Japan's First Osmotic Power Plant Turns Desalination Waste Into Steady Electricity

Fukuoka puts two waste streams to work

At the Uminonakamichi Nata Seawater Desalination Center in Fukuoka, Japan, concentrated salt water and treated sewage have become ingredients for electricity. An osmotic power facility that officially began operating with an opening ceremony on August 5, 2025, is designed to generate roughly 880,000 kilowatt hours a year by capturing energy from the difference in salt concentration between the two streams.

The installation is the first operational osmotic power plant in Asia and the second continuous commercial plant of its kind worldwide, following a Danish facility that opened in 2023. Located within the desalination center, also known as Mamizupia, it sends its electricity back into the work of making seawater suitable for drinking.

Its annual production is equivalent to the electricity used by about 220 to 300 average Japanese households. Operators report a utilization rate approaching 90 percent. The attraction is steady operation: unlike solar panels or wind turbines, the plant does not depend on daylight or wind, and its operation does not depend on seasonal rainfall.

Japan imports most of its fossil fuels and faces growing pressure to reduce carbon emissions. This small installation tests whether the seas surrounding the country, combined with existing water infrastructure, can support a dependable source of clean power. It burns no fuel and emits no carbon dioxide during generation. Beyond a single turbine, it has virtually no moving parts.

How a salt difference becomes electricity

Osmosis is the natural movement of water through a membrane that separates liquids with different salt concentrations. It is also part of the process that lets tree roots draw water from soil and helps human cells maintain their condition in surrounding fluids.

A semipermeable membrane acts as a selective barrier. Water molecules move from the less salty liquid toward the saltier liquid as the concentrations move toward balance. The membrane keeps salt ions from crossing, allowing water to pass instead.

Inside a sealed chamber, water entering the salty side increases its volume and pressure. Engineers direct that pressure through a turbine, which turns a generator. The energy comes from the chemical difference between the two liquids.

Fukuoka uses a method called pressure retarded osmosis, or PRO. The salty side is kept under pressure while water continues to enter through the membrane. The resulting pressurized flow provides the force needed to generate electricity.

For a standard system pairing seawater with fresh water, the chemistry works best at a pressure difference of about 26 bar. That is roughly equivalent to the pressure beneath a column of water 270 meters tall. Fukuoka uses a stronger salt contrast by replacing ordinary seawater with concentrated brine left over from desalination.

Sandra Kentish, a chemical engineer at the University of Melbourne, explained that pairing this brine with treated wastewater widens the salinity gap and makes more energy available than a standard seawater arrangement. Dr Ali Altaee, a specialist in alternative water sources at the University of Technology Sydney, noted that the two streams were already being mixed before discharge. The membrane and turbine capture energy that would otherwise dissipate into the sea.

Why Fukuoka already had the ingredients

The greater Fukuoka metropolitan area serves roughly 2.6 million residents without major nearby rivers. That geography makes rainfall an unreliable sole source of drinking water and has pushed the region to find additional supplies.

Since 2005, the Uminonakamichi Nata Seawater Desalination Center has helped stabilize those supplies by producing about 50,000 cubic meters of fresh water a day. Desalination removes salt from seawater, leaving a more concentrated salty stream behind.

The resulting brine can have salt levels near 8 percent. Disposing of it requires care because concentrated salt water can damage local marine ecosystems. The center also has access to treated wastewater piped from a nearby sewage treatment facility, providing the less salty side of the osmotic process.

Operators now pass those streams through membrane modules before discharge. Water crossing into the brine increases pressure, drives the turbine and dilutes the concentrated salt water. The diluted output can then be released with less ecological stress.

The electricity returns to the desalination center, reducing its draw from the conventional grid and trimming operating costs. The facility therefore links water production, wastewater management and energy recovery within an existing operation.

Kenji Hirokawa, who heads the Seawater Desalination Center, has described the installation as a modest first step rather than a finished solution. Its broader appeal lies in a problem shared by coastal cities: growing energy demand alongside the need to manage brine. Fukuoka brings those concerns together in one working system.

