The plant, operated by the Fukuoka District Waterworks Agency, marks Japan as only the second country in the world to deploy this technology after a Danish company pioneered it in 2023, according to Kyodo News. Local officials and experts present the project as a breakthrough for renewable energy that can run around the clock, unaffected by weather or sunlight, and without emitting carbon dioxide. “I feel overwhelmed that we have been able to put this into practical use. I hope it spreads not just in Japan, but across the world,” said Akihiko Tanioka, professor emeritus at the Institute of Science Tokyo, in comments to Kyodo News.
Beneath its understated exterior, the Fukuoka facility relies on a process known as salinity gradient power. Treated sewage water and highly concentrated seawater are separated by a special permeable membrane, triggering osmosis as water molecules move toward the higher salinity side. The resulting water pressure spins a turbine, which in turn powers a generator.
The plant is expected to generate approximately 880,000 kilowatt-hours of electricity annually, according to the Fukuoka District Waterworks Agency. That energy will directly support a local desalination plant supplying fresh water to Fukuoka City and neighboring areas—tying the renewable energy project closely to local water infrastructure.
Could you please explain this part? My first thought was “hey cool, they can use the brine from the desalination plant,” but I guess I was wrong. I thought they would just dilute the brine with treated waste water, then dump it in the ocean, refill the PRO plant with brine from the nearby desalination plant.
Basically in areas where desalination is used for mass water production pure water is the resource in shortest supply and therefor osnhigher value than electricity. Wastewater is a potential source to feed RO systems to generate pure water, any water source with a lower concentration of solute than seawater is a potentially more efficient feedstock for the RO purification than seawater. Given that RO purification takes more energy than the energy generation system discussed here, it would be more efficient and therefore cost effective to mix the wastewater into the feed stream and produce a less concentrated brine waste output rather than use it to dilute the brine waste after the fact. The more concentrated the brine output from an RO plant, the more energy it takes to produce due to diminishing returns as the osmotic pressure increases with solute concentration.
There are exceptional cases, as I mentioned, where the composition of the wastewater stream will make it so that concentrating the solute in that stream has an economic incentive greater than the demands for RO water. In those cases diluting the brine may make economic sense. The location of the plant impacts this as well, there may be some edge cases where transporting the brine to the source of wastewater and processing it there on site may have an economic advantage. I’m thinking of nuclear waste and high toxicity chemical plant waste here where release is not an option and storage is mandated, so the waste stream will need to be both segregated from other less toxic waste and stored long term.