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Clean Water, Cleaner Energy: Wastewater is Renewable
Pennsylvania Ag Connection - 06/10/2019

Penn State researchers are discovering ways to turn wastewater treatment into a renewable source of clean energy.

Among renewable sources of energy, humans have long recognized the power of water. At the places where freshwater and saltwater meet, for example, there are vast amounts of potential energy to be tapped--enough, in theory, to supply 40 percent of global electricity needs.

The process is called salinity gradient energy, and it's a new area of focus for environmental engineer Bruce Logan. But it's also a continuation of the work he's been doing at Penn State for 20 years.

Around the time he arrived at Penn State, Logan was inspired by the words of Nobel laureate Richard Smalley, who famously called the need for energy humanity's greatest challenge.

"We use about five percent of global electricity just to maintain our water infrastructure," he says. "Treatment, distribution, collection, conveyance. That's not sustainable globally when you consider that a billion people lack sanitation, and two billion are really struggling to have access to potable water."

Drawing on his training in environmental biotechnology, Logan set about trying to develop a method of treating wastewater that would not require energy. What he came up with--the microbial fuel cell--is even better: a system that actually generates electricity from wastewater while the water is being treated. Bacteria liberate electrons as they digest the organic matter in the water during the treatment process. When these free electrons are fed into an electrochemical circuit--a fuel cell--they can be used to generate power.

Logan has long since proven that microbial fuel cells can produce electricity from ordinary domestic wastewater, as well as from farm, food processing, and industrial wastewater streams. "Virtually any biodegradable material can be used to produce power," he says. "Today, there's probably not an environmental engineering program in the world that hasn't experimented with this."

But back in 2003, when he set up his first demonstration cell, "people looked at me like I was crazy." He credits the Kappe endowed professorship, and the wealth of facilities Penn State made available, with freeing him to explore these uncharted waters. "It's really made a difference, to me and to the people I've trained," he says. "There are a lot of people doing this now, because we established the field."

From wastewater and fuel cells, Logan progressed to desalination, a logical extension of his expertise. "More and more across the U.S., we've drawn down our aquifers and we're increasingly relying on brackish, or slightly salty, water," he says.

There's a growing need for cheaper, more efficient desalination techniques, both here and around the world. To address this demand, Logan devised a new, low-energy method using electrochemistry. "It's basically pulling salt ions out of solution using sodium-specific electrodes," he says. As it happens, this same technology, run in reverse, can be used to produce salinity gradient energy.

"Instead of using electricity to pull ions out," he explains, "you're moving those ions through a flow cell, creating current.

"It's like water flowing over a dam 270 meters high--that's how much energy you could get from a freshwater-saltwater interface. But, of course, it's more complicated than that." Earlier approaches by other researchers have run into problems including fouling of the fine mesh membranes used to remove salt and failure to withstand the pressures of mixing. None of these approaches have been able to demonstrate the level of efficiency that would make its use worthwhile.

"Realistically, the best place to implement this would be in wastewater treatment plants that discharge into the ocean...Boston, New York, Philadelphia, Los Angeles, San Francisco. These are places where we treat water to very high quality, and then we just throw it away."

Logan's flow cell approach, however, combines the best aspects of these earlier methods in a system that works something like a battery. A prototype constructed with assistant professor Christopher Gorski and postdoctoral scholar Taeyoung Kim in 2016 achieved twice the power density of previous techniques. Gorski recently received an NSF CAREER grant to look for ways to boost that output even further.

The next step would be to scale up the technology in a way that is cost-efficient, no small feat. But even if this can be achieved, Logan says, it's not likely we'll soon see gigantic flow cells deployed on our river-seawater shores.

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