[HN Gopher] Why is desalination so difficult?
___________________________________________________________________
Why is desalination so difficult?
Author : mrzool
Score : 311 points
Date : 2023-07-05 16:37 UTC (6 hours ago)
(HTM) web link (practical.engineering)
(TXT) w3m dump (practical.engineering)
| ejz wrote:
| One thing that this article missed was that it was San Diego
| centric. In Israel, desalination is a much bigger part of the
| ecosystem. Over half of its domestic water comes from
| desalination. Quite a bit of the problem in California, as in
| almost every industrial application, is just that we make it hard
| to do anything with atoms.
| fwlr wrote:
| Relatively high energy cost since you're undoing an endothermic
| reaction, you need to do a lot of it since we use water in large
| quantities... but most of all, the planet naturally does a lot of
| desalination for us already through various geological processes,
| so our "price point" for desalination is $0 per liter
| (infrastructure to capture rain, dam rivers, or tap groundwater
| isn't literally free, but it's pretty close - especially when it
| comes to the marginal cost for the next liter). It's not
| difficult to desalinate _per se_ , it's difficult to desalinate
| extremely cheaply and at huge scale.
| nerbert wrote:
| In 2022, 85% of the country's drinkable water was produced
| through desalination of saltwater and brackish water. If there
| is a real need, and a will to address it, we have everything we
| need to to it.
| https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in...
| LordDragonfang wrote:
| >85% of the country's drinkable water
|
| "the country" in this context is Israel, since HN truncates
| the wiki link right before that.
| ramesh31 wrote:
| >In 2022, 85% of the country's drinkable water was produced
| through desalination of saltwater and brackish water.
|
| Israel does this by burning massive quantities of fossil
| fuels: https://en.wikipedia.org/wiki/Energy_in_Israel#/media/
| File:E...
|
| It's not even remotely economical without huge government
| subsidies. Completely untenable with current technology for
| poorer countries, or anyone that cares at all about carbon
| emissions.
| ALittleLight wrote:
| Couldn't they just replace fossil fuels with nuclear and
| solve the emissions issue?
| tguvot wrote:
| Not sure that largish, easily targetable, nuclear
| facility on a tiny spot of land is a good idea.
| zirgs wrote:
| They already have one: https://en.wikipedia.org/wiki/Shim
| on_Peres_Negev_Nuclear_Res...
| tguvot wrote:
| officially it's a textile factory :)
|
| on a serious note, it's a smallish research/etc facility.
| probably most of things are as deep underground as
| possible. it's not same thing as full blown nuclear power
| plant
| jbm wrote:
| Is there a practical reason why it would be difficult to do
| this with solar power? Is this a process that does not
| adapt well to intermittent power sources?
| Gibbon1 wrote:
| I feel like I never get enough of the operational details
| to know. But intermittently running a capital intensive
| thing has bad economics. If the capital cost per m3 is
| $0.50 when the plant runs 24/7. It'll be $2.0/m3 if you
| cut it back to 6 hours a day.
|
| However the details are important. You'd need to do a
| deep operations analysis to get an answer. That also
| would include energy storage as well.
| ramesh31 wrote:
| >Is there a practical reason why it would be difficult to
| do this with solar power?
|
| Same reason it's hard to do anything with solar. Grid
| scale storage is an unsolved problem.
| jbm wrote:
| I wasn't too clear about what I meant, but is storage
| even necessary here? For example, would there be an issue
| if the desalination process was left in an intermediate
| state for X hours / days while power is intermittent?
|
| I wonder if there are more energy-expensive
| desalinization processes that are better to use with
| intermittent power sources, like solar.
| tguvot wrote:
| desalination plants are built to produce specific amount
| of water in order to cope with demand. if you going to
| stop desalinating while there is no solar, you need more
| plants in order to desalinate more water during the day.
| also, in general, even during day, solar not always
| available.
| janalsncm wrote:
| This doesn't make sense to me. You don't need to store
| energy to desalinate water. You can store the final
| product. And water storage _is_ a solved problem. At
| times where supply exceeds demand, use excess energy to
| desalinate more water. When energy demand is high,
| desalinate less.
| tguvot wrote:
| One of desalination facilities I think is actually solar
| powered. It mix of evaporative desalination with power
| generation (giant tower that a bunch of mirrors focus
| light one). Not sure if it's in production now.
|
| But in general in Israel solar has a couple of problems:
| very dusty (sand storms) and local electrical company
| which tends to create problems
| mcpackieh wrote:
| > _It 's not even remotely economical without huge
| government subsidies._
|
| Why shouldn't _drinking water_ of all things receive
| subsidies? Why must drinking water be a for-profit
| enterprise?
| ramesh31 wrote:
| >Why must drinking water be a for-profit enterprise?
|
| It's about sustainability, not profit. Of course wealthy
| nations can (literally) burn enough money turning fossil
| fuels into water to make their population comfortable and
| happy. But most can't, and the externalized cost is
| unimaginable at a global scale.
| mcpackieh wrote:
| Clean energy exists, it would require more subsidies but
| so what?
| dylan604 wrote:
| Wind powered desalination seems like a perfect combo
| seeing as it's pretty much always windy on the coast.
| California's May gray and June gloom makes me thing would
| keep solar from my first option.
| bsder wrote:
| HVDC links to El Centro and Arizona would make solar just
| fine for Southern California.
| ramesh31 wrote:
| >Clean energy exists, it would require more subsidies but
| so what?
|
| Easy to say when your government can afford the
| subsidies. But the vast majority of freshwater-insecure
| nations will never be able to do this without a 10x
| technological breakthrough.
| mcpackieh wrote:
| Well if they can't afford to even subsidize fresh water,
| their choices are move or die. What's your point then?
| coryrc wrote:
| Because the US has plenty of free water that's growing
| alfalfa in the desert. Subsidies are unnecessary.
| mcpackieh wrote:
| Some parts of the US have too much fresh water, other
| parts have too little. Fresh water is a regional matter;
| you're talking about the American Southwest, particularly
| California, specifically. But this conversation is about
| desalination generally, and particularly Israel.
| sophacles wrote:
| Are you sure? The alfalfa itself is subsidized. (I mean
| subsidies besides giving all the water to the farmers
| thing).
| janalsncm wrote:
| A lot of public goods are that way. What you've stated is
| practically a tautology. "Government subsidy" just means
| the public is paying for it. Other things that fall into
| that category are universal education, the military, and
| police.
| tguvot wrote:
| You can't drink hydrocarbons.
| SillyUsername wrote:
| Stupid question time, why not expose the sea water over a larger
| area and expose to sunshine? There's plenty of room in the UAE
| for this, somebody is even building a wall that could hold the
| water which could be dual purpose and act as cooling via
| evaporation...
| djent wrote:
| Larger area than the oceans?
| SillyUsername wrote:
| We'll no but a super long wall facing south might do the
| trick...
| 0xbadcafebee wrote:
| I mean basically you're describing a solar pond
| (https://en.wikipedia.org/wiki/Solar_pond) or salt evaporation
| pond (https://en.wikipedia.org/wiki/Salt_evaporation_pond).
| There isn't an easy way to recover the evaporated water from
| that process; you would effectively have to build an artificial
| enclosed environment and it wouldn't work as fast as exposure
| to the elements. Basically you would need a still (as in for
| alcohol) the size of 20 football fields. But sun power wouldn't
| be enough to power it fast enough to be practical. Maybe if you
| moved it into outer space...
| justsomehnguy wrote:
| This is how it is solved everywhere... but now you need to ship
| all this desalinetad water. And surprise! it's the second the
| most expensive part in the process.
| barelyauser wrote:
| Search for OTEC (Ocean Thermal Energy Conversion). Not
| necessary to build wall, just a big enough pipe to collect cold
| water from the depth. Then a special contraption will yield
| pure water from evaporation. Can be done virtually everywhere
| there is a deep enough ocean.
| profsummergig wrote:
| While on the issue of water:
|
| Some countries have floods in one part, and drought in the other.
|
| The floods are so bad that large numbers of people die.
|
| Here's a great challenge that should be worked on: how to capture
| the flood water and use it to mitigate the droughts. Could
| something like Elon Musk's Boring company concept fix this?
| falcolas wrote:
| That's basically how dams and reservoirs work. It just requires
| a lot of space to store water.
| NegativeK wrote:
| People are constantly talking about using pipes to solve the US
| desert southwest's water crisis, as if building a water
| pipeline for thousands of miles isn't some gigantic logistical
| and engineering nightmare -- as well as a bureaucratic
| nightmare, since most places with fresh water don't want people
| in the desert literally sucking them dry.
| seunosewa wrote:
| The cost of pumping dirty water uphill ...
| fareesh wrote:
| I remember reading that it causes some problems related to
| fishing because the fish all migrate further out.
| photonerd wrote:
| That's less the fault of the desalination, more
| improper/shortsighted disposal of waste byproducts
| bhhaskin wrote:
| The worry is a local increase of salinity levels, due to
| putting the brine back in the ocean. I think that is a solvable
| issue though. We can harvest some of the salt and then mix the
| resulting brine with reclaimed water. Then spread that out
| instead of dumping it in one area. We could also create inland
| salt deposits.
