Current wastewater treatment technology involves a number of steps designed to separate the solid and liquid components of sewage and clean the wastewater before it is released into a waterway. This often involves settling tanks, macerators that break down larger objects, membranes to filter particles, biological digestion steps and chemicals that kill harmful microbes. One estimate puts their energy use at 2 percent of overall consumption in the U.S.
A team at the J. Craig Venter Institute is developing a version of a so-called microbial fuel cell (MFC), which uses organic matter, such as the material in sewage, as fuel, and microbes break down the organic matter. In the process of doing so, the bacteria produce electrons, which have a negative charge and are the basic units of electricity. Electricity consists of a flow of electrons or other charges through a circuit.
An MFC consists of a sealed chamber in which the microbes grow in a film on an electrode, which receives their electrons. Meanwhile, positively-charged units termed protons pass through a membrane to a second, unsealed container. In that container, microbes growing on another electrode combine oxygen with those protons and the electrons flowing as electricity from the electrode in the sealed chamber, producing water or other products like hydrogen peroxide.
Their new MFC uses ordinary sewage obtained from a conventional sewage treatment plant. Microbes that exist naturally in the sewage produce electrons as they metabolize, or digest, organic material in the sludge. Bretschger found that microbes exist in the MFC community that might even break down potentially harmful pollutants like benzene and toluene that may be in the sludge.
"Our prototype incorporates innovations so that it can process five times more sewage six times more efficiently at half the cost of its predecessors," said Orianna Bretschger, Ph.D., who presented a report on the improved technology at the latest ACS meeting in San Diego. "We've improved its energy recovery capacity from about 2 percent to as much as 13 percent, which is a great step in the right direction. That actually puts us in a realm where we could produce a meaningful amount of electricity if this technology is implemented commercially. Eventually, we could have wastewater treatment for free. That could mean availability for cleaner water in the developing world, or in southern California and other water-short areas of the United States through the use of more wastewater recycling technologies."
Bretschger said the MFC also is quite effective in treating sewage to remove organic material, and data suggest a decrease in disease-causing microbes. "We remove about 97 percent of the organic matter. That sounds clean, but it is not quite clean enough to drink. In order to get to potable, you need 99.99 percent removal and more complete disinfection of the water." Still, she suggested their MFC might one day replace some of the existing steps in municipal wastewater treatment.
In the last year, they increased the amount of waste their device could handle each week from 20 gallons to 100 gallons, trucked in from a local treatment plant near San Diego. They also replaced the titanium components with a polyvinyl chloride (PVC) frame and graphite electrodes. Because of that, the new fuel cell costs about $150 per gallon, half as expensive as their previous prototype. The group hopes eventually to bring the cost under $20 per gallon or less to be cost competitive with existing water treatment technologies.
Bretschger reported that the new device is also more than six times as efficient as its predecessor, turning 13 percent of the usable energy in the sludge into electricity. While this only generates a small current, Bretschger explained that a large device running at 20-25 percent efficiency could produce enough power to operate a conventional wastewater treatment plant. A typical sewage treatment plant may consume enough electricity to power 10,000 or more homes, according to some estimates.