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First posted June 17, 2011. Revised April 16, 2020. This article concerns microbial fuel cells, and I wrote an explanation of how they work a little later.

American society uses scads of energy. Most of it comes from fossil fuels. So how much energy are we flushing down the toilet?

It may seem like I’m asking how much energy we waste and get no benefit from. That’s a good question, but it turns out there is ample energy in wastewater.

When gas lights lighted street, gas was collected from rotting sewage. As it turns out, at least some of the germs in it produce electricity. A device called a microbial fuel cell puts it to work in a process called bioelectricity.

These cells generate electricity from wastewater. They take advantage of chemical processes driven by bacteria as they feast on the sewage. In other words, these microbes, which purify water anyway, can give us electricity in the process.

Before the development of the microbial fuel cell, it was possible to extract methane in sewage or garbage using an anaerobic digester. That methane could be burned to operate a turbine the same as natural gas or any other fossil fuel.

Microbial fuel cells have at least one important advantage over anaerobic digestion: they make electricity directly. There’s no need to extract or burn methane. Fuel cell wastewater treatment can probably extract more energy. It can certainly avoid the air pollution that burning methane produces.

A brief history of bioelectricity

The electrochemical activity of bacteria was first described in the Proceedings of the Royal Society of London in 1911. It remained a curiosity until the 1960s. The American space program needed ways to generate electricity and treat human wastes in space.

Research on microbial fuel cells began in earnest. A paper published in 1990 described one that had operated continuously for five years. It used municipal wastewater for fuel. Now, thirty years later, the research literature has described more than a thousand of them.

In 2004, scientists tested raw sewage from the Toronto wastewater treatment plant. They calculated that the world’s wastewater could provide a continuous supply of energy roughly equivalent to the annual output of 70 to 140 large nuclear plants.

More recently, an article in Scientific  American concluded that the methods used in the 2004 study seriously underestimated the amount of energy in wastewater. The newer study also investigated industrial wastewater, which has even more energy potential than municipal wastewater.

Sewage as an energy source

Sewage treatment plant. Some rights reserved by eutrophication&hypoxia

Currently, wastewater treatment involves, among other things, allowing sewage to stand in tanks while solids to settle to the bottom. Then the plants must deal with resulting sludge. They can compost it, burn it, or spread it out on large tracts of land.

Whatever they do, it’s expensive and has some kind of environmental impact. Using microbial fuel cells in wastewater treatment would produce energy while consuming matter that would otherwise become sludge.

Sewage is an infinitely renewable energy source. After it passes through the microbial fuel cell, the effluent is cleaner than the input. So far, it doesn’t meet standards for releasing treated sewage into lakes or streams, but it doesn’t need to. It passes from the fuel cell through all the ordinary and proven treatment procedures.

In the meantime, the cells produce electricity. Water treatment and wastewater treatment facilities now use about 5% of the nation’s total energy consumption. Can using fuel cells in wastewater treatment supply that much power?

We’ll never stop flushing sewage down the toilet, but using microbial fuel cell technology for bioelectricity can turn it from a disposal problem to a resource.

Some technical details about microbial fuel cells

Microbial fuel cell diagram. Source unknown

Fuel cells, like batteries, have two electrodes and an electrolyte. They need some kind of fuel. A catalyst causes a chemical action that produces an electric current. The current flows from the negative to the positive electrode through an external circuit. The system operates so long as it has a continual supply of fresh fuel.

Wastewater is one possible fuel for microbial fuel cells. Microbes serve as the catalyst. As they digest the fuel, some bioelectric microbes can “donate” electrons. Others accept them.

Although the earliest discovery of bioelectric bacteria used common E. coli, most microbial fuel cells depend on anaerobic bacteria. They thrive in the absence of air.

The literature reports both single chamber microbial fuel cells and two-chamber cells with some kind of membrane keeping them separate.

Single-chamber microbial fuel cells are the newer technology. The first article to report one appeared in 2003. They have no need for a separate chamber for the positive electrode. It is exposed to the air. Being simpler in design, they are easier to scale up. They also have a higher output than two-chamber cells, but less current efficiency.

Whatever the design, certain factors limit the cells’ ability to reach the theoretical outputs. Some materials commonly used in fuel cells inhibit bacterial growth. On the other hand, bacteria growing too fast can also cause trouble. Scientists are working hard to find cost-effective materials that work well and promote optimum growth of the microbes.

Let’s wait patiently. What may seem to scientists like rapid growth in knowledge remains slow and halting progress toward developing practical technology. Eventually, I’m sure, the microbial fuel cell technology will advance to a point where we can actually start using them.

Sources:

Microbial fuel cells: an overview of current technology / A.J. Slate et al., Renewable and Sustainable Energy Reviews 101 (March 2019): 60-81, via Science Direct
Paying waste: sewage contains more usable energy than scientists thought / Mike Orcutt, Scientific American. January 11, 2011.
Waste not: energy from garbage and sewage / Michael Schirber, Live Science. November 3, 2004