmrco.com/docs/pub93_Pers..._MarPolBul.pdf

    It says the amount of evaporation from the gulf is 2 meters of water a year. Given the gulf surface area of 239000 sq km, that means 131 trillion gallons of water leave the gulf every year due to evaporation. If you want to keep human influence to just 5% of the natural process, you can remove 8 trillion gallons of fresh water a year (24 billion cubic meters).

    Another thing to consider: Some desal methods do not discharge salt. The water is recovered until the remaining solution is saturated with salt. This is then discharged into evaporation ponds. The salt crystallizes out, is collected and sold as sea salt.
    Are you a Frozen fan? Frozen fanfiction:
    https://www.fanfiction.net/s/10952902/1/Rain-of-a-Child-s-Tear

    Let the storm rage on!

  9. #9
    The heavy brine should be dried and used to backfill salt mines.

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