Everyone Was Wrong About Reverse Osmosis–Until Now | Hacker News Reader

Everyone Was Wrong About Reverse Osmosis–Until Now

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208 pointssohkamyung79 comments

I read the headline and suspected it was overstated, but the content of the article...for once, seems to match the headline.

That is, the long-accepted “solution-diffusion” theory that the water dissolved and diffused across the membrane is wrong. That instead, the membrane has spontaneous tunnels created through the pressure/heat/friction. And the tunnels are ~5 angstroms in diameter, allowing clusters of ~1.5 angstrom water molecules through.

That's fairly exciting, as material designers can now target and optimize for that behavior.

I was hoping for more detail on how it keeps salt from following the same path. Individual NaCl bonded clusters are larger than water molecules, but searching around suggests they aren't always much larger.

i always wondered why membranes. can we not drill 0.2nm holes in silicon for instance?

If you had a thin sheet of silicon filled with 0.2nm holes, wouldn't that be a membrane?

Not if it’s thick and/or rigid, right?

What would be the advantage? Presumably it would have to be very thin, so rigidity seems like it would just mean fragility. Are you seeing it differently?

Just speculating!

Yes, that would be a membrane. There a number of membranes with larger pores that are fabricated like above, but out of polycarbonate (track etch membranes).

Some people have also experimented with single and double layers of graphene with a few very small pores. Because they are so thin, you don't need as much porosity for high transport rates.

You still have fouling issues, though.

What you're picturing has a name: https://en.wikipedia.org/wiki/Fritted_glass. It's used in a https://en.wikipedia.org/wiki/B%C3%BCchner_funnel instead of filter paper, to efficiently separate liquids from solids.

Also, if you take some fritted glass and wrap it around a solid core, such that the holes don't go anywhere — i.e. they're just pits that water molecules (but not anything larger) can enter, and then get stuck there by surface tension — then you've get https://en.wikipedia.org/wiki/Molecular_sieve s.

> If you had a thin sheet of silicon filled with 0.2nm holes

That's literally individual silicon atoms removed

"The nearest neighbour distance is 0.235 nm" https://www.princeton.edu/~maelabs/mae324/glos324/silicon.ht...

Silicon lattice spacing is ~0.5 nm - you're talking about a single atom

> can we not drill 0.2nm holes in silicon for instance?

I think that this is interesting question, and should not be waved away with "just drill some 0.2nm holes, use a real small drill bit"

0.2nm is tiny, given that it's on a scale that impacts how H2O molecules behave.

It's also smaller than our ability to make the circuits in computer chips, which are currently > 1 mn (1).

Use a laser? Light with a comparable wavelength is x-rays. As used in the chip manufacturing (2)

This is about size of the spacing between individual atoms (3)

So it seems like a material with 0.2nm holes "drilled" into it would be a major engineering feat at least.

1) https://research.ibm.com/blog/1nm-chips-vtfet-ruthenium

2) https://en.wikipedia.org/wiki/X-ray_lithography

3) https://en.wikipedia.org/wiki/Atomic_spacing

I’m not sure we can do that. Even if we could, drilling millions of holes is a lot of work.

Also, for membranes, it’s not only the size and shape of the ‘holes’ that count. Many have specific chemical properties that do not solely filter on particle size.

That would cost 10000000x as much to manufacture. Think making a net versus hollowing a solid cube of steel into a net.

no shade but this is peak "can't we just" hacker news energy

What's wrong with "can't we just" questions?

Lots of curious laypersons here, and it's often a useful way to learn. Usually it's pretty easy for an expert to answer such questions.

nothing wrong with it that's why i put no shade

It's only wrong if you push the follow-through onto others. That's lazy. If you're really curious, spend 5 minutes on it.

In this case, if you ask "can we just do this" you should follow through with "how big is that actually, what's of comparable size?" We can all google this, or learn how to. It's a useful way to learn. You won't learn how to find things out, by always just asking "can't we just" questions and walking away.

In this case, soon you'd find out that 0.2nm is on the same scale as the spacing between individual atoms in a silicon crystal.

Literal "drill" bits for removing 1 atom at a time are obviously out of the question.

It is 10x smaller than the 2nm, the scale of the best x-ray lithography computer chip manufacturing processes.

