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Comment by tempestn | Hacker News Reader

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.

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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.

Ah, so a slightly more efficient desalination plant then.

Slightly more efficient, with less waste.

Pretty solid win-win

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.

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.

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/)

> 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.

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.

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

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

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