Microorganisms are multitalented. The same kinds of bacteria used in wastewater treatment plants can also generate small amounts of electricity. But these two abilities have rarely been combined so cleverly.
In a study published last week, Penn State University researchers outlined a system called the microbial reverse electrolysis cell (MRC) that both cleans water and creates electricity. Today it's too expensive for practical applications in the field, but it opens up the possibility not only for more efficient wastewater cleanup, but also for hybrid water-treatment and power plants that can do both.
The MRC is a combination of two generating systems: a microbial fuel cell and a reverse-electrodialysis system (RED). A microbial fuel cell generates an electrical current by gathering electrons that are given off as bacteria to oxidize plant and human waste. A RED stack pumps a combination of fresh and salt water through a series of membranes that separate positively and negatively charged ions, and as the ions migrate toward their respective electrode, a usable electric current is created.
On their own, both systems are inefficient. Microbial fuel cells generate too little energy to be used in wastewater treatment, while RED systems require multiple costly membranes to generate any significant power. However, Penn State researcher Roland Cusick says, when paired together, the two systems can generate surprising levels of electricity.
To combine the two technologies, PSU researchers placed a RED stack into the middle of a microbial fuel cell filled with acetic acid as the wastewater solution. For their tests, they pumped fresh water and ammonium bicarbonate (a baking soda-like solution) into the stack, which generates a current. That current raises the electrical potential inside fuel cell, which excites the bacteria and causes them to up their consumption rate of the acid. And when they treat the wastewater faster, they generate electricity faster. "It's kind of like a turbocharger," Cusick says. "The bacteria are creating the energy, but the stack is creating the turbocharging effect."
The researchers say they generated 0.94 kilowatt-hours per kilogram of waste material. Conventional treatment processes—such as activated sludge—consume 1.2 kilowatt-hours per kilo of waste. To put those figures into perspective: Brooklyn's Newton Creek wastewater treatment plant consumes about 700,000 kilowatt-hours of per day. Based on Cusick's estimates, if fitted with an MRC system, the plant could generate roughly 350,000 kilo-watt hours a day instead. "That's enough energy for about 11,000 people per day," he says.
The reason Cusick's team can't do that now is the cost of the membranes—they run in excess of $100 a piece, and tens of thousands of membranes are required to treat the massive volume of water plants receive. To put it in perspective, it cost researchers over $1100 to build their experimental cell, which only had a capacity of just 16 milliliters. But if the cost came down, an MRC-equipped water-treatment plant would be ideal for developing countries, he says, solving two problems—water sanitation and power generation. "Twenty years from now, it makes economical sense for me to build something like this," he says.