I’d say that peak “can’t we just” comes with an assumption that we could just, which is quite different from this comment which comes off to me as hoping to learn why we can’t just.

I often want to honestly ask "why we can't just do X", but I feel like I end up coming off as asking "can't we just do X" no matter how hard I try to phrase it otherwise. I feel like this is a place where English grammar just doesn't give you any good options.

I’ll often ask “what are the problems with doing X?”.

It doesn’t imply that there should be a solution but you still want to learn why it might not work. Often you can continue the conversation enough to either exclude it or think some more about the problems that are described.

i don't think you quite grasp how small that is

Yeah, the smallest drill bits regularly commercially available are on the order of 1mm. That’s 6 orders of magnitude.

Individual silicon atoms in a crystal are around 0.2nm apart.

https://www.princeton.edu/~maelabs/mae324/glos324/silicon.ht...

So no, you don't get drill bits that are 1-2 atoms wide.

Also it would take a long time to "drill" a useful number of holes if you do it one by one.

Use X-ray lithography maybe? The wavelength can be short enough and it might be more feasible to do in bulk.

You can "drill" 5nm holes in aluminum oxide[1]. Which is pretty sweet, but calling that process "drilling" is a gross misnomer. The finest drills[2] out there are measured in hundreds of nm, and still take several seconds to get through relatively thin material. To drill the billions of holes you'd need for a reverse osmosis membrane, you'd die before it was finished.

[1] https://en.m.wikipedia.org/wiki/Anodic_aluminium_oxide

[2] https://www.tpc.toray/en/technology/hole/hol_005.html

NaCl isn't bonded in water, it's ionized and dissolved. Each ion (Na+ or Cl-) is surrounded by a large number of water molecules due to the electric field of the ion.

That's not completely helping me visualize why it doesn't pass through the membrane. Are you saying that each water molecule surrounded (Na or Cl) ion stays intact as a group, and is too collectively too large to pass?

The water molecules aren't chemically bonded to the ions, but they are "bonded" by intermolecular forces. Although weaker than a chemical (intramolecular) bond, the intermolecular forces are still strong enough to "bond" water molecules to the ion. So either only water molecules not "bonded" to ions can pass through these channels, or the pressure differential across the channel can free the water molecules from the ions.

It's not quite a pressure differential, but a chemical potential differential (but thinking of it as a 'pressure' gives a useful mental model).

It’s force per area so it’s definitely a pressure, the difference is just that the driving mechanism is chemical potential instead of a difference in number of particles.

Tough a system put into contact with a resevoir of a constant concentration should try to increase in volume until the concentrations on both sides are equal.

It's probably the 'constant concentration' part that is confounding the two. It is connecting number of particles with volume, whereas the canonical ensemble has them as separate terms.

Had a skim, this seems relevant [0]

> The Na+ hydration shell, which is formed by three water molecules, is quite stable while the K+ ion can only perturb the water structure in its immediate neighborhood so that it is unable to attract the water molecules. As a result the diameter of the hydrated Na+ is about 0.5 nm while the K+ diameter is equal to its ionic diameter.

Basically in water Na+ and Cl- are surrounded by water molecules attracted by electrostatic forces. This will add bulk to the naked ion and they are reasonably stable/robust

You can get an intro into solvation shells here[1]

[0] https://www.sciencedirect.com/science/article/pii/0005272896....

[1] https://en.m.wikipedia.org/wiki/Solvation_shell

Trashcan-sized RO filters that you can install at home always felt like magic, yet I strongly feel they should be a commodity in the developed world, something you'd expect out of wherever you live, just like stable AC power. A friend used to be a salesman for those; they'd bring an electrolysis device to a presentation at your home, and show what kind of stuff was floating in your tap water. It made for a cool show but also did help spread awareness of just how bad the water was.

Aren't they a commodity? You can stop by any hardware store and grab a system and a bag of salt. Not much need to consider brand names, they more or less work the same.

Edit to add: Apparently I'm easily confused. I thought water softening was happening by reverse osmosis, but it's a different mechanism.

That's not a reverse osmosis system, but a water softener that swaps ions like calcium with ions like potassium or sodium.

Most RO systems have multiple layers, including a water softener.

all RO systems I've found have a carbon filters, sediment (paper) filters and perhaps a VOC (carbon) filter. I've never seen a salt-based water softener stage for a home RO system.

