March 1, 2012 at 3:01 PM ET
Microbes that digest wastewater in a fuel cell to produce electricity are getting a boost from a technology that captures energy from the difference between salt and fresh water, scientists report in a new study.
"It is like putting another battery into a flashlight, you get more voltage and power out," Bruce Logan, an environmental engineer at Pennsylvania State University, told me Thursday.
The technology could lead to wastewater treatment plants that generate electricity, instead of consuming it.
The system produces 0.94 kilowatt hours of electricity per kilogram of wastewater organic matter, Logan and his colleagues reported online Thursday in the journal Science.
That compares to 1.2 kilowatt hours per kilogram of organic matter consumed in the treatment of activated sludge.
The potential energy in wastewater today in the U.S. is between 15 and 20 gigawatts, Logan noted in a Science podcast. One large nuclear power plant can put out 1 gigawatt of energy.
"We are talking about something around 15 to 20 equivalent nuclear power plants just in wastewater," he said.
What's more, "anything that is biodegradable, the bacteria can break down and make a current from," Logan told me. Think agricultural waste, for example.
How it works
The technology is called a microbial reverse electrodialysis cell (MRC).
It starts with a microbial fuel cell, which uses bacteria to turn plant and human waste into electrons that are turned into a electric current.
These types of fuel cells, however, are limited to the voltage bacteria can make, which amounts to about half a volt per cell, Logan explained.
"With a reverse electrodialysis system, you can actually add in more voltage," he said.
These systems capture the energy difference between saltwater and freshwater. Logan explained:
"If you have saltwater and you try and squeeze water out of it, that takes energy and the reverse of that is what gains you back energy. So if I have a saltwater and a freshwater solution, there's a lot of energy there."
The trick is to capture that energy, which reverse electrodialysis (RED) accomplishes by pumping solutions of salt and fresh water across specialized membranes that let only positively and negatively charge ions through.
"All the anions go in one direction and all the cations go in the other direction," Logan said. "And that is like a battery, it is creating electrochemical potential."
The problem with the technology is requires a lot of membranes to get an electric current going and membranes are expensive.
By combining a few membranes with microbial fuel cell, which is already generating an electric current, "the RED stack acts to boost the voltage and also seems to help produce a higher current as well," Logan said.
In previous work, researchers have shown that an MRC can work with natural seawater, but found that organic matter in the water will quickly foul up the membrane without extensive pre-cleaning and treatment.
Use of seawater also restricts the technology to coastal areas.
In the new study, Logan and his colleagues added ammonium bicarbonate, a combination of ammonia which is common in household cleaning products, and carbon dioxide, to the waste water.
The salty solution works similar to seawater, but with the added advantages of not fouling the membranes and it is easily removed from the water with moderate heating.
"At lower temperatures than boiling, the ammonium bicarbonate comes off and you are left with a low salt solution," Logan said.
"Then you capture that ammonium carbonate in a high concentration solution and you've regenerated your salinity gradient. Then you throw that in your RED stack, you capture that energy as electricity."
The waste heat generated by wastewater treatment plants and other industrial processes, he added, is sufficient to recapture the salty solution.
So, if you are treating wastewater, you are getting all the heat you need.
Full scale MRCs at our wastewater treatment plants churning out electricity and drinkable water is feasible within three to five years, Logan said, if all goes well.
Hurdles include preventing crossover of ammonia from the stacks into the wastewater, which is undesirable, and lowering the cost of the membranes.
"Membrane sells at what it sells for," Logan noted. "If we suddenly need a lot more of it, the price could come down. But we think we can also design some cheaper membranes and that could help."