Feb 28, 2013
Microbes could harvest power from wastewater
New "outside the box" ideas typically go through three stages of comment by sceptics, according to Bruce Logan of Pennsylvania State University, US. The first stage is "it can't be done", followed, when it has been shown to work, by "it's possible, but too expensive" and, once it has been shown to work at reasonable cost, by "I said it was a good idea all along". Logan, who specializes in water-resource issues, said his research has weathered this scenario repeatedly.
"One billion people lack adequate drinking water," he told participants at the New Horizons in Science meeting in Raleigh, North Carolina, US. "Two billion people lack adequate sanitation. By 2025, due to population growth and climate change, about a quarter of the world is going to live in severe water-shortage areas."
Logan went on to note that in the US, the water infrastructure uses at least 3–5% of all electricity generated, some 20 to 30 gigawatts (GW), or the output of 20 nuclear power plants. The energy used both to produce drinking water and to treat wastewater is increasing because modern technologies consume more energy.
"And yet there's energy in that wastewater," he said. US wastewater from domestic use, food processing and animal waste alone contains about 17 GW. Wastewater treatment uses 10 to 20 GW, roughly 1.5–5% of all electricity production, said Logan, acknowledging that both the data and estimates vary widely. The point is, he said, "there is almost 10 times more energy in the wastewater than the energy it takes to treat it." Logan reckoned "this is sort of backwards; we're throwing this energy away."
Tapping that resource is the goal of Logan's research. Put simply, he asked, are there energy production methods found in nature that could be mimicked in an engineered process to recover energy from wastewater? Promising solutions involve two categories of microbes: exoelectrogens, which make electric current, and electrotrophs, which consume electrons and produce hydrogen and methane.
Logan has put exoelectrogens to work in microbial fuel cells (MFCs). The test device he has created has no moving parts, just the bacteria already present in wastewater. They congregate on an anode, oxidizing biomass in the wastewater, and create an electric current to a cathode. In a small-scale demonstration viewable on the internet, the bacteria create enough energy to power a fan.
MFCs have weathered the three-stage scepticism process, Logan said, as have other approaches developed in the course of his research. These include reverse electrodialysis (RED), which produces an electric current in a saline solution such as seawater. Logan has coupled RED with MFC technology to make a microbial fuel reverse electrodialysis cell (MRFC), producing both higher voltages and higher power.
The bottom line for Logan and his colleagues is that microbes, unaided by chemical additives, can help solve an impending crisis, once these test projects can be scaled up to real-world capacities. Instead of draining electrical resources, wastewater treatment plants could be self-sufficient and may even contribute energy to the grid.
About the author
Harvey Leifert is a contributing editor to environmentalresearchweb.