In RO context it's called remineralization (or alkalinization, I guess), not softening, and a lot of home RO systems have a final stage for it.

Googling for reverse osmosis minerals will bring you to a large number of probably fabricated claims about water pH and health that ultimately align on taste preference.

> “Remineralization is typically to enhance the flavor of the water, because reverse osmosis reduces the level of minerals in the water significantly, and some consumers prefer the taste of the water when there is enhanced mineral content,” explains Rick Andrews, of the National Sanitation Foundation’s Global Water Program. - https://www.thespruce.com/best-reverse-osmosis-systems-45868...

https://www.homedepot.com/p/ISPRING-Premium-10-in-Universal-...

https://www.homedepot.com/p/Express-Water-Alkaline-Water-Fil...

https://thewatergeeks.com/how-to-remineralize-ro-water/

etc

It depends on how hard your water is, but if it's very hard and the RO system gets lots of use, you need a softener or anti-scalant injector that injects a small amount of chemical which prevents scale buildup.

If you don't do this and have particularly hard water the membranes foul pretty rapidly.

In the case of undersink systems pumping out two gallons or so a day from standard municipal water, it's probably less important.

I only know this because I've been playing with osmosis systems for a few years and have built one that does around 150 gallons a day from undersink system parts with 2 membranes, some filters and pumps for my garden (otherwise my water is simply too hard to grow all the things I want to grow).

There's no free lunch here. Typical reverse osmosis filters produce one unit of clean water for every three units of intake, functionally tripling the cost of water (not including the price of the filters themselves, which should be replaced annually).

Wait, where do the other two units of water go?

They’re usually discarded into the drain line, so into the sewer system

God, what a waste.

Why? most people only have it attached to their drinking/cooking water, it ain't that bad. that's worrying about the 1% problem when the 99% problem is stuff like industrial usage and watering lawns in areas that were once arid scrub lands.

I've an aversion to paying for potable water and then running it right down the drain with a very slight increased concentration in contaminants, other conversations regarding filter efficiency in this thread were rather eye opening. It feels like basic "turn the tap off while you brush your teeth" wastage writ large. At the very least, plumb it to grey water uses first.

Yes, in the big scheme of things its a rounding error against industrial uses but that still doesn't mean I'm going to leave my hose bib running 24/7.

Ideally into your sewage system, as they are now concentrated wastewater (although if you live in a developed region with centralized water treatment, there won't be that much waste in the water to begin with). RO systems designed for processing seawater have a much harder time of it, as their waste is ultra-salty brine which will kill any local marine ecosystems if you decide to just dump it back into the sea.

Since the flow of deionized water is only 1/10 of total water flow... I think the increase in "waste" concentration is also only about 10% even of the filtered water is perfectly pure. So if you had 400ppm total dissolved solids it would now be *10/9=444ppm. Not much of a concern.

That would not be ideal for me (Bay Area resident) where water is crazy expensive. I'd want to keep it for watering plants, flushing toilets, etc.

Yes, the only problem is that most homes are not plumbed for grey water and it's a very expensive retrofit. If you're building a new home, ask about that and structured wiring. It's unfortunately usually a way more economical idea to just get those toilets that have a sink built into the top (popular in countries like Japan) that use the hand washing water to flush the toilet vs trying to store and/or pump it around.

Seems like thie brine from desalination plants should be pumped into tailing ponds and dried out to get useful minerals out of it that could then be sold to help offset the cost and environmental impact of desalination. https://news.mit.edu/2019/brine-desalianation-waste-sodium-h... looks like this is not an original idea :)

Can’t they be used as a source of non-potable water? Like toilets, irrigation, maybe laundry?

Sure. I've lived on the countryside as a kid, it was before the village had a sewage system built. We not only had RO, but also a tiny waste water processing plant in the backyard, you'd put a spoonful of some powder in the tank once a month or so, and the "cleaned up" grey water would slowly trickle out and irrigate the vegetables in the garden. This was around 2002. I'd like to live like that one day again.

Essentially you can think about them as "water which washes the filter", i.e. contaminants removed from the drinkable water are in this waste stream. Without this filter this fine would clog up in couple days. In typical home install this water is just dumped into the sewer, if you have use for gray water, you can capture it and re-use it.