|
| The biggest issue is cost for these plants and it isn't worth
| spending the political capital to build them yet.
| emptybits wrote:
| "Turning sea water into clean, drinkable water costs $2 to $5 for
| 1000 gallons.
|
| Less than half a penny per gallon is obviously absurdly cheap."
|
| - Elon Musk, 2023-05-07
| https://twitter.com/elonmusk/status/1655262008898383872?ref_...
| tibbon wrote:
| I'm looking at this and unclear what he's talking about.
| Essentially, citation needed Elon?
| emptybits wrote:
| Musk made the same "absurdly cheap" claim in a recent
| interview with Bill Maher.
|
| https://www.youtube.com/watch?v=oO8w6XcXJUs [skip to 14:45]
|
| I don't think he gave any substantiation to the claim then
| either. (He dismissed delivery of clean water to the world as
| a problem not interesting enough for him to spend time on.)
| Veserv wrote:
| Israel desalination plant profitably offering a fixed price
| of 1.45 NIS per cubic meter. At current exchange rates that
| is around 0.40$ per cubic meter (1000L).
|
| https://www.gov.il/en/departments/news/desalination_260520
| infogulch wrote:
| Just look at this thread for price estimates. $2-5 is pretty
| reasonable, even assuming a cost breakdown with energy at 10%
| and capital/infrastructure at 90% of the final cost.
| iudqnolq wrote:
| It's very easy to forget that we use orders of magnitude more
| water than comes out of our tap. A pound of beef is about 2,000
| gallons, so that would be a nontrivial price increase.
| sbussard wrote:
| What? Are they drowning cows now??
| jjk166 wrote:
| Probably shouldn't be raising cattle in areas that would need
| desalination to produce those 2000 gallons.
| emptybits wrote:
| Or maybe such water-intensive food production _should_
| require consumption of the planet 's nearly unlimited
| seawater. Pass the greater expense to the cattle producers
| and/or consumers. Leave the planet's relatively limited
| freshwater sources for direct human consumption, or food
| production that's more sustainable, or other urgent and
| necessary activities. The beef lobby may not agree.
| infogulch wrote:
| And what of areas where just having water coming out of a tap
| at all would be a huge improvement?
| leecarraher wrote:
| I like the analogy at the end regarding nuclear power vs piping
| in water from freshwater sources. The upfront costs are high, but
| over time it would be cheaper than desalination, but due to short
| term governments and borders, it's hard to justify the upfront
| costs. So instead we are somewhat stuck with more expensive short
| term solutions.
| flybrand wrote:
| I work in microfiltration (a pre-filter step for RO), and my view
| is:
|
| 1/ it's as much an energy and water storage problem as it is a
| technical problem.
|
| 2/ commercially, because of 1/, RO is a municipal sale. It is a
| civil initiative, rather than a commercial one, which means it
| gets crowded out by other civil decisions.
| alana314 wrote:
| Could we make a deep sea elevator similar to a space elevator?
| And if so, could we use deep sea pressures to facilitate cheaper
| desalination?
| User23 wrote:
| Solar stills are one of those basic survival tools anyone living
| in an arid region near a salty body of water should understand
| how to rig. They're dirt simple and as easy to build as a prison
| still with the added bonus of only requiring the sun as an
| external power source.
|
| Similarly everyone should know how to rig a basic water
| purification system using gravel, sand, and charcoal in series.
|
| Even just as applied science experiments to do with kids they're
| worthwhile.
|
| Edit: Water based solar power is generally an area I think that
| deserves more research. While photovoltaics have their
| advantages, water is cheap, clean, and reliable. Heating water
| with sun during the day and using it for household heating at
| night is the simple application that I'm most familiar with, but
| I wouldn't be shocked if there's some scale where an economically
| interesting Carnot cycle becomes possible.
| [deleted]
| photonerd wrote:
| Water isn't the most efficient way of doing that but the
| principle is sound. There are molten salt solar arrays that
| work similarly.
| kjellsbells wrote:
| Idle observation: do people in Materials Science feel like
| this is the Golden Age for their field?
|
| So many incredible paths to follow: Battery tech. Solar
| cells. Desalination. Carbon handling. What a time to be
| alive.
| zokier wrote:
| Probably very dumb idea, but would it be feasible to just pump
| and sprinkle sea-water on hot, dry desert and let it naturally
| evaporate, and then collect it as fresh rainwater back? i.e. how
| much water would you need to evaporate to have a noticeable
| increase in rainfall?
| ars wrote:
| Israel does this for a different reason: To collect minerals
| from the dead sea.
|
| It doesn't increase rainfall.
|
| Israel is actually desalinating water and rejuvenating a river,
| which eventually will reach the dead sea (although that's not
| why they are doing it, but it makes the point that evaporating
| water isn't doing much).
|
| Maybe you mean on a far larger scale?
|
| Well - the ocean itself is much larger, and water evaporates
| from its surface all the time. Humans aren't going to make an
| evaporation zone larger than the ocean.
|
| Also: Water does not (mainly) evaporate because of heat, but
| rather because of wind. There's lots of heat in the desert, but
| not much wind. The ocean has a ton of wind.
| eklitzke wrote:
| Actually deserts can be quite windy, especially deserts near
| the ocean. Wind forms when there's a pressure differential,
| and the most common reason for a large pressure differential
| is when there are two adjacent areas with a significantly
| different temperature. So when you have a desert next to the
| ocean, the desert cools at night and then during the day, as
| the air in the desert heats up, it lowers the air pressure on
| land and pulls in air from the cooler air over the ocean.
| This phenomenon is why the SF Bay Area consistently has high
| wind and good sailing conditions during the summer,
| especially in the afternoons.
|
| That said, none of this contradicts the overall point you
| were making.
| Terr_ wrote:
| My guess is that no matter how much water you manage to
| evaporate, the scheme will be doomed by the original causes for
| why the area is a "hot dry desert", the wind and geography
| patterns.
|
| So even if you evaporate a _lot_ of water, it won 't fall where
| you need it or where you can collect it.
| samstave wrote:
| You know this is how the Phoenecians became the dominant
| culture, its also where the term "salary" comes from and "worth
| his weight in salt" -- as salt was the only known preservative
| of the massive amounts of Tuna the phoenecians were catching
| and shipping throughout the mediterrainian - and made them a
| super-power - they had control of the preservance of food over
| shipping distances...
|
| Salt was used as money.
|
| EDIT: Only a fn idiot without knowledge of history would
| downvote a comment... Jimminy Carter, what type of stupid are
| you trying to promote?
| eichin wrote:
| Probably (not me, I don't have downvote bits) because it
| turns out there's a lot of "game of telephone"/"folk
| etymology" about this legend which apparently started in the
| 1800s in english; it's not actually historical as you've
| expressed it.
| soperj wrote:
| > worth his weight in salt
|
| Never heard this saying before. I've heard "worth his weight
| in gold".
| jareklupinski wrote:
| i'd like to see the xkcd author tackle this as a "What If?"
| kneebonian wrote:
| He already has. https://what-if.xkcd.com/152/
| lambdasquirrel wrote:
| I think the key difficulty is condensing a very large quantity
| of water out of an even larger quantity of air, in the desert.
| The thermodynamic equilibrium of water vapor vs water-in-
| condensed-form isn't going to work well for you here, even
| after nightfall. The very reason that it was possible to
| evaporate the water out (i.e. the air is very dry) cuts back
| the other way.
| jononomo wrote:
| In other words, you want to salt the land? Isn't that what
| marauders do when they want to destroy an area permanently?
| qorrect wrote:
| Presumably this is the desert where no plants are growing.
| caseyohara wrote:
| Deserts are far more alive and biodiverse than people
| think.
|
| > The Sonoran Desert encompasses 120,000 square miles of
| southwestern Arizona, southeastern California, and in
| Mexico, northwestern Sonora and most of the Baja Peninsula.
| With nearly 3500 species of plants, 500 species of birds,
| and 1,000 species of bees, the Sonoran is the most
| biodiverse desert on earth.
| jtriangle wrote:
| You do have to consider though, how important are those
| ecosystems?
|
| Because I see this often used as a reason we can't do
| something, but they never qualify it with reasons why one
| should care.
|
| I also pose this as a completely honest question, as I
| don't really know if you wiped out every desert ecosystem
| with solar/desal/etc if it'd actually affect anything
| else.
| lovemenot wrote:
| >> I also pose this as a completely honest question, as I
| don't really know if you wiped out every desert ecosystem
| with solar/desal/etc if it'd actually affect anything
| else.
|
| Presumably, you believe that there is _something_ amongst
| all that "anything else" that has intrinsic importance.
| Perhaps for you it is humanity in general. Perhaps just
| yourself and your own family.
|
| Whatever _something_ you might value intrinsically is
| fundamentally arbitrary. Why not me and my family? Why
| not other great apes besides humans?
