Coastal wastewater treatment plants may be a nasty but necessary way to handle the effluent from our cities, but a new study by Stanford University indicates that they could also double as power plants to make them energy independent and carbon neutral. By mixing freshwater from the plants with seawater, the researchers say they have the potential to recover 18 gigawatts of electricity worldwide.

This is because when freshwater and saltwater mix, it produces what is called a salinity gradient. In the case of wastewater coming out of a treatment plant, the discharge contains 20 times less salt than the seawater it mixes with. Skipping over a lot of chemical details, what this means is that every cubic meter of freshwater can theoretically produce a little bit of energy. Since plants are continually discharging freshwater into the sea after treatment, capturing this energy and using it to power these energy-intensive wastewater treatment plants would be huge for the environment.

After all, running these wastewater plants takes up three percent of the electrical power output in the US alone.