Japan has opened its first osmotic power plant

They really fail to explain a key point here. The reason you colocate this with a desalination plant is because you use the super-salty wastewater from desalination as the salty side of the osmosis power plant. Then you find some wastewater which is low in salt (such as semi-treated sewage), and use that as the fresh side of the osmosis power plant.

The end result is that the salty wastewater is partially diluted, which means it has a lower environmental impact when it is discharged to the ocean.

explains your username, explain like im like 17

thanks

Basically this is like the recouperator on early heat engines, but with a liquid gradient instead of a thermal one.

It's making desalination more efficient and the effluent a bit easier on the ecosystem.

> They really fail to explain a key point here. The reason you colocate this with a desalination plant is because you use the super-salty wastewater from desalination as the salty side of the osmosis power plant. Then you find some wastewater which is low in salt (such as semi-treated sewage), and use that as the fresh side of the osmosis power plant.

They do hint at it at end:

> “It is also noteworthy that the Japanese plant uses concentrated seawater, the brine left after removal of fresh water in a desalination plant, as the feed, which increases the difference in salt concentrations and thus the energy available.”

And the "fresh" water is also "treated wastewater". That could mean a bunch of things but in most cases it's water that's released into the environment by the water treatment plant. Its quality can be as good as clean water, but most municipalities wouldn't feed that right back to the consumer, they dump in a river or lake instead.

Goes nicely with the "pressure exchanger" which recovers the pressure of the high pressure brine waste stream. Lots of heat exchanger analogies!

https://en.m.wikipedia.org/wiki/Pressure_exchanger

Yeah, this is the coolest part. The leftover brine from desalination is generally just a problem. It's harmful to the marine habitat if you just put it back into the ocean, and there isn't a lot else good to be done with it. (Basically you have to dilute it first.) But this way you get useful work out of the dilution!

The article also doesn't say if it produces more power than the attached desalination plant requires. I doubt it as you'd be getting close to a perpetual motion machine if so. In which case basically what you've got is a very energy efficient desalination plant, more than a power plant.

Fukuoka's desalination plant treats about 16400 m^3 of water per day. Assuming 3kWh per m^3 of water, this works out to a time-averaged power consuption of ~2000kW.

The osmotic power plant generates about 100kW, so it's about 5% of the total desalination energy requirement.

Why are we assuming 3kWh per cubic meter of water?

I did some cursory research and that seems to be a common estimate for modern osmosis-based desalination energy costs.

If you have a better estimate, feel free to supply it.

Yeah those are funky devices. I'll be curious to see what the production cost per cubic meter of fresh water ends up at.

Ah, so a slightly more efficient desalination plant then.

Slightly more efficient, with less waste.

Aren't there better uses for the treated water than this? Can't you use it instead of desalinating salt water? Or just run this treated water through the same RO and you won't produce any brine and the result will be just as pure.

Yeah I think that's the difficulty here, the technology needs just the right kind of placement and the right surrounding setup. There is a desalinization plant nearby, they feed the water into the city, the city uses the water, the sewage comes to the water treatment plant, they clean it up to environmental release standards and instead of dumping it back into a river or the ocean, they use it together with stronger brine from the desalinization plant to produce some electrical power.

From what I understand most municipalities do not directly feed sewage treated water right back to the consumer, normally they dump it into a lake or river first. A lot of that may just be an informal "yuk" factor not necessarily not having the technology.

It's cool but everything sort of has to be aligned for it to work well.

Pretty solid win-win

According to this delightful overview [1] of the desalination plant, the capacity overall is 12000kW so that's definitely close enough.

1. https://www.niph.go.jp/soshiki/suido/pdf/h21JPUS/abstract/r9...

Yeah it's like regenerative braking vs losing that energy as waste heat.

Wow, that completely changes my opinion.

In fact, it almost seems that you could simple pull in sea water as the “low salt” water and still have a large enough delta against a brine solution.

Really interesting that it also solves the brining issue.

Great analogy. This is similar to producing gas from sewage to feed back into the gas network, which is used to cook food, which produces sewage. (i.e. https://www.hofor.dk/english/hofor-utilities/town-gas/)

Usually you loop the treated waste water through nature for dilution (and more filtering if you use ground water) in case there is some problem with the treatment process.

Also it is kinda hard to sell to people the concept of “you are drinking literal shit/piss” even though if you stop and think about it all lake/river/reservoir water is full of fish, bird, etc shit.

Can't we just process it into salt/lithium and whatever is there? Since its already concentrated?

This sounds pretty awesome, recycling the waterwaste in a sort of feedback loop resulting in drinking water and power.

I am kind of curious on how much you can/should optimize this process until it becomes dangerous or unmaintainable. And can we do this on more places on this planet? For instance somewhere on a desert coast or something? Could be cool to build some of those between Sahara desert and the ocean, combined with solar panels or something.

Depends on the CAPEX and OPEX requirements. If it is cheap to do, it could be a solid win, but if the plant requires a lot of capital, it might be cheaper to just take the hit on efficiency

Yes the brine could just be diluted wih gray water to reduce the environnemental impact without the energy recovery of the osmotic plant and the capital can be invested in other renewable with better efficiency.

That being said it's a first so it's a pilot project needed to have feedback on a real plant in operation and not just back of the enveloppe calculations and suppositions. Sometime you need to just build the thing to encounter problems, issues or non-issues.

> The article also doesn't say if it produces more power than the attached desalination plant requires. I doubt it as you'd be getting close to a perpetual motion machine if so.

Not really. Even if it would generate enough to power the plant, it would still rely on work being done outside of the plant, i.e. the flow of semi-treated waste-water and possibly the brine itself.

If you want that much salt, then yes. But you probably don't

The amount of power consumed by the desalination process >> the amount of power produced by the osmosis power plant

It is more like it slightly reduces the power required to run the desalination process

It's harmful to a tiny watershed of marine habitat immediately downstream of the discharge pipe, and dilutes rapidly. With that said - if you can harvest a meaningful amount of energy from desal anything helps. I don't know that 5% is a meaningful amount, however.

Ok so if I'm understanding your comment correctly, this "power plant" is actually reclaiming some of the energy from the desalination process?

Wow, missing this point in the article is pretty silly - thanks for pointing that out, that’s a pretty cool aspect.

But, if it’s in any way efficient, why not just use some of the fresh water you produce? Doesn’t that kinda become free power in a sense?

Using fresh water to power a desalinization plant just seems counterintuitive to me, I guess, but maybe (certainly) I’m misunderstanding something