|
| Arbitrarily, I value (desert) ecosystems as having value
| in their own right. Biodiversity is an intrinsic good,
| with no further justification required.
| q845712 wrote:
| it's probably a good starting position to assume that
| since we all the share the same closed-ish system called
| planet earth, there's interconnections between different
| systems. Certainly the border areas between desert and
| not-desert aren't very crisply defined. Certainly
| (reference in other threads) nutrients can be blown by
| the winds from desert into non-desert areas far away.
| Certainly there exist some animals who go in and out of
| desert regions (birds, butterflies, ...). It's a really
| good idea to assume that things on this planet are
| connected to each other.
| jtriangle wrote:
| Sure, I mean, that's my assumption, the question is more
| of "how important is that connection" in a given
| instance. And, I suppose down that line of questioning,
| do we have the knowledge/systems/etc to overcome any
| losses?
| micromacrofoot wrote:
| deserts are actually functioning ecosystems with life
| though, contrast to say, a salt flat
| stOneskull wrote:
| seems you may as well put nuclear power plants in the desert
| josephjrobison wrote:
| There is talk about pumping sea water into the Saharan desert
| in certain parts, which gets at that.
|
| https://www.youtube.com/watch?v=V2b7ztWvFOg
| samstave wrote:
| Was going to say this, there are/have been many ideas to do
| just this - one was to build a huge canal and just then let
| the sea flood the area...
|
| Also - there are lots of un-earthed treasures to be found
| under the saharan which was once a lush environ and have been
| covered with sand - so prior to flooding it, we need to lidar
| and excavate it.
|
| What if we vaccuumed up all the sand and built a new
| island/continent with the material and just revealed
| everything underneath - then flooded it. (I believe UAE is in
| the market for more sand-built-land-masses)
| hanniabu wrote:
| I wonder how that will affect the Amazon rainforest since it
| receives phosphorus and other nutrients blown of the Atlantic
| from the Sahara desert. If salt starts getting blown over too
| I wonder if that will ruin the soil fertility there.
| jtriangle wrote:
| Using a desert isn't a great idea, but, using the sun to
| evaporate water works just fine. You're just replacing an
| expensive heat source with a 'free' heat source.
|
| That's not unusual in desalination however, many facilities are
| combo plants, they're producing power and then using waste heat
| for desalination.
| cratermoon wrote:
| A desert is a living ecosystem. Evaporating salt water would
| leave behind substantial salt solids. Think Great Salt Lake.
| LordDragonfang wrote:
| I mean, that's actually not too much of a downside, since
| salt is a fairly valuable economic good - Indian salt farmers
| do exactly that to harvest the salt crystals.
| azernik wrote:
| And that is the heart of a decently profitable economy
| around the Dead Sea, on both the Jordanian and Israeli
| sides.
| cratermoon wrote:
| Destroying existing ecosystems is kind of part of how we
| got where we are now.
| justinator wrote:
| We're coming to the inevitable conclusion that we can't
| keep doing this, as there's little existing ecosystems to
| destroy - I mean: we're looking at deserts and thinking:
| "why not?". Slightly more outlandish, we're looking at
| whole planets and thinking the same.
|
| Some call that par for the course, others unsustainable.
| Still others don't make a distinction and see them as the
| same.
| gamegoblin wrote:
| I was surprised how _cheap_ it is. Desalinated water costs ~50
| cents per 1000 liters [1]. That 's about the same amount of water
| as a typical American household uses per day.
|
| 50 cents per day for a fully desalinated water supply is...
| incredibly cheap.
|
| If you're interested in water policy and water management /
| engineering, I cannot recommend enough reading the book "Let
| There Be Water: Israel's Solution for a Water-Starved World".
|
| [1] https://en.wikipedia.org/wiki/Desalination#Costs
| bsder wrote:
| Agreed. Desalinization is cheap. It works just fine for
| providing water for coastal _people_. As California is proving.
|
| What it does _not_ do is provide freshwater for argibusinesses.
| As California is _also_ proving in the Central Valley. :(
| Zetice wrote:
| To be fair to the author, difficult here could just mean "more
| complex than it seems" which he does a good job of
| illustrating, specifically around the additional concerns that
| go in beyond the actual processing of the water.
|
| He says it's viable for many applications.
| BeetleB wrote:
| > That's about the same amount of water as a typical American
| household uses per day.
|
| I assume you meant per year?
| timerol wrote:
| No, 1000L/day is about right for a typical American
| household. [1] claims "The average American family uses more
| than 300 gallons of water per day at home." 300 gal is 1136
| L.
|
| https://www.epa.gov/watersense/how-we-use-water
| lexicality wrote:
| I know American toilets are comically big but 250L a day on
| flushing them seems insane to me. A cursory google suggests
| the average UK household uses 350L a day in total!
| weaksauce wrote:
| That's not too bad. the normal baseline consumer costs are
| actually more expensive than that. normal base use in irvine ca
| is 1.78 per 748 gallons which is almost 3000 liters. (2831.488
| liters)
|
| I assume that's cost to make and not total cost to consumer
| post treatment plant distribution and maintenance so it would
| be more expensive than that but still in the ballpark of
| reasonable.
| iudqnolq wrote:
| *typical American household uses directly.
|
| Don't forget food, industrial, etc
| [deleted]
| bufferoverflow wrote:
| Yes, but you pay for that when you purchase the products.
| These costs are baked in.
| iudqnolq wrote:
| But we're talking about increasing the per-gallon cost of
| everything. So the baked in cost would increase.
| jjk166 wrote:
| Or moving water intensive operations away from areas
| where water is expensive. No one is going to be doing
| desalination in Michigan.
| dylan604 wrote:
| why not? we're growing heavy water using crops in places
| with little water. logic is not always the deciding
| factor if involved at all in a lot of modern things
| sophacles wrote:
| There are a few very big salt mines below the great
| lakes. We could totally get a desalinization plant in
| Michigan by pulling water from the lakes, salt from the
| Detroit salt mine, and combining them in the input stream
| to the plant!
| dylan604 wrote:
| someone has been playing in the perpetual motion machine
| sandbox again i see
| cyanydeez wrote:
| Certain Michigan towns may need to do so...
| janalsncm wrote:
| I think the implication is that the costs will be higher
| than 50 cents if 50 cents does not cover non-household use.
| Veserv wrote:
| Freshwater withdrawals per capita in the US (which includes
| agricultural exports and animal feed such as alfalfa sent to
| Saudi Arabia) are around 1550 cubic meters per year.
|
| https://ourworldindata.org/grapher/water-withdrawals-per-
| cap...
|
| So that is around $775 per person per year assuming no net
| change in water use. In contrast, Germany uses around 410
| m^3, France around 475 m^3, and Australia around 724 m^3, so
| the US is a significant outlier.
| iudqnolq wrote:
| That equates to around 4,000 liters per person per day.
|
| Freshwater withdrawals is a very broad category, it also
| includes water released to turn hydropower turbines. But
| it's also unfair to compare across countries without taking
| into account water sent between countries in the form of
| produce and products.
| Veserv wrote:
| You were quibbling about how 1,000 liters per day does
| not adequately account for all usage. Freshwater
| withdrawals is on average going to be a overestimate and
| the US is one of the worst outliers with significant
| agricultural exports which are one of the largest
| contributors to differential water withdrawals. Despite
| this, it would only be $2.00 per day for a fully
| desalinated water supply even if we safely overestimate
| usage. Despite it being 4x higher than the person you
| replied to said, that is still incredibly cheap.
| Hxnd wrote:
| >Despite it being 4x higher than the person you replied
| to said
|
| $0.5 was per household. $2.00 was per capita. So 10x
| higher.
| iudqnolq wrote:
| I agree with everything you say (with the assumption the
| person I replied to has the correct price, which I know
| nothing about)
| aurizon wrote:
| several better ways, multi stage stills and reverse osmosis are
| leaders of the pack,
|
| https://www.veoliawatertechnologies.com/en/technologies/mult....
|
| https://en.wikipedia.org/wiki/Reverse_osmosis
| mplewis wrote:
| The author specifically mentions reverse osmosis.
| aurizon wrote:
| Yes, RO is energetically the best, but places with geothermal
| excess, like iceland might choose the heat based method as
| they have free heat = the still warm water discharged after
| power still is warm enough for distillation.
| ZeroGravitas wrote:
| He also covers multi-stage stills:
|
| > My garage demo has very little going for it in terms of
| efficiency. It's about as basic as distillation gets. There's
| lost heat going everywhere. Modern distillation setups are
| much more efficient at separating liquids, especially because
| they can take advantage of waste heat. In fact they are often
| co-located with coal or gas-fired power plants for this exact
| reason. And there's a lot of technology just in minimizing
| the energy consumption of distillation, including reuse of
| the heat released during condensation, using stages to
| evaporate liquids more efficiently, and using pumps to lower
| the pressure and encourage further evaporation through
| mechanical means.
