<p>Starting with question 2): you can use heat to evaporate the water but you need some sort of energy or thermal exchange to cool and condense it back to liquid. My understanding is that reverse osmosis (RO) is more energy efficient overall.</p>

<p>Question 1) Brine is only put into the oceans from system that are near the sea, which usually use seawater as a water source. The brine has an insignificant effect on the ocean salinity once it is mixed in but can have a local effect because of poor mixing and because differences in density between the brine and seawater cause it to sink to the bottom. Systems can be engineered to minimise the impact.</p>

<p>Inland, the brine can degrade groundwater resources if it is disposed to seep into the ground. Even in hot, dry climates, it may be difficult to completely evaporate it to dryness in lined basins so it doesn't seep in. Seepage basins are a viable option in some places where the groundwater is already to salty to use as a resource (for example where the salty water is extracted, put through an RO system and then reinjected to the same aquifer).</p>

<p>RO systems are designed so that the salty concentrate is removed during operation so the system doesn't need to be cleaned to remove the salt. However, impurities may still require cleaning of the membranes. There are ways to minimize the amount of cleaning needed.</p>

CC BY-SA 4.0
2019-10-18T17:07:09.297Z
2) yeah that makes sense. every paper/article i've found online says that it is more energy efficient, I guess condensation energy required is why!
Joe
CC BY-SA 4.0
2019-10-18T17:07:45.113Z
1) yeah that makes sense, I didn't think of groundwater requirements. Thanks for including the explanation about cleaning the membranes too!
Joe
CC BY-SA 4.0
2019-10-18T02:38:18.617Z
2019-10-18T02:38:18.617Z
haresfur
<p>One factor that makes distillation expensive is the alloys necessary for equipment to handle hot brine are very expensive. For ambient temperature osmosis fairly common stainless like 316 is satisfactory although care must be taken in design to avoid crevices and other considerations.  </p>

CC BY-SA 4.0
2019-10-17T21:22:51.473Z
2019-10-17T21:22:51.473Z
blacksmith37
<p>Dumping the brine in a desert would probably be a viable solution in a few places, but many towns don't have a handy desert nearby. It would need to be a really barren desert,almost devoid of life, which few deserts are. Environmentalists would be aghast if you flooded the Sonoran Desert with brine. Bonneville Salt Flats might be better, but they are used for motor sports and are a prime location for breaking land speed records. Great care would have to be taken not to contaminate the water table which underlies most deserts.</p>

<p>I think the ocean will have to remain the main means of disposing of brine. You would need to be careful where you pumped it as some marine ecosystems are much more sensitive to increased salinity than others, but I have not yet heard of any serious harm done by brine disposal at sea.</p>

<p>As you suggest, solar energy would in theory be a good way to purify water. There are some very small scale solar stills which do just that, but they are only practical in countries where there is hot sunshine almost every day. The difficulty comes when you try to scale it up so that it could supply a city. This has never been done successfully, partly because it would have to compete with established methods like reverse osmosis.</p>

CC BY-SA 4.0
2019-10-17T22:43:56.757Z
2019-10-18T06:32:29.940Z
2019-10-18T06:32:29.940Z
Michael Walsby
<p>So I was recently watching <a href="https://www.youtube.com/watch?v=bfr82RB72U8" rel="nofollow noreferrer">this video</a> regarding desalination efforts around the world and the viability of desalination as a means to provide humans with freshwater. I had a few questions:</p>

<p>1 - They mention that the brine that is released as a byproduct of the process is potentially harmful to marine life, so why not just dump it in the desert? Why do we need to release back in the ocean? I'm sure the effects of this won't be that big given that desalination is a tiny percentage of the world's water production. And if anything, given that global sea levels are rising, this might help things, right?</p>

<p>2 - The video states that the vast majority of desalination plants these days use reverse osmosis desalination and not heat-based methods. Why is that so? How is a process that requires energy better than using solar energy to evaporate the water? </p>

<p>Heat-based methods require energy from the sun, yes. But the way I imagine things, we can passively let in water from the ocean, put it under some glass panel that magnifies sun energy, and then let the water evaporate out through some piping to condense somewhere else. How is powering a pump through a membrane (that requires cleaning after) better?</p>

<hr>

<p>A lot of these thoughts are just me thinking out loud with no data to back me up because honestly, I wouldn't know what to look up regarding things like this. I'd love some science-based replies to this! I'm definitely missing something big.</p>

CC BY-SA 4.0
2019-10-17T22:21:36.767Z
This would be a better fit for [Engineering SE](https://engineering.stackexchange.com/) or [Sustainability SE](https://sustainability.stackexchange.com/).
Spencer
<oceanography><water><environmental-protection><desert>
Why is reverse osmosis desalination the dominant method used and not heat-based methods? Also why is brine being dumped back into the ocean?
Joe
# Why is reverse osmosis desalination the dominant method used and not heat-based methods? Also why is brine being dumped back into the ocean?

