The term "renewable energy" may only seem like a catchphrase of the 21st Century, yet it has been a part of human history for millennia. Prior to the discovery of fossil fuels, humans only had a few options to generate power, namely sun, water and wind. From the Ancient Greeks to the Romans to pre-industrial Europe, the use of these three sources led to improved heat production, food production, manufacturing, and transportation. Even today, these three resources are used to help keep us living comfortably.
With an increase in population and the continuing threat of the end of the fossil fuel era, researchers have looked in other directions to help keep the lights on. In particular, one incredibly abundant resource on the planet, dead organic material collectively known as biomass has been identified as the future or renewable energy. However, while biomass may be considered to be a resource, converting it into energy has been an entirely different problem. For that, there was no other option than to turn to germs, more specifically, bacteria.
In the last few decades, a large number of bacteria have been found to not only utilize biomass, but also produce energy, including electricity. The result has been the creation of a new kind of energy technology, the microbial fuel cell, or MFC. The concept is similar to a battery. The positive end, the cathode, is made of a metal, such as copper or silver. The negative end, the anode, is comprised of what are known as exoelectrogenic bacteria, which produce the electrons needed to make electricity. In between the two is a solution capable of transferring the electrons from negative to positive. That solution can be anything from liquefied dead plants to industrial waste. The sheer diversity of potential solutions continues to grow allowing researchers to develop and commercialize an entire line of MFCs.
In the last few years, a number of researchers have explored another highly abundant resource chock full of bacteria and solutions that can transfer electrons. It is well known to each and every one of us: wastewater. The combination of urine, fecal matter, dead skin and sweat may be the perfect matrix for MFCs. They have high levels of organic content, they are rich in bacteria, and there are billions of litres made each and every day.
In 2008, a group at Penn State University were one of the first to find electricity in wastewater. The team chose not human but pig wastewater and the outcome was more than promising. The team found that not only was energy produced, but the smell associated with the waste was reduced by almost 85 percent. There was no reason not to suspect that the same would be found in the human counterpart. This provided more than enough reason to look into human wastewater and find out whether the same potential existed.
Last year, scientists from the J. Craig Venter Institute examined the potential of MFCs for wastewater remediation. In their paper, the team found a number of microbes that could both treat waste and produce electricity. However, there was a significant biodiversity that could lead to fluctuations in the performance of the batteries. A more stable bacterial anode was needed to ensure that MFCs would be functional year round.
This week, a group from Stanford University generated just that stable anode. The team published an article describing how they tested an MFC battery using a defined bacterial population made up of exoelectrogenic bacteria. Using both salt solutions and also municipal wastewater as the matrix, the team found that electricity was not only produced but could also be sustained with minimal maintenance. What was even more interesting was the electricity output. Considering the size of the battery (a few centimetres) and the amount of wastewater used, only two millilitres, the amount of voltage produced was comparable to a regular battery.
The findings, while small, reveal an incredible potential. By increasing the size of the battery from centimetres to metres, the voltage output would no doubt increase as well. But the true value comes in the realization that each and every day, billions of litres of wastewater are produced. The resultant energy production would not only be a boon for the electricity industry, it would also be inexpensive. Moreover, based on the studies with the pig wastewater, there is even the potential for odor remediation while increasing energy stores.
Large scale wastewater MFCs are still a few years away as researchers continue to optimize their technologies and find ways to acquire funding and partners. Yet there is little doubt that soon, the discharge of sewers will be yet another resource in our continual hunger for energy. When that time comes, we will be able to view every visit to the toilet with pride as not an elimination of waste, but as a contribution to sustainable energy production.