| FreshStart wrote:
| Should heat and cool it with standing soundwaves. Salty water
| takes the heat, cools and falls (brine-fall) rest is less
| salty.add membranes at intersection points..
| MarkusWandel wrote:
| Some years ago I toured a maple syrup operation that has the
| opposite goal: Concentrate the dissolved stuff in the water.
| Their first stage was reverse osmosis, but only to a point.
| Second stage is boil, but with aggressive heat recovery from the
| steam to preheat the incoming liquid. All this to keep the energy
| cost under control.
| ftxbro wrote:
| Does this mean I can buy watery maple syrup that is 2x watery
| for less than half the normal cost like maybe a quarter of the
| cost? Or does the volume and weight for shipping and handling
| offset that savings or does one want watery syrup so there is
| no market so they don't make it?
| jeffbee wrote:
| In an operation like that, do they heat the syrup under low
| pressure to reduce the boiling point of water?
| philote wrote:
| I'd guess that if anything they'd increase the pressure to
| raise the boiling point. That way things dissolve in it
| faster and the water doesn't evaporate away.
| tylerag wrote:
| They're concentrating maple syrup. It comes out of the tree
| with the sugar and flavors already dissolved in it.
| giantrobot wrote:
| Part of the flavor of maple syrup is due to
| caramelization and the Maillard reaction of components of
| the sap. Just concentrating the sap would get you a
| syrupy substance sourced from a maple tree but it
| wouldn't be _maple syrup_.
| blamazon wrote:
| On this topic the 14 minute episode "How Do They Make Maple
| Syrup?" from the PBS chemistry show 'Reactions' may be of
| interest:
|
| https://youtu.be/nSRCDiKMEJc
| kevin_thibedeau wrote:
| I'll add Adam Ragusea's Hickory syrup experiment:
|
| https://www.youtube.com/watch?v=rT9IJXuHbKs
| JshWright wrote:
| The goal is to evaporate the water away. For every liter of
| finished syrup, you need to get rid of approximately 40
| liters of water.
| jtriangle wrote:
| That's more or less the correct way to run anything energy
| intensive, scavenge as much of the waste heat as you reasonably
| can.
|
| Theoretically, continuous distillation can be extremely
| efficient, as, you're removing as much heat as you're putting
| into the system. In reality, you get into diminishing returns
| fairly quickly, because insulation, pumps, heat exchangers,
| etc, are all far from free, especially at scale.
| 1letterunixname wrote:
| Pick one
|
| 1. Steam distillation + Product can be perfect DI type 1 water -
| Expensive: 300+ kJ/L
|
| 2. RO membrane + Cheaper - Slow - Wastes more water - Requires
| regular changing of membranes
|
| The end.
| cratermoon wrote:
| Googly eyes on the flow meter
| luxuryballs wrote:
| Can I shower in salt water?
|
| Then all I need to do is desalinate drinking water.
|
| "Distributed" home desalination for drinking water seems like the
| best approach in my mind, then people can pay as much or as
| little they need, but I have no real data to back this up.
| jakear wrote:
| No. Unless you want salt-caked hair. And body. Curious, have
| you ever been to a beach?
| luxuryballs wrote:
| yeah but I usually feel quite refreshed after coming out of
| the waves, but keep in mind we don't need clean feelings to
| survive, we do need fresh drinking water
| jakear wrote:
| Salt overexposure is liable to dry up your skin and cause
| rashes/cracking leading to vectors for infection.
| stOneskull wrote:
| you see that with surfers. at 30, they look like they're
| 50
| smolder wrote:
| And you're attributing this all to sea water exposure? My
| intuition is that sun exposure would be a bigger factor.
| soperj wrote:
| Is it? My parents have a salt water pool and swim in it
| daily through the summer.
| jjk166 wrote:
| You're likely better off treating and recirculating the same
| fresh water for bathing. You can also potentially save energy
| on heating the water with a proper setup.
| tibbon wrote:
| I've been wondering if one solution is having dual plumbing in
| some houses, that includes a parallel system for non-drinkable
| (but otherwise clean) water.
| smolder wrote:
| I believe that's called a grey water system.
| bobbean wrote:
| I'm not even remotely knowledgeable about this, but I'd assume
| saltwater would wreck plumbing. The connection in and out would
| probably be degraded much quicker. Then the water treatment
| plants would have to deal with dirty salt water, which is
| probably more difficult.
|
| On top of that all the brine that people produce in their homes
| would have to be disposed of, and I'm sure many people would
| just end up flushing it down the drain. So the water treatment
| plant would have to deal with highly concentrated, contaminated
| saltwater.
| beembeem wrote:
| Bingo. Corrosion. Everything would need to be "marine-grade"
| aka expensive to plumb :)
| dghlsakjg wrote:
| Having worked on and plumbed boats, the bigger issue is
| actually growth and especially mineral deposits. Corrosion
| is less of an issue since most plumbing is actually plastic
| at this point. Although any water that goes into an
| appliance needs to be fresh water, so it would really ONLY
| be for showering.
|
| Sewage in particular will create hard deposits in plumbing
| that needs to be dealt with every few years at a minimum.
|
| Frankly, unless you are in a rather extreme environment,
| like a desert, or a boat where you have to carry or make
| all your own fresh water, saving a few gallons on showering
| and washing is pretty inefficient. You could have a far
| larger impact by changing habits, and ensuring low flow
| appliances.
| smolder wrote:
| Interestingly, there is one house that I know of with both
| hot and cold freshwater plumbing as well as hot and cold salt
| water plumbing: the Breakers mansion, built by the
| Vanderbilts. I'm sure they spent fortunes maintaining that
| plumbing and think the tour guide said something to that
| effect, but everything was a show of wealth there. One room
| featured platinum wallpaper, because, why not?
| omoikane wrote:
| Related, it seems inconvenient that we haven't evolved to be
| able to just drink salt water, like cats can.
| graphe wrote:
| Insightful. Have we been attacking the problem the wrong way?
| Gene editing is another field, in the future perhaps we'll
| see edits to the kidneys to save water.
| mschaef wrote:
| > Can I shower in salt water?
|
| You at least need special soap:
| https://en.wikipedia.org/wiki/Saltwater_soap
| mcculley wrote:
| Yes, I do this on boats quite often. It is unpleasant compared
| to showering with fresh water.
| nickff wrote:
| Yes, you can shower in salt water, but you won't feel as clean
| in the end. You also use water for washing clothes and dishes,
| as well as washing your hands, watering plants, and various
| other tasks, for which salt water is unsuitable.
| dec0dedab0de wrote:
| I had a dream the other day that we started generating
| electricity with steam engines using ocean water, then collected
| the steam for potable water
| narag wrote:
| Tell that to israelis: https://www.israel21c.org/how-israel-used-
| innovation-to-beat...
| hammock wrote:
| Because water is the universal solvent and NaCl is probably the
| most readily water-soluble salt. Reversing that state is going to
| take a lot of work
| abotsis wrote:
| What ever happened to desalination via ICP that the mit postdocs
| had working? 20wh/L iirc...
| egberts1 wrote:
| MIT solar distiller in 2020 demonstrated a gallon and a half of
| fresh water in one hour using a square meter of close-quarter
| membrane distillation process.
|
| So, 10,000 sq meter of this baby could pump 150,000 gallons of
| fresh water over a ten-hour solar shift.
|
| Seems like the secret sauce is 1.2cm (or is that 80mm) separation
| between diffuser plates thus taking advantage of solar
| heating/condensation/collection in one area.
|
| Of course, there remains an collection issue of brine discharge
| which could be removed gradually instead but in same but 3-peat
| manner (down to 2-3 permille, or 0.2-0.3% salinity level.)
|
| At any rate, this MIT method has leapfrogged the passive solar
| method ahead of reverse osmosis (RO) method by quite a bit, in
| terms of energy required to extra fresh water. RO still holds the
| insurmountable lead in base (non-fluctuating) water output rate.
|
| https://news.mit.edu/2020/passive-solar-powered-water-desali...
| Animats wrote:
| It's not difficult. It's just energetically uphill. Undoing
| entropy costs energy. Second Law of Thermodynamics.
| jillesvangurp wrote:
| Exactly, desalination is very popular in arid countries. With
| renewable prices trending down, it's getting cheaper too.
| Basically, don't use consumer grid prices because those still
| include a fat profit margin for the energy suppliers and their
| sunk investment in legacy expensive generation using gas, coal,
| or nuclear. If you are within 40 degrees of the equator, which
| is where you'd find most arid places, solar is a very good
| option for generating lots of energy cheaply. Cents per kwh
| basically, possibly dipping below 1 cent per kwh in the not so
| distant future. And since you can store water in reservoirs,
| it's OK to not be desalinating 24x7.
|
| A thousand liters takes about 3kwh. It's not really that
| expensive. If you run a very inefficient house in the US,
| that's actually what you'd need per day. You might consider
| some cost/water saving solutions if that worries you. But,
| either way, we're talking cents per day per household
| basically.
