Using a new chemical-bioelectric process known as “microbial reverse electrodialysis,” researchers at Penn State are able to generate electricity from wastewater treatment. Initial tests show the process is capable of producing 0.94 kilowatt-hours of electricity per kilogram of organic waste.
Wastewater treatment accounts for 5 percent of the total electricity generated in the United States. A typical municipal treatment plant serving a large urban center typically consumes millions of kilowatt-hours of electricity every month at a cost of hundreds of millions of dollars. It may soon be possible to turn those numbers on their head, making wastewater treatment an energy producer instead of a voracious energy consumer.
Through a combination and refinement of two proven technologies, researchers at the Hydrogen Energy Center and the Engineer Energy and Environment Institute at Penn State have already proven the concept. Essentially, it’s producing electricity through a process of biochemistry with wastewater as the feedstock.
Reverse Electrodialysis (RED)
RED generates electricity through differences in salt concentrations. Pumping freshwater and saltwater through alternate cells separated by an ion-exchange membrane produces an electrical current, but generating costs are high, as much as $6.70 per kilowatt-hour, due to the need for many cells and the high cost of the ion-exchange membranes.
Microbial fuel cell (MFC)
Like RED, a microbial fuel cell (MFC) uses compartments separated by an ion-exchange membrane. Decomposed organic material in one compartment creates hydrogen gas that passes through the membrane into the other compartment, separating electrons that flow through an electric circuit.
Microbial reverse electrodialysis (MRC)
By combining RED with MFC, researches created a new system known as a microbial reverse electrodialysis cell, or MRC for short. Adding the fuel cell to the RED process is like “putting another battery into a flashlight,” says Penn State environmental engineer Bruce Logan, “you get more voltage and power out of it.”
The requirement for the salt gradient in MRC requires access to areas where both seawater and river water are available, but researchers are working on substituting seawater in the RED stack with ammonium bicarbonate, a salty solution of ammonia and carbon dioxide. Ammonium bicarbonate increases the efficiency of the MRC process, producing 0.94 kilowatt-hours of electricity per kilogram of organic waste.
Not only does MRC generate power, it cleans the wastewater “faster and better,” says Logen.
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