ON SALT AND BATTERIES: Seawater Energizes Wastewater Treatment

We’ve all heard of green energy, but what about blue energy? Blue energy is a term associated with generating “osmotic power”, or energy resulting from the osmosis that occurs between two solutions of water with different concentrations. Traditionally, blue energy is derived from a dissolved substance like salt being separated from water by a thin semi-permeable membrane that lets the water naturally pass through. That flow of water builds up pressure which can be used to drive turbines and generate power. Because of this phenomenon, places where ocean water and freshwater mingle — such as coastal locations that include wastewater treatment plants — could also be locations that self-generate sustainable power. Even more exciting is new research out of Stanford University that could lead to the manufacture of special batteries that would not only generate power, but recharge themselves at the same time while eliminating the need for membrane technology.

The American Chemical Society recently released a paper in their ACS Omega journal that describes such a battery and suggests that it could be used to make coastal wastewater treatment plants completely energy-independent. Blue energy is considered an “immense and untapped source of renewable energy” and it’s possible that the battery could successfully capture huge amounts of energy for coastal wastewater treatment plants “without membranes, moving parts or energy input”. 

A prototype of the battery has been tested at the palo Alto Regional Water Quality Control Plant umixing the site’s wastewater effluent with seawater obtained from nearby Half Moon Bay. Results were enormously promising.  After more than 180 cycles, the battery maintained nearly one-hundred percent effectiveness in capturing the energy created from the differences in salt concentration between the two fluids, also known as salinity gradient energy.

Theoretically, this renewable energy technology could be installed at any location where fresh and saltwater mix, but wastewater treatment plants are especially interesting for one major reason: they use incredible amounts of energy. Wastewater treatment plants account for about three percent of total U.S. electrical usage. Wastewater treatment is critical for the well-being of any community, but it’s also vulnerable to power grid emergencies. If wastewater treatment plants were entirely energy independent, they would not only experience huge decreases in emissions, they would also be virtually invulnerable to power outages.

Even more exiciting is the global implications of blue energy techonology. For every cubic meter of freshwater that mixes with seawater, close to .65 kilowatt-hours of energy is produced. That amount could power the average American household for nearly 30 minutes. If one then considers the approximately 18 gigawatts theoretically generated and recovered from coastal wastewater treatment plants, that number rises to 15 million households – continuously.

This is the first technology to succeed in capturing blue energy electrochemically. The process begins by releasing sodium and chloride ions from the battery electrodes into the solution, which causes the current to flow from one electrode to the other. Then, a rapid exchange of wastewater effluent with seawater causes the electrode to reincorporate sodium and chloride ions awhich reverses the flow. As this happens, energy is recovered without an intial input of energy or charging requirement. The result is a battery that is awlays discharging and recharging in a self-sufficient loop.

Backup batteries are also unnecessary, as the electrodes are coated with durable materials that protect them from corrosion. The technology could also provide enough voltage for both a wastewater treatment plant and other nearby industrial applications. Further testing is required to explore the full potential of blue energy on a global scale, but recent results cseem to suggest a sea change in renewable energy sources. 

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