|
| Not nothing. But cheap enough that it is a common solution to
| get water in places that have average incomes far below those
| common in places like the US where desalination is mostly
| science fiction.
| idoubtit wrote:
| Focusing on the financial side is okay, though I would
| mention that the energy cost is only half of the total cost,
| according to the literature. But desalination is not only
| about money. It has an impact on its environment, because it
| pumps fresh sea water and reject brine. These operations have
| a high cost for the local marine life.
|
| A large desalination plant means large patches in the sea
| where life is not sustainable.
|
| BTW, if an average US house really needs 1m^3 per day, that's
| appalling. These past years, my house has used less than
| 10m^3 per year and per person. 30x less. I'm afraid most US
| homes will keep wasting drinkable water and pressure society
| on building desalination plants, rather than halve (at
| least!) their water usage and protect the environment.
| jillesvangurp wrote:
| True in shallow waters, a literal drop in the ocean if you
| pump the salty water a bit further out where it is deeper.
| The Pacific coast in the US is pretty deep even close to
| the coast. The Atlantic is pretty deep as well. Of course
| pumps and pipes cost a bit extra so there is a tendency to
| cut corners there. But it's not a challenging problem
| technically.
| jonny_eh wrote:
| Separating two things isn't necessarily expensive. Think of
| coin sorters that just use gravity and different sized holes.
| 4rt wrote:
| You still have to lift all the coins.
| jonny_eh wrote:
| Slide the coins off the table into the sorter, done!
| 11101010001100 wrote:
| You need to bring the coins uphill in the first place.
| tshaddox wrote:
| Right, but that's easy. If desalination only required
| lifting each unit of water a few inches we probably
| wouldn't say it's particularly difficult.
| 11101010001100 wrote:
| Easy as in moving mass against a gravitational potential?
| Didn't know building a rocket was easy.
| WJW wrote:
| Moving water against a gravitational potential has been
| done for literal millennia and no rocketry is required. A
| water pump will suffice.
| jonny_eh wrote:
| My point is, we shouldn't assume that fighting entropy
| isn't necessarily _expensive_. There 's always a cost, and
| it may in fact be high, but we shouldn't assume it is.
| cyberax wrote:
| The thing is, it doesn't have to be. The energy released by
| mixing salt into water is small, around 3.9kJ/mol.
|
| Molar mass of salt is 58g/mol, and the average sea water
| salinity is around 3.6%
|
| So a cubic meter of sea water will have 1000*0.036/0.058=620
| moles of salt, and it'll require 2.4MJ of energy to remove the
| salt in a perfect desalinator.
|
| In more common units, 2.4MJ is about 0.75 kWh. Around here
| electricity is ~10 cents per kWh, so the absolutely lowest
| price of one cubic meter of desalinated water would be around 8
| cents.
| Animats wrote:
| > cubic meter of sea water will have 1000*0.036/0.058=620
| moles of salt, and it'll require 2.4MJ of energy to remove
| the salt in a perfect desalinator. In more common units,
| 2.4MJ is about 0.75 kWh.
|
| > A thousand liters takes about 3kwh.
|
| So, if those numbers are right, desalination is currently at
| about 25% of theoretical energy efficiency. Is that correct?
| cyberax wrote:
| I made a small arithmetic mistake at the end, 2.4MJ is
| about 0.66kWh
|
| But otherwise it's correct, we're at about 20% of the
| theoretical maximum. The best RO systems are right now
| working towards 2kWh per cubic meter: https://uh.edu/uh-
| energy/educational-programs/tieep/content/...
| [deleted]
| olejorgenb wrote:
| According to the paper [1] he talks about in the video, the
| theoretical limit is a function of %salt removed and %waste-
| water.
|
| For 90% salt removal with 50% waste-water, they say the limit
| is 1.09kWh per cubic meter (3.924 MJ)
|
| NB: It is not 100% clear to me if the result is independent
| of the type of technology, but they do claim:
|
| > We first derive the general expression of the thermodynamic
| minimum energy of separation determined by the Gibbs free
| energy, which is independent of the method of desalination
|
| [1] https://pubs.acs.org/doi/10.1021/acs.jchemed.0c01194
| cyberax wrote:
| Yes, their result will approach mine if the amount of
| rejected water approaches 100%.
|
| Their result is independent of technology, it's derived
| from fundamental thermodynamic principles.
| tln wrote:
| The article / video mentions a paper discussing theoretical
| minimum.. I think it's this paper:
| https://pubs.acs.org/doi/10.1021/acs.jchemed.0c01194
|
| > desalinating 35 g L-1 seawater at 50% water recovery has a
| theoretical minimum energy requirement of 1.1 kWh m-3 and a
| practical minimum of 1.6 kWh m-3.
|
| SOTA is apparently ~3.7 kWh m-3. That's not a huge factor
| lagolinguini wrote:
| Correct me if I am wrong as I am not a physicist. I see a
| point that is important to consider, that you have
| potentially overlooked. First, you assume that dissolution of
| salt is a completely reversible thermodynamic process, which
| is fine. But considering it a reversible process, in order to
| reverse the process we need to do a certain amount of work
| which you have calculated. In order to do work we need an
| engine. The most efficient possible engine is a Carnot
| engine. It is known that a Carnot engine can never be 100%
| efficient (unless we can achieve infinite or zero
| temperature). Given that you calculated the amount of work
| needed to reverse the process, you still need to bound the
| efficiency by the efficiency of a Carnot engine.
| Alternatively you need to factor in the efficiency of a
| Carnot engine to get the minimum required energy input.
| cyberax wrote:
| You are correct. Although technically, dissolution is not a
| reversible process. That's why you need to input energy to
| reverse it.
|
| Carnot cycle, technically, doesn't apply to all energy
| sources directly.
|
| For example, solar panels have their "hot side" at around
| 6000K, so Carnot efficiency would be close to 100%. Real
| solar panels have other limiting factors, and I believe the
| absolute achievable theoretical maximum is around 80%.
|
| On the other side of the spectrum, wind turbines have very
| lousy Carnot efficiency because they're exploiting a
| temperature difference of just a few degrees. However, the
| "Carnot tax" is not paid by us directly, so we don't really
| care about it.
| delecti wrote:
| It's not _tricky_ , but it's a lot of work, which is a kind of
| difficulty.
| [deleted]
| vl wrote:
| Dissolving common table salt in water is endothermic. I.e. it
| consumes energy, not produces.
| dahfizz wrote:
| Conceptually simple things can be difficult to achieve. Like
| lifting 500lbs or running a marathon.
| agnosticmantis wrote:
| Given the countless environmental challenges we are facing (and
| causing), we should more seriously and openly consider putting a
| stop to exponential population growth as an (at least short-term)
| solution. It's astonishing how some people preach blind faith in
| our ability to just find solutions for problems caused and
| exacerbated by never-ending population growth without identifying
| it as the root cause. Why is it a given that the earth can just
| withstand whatever we throw at it?
| mr-ron wrote:
| Its not exponential. In fact its estimated to level off over
| the next century: https://ourworldindata.org/world-population-
| growth
| dstainer wrote:
| Slightly off topic, however, the post references the Carlsbad
| desalination facility. If you find yourself in San Diego and like
| oysters, I would highly recommend you checkout the Carlsbad
| Aquafarm. Take the tour and pick up some oysters.
|
| What's really interesting and relevant to the topic is that the
| oyster farm serves as a pre-filter to the desalination plant and
| there's an symbiotic relationship between the plant and the
| oyster farm.
| aeonsky wrote:
| Pick up some oysters, for eating? If these oysters serve as a
| pre-filter for the plant, would you not want to eat them as
| these oysters would contain all sorts of pollution?
| the_sleaze9 wrote:
| A strong case for not eating any oysters at all -- but don't
| forget they're totally aphrodisiacs
| js2 wrote:
| They clean the oysters before selling them:
|
| > This was the first oyster farm to feature an inventive
| "depuration and purification" process, which involves
| immersing the oysters in triple-filtered seawater once they
| reach full size. This ensures that the oysters are a
| completely safe, top-quality delicious shellfish product.
|
| https://www.usatoday.com/story/travel/10best/2022/08/04/how-.
| ..
|
| https://en.wikipedia.org/wiki/Depuration
|
| TIL.
| xigency wrote:
| That's a really convoluted way to say they rinse them off
| in clean water.
|
| Anyway, seafood comes from the ocean. I don't see why they
| would be worse than other oysters.
| js2 wrote:
| > That's a really convoluted way to say they rinse them
| off in clean water.
|
| It's more than rinsing them off. Oysters are filter
| feeders. They need to spend enough time in clean water to
| pump out any contaminants. It's an FDA regulated process:
|
| https://www.fda.gov/food/federalstate-food-
| programs/national...
|
| > Anyway, seafood comes from the ocean. I don't see why
| they would be worse than other oysters.
|
| It depends on the cleanliness of the water. These oysters
| are raised in a lagoon surrounded by the city of
| Carlsbad:
|
| https://www.google.com/maps/place/Carlsbad+Aquafarm/@33.1
| 419...