This wastewater isn't even gray water, it could be simply reused as non-potable. I think of it as "light gray water" - you wouldn't want to drink it but it would be better than gray water for plants, etc.

any relevant research on how much of that higher-than-tap-water TDS content makes it into the plants? I'm not sure I'd want to use this brine (plants also aren't huge fans of saltwater) to water an edible herb garden, for example.

It's only about 50% more concentrated than normal tapwater. It's taking the junk dissolved in 3 liters and condensing it down to 2 liters. Different cities probably vary more than that. Probably still pretty safe to drink, although that defeats the point of having a filter.

>Typical reverse osmosis filters produce one unit of clean water for every three units of intake

That's actually a very optimistic figure IME:

I recently started using a consumer under-sink RO system from GE Appliances at my off-grid cabin, the kind you find at Home Depot.

If you use the included pressurized accumulator tank, the stated efficiency is ~10%.

Since I'm off-grid, the reject line is run to a 5-gallon water cooler bottle, making product:reject water ratio comparisons trivial. Even 10% is somewhat optimistic.

The efficiency is about double if you disable the pressurized accumulator, that way there's no pressure on the other side of the membrane (except when stopped). It's not a simple fixed ratio since the efficiency decreases as the accumulator fills and the pressure difference across the membrane approaches zero. It's actually pretty insane how wasteful the system is as-delivered if you use the accumulator, towards 100% full it's mostly just rejecting water for an hour+.

Without the accumulator, but using the included reject water restriction orifice, it's more like 20-25%. I've ended up adding an adjustable restrictor valve on the reject water line to keep it closer to the 3:1 you described, which shouldn't damage the membrane AIUI.

I guess it's just safe defaults they ship in an unsophisticated system connecting to potentially high inlet pressures. This combined with a luxurious pressurized accumulator tank, makes it spectacularly wasteful of perfectly good water. Most wouldn't even realize the waste volumes having the reject line plumbed into the sink drain.

I've been making huge hexagonal concrete pavers with the reject water...

Your "pressurized accumulator" sounds like a "permeate pump", which are readily available, not too expensive (~50 USD), and easily installed. It took me longer to move the filter assembly out from under my sink than to splice the device into my RO system!

Anyone with an RO system living in a water-scarce region should install one! It'll save you many gallons a day, and has no downsides, unlike most other water reduction devices (like weak-flushing toilets or barely-misting shower heads).

I don't follow.

The "pressurized accumulator" I'm referring to is just a water storage tank in the form of a bladder inside an air-pressurized steel vessel. A smaller version of the tank you find next to practically any well-pump to conserve pump cycling/wear and tear/smooth pressure spikes... Its participation in this system post-membrane is wasting water, not conserving it.

It's included from the factory just so nobody has to wait for filtered water to pass the membrane at time of use, instead experiencing an instant powerful jet of water out of the filtered water faucet.

Seems like they misunderstood you but also gave you good advice. Using a post-RO pump removed the pressure from the RO output side. I used this setup when I had a salt-water reef aquarium and needed perfect water. The pump made a large difference in the ratio.

There can be downsides to the permeate pumps, but not many.

Installed with a standard hydraulic shut off valve (ASO) you won't see any issues but you won't get much in the way of improved storage pressure. You will just see improved product/waste ratio as the pump negates the back pressure from the tank. The downsides are the periodic thumping noise and the added complexity of the extra tubings and fittings.

Installed without an ASO valve the permeate pumps will cause some TDS creep and bleed high TDS product water into the storage tank each time the system stops and starts. This will mix with the low TDS product water and raise the average up somewhat. The amount will vary on usage behavior but it's a noticeable TDS bump for most users.

You can double up the RO stages to roughly half wastage, with the side effect that you have to change that second filter twice as often.

What would electrolysis reveal about your tap water? That is not a normal test for water quality. What happened to the water when the machine was turned on?

https://www.youtube.com/results?search_query=tap+water+elect...

So... it's a scam where they put iron from the electrodes into the water, then show you the discoloration?

I don't understand the physics, but as I said, it was a cool show.

> Trashcan-sized RO filters

I think the RO filter is still a small cartridge, the thing that looks like a white bbq propane tank is just a water reservoir since the filtering is very slow.

https://archive.is/TGkwS

Sounds like a pitch, but it's more of where to get the goods at a reasonable price than a deep, vertical supply chain intent on creating an inkjet economy.