A long route from an idea to operation

The idea of generating power from differences in salt concentration dates back decades. An American researcher proposed the concept in the Journal of Membrane Science in 1976. More than three decades passed before Norwegian utility Statkraft began testing serious hardware.

In November 2009, Statkraft opened the first PRO prototype at Tofte on the Oslo Fjord. The project aimed to show that osmotic electricity could compete in energy markets. It targeted a design capacity of 10 kilowatts but generated only 2 to 4 kilowatts in practice.

Statkraft shut the project down in January 2014, saying membrane efficiency was still too low for commercial viability. Over the following decade, osmotic power remained largely in pilot projects and academic research while solar and wind costs fell sharply.

The milestones show how slowly the technology moved from a published idea to continuous commercial operation:

• 1976: An American researcher proposed salinity gradient power in the Journal of Membrane Science.
• 2005: Fukuoka's desalination center began providing an additional source of drinking water.
• November 2009: Statkraft opened its PRO prototype at Tofte, Norway.
• January 2014: Statkraft closed the project because membrane efficiency was too low.
• 2023: Danish company SaltPower opened a continuous commercial plant in Mariager.
• 2024: Research in Chemical Engineering Science described membrane modifications for generating sustainable power from salinity gradients.
• August 5, 2025: Fukuoka officially began operating its osmotic power facility.

What the Norwegian experiment taught engineers

Membrane power density became a central measure of whether osmotic generation could pay its way. It describes how much electricity a system can produce for each square meter of membrane, linking the size and expense of the equipment to its useful output.

Research published in academic journals has put the level needed to approach financial viability at roughly 5 watts per square meter. The Tofte installation managed only 1 to 3 watts per square meter. Its membranes cost too much, became fouled too quickly and passed too little water to justify the energy needed for pumping.

Fouling occurs when material accumulates on or within a membrane and interferes with its work. Organic matter and minerals are among the substances that researchers want future membranes to resist. A barrier that performs well initially must also keep working during prolonged operation.

Fukuoka's response is to make better use of its location and water streams rather than wait for a breakthrough in materials. Its concentrated brine produces a steeper salinity gradient than ordinary seawater paired with fresh water. That stronger difference lets existing membrane technology extract enough net energy to make the installation worthwhile.

The Danish plant provided an earlier example of continuous commercial operation. The larger Japanese installation adds an Asian example and shows how PRO can be fitted into municipal water infrastructure already in use.

Akihiko Tanioka, an expert in osmotic power, welcomed the achievement and expressed hope that it would be adopted elsewhere.

This successful implementation is a major achievement. I hope it will be replicated globally.

Small output, dependable production

Fukuoka's annual generation of about 880,000 kilowatt hours translates into an average continuous output of approximately 100 kilowatts. Kilowatts describe the rate of electricity production; kilowatt hours measure the amount produced over time. The distinction explains how a small generator can accumulate a useful annual total through steady operation.

The installation is tiny beside offshore wind farms or gas power stations producing hundreds of megawatts. Nearly all of its electricity goes back into the desalination center. Its main role is an energy recovery loop within a water facility, rather than a standalone station supplying a city.

The project cost roughly $4.38 million, underscoring that osmotic electricity is not yet cheap power. Its value comes from recovering energy from existing waste streams and producing it predictably.

A utilization rate near 90 percent means the system can keep generating through calm nights, cloudy winters and typhoons. Solar panels stop producing after sunset, wind turbines depend on moving air, and hydroelectric dams can lose output during prolonged dry weather. Osmotic power draws on a different resource: the continuing flow of liquids with different salt concentrations.

About two football fields of solar panels might match Fukuoka's annual output on paper. Their production would still vary with clouds, daylight and seasonal sun angles. Grid operators balance such changes using reserves kept ready to generate or battery storage. The osmotic plant instead offers a relatively flat generation pattern that can be scheduled with confidence.

The contrast helps explain the interest in baseload clean power, meaning electricity available steadily rather than only when the weather permits. Solar and wind installations need costly battery backup to approach the same consistency.