|
| I imagine that lagoon is subject to runoff and not nearly
| as clean as oysters harvested in open waters.
| beembeem wrote:
| That's really cool! Thanks for pointing this out.
| [deleted]
| csours wrote:
| I wonder if the brine could ever be valuable enough to extract
| minerals from. I keep hearing about how many tons of x mineral is
| in seawater.
| michael_vo wrote:
| You could use simple pricing to influence behavior. 1100 litres a
| day for each American is so damn much. When you hike and stay in
| the mountain huts you are charged 3$ for a 4 minute shower.
|
| You could probably fix the drought situations by reducing
| consumption.
| jeffbee wrote:
| The energy costs are a bit of a red herring depending on local
| conditions. In California we currently "curtail" i.e. discard a
| huge amount of renewable energy in the spring season. If we can
| seasonally apply that energy to desalination, and store the fresh
| water for later, it is essentially a huge time-shifting battery
| that stores excess spring energy for the summer.
| Maxion wrote:
| There's lots to unpack here why this isn't workable at scale.
|
| 1) Renewable energy product still has a cost associated with
| it, even if it is at times, excess.
|
| 2) That excess capacity, and the times when there's more energy
| produced than consumed might not match with water demand.
|
| 3) There definitely isn't, and won't be, enough excess
| renewable capacity to distill even a fraction of the fresh
| water consumed.
|
| 4) This means that you still have to calculate a per kWh cost
| for the energy consumed to distill salt water to fresh water.
| The average kWh might not be the same as the market average kWh
| price, since if you make your distillation plants oversized so
| you can utilize any spare energy production, but there will
| still be a price.
|
| 5) This price will most likely mean that the per gallon cost of
| distilled water will be higher than RO, or water pumped through
| a pipeline.
|
| Desalination is still an extreme measure taken when all other
| forms of fresh water are cost prohibitive.
| jeffbee wrote:
| With the amount of curtailed energy this year in California,
| using a state-of-the-art RO process, we could have
| desalinated about 880k acre-feet of water. This is roughly
| enough water for all domestic urban water use statewide for
| about half the year. It is already close to penciling out and
| our energy resources are still expanding.
| jjk166 wrote:
| > 1) Renewable energy product still has a cost associated
| with it, even if it is at times, excess.
|
| This cost is already paid for in the infrastructure. You're
| not going to tear down extra solar panels when demand is low
| just to reinstall them an hour later.
|
| > 2) That excess capacity, and the times when there's more
| energy produced than consumed might not match with water
| demand.
|
| Water can be stored very easily in large quantities and over
| long periods of time. Replenishing an aquifer in the summer
| will still help you even when the dry season is winter.
|
| > 3) There definitely isn't, and won't be, enough excess
| renewable capacity to distill even a fraction of the fresh
| water consumed.
|
| You don't need to distill 100% of freshwater, you just need
| to make up the difference between what is naturally available
| and what is used. The difference is generally small,
| especially when combined with water conservation methods.
| California's water shortfall could be covered by using just
| 6% of it's current annualized electricity generating capacity
| for desalination.
|
| > 4) This means that you still have to calculate a per kWh
| cost for the energy consumed to distill salt water to fresh
| water. The average kWh might not be the same as the market
| average kWh price, since if you make your distillation plants
| oversized so you can utilize any spare energy production, but
| there will still be a price.
|
| You would presumably locate your desalination plant in an
| appropriate location and operate it at appropriate times such
| that your cost per kwh is substantially below normal market
| rate.
|
| > 5) This price will most likely mean that the per gallon
| cost of distilled water will be higher than RO, or water
| pumped through a pipeline.
|
| RO would be desalination. A pipeline is still taking water
| from somewhere else, the price depending heavily on where
| you're getting it from and the geography between you and the
| source. In many cases there isn't a suitable freshwater
| source to pull from. Certainly there are no fresh water
| sources so limitless and readily accessible as the world's
| oceans.
| pornel wrote:
| I assume this isn't done due to the large cost of building a
| desalination plant, which isn't paying for itself when it's not
| running. The money could be invested in something else that
| runs 24/7.
| adgjlsfhk1 wrote:
| The water storage might be tough here. Especially in
| California, there will be a ton of evaporation (which will
| raise salt levels) and make it less efficient.
| akiselev wrote:
| Is it just me, or did this article dance around the question?
|
| I am not a physicist but let me give it a stab: except for a few
| specialized steps like UV or oxidizing heavy metals, most
| filtration is mechanical. A series of filters with smaller and
| smaller pores capture more and more of the mess in the water like
| bacteria and particulates while UV breaks down viruses, the
| oxidizer precipitates out metals, and so on.
|
| None of those methods work with salt. Salts in general
| disassociate through ion-dipole interactions - the water dipoles
| essentially rip the ionic compound apart and surround each ion in
| what is called a hydration shell. They're bigger than bare water
| molecules but not much bigger - much too small to target with
| pore size. This shell also puts them in a thermodynamically
| stable state and it takes energy to "jostle" the water molecules
| away from the ions either through evaporation, distillation, or
| through another chemical reaction that precipitates out the ions.
|
| As it turns out, doing that takes a _lot_ of energy, so we use
| reverse osmosis as a cheaper alternative: we exploit the
| hydration shell of the ions by putting them behind a semi-
| permeable membrane with _very_ small pores, "nanopores" if you
| will. The pores are too small for water to cross normally, but
| under high pressures bare water molecules can be forced through
| the pores while the ions trapped in their shells remain and
| concentrate into a brine. It takes less energy but produces a
| concentrated liquid waste stream that must be disposed of.
|
| Someone please correct any mistakes I've made
| eutectic wrote:
| I think it's more a problem of entropy; You're taking a high-
| entropy mixture and trying to extract a pure substance. Think
| sorting red and blue lego.
| aidenn0 wrote:
| The comment is still useful; if the red legos and the blue
| legos are different sizes it's pretty easy to sort them
| mechanically.
| eutectic wrote:
| Yes, but there will always be some necessary energy input.
| gabereiser wrote:
| It's important to note that the energy needed for RO to work is
| due to the high pressures needed to ram that little H2O
| molecule through that virgin nanohole. 700-900wh for a trickle
| of 14gal/h. At least that what I'm getting on my sailboat.
| theresistor wrote:
| That's quite inefficient if you're using a Clark pump. I'm
| currently spending about 1000W to making ~40G/h with a
| Schenker Zen150.
| marcosdumay wrote:
| > As it turns out, doing that takes a lot of energy
|
| The change in entropy between a batch of saline water and a
| batch of fresh water and enough saline water that its
| concentration don't change is about the same as letting that
| same fresh water fall for 200m and converting the resulting
| energy into heat (at 300K).
|
| What means that desalination will take a lot of energy whatever
| method you use. There are distillation procedures close to
| perfect efficiency that wouldn't take much more energy than
| reverse osmosis; and of course, electrical separation is that
| one method with lots of promise but that stops due to material
| related problems every time it's tried. It just so happen that
| we know how to scale reverse osmosis up cheaply and reliably;
| but this looks like a feature of our technology and not
| anything intrinsic.
| lisper wrote:
| You've pretty much nailed it except for one minor nit:
|
| > It takes less energy but produces a concentrated liquid waste
| stream that must be disposed of.
|
| This implies that creating a concentrated waste stream is a
| problem unique to reverse osmosis. It isn't. No matter what you
| do you're going to end up with a bunch of salt that you have to
| get rid of somehow.
| darkclouds wrote:
| Its always been my understanding that any treatment to remove
| stuff from water is going to produce waste which needs
| disposing, just look at the Brita water jug filters, they
| need disposing.
|
| I've often wondered why dont we have more pure water pumped
| through the water mains in various countries, and I think
| after reading about Super K the Japanese Neutrino detector
| [1] and how the water in the tank was so pure it had
| dissolved a spanner/wrench that was left in the bottom, years
| ago, I might have the answer.
|
| Firstly there is health implications for drinking pure water,
| and whilst it probably wont dissolve your guts [2], it will
| drastically and quickly alter your chemistry [3] which in
| moderate doses may be a good way to calm down, I havent tried
| personally yet, but there is another problem.
|
| The ultra pure water would probably dissolve the older
| ceramic and metal pipes used to deliver water around the
| countryside, from the inside out.
|
| In fact I would even go so far to guess that water mains
| pipes last longer if its delivering hard water compared to
| soft water, and probably explains the pub culture as the
| water is standardised in various alcohol brands.
|
| Either way I prefer soft water, its more relaxing and could
| well help to reduce a certain amount of anxiety in the
| population along with stress levels, that could be useful for
| built up cities, but watch the GDP levels of the region go
| down if that happened and the profile of crimes change [5],
| not to mention health conditions!
|
| [1] https://www.businessinsider.com/super-kamiokande-
| neutrino-de...
|
| [2] http://physicsandphysicists.blogspot.com/2018/06/super-
| kamio...
|
| [3] https://chemistry.stackexchange.com/questions/30754/effec
| t-o...