(Asked by: username_0 on 2019-10-17)

So I was recently watching [this video](https://www.youtube.com/watch?v=bfr82RB72U8) regarding desalination efforts around the world and the viability of desalination as a means to provide humans with freshwater. I had a few questions:

1 - They mention that the brine that is released as a byproduct of the process is potentially harmful to marine life, so why not just dump it in the desert? Why do we need to release back in the ocean? I'm sure the effects of this won't be that big given that desalination is a tiny percentage of the world's water production. And if anything, given that global sea levels are rising, this might help things, right?

2 - The video states that the vast majority of desalination plants these days use reverse osmosis desalination and not heat-based methods. Why is that so? How is a process that requires energy better than using solar energy to evaporate the water?

Heat-based methods require energy from the sun, yes. But the way I imagine things, we can passively let in water from the ocean, put it under some glass panel that magnifies sun energy, and then let the water evaporate out through some piping to condense somewhere else. How is powering a pump through a membrane (that requires cleaning after) better?

---

A lot of these thoughts are just me thinking out loud with no data to back me up because honestly, I wouldn't know what to look up regarding things like this. I'd love some science-based replies to this! I'm definitely missing something big.

**Tags:** oceanography, water, environmental-protection, desert

> **Comments:**
> **username_1**: This would be a better fit for [Engineering SE](https://engineering.stackexchange.com/) or [Sustainability SE](https://sustainability.stackexchange.com/).  

---

## 3 answers

### Answer by username_2 on 2019-10-18. Score: 4 | Accepted answer

Starting with question 2): you can use heat to evaporate the water but you need some sort of energy or thermal exchange to cool and condense it back to liquid. My understanding is that reverse osmosis (RO) is more energy efficient overall.

Question 1) Brine is only put into the oceans from system that are near the sea, which usually use seawater as a water source. The brine has an insignificant effect on the ocean salinity once it is mixed in but can have a local effect because of poor mixing and because differences in density between the brine and seawater cause it to sink to the bottom. Systems can be engineered to minimise the impact.

Inland, the brine can degrade groundwater resources if it is disposed to seep into the ground. Even in hot, dry climates, it may be difficult to completely evaporate it to dryness in lined basins so it doesn't seep in. Seepage basins are a viable option in some places where the groundwater is already to salty to use as a resource (for example where the salty water is extracted, put through an RO system and then reinjected to the same aquifer).

RO systems are designed so that the salty concentrate is removed during operation so the system doesn't need to be cleaned to remove the salt. However, impurities may still require cleaning of the membranes. There are ways to minimize the amount of cleaning needed.

> **Comments:**
> **username_0**: 2) yeah that makes sense. every paper/article i've found online says that it is more energy efficient, I guess condensation energy required is why!  
> **username_0**: 1) yeah that makes sense, I didn't think of groundwater requirements. Thanks for including the explanation about cleaning the membranes too!  

---

### Answer by username_3 on 2019-10-17. Score: 2

One factor that makes distillation expensive is the alloys necessary for equipment to handle hot brine are very expensive. For ambient temperature osmosis fairly common stainless like 316 is satisfactory although care must be taken in design to avoid crevices and other considerations.


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### Answer by username_4 on 2019-10-17. Score: 1

Dumping the brine in a desert would probably be a viable solution in a few places, but many towns don't have a handy desert nearby. It would need to be a really barren desert,almost devoid of life, which few deserts are. Environmentalists would be aghast if you flooded the Sonoran Desert with brine. Bonneville Salt Flats might be better, but they are used for motor sports and are a prime location for breaking land speed records. Great care would have to be taken not to contaminate the water table which underlies most deserts.

I think the ocean will have to remain the main means of disposing of brine. You would need to be careful where you pumped it as some marine ecosystems are much more sensitive to increased salinity than others, but I have not yet heard of any serious harm done by brine disposal at sea.

As you suggest, solar energy would in theory be a good way to purify water. There are some very small scale solar stills which do just that, but they are only practical in countries where there is hot sunshine almost every day. The difficulty comes when you try to scale it up so that it could supply a city. This has never been done successfully, partly because it would have to compete with established methods like reverse osmosis.


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