Why pairing power with desalination matters

A techno economic analysis of PRO published in Frontiers in Energy Research identified integration with desalination plants as one of the technology's more commercially viable routes. Such analysis examines engineering performance alongside costs to assess whether a system can make economic sense.

Fukuoka illustrates the reason. The concentrated brine is already produced by the desalination process, and the treated wastewater is already flowing. The power installation does not need to buy fuel or build pipelines across wilderness to assemble its two main inputs.

The two liquids were already destined to meet before discharge. Adding membranes and a turbine makes that mixing process useful for electricity generation. Energy recovery can then help offset some of the power consumed by the desalination center itself.

The 2024 Chemical Engineering Science study described novel membrane modifications that could advance sustainable generation from salinity gradients. That research addresses a different part of the same economic challenge: improving how much useful energy the equipment can extract.

Fukuoka combines favorable water chemistry with an existing industrial setting. Better membranes could improve performance further, while the current plant tests how existing equipment handles continuous use. Its practical importance rests on both the electricity it recovers and the operating experience it can provide.

The membrane challenge and the global opportunity

The International Renewable Energy Agency has estimated the global technical potential of salinity gradient power at 647 gigawatts and 5,177 terawatt hours a year. Geography and engineering limits will keep the practical amount far below that estimate.

IRENA has identified membrane costs as a major barrier. Researchers are seeking materials that are cheaper, more durable and able to pass water faster while resisting the buildup of organic matter and minerals.

Japan has an industrial reason to pursue those improvements. According to the Japanese government, domestic companies hold roughly 60 percent of the global desalination membrane market. Progress in membranes for osmotic power could strengthen that position and create additional export markets.

Kyowakiden Industry, the engineering company that supplied Fukuoka's membranes and turbine package, has stated an ambition to build plants five to ten times larger if the installation performs well during testing over several years. Expansion is conditional on that performance; no date has been given for those larger facilities.

Regions that already depend heavily on desalination are likely customers. In the Middle East, individual desalination plants can produce more than 900,000 cubic meters of fresh water a day. Compared with Fukuoka's roughly 50,000 cubic meters, that is more than 18 times as much daily production, showing the larger scale of water infrastructure where osmotic energy recovery might be considered.

What continuous operation needs to show

Prototypes and demonstrations have appeared in Norway, South Korea, Australia, Spain and Qatar. The interest extends beyond coastal desalination: Dr Altaee noted that the University of Technology Sydney maintains a prototype that could restart with government funding, using salt lakes in New South Wales as a resource base.

Fukuoka now provides a working test of membranes, pumps and economics over sustained operation. The central task is to establish whether those elements can perform long enough to attract investment in larger facilities.

The installation's importance therefore reaches beyond its roughly 100 kilowatts of average output. It brings together concentrated desalination brine, treated sewage and electricity generation at a municipal facility, using streams that were already being handled there.

Coastal cities with desalination infrastructure could gain another source of steady clean electricity if that model proves durable and affordable at greater scale. For now, Fukuoka's achievement is concrete: a small operating plant is recovering energy from waste water streams and returning that electricity to the production of drinking water.

What to Know

• Fukuoka's osmotic power facility officially began operating on August 5, 2025, becoming Asia's first operational plant of its kind and the world's second continuous commercial installation after Denmark's 2023 plant.
• Pressure retarded osmosis uses concentrated desalination brine and treated wastewater to produce roughly 880,000 kilowatt hours annually, equivalent to the use of about 220 to 300 average Japanese households.
• Operators report utilization approaching 90 percent, with electricity returned to the desalination center.
• The project cost roughly $4.38 million and has an average continuous output of approximately 100 kilowatts.
• Membrane costs, performance and fouling remain central challenges, as the abandoned Norwegian prototype demonstrated.
• Japanese companies hold roughly 60 percent of the global desalination membrane market, according to the Japanese government.
• Kyowakiden Industry aims to build facilities five to ten times larger if testing over several years succeeds.