Schematic of the MEB and the energy recovery cycle. Source: M. Ye et al.

Coastal wastewater treatment plants can attain some energy independence by tapping into blue energy, or the salinity gradient formed when treated wastewater discharges mix with seawater. Stanford University researchers demonstrated the potential of this energy resource with a mixing entropy battery (MEB) system, which uses battery electrodes to convert salinity gradient energy into electric power.

Each cubic meter of freshwater that mixes with seawater yields about .65 kWh of energy, sufficient to power an average house in the U.S. for about 30 minutes. Globally, the theoretically recoverable energy from coastal wastewater treatment plants is about 18 GW, which could power more than 15 million homes continuously.

The electrodes are made with Prussian blue, a material widely used as a pigment and medicine, and polypyrrole, an experimental battery material. A prototype MEB was tested by flushing the system with alternating hourly exchanges of wastewater effluent from the Palo Alto Regional Water Quality Control Plant and seawater collected nearby from Half Moon Bay, California. The average power output was 16 mW/m², and battery materials maintained 97% effectiveness in capturing the salinity gradient energy during 180 cycles.

A research paper on the experimental MEB is published on ACS Omega.