|
| [4] https://nuscimagazine.com/water-so-pure-it-will-kill-
| you-261...
|
| [5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7576670/#:~:
| tex....
| ars wrote:
| > the water in the tank was so pure it had dissolved a
| spanner/wrench that was left in the bottom
|
| Once it dissolved a tiny bit of the metal it would no
| longer be so pure. So this sentence makes no sense. It
| takes just a minuscule amount of mineral to replicate
| regular well-water.
| rightbyte wrote:
| How did they know that there was a wrench there in the
| first place? Maybe the mechanic forgot it somewhere else.
|
| I thought salt and impurities in water was the driving
| factor in rust.
| lisper wrote:
| > the water in the tank was so pure it had dissolved a
| spanner/wrench that was left in the bottom
|
| "Apparently somebody had left a wrench there when they
| filled it in 1995," he said. "When they drained it in 2000
| the wrench had dissolved."
|
| I dunno, I think if you left a wrench soaking in regular
| water for five years there wouldn't be much left of it
| after that either.
| DayDollar wrote:
| It would rust and crusted with chalk and beneath that
| layer untouched.
| m463 wrote:
| People who want to recover old coins such as encrusted
| roman coins will soak them in distilled water to dissolve
| the minerals in a pretty short time. The metal usually is
| not affected.
| renewiltord wrote:
| What's the process, though? The "dissolving" is
| presumably rusting and then motion of water washing away
| the rust, but rust requires an oxygen source for the
| chemical reaction, and apparently Super Kamiokande has
| dissolved oxygen specifically removed using a vacuum
| degasifier to prevent interference and growth of
| bacteria.
|
| I'm not feeling particularly convinced by this anecdote.
| It sounds a bit urban legendy. Still, I won't claim more
| than a high-school knowledge of Chemistry so I'm eager
| for someone to correct me and supply an explanation.
| jhoechtl wrote:
| > I dunno
|
| During WWII Entire tanks drowned in Siberian sumps and
| were brought back to life with their engines running.
|
| After five years in regular water I bet the wrench would
| be in working condition.
| chiefalchemist wrote:
| You have that problem only if you decide to remove all the
| salt. You could decide to remove say 20% of the water and
| make that only H2O; effectively getting H20 and a higher
| concentration of salt water as outcomes.
|
| I say this having recently read about the desalination plant
| in Dubai.
| tiku wrote:
| salt battery's.
| ericlewis wrote:
| good thing people need salt!
| Loquebantur wrote:
| The people who need that water tend to shed it after some
| time.
|
| Discarding waste water into the oceans via rivers is a huge
| idiocy. You essentially rely on the environment to
| "magically" sort it all out. Naively so and fraught with
| huge inefficiencies.
|
| Proper treatment of that waste in the sense of recovering
| usable matter streams is the logical way to go.
| daveslash wrote:
| I know that generally speaking, disposing of waste in the
| ocean and expecting it to disperse enough to be harmless
| is foolish and wrong. But in the case of salt, It would
| seem to me that the ocean can handle that amount of salt.
| Course, I haven't done the math. But it would seem to me
| that the back in == salt taken out. We'd only be changing
| the net salinity by the amount of water subtracted.
| Without having done the math, my gut reaction is to think
| that's something the ocean can handle.
|
| The problem, as I see it, is _localized_ concentrations.
| While the ocean at large might be able to absorb it, the
| localized concentrations can be very problematic.
| londons_explore wrote:
| There are potentially some good reasons to just spray the
| brine into the air.
|
| Seawater sprayed into the air becomes tiny salt crystals,
| which in turn help clouds to form, and cause increased
| rainfall. The rain produced has negligible levels of
| salt.
|
| In places with dry climates, this often can turn desert
| land into farmland across an area hundreds of kilometers
| wide.
| singleshot_ wrote:
| Surely the salt falling on the farmland would have some
| kind of deleterious effects, right? I imagine you're not
| trying to grow sea cucumbers in this scenario, and I'm
| not sure most land plants would be crazy about a ton of
| salt.
|
| Just ask the Carthaginians...
|
| Guessing that you know something I don't here, though.
| londons_explore wrote:
| I believe that when every spec of salt creates a far
| larger water droplet - and the overall concentration in
| rain ends up tiny.
|
| If anything, doing lots of this might _reduce_ the salt
| concentration in soils, due to increased rainfall.
| FredPret wrote:
| It might just cause earlier rainfall, not more of it.
| There's x amount of water in the atmosphere, and you
| can't add more by spraying salt crystals.
| stouset wrote:
| If people needed as much salt as was contained in the water
| to begin with, we wouldn't need to remove it in the first
| place.
| scythe wrote:
| Not _quite_ that simple. Even isotonic water would
| probably be too corrosive for existing pipework.
| suddenclarity wrote:
| No reason to keep the 1:1 ratio. Use the salt to replace
| our current salt mines/outtakes and then use the water as
| an addition to our current freshwater usage.
| Retric wrote:
| The waste stream doesn't contain that much more salt than
| seawater. Extracting salt from mines is much cheaper than
| extracting it from slightly brackish wastewater from
| water treatment plants.
| [deleted]
| mschuster91 wrote:
| The problem is that salt brine from seawater contains a
| lot of side stuff - you still need purification at a
| scale you don't need mining rock salt.
| giantrobot wrote:
| The brine waste from RO is still mostly water. In order
| to extract the salt you'd need to evaporate the water
| which still takes a lot more energy. You could use
| evaporative ponds to let the Sun do the work but that
| takes a lot of space. In either case you're spending a
| lot more money per pound than just digging the salt out
| of a mine.
| eropple wrote:
| Is that true when we take into account environmental
| externalities? I am not an expert in this field; I know
| that many forms of mining are capital-B Bad for the
| environment but I don't know how salt mines impact the
| area around them.
| [deleted]
| pas wrote:
| well, mines are bad on the short term. the long term
| damage is not of the actual digging, but of the
| separation processes. which all can be done as clean as
| we wish it just costs more.
| dfxm12 wrote:
| _It takes less energy but produces a concentrated liquid waste
| stream that must be disposed of._
|
| I've heard that this brine is toxic. Does this make disposal an
| issue? Is the toxicity true or hyperbole? I mean, do we know
| how bad it is, and if we can do anything safely with it? It
| seems like "salt" is useful in a lot of contexts, including
| industrial, so can we do something with the brine besides
| disposing it somewhere?
| tivert wrote:
| > I've heard that this brine is toxic. Does this make
| disposal an issue? Is the toxicity true or hyperbole? I mean,
| do we know how bad it is, and if we can do anything safely
| with it? It seems like "salt" is useful in a lot of contexts,
| including industrial, so can we do something with the brine
| besides disposing it somewhere?
|
| Too much salt can kill stuff (e.g. people, plants), so I
| suppose that makes it "toxic." Maybe there's a tiny amount
| old industrial pollution from anywhere an everywhere that
| concentrated in there, too.
|
| However, if you're desalinating seawater, what's the problem
| with just dumping the brine back in the sea? Unless you
| introduced new stuff into it during the desalination process,
| you wouldn't be making anything worse.
| grogenaut wrote:
| Because the brine is hyper-salty compared to ocean water
| and takes a while to mix back in, essentially creating a
| new ecosystem, brackish, where the outlets are.
|
| What's wrong with pumping 10% CO2 into your office
| constantly from a compressed gas plant extracting oxygen
| and argon next door?
| tivert wrote:
| > Because the brine is hyper-salty compared to ocean
| water and takes a while to mix back in, essentially
| creating a new ecosystem, brackish, where the outlets
| are.
|
| How large would those brackish areas near the outlets be?
| It seems to be that would be a big problem in an enclosed
| bay, but much less so on a shore facing open ocean.
|
| Could they run pipes out a few kilometers with small,
| regular holes (maybe some modification of oil pipeline
| technology) to spread the discharge out and mitigate the
| concentration problem?
|
| Could they make the waste output less concentrated? Maybe
| by either running the desalination process less (would
| that also increase energy efficiency?) or by pre-mixing
| the waste with some un-desalinated intake water?
| rtkwe wrote:
| The pipe with many holes is one of the solutions used
| today it's just imperfect and requires a lot of pipe to
| make sure the waste output isn't too concentrated in a
| single area.
| jerf wrote:
| There are several practical problems dumping the brine back
| in the sea.
|
| If you dump it on a living ecosystem you tend to kill it.
| Living ecosystems are, unfortunately, concentrated right
| where we are desalinating and is cheap to dump.
|
| Compounding this problem is that water mixes _much_ more
| slowly than your intuition suggests. It can stay a coherent
| mass of high-salt water _way_ longer than you 'd think,
| killing as it goes. This is one of the more surprising
| things I've learned in the past few years, honestly. Your
| kitchen-scale-based intuition of how long it takes for
| liquids of different characteristics to blend together
| turns out to be _way_ off.
|
| Trying to pipe it away to somewhere where it is less of a
| problem is expensive.
