Harnessing the Power of Salt: How Japan’s First Osmotic Power Plant Creates Energy from Seawater

Imagine a world where mixing fresh and saltwater generates electricity. Japan began operating its first osmotic power plant in Fukuoka in August 2025, becoming the second country to deploy the technology at this scale. The plant uses the salinity difference between water streams to generate power.

An osmotic power plant is a renewable-energy facility that captures energy from the difference in salt concentration between two water streams. It does not burn fuel or rely on waves, tides, sunshine or wind. Instead, it uses controlled water flows, membranes and pressure to turn a naturally occurring salinity gradient into electricity.

What is Osmotic Power (Salinity Gradient Power)?

Osmotic power, also known as salinity gradient power, exploits the natural difference in salt concentration between freshwater and saltwater. When these water streams are brought together on either side of a selectively permeable membrane, water molecules move from the less concentrated freshwater side to the more concentrated saltwater side. This phenomenon, called osmosis, drives osmotic power generation.

The process is commonly described as pressure-retarded osmosis (PRO). In a PRO-based plant, freshwater moves through a membrane into a pressurised saline stream. The resulting pressure increase is recovered through a turbine or other energy-recovery equipment. A greater difference in salinity creates greater potential osmotic pressure.

Because its water streams can be supplied independently of weather and daylight, the Fukuoka facility is designed for a high operating rate. The Fukuoka District Waterworks Agency estimates an operating rate of about 90%, including maintenance stoppages, compared with roughly 10–20% for solar installations. This is a planned figure, and the facility is subject to a five-year verification period following its August 2025 start-up.

That operating rate does not remove the energy costs of treating, pumping and circulating water, or the need to maintain membranes. The project’s stated output is therefore a net figure: electricity generated minus the electricity used by its pumps.

The Core Mechanism: How Fukuoka's Plant Generates Electricity

Japan's osmotic power plant in Fukuoka began operating in August 2025. It uses pressure-retarded osmosis to convert a salinity difference into electricity.

The Role of the Semi-Permeable Membrane

At the heart of the plant is a PRO membrane, which allows water to pass while retaining dissolved salts. Publicly available project information does not state the membrane’s water permeability, operating pressure or cleaning regime, so those values cannot be assumed.

Published design figures provide operating context: the plant is planned to use about 10,000 tonnes of concentrated seawater and 9,000 tonnes of treated wastewater each day, and to deliver about 110 kW of net electrical power. Its maximum planned annual generation is about 880,000 kWh. Using the stated annual water volumes, this corresponds to approximately 0.13 kWh of planned net electricity per cubic metre of combined feedwater. Pretreated wastewater is the lower-salinity stream, helping limit the solids and contaminants presented to the membrane.

The Osmotic Process in Action

1. Water Reservoirs: The plant maintains two water streams: treated wastewater from Fukuoka City’s Wajiro Water Treatment Centre and concentrated seawater discharged from the desalination centre.

2. Pressure Build-Up: When the treated wastewater and concentrated seawater are placed on either side of the semi-permeable membrane, water moves into the saline stream. This increases the volume of the pressurised saline stream.

3. Turbine Rotation and Electricity Generation: The increased flow of pressurised saline water drives a water turbine and generator. The reported output is net of the electricity used by the plant’s pumps.

After energy recovery, the streams are diluted and discharged to Hakata Bay, as they were before the osmotic-power installation. Locating the plant within the existing desalination centre uses established water streams and discharge arrangements.

Leveraging Desalination Byproducts

A key feature of the Fukuoka plant is its use of concentrated seawater. Rather than relying on ordinary seawater, the plant uses brine discharged by the nearby desalination facility, which removes freshwater from seawater for Fukuoka and surrounding areas. This concentrated seawater has a higher salt concentration than ordinary seawater, increasing the osmotic pressure difference and the energy available for generation.

Ordinary river water and seawater offer a smaller salinity contrast than treated wastewater and desalination brine. By using a concentrated stream already present at the site, the Fukuoka plant uses a stronger pressure gradient without producing brine solely for electricity generation.

The Significance of Japan's Osmotic Power Breakthrough

The Fukuoka plant is Japan’s first practical osmotic-power facility. It follows SaltPower’s 100 kW installation at Nobian’s Hvornum brine field in Denmark, which began operating in 2023.

A Next-Generation Renewable Energy Source

The Fukuoka District Waterworks Agency describes osmotic power as a next-generation renewable energy source. Its planned output is based on water streams available from adjoining desalination and wastewater systems, rather than on solar irradiance or wind conditions.

Osmotic power is best viewed as a complementary source of firm renewable electricity. Its output can be steady when water supplies and plant operations are steady, making it useful for facilities with continuous electricity demand, including desalination plants, wastewater-treatment works and coastal industrial sites.

Powering Essential Services

The plant is expected to generate up to approximately 880,000 kilowatt-hours of electricity annually, enough to power about 220 Japanese households. The electricity is intended to help power the desalination facility that supplies fresh water to Fukuoka and neighbouring areas.

The installation is located at the Uminonakamichi Nata Seawater Desalination Centre and uses the centre’s concentrated-seawater discharge alongside treated wastewater from the Wajiro Water Treatment Centre. This arrangement integrates the generating equipment with existing water infrastructure.

Global Potential

Operating projects provide clearer evidence of the technology’s current status than broad estimates of future deployment. SaltPower’s plant at Nobian’s Hvornum brine field in Denmark began operating in 2023 with a stated capacity of 100 kW, using freshwater and brine from salt production. Fukuoka began operating in August 2025 with a planned net output of about 110 kW and maximum annual generation of about 880,000 kWh.

Suitable locations need dependable freshwater, wastewater or desalination-brine streams; available land within existing water infrastructure; effective pretreatment; and environmental controls for water discharge. The strongest opportunities may be coastal cities and industrial water hubs where the relevant streams are already collected and treated.

Osmotic power remains an emerging technology operating at a modest scale. Fukuoka will assess the installation over its first five years of operation, including the reliability of its water streams, membranes, pumps and net output.