If you (US-based) want some residential/commercial RO membranes or water softeners for a reasonable price, I've had these at all the homes and apartments of family and self for 20 years.

Retail channel: https://wateranywhere.com

Commercial channel: https://www.appliedmembranes.com

My mom's house in Texas had the following freighted together:

- Whole house filtration (3x 20" big blue + 2 bypass valves)

- Water softener of sufficient grain reduction capacity - uses rock salt for cation exchange sodium for calcium in the presence of the catalyst bed. Creates water that's nicer to bathe and wash with, but it's probably not potable and can't be used to water plants.

- RO - 100 GPD - 3x 10" filters + RO membrane + UV + 1 post filter + permeate pump (reduces waste and transfers pressure from waste to feed). It's a PITA to run the tubing to refrigerator, sink, and wherever else you want it, but it's a one time thing. I used oversized tubing 3/8 ID to increase flow rate and reduce loss over a long run.

Initial cost is about $4k. Ongoing cost is about $40-50/year for whole house filtration and $60-80/year for RO depending on water quality. The RO and whole house filter setup should be sanitized every 6 months using appropriate supplies and methods.

Great care and research has to be taken choosing effective whole-house filter and RO filter cartridges to trap potential contaminants. At a minimum, the water should be sampled and tested by a lab and the local health department. There are specialized filters for heavy metals, chloramines, nitrates, and such. Here are some: https://wateranywhere.com/filters-and-housings/kx-extruded-c...

Mom's RO setup produces water <2 ppm at a resistance of 1 MΩ/cm. It won't be as good elsewhere without sufficient pressure and if the water has higher ppm of dissolved solids.

Bottom line: Never drink US tap water. Flint, MI isn't an aberration. (My mom's extended family lost several members due to cancer from polluted water at a house they shared around Durango, Colorado in the 4 corners area.)

this is an overreaction and there are much more dangerous things out there. If you are concerned about your water then get it tested, this is by far overkill. If you're super concerned get a RO system for your kitchen sink not the whole house which would be extremely wasteful.

Hey there fellow RO hacker!

I'd like to plug https://www.freshwatersystems.com/.

I built a system that does around 150 gallons a day from undersink parts, some filters, pumps, an IBC tote tank I got at the feed store and a softener. Excluding the softer total cost was under $500. It's for my garden and the animals.

TL;DR

"In a study published in April, Elimelech’s team proved that the once-frustrating assumption about how water moves through a membrane is, indeed, wrong. They replace it with a “solution-friction” theory that water molecules travel in clusters through tiny, transient pores within the polymer, which exert friction on them as they pass through. The physics of that friction matter, because understanding it could help people design membrane materials or structures that make desalination more efficient or better at screening out undesirable chemicals, Elimelech says."

Cool engineering story.

Phys.org had an article on this a few weeks ago https://news.ycombinator.com/item?id=35606682

Wonder if one could make the membrane out of something like stainless steel to support higher pressures now that we know how this process works

Ignoring the "how impossible is it to drill 0.2nm holes in anything" conversation above for a moment, what do you get out of stainless steel? My gut tells me you're looking for something with more permanence, but even stainless steel seems like a bad choice for something whose job requires constant contact with salt water (ergo, rust)

Just drill some 316 and it'll be good for a while :P

Supporting higher pressures or higher flow rates through the same area is one way to scale up reverse osmosis, but it requires some serious engineering to do (from the discussion above, very nearly at the “controlling placement at the individual atom” level). Meanwhile another way to scale it up is just make more of these membranes that we know how to easily make, and have bigger areas of membrane.

There are ceramic membranes and nickel membranes but to be efficient they have to be so thin that they are very fragile anyway.

Suggestion: Add selected chemicals A to the water. Select chemicals A to add that will attach fairly strongly to the chemicals B want to remove from the water. Now the chemicals B don't want in the pure water have joined (bonded) with added chemicals A to form larger molecules C that have a harder time than chemicals B flowing through the pores in the membrane. So, can use membranes with larger pores that the pure water can more easily flow through and still block relatively large chemicals C.

Anyone try any such thing so far?

One thing to know if you're a coffee snob is which cafes near you have an RO filter. The ones that do make much better espresso. https://www.goodreads.com/book/show/29427961-water-for-coffe... is the definitive guide apparently.

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