|
| In the long term just dropping it back into the ocean is
| not a big deal, but that short term is surprisingly
| destructive. You'd think you could just drop it in and
| maybe a few hundred feet from the outlet it would be all
| dissipated and harmless, but unfortunately the physics
| don't work out that way.
| scythe wrote:
| The "clever" version I've seen in some papers is to use
| reverse electrodialysis to recover energy as you dilute
| the waste brine with seawater. AFAIK, this has not been
| incorporated into any existing installations.
| Robotbeat wrote:
| > If you dump it on a living ecosystem you tend to kill
| it.
|
| This is false and somewhat dishonest. This is simply a
| choice of not diluting it enough. There is nothing about
| the discharge from reverse osmosis that is any more
| fundamentally toxic than the natural process of
| evaporation.
|
| Proper dilution is essential, and treating the discharge
| as fundamentally toxic actually undermines the
| engineering to do this proper dilution because people
| will figure "oh well, I guess there's nothing we can do
| as it's going to tend to kill no matter what."
|
| People need to stop misleading about discharge toxicity.
| cyanydeez wrote:
| This is dishonest: toxicity, dosage are fundamental
| properties.
|
| You can't just wave a word at it like dilution and think
| you're solving an engineering problem.
|
| If you got 1 gallon out of sea water, what do you think
| you need to dilute it to be safe? Typically, it's 99%[?].
| schiffern wrote:
| >This is simply a choice of not diluting it enough.
|
| If that choice were up to _engineers_ , it's fine.
|
| In practice the choice is actually made by MBA types, a
| field where harmful short-termism is almost a religion.
| cyanydeez wrote:
| It's made by practical money and people who don't want to
| consume local resources.
| margalabargala wrote:
| > water mixes much more slowly than your intuition
| suggests
|
| As a visceral example of this in the other direction, the
| freshwater plume of the Amazon River extends more than
| 60km into the ocean. [0]
|
| I would love to see these plants placed in areas where
| there's a nearby dry below-sea-level basin, into which
| the brine may be discharged. The Salton Sea in CA is one
| example, there's another similar location in Egypt I'm
| aware of. The advantage of such locations is they are
| usually extremely hot and arid, which means there's
| generally not much of a local ecosystem or human
| population, and there is ample solar power availability.
|
| [0] https://eos.org/science-updates/the-amazon-rivers-
| ecosystem-...
| AnimalMuppet wrote:
| The Salton Sea is over a hundred miles from the ocean,
| though, with at least one mountain range in the way. It
| _might_ be a good dumping ground (or maybe not - there at
| least were fish in it, if there still are, this would
| almost certainly kill them). But it would definitely take
| a lot of energy to pump the water there.
| cyanydeez wrote:
| Great movie: Salton Sea
| bjelkeman-again wrote:
| The extreme volumes produced in the Arabian Gulf is
| subject to a lot of studies (some plants produce one
| million m2 per day). This does cause issues. An in-depth
| look at possible futures can be found here.
|
| http://essay.utwente.nl/79579/1/Dols%2C%20F.J.%201862227%
| 20_...
| function_seven wrote:
| It's toxic only because of concentration. AKAIK, there aren't
| any compounds in the brine that weren't present in the
| seawater to begin with. The solution is dilution, but I'm
| sure it's easier for me to type that than it is to achieve in
| a desalination plant.
|
| But, why not really-long-pipe-with-small-holes-along-the-
| length? That seems to me like a simple mechanism to send the
| brine back into the ocean without causing a local disaster on
| the sea floor. Is there maintenance required that makes it
| more expensive than I realize?
| kibwen wrote:
| _> But, why not really-long-pipe-with-small-holes-along-
| the-length?_
|
| The OP mentions that this is common in practice, although
| it's easier to tell in the video that this is what is being
| described.
| Timshel wrote:
| I believe I had read just dilute it until concentration is
| ok then release. But might be more tricky than that ^^ : ht
| tps://www.sciencedirect.com/science/article/abs/pii/S00489.
| ..
| dylan604 wrote:
| dilute it with what? the clean water you just removed the
| salt from?
| toast0 wrote:
| Sewage treatment outflows maybe? Wouldn't be enough in a
| community where all the freshwater is coming from
| desalination, because not all of the freshwater goes into
| the sewage system, but it might be workable in
| communities where desalination is augmenting other
| sources of freshwater.
| VintageCool wrote:
| Or a large amount of regular saltwater.
| function_seven wrote:
| No, the salt water you're about to discharge it into.
|
| Have a pump that draws in 10L of ocean water for every 1L
| of brine you need to dispose of, mix 'em up, and
| discharge the 11L of only-slightly-saltier water back
| into the sea.
|
| Not sure when it makes more sense to do that vs. having a
| leach-field type of brine discharge. They both ultimately
| do the same thing, but one requires more mechanicals, the
| other requires more piping and "passive" infrastructure.
| xenadu02 wrote:
| No, fresh water just enters the water cycle. It will
| eventually evaporate or end up in a river and back into the
| ocean where it will be reunited with the salt. The overall
| salt concentration of the ocean would not be changed unless
| we sequestered the fresh water permanently. Even then it
| would take a tremendous effort to make even the tiniest
| difference in salinity.
| rtkwe wrote:
| It will change the salinity on the short term though at the
| release location and the amount a large plant will be
| discharging is enough to alter the local salinity so long
| as the plant remains operational which will negatively
| affect sea life in that area.
| Robotbeat wrote:
| Only if the discharge is not diluted sufficiently.
|
| There is nothing about reverse osmosis that is
| fundamentally more toxic or harmful than the typical
| evaporation that takes place naturally in the ocean. And
| it's pretty dishonest to claim otherwise. If there's a
| problem with too high salinity of discharge, that's an
| engineering problem that should be fixed with greater
| dilution.
| 8n4vidtmkvmk wrote:
| What would you dilute it with? Surely not the water we
| just extracted?
| strken wrote:
| If you're drawing water from a bay and not putting all
| that water back in, then the salinity of the bay must
| depend on the rate it mixes with the outside ocean, since
| you're removing water from the system.
|
| I assume that in practice the amount of water taken by a
| desal plan is tiny and most bays have high tidal inflow
| and outflow, but it's obvious that more than just
| dilution should be considered.
| emodendroket wrote:
| It's full of contaminants from the process that make it
| unsuitable for a lot of uses, as I understand.
| londons_explore wrote:
| > It takes less energy
|
| Distillation and reverse osmosis theoretically use the _same_
| amount of energy.
|
| Practically, reverse osmosis tech is far closer to that ideal
| efficiency level, especially if electricity is your starting
| energy source.
|
| But it doesn't seem out of the realm of possibility that
| someone will figure out efficient distillation in the future.
| distillation has the big benefit that it can make use of low
| grade heat which is waste from lots of industrial processes.
| at_a_remove wrote:
| I only have a BS in Physics but you're basically correct. But
| to make it even more simple and divorced from method:
|
| 1) There's a large difference in energy and entropy between
| seawater and drinkable "fresh" water. This represents a _bare
| minimum_ expenditure, below which you can never go, lest you
| attempt to create a perpetual motion machine.
|
| 2) No matter how you do it: Well, now you have a bunch of
| previously dissolved solids covering everything. How do you get
| them off of your surfaces and out of your tubes and "away" from
| everything else?
|
| Once you stare at the first factor, then look at the second
| factor, then go back and forth, you come to your senses and
| realize that the dream of a jeroboam of colorless, tasteless
| water next to a little pile of fine powder is just not going to
| happen, and that the more sensible thing is to release some
| extra briny water _back_ to your source and hope it doesn 't
| kill too many fish.
| vladraz wrote:
| A sensible thing to do is to turn the waste brine water into
| a resource. Since it's already been pumped up, pour it out
| into an evaporation pond to increase humidity in an area that
| could benefit from it, and then scoop up the salt to extract
| valuable minerals.
| Bost wrote:
| I guess there aren't that many valuable minerals in
| seawater. For example, Fritz Haber, a German Nobel Prize
| winner in chemistry, tried to extract gold from seawater
| after WWI to pay for the war reparations... long story
| short, the concentration of gold in seawater is too small.
|
| Also, the phase transition for H2O from liquid to gas
| requires a lot of energy and space (evaporation surface).
| In other words, it takes ages to evaporate all the water.
| Also, the larger your pond is, the more expensive it is to
| scoop up the salt. And then just one rainy afternoon can
| set you back a lot.
| kylehotchkiss wrote:
| If you built a desalination system say... 500 feet under the
| ocean and have the pressure above pushing water through the
| filters, is it possible to lower the amount of required energy
| just a little? Then you're more pumping water out of the system
| than pumping it through heavy filters
| rootusrootus wrote:
| Wouldn't the energy of pumping the water all the way back up to
| the surface completely balance out the energy provided by the
| weight of that water to push it through the membrane?
| function_seven wrote:
| Maintenance of those filters and associated infrastructure will
| become crazy expensive.
|
| And now you need to pump the final product up to the land
| surface, adding cost there as well.
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