Beyond bio-methane

For thousands of years, livestock manure has been used as fuel for fires to provide warmth and a means to cook. But only in the last fifty years has manure's potential as a source of electricity been realised through the use of bio-methane digester systems in Asia, Europe and, increasingly, North America. However, the generator systems used to collect the methane from manure - which is used to produce electricity - are relatively costly and energy inefficient. As an alternative, scientists have been investigating the natural ability of microbes - found in manure and rumen fluid - to produce electricity. If successful, microbial fuel cells have the potential to provide farmers worldwide with a reliable, steady and economical method of powering applications.

Since the early 1960s, scientists have known that electricity can be produced directly from microbes. But a major breakthrough was achieved in 2002, when electrical current was drawn from microbes in a fuel cell without the need for a 'reduction-oxidation mediator' - an added substance which transfers electrons across the microbe's cell wall. Now American scientists at the Ohio State University in Columbus are taking microbial fuel cell research one step further towards practical application by measuring the electricity-producing abilities of microbes found in manure and in the rumen of cattle. As the bacteria found in these environments digest cellulose - the primary component of roughage in the diet of ruminants - they naturally release electrons, creating a small but steady electrical current.

Laboratory microbial fuel cells

credit: Ann Christy

The small laboratory fuel cells created by the team of Ann Christy, Associate Professor of food, agricultural and biological engineering at Ohio State, contain two chambers. The negative (anode) chamber is filled with manure or rumen fluid, which contains microbes and cellulose "fuel". The other chamber, the positive cathode, is filled with potassium ferricyanide, a chemical that acts as an oxidising agent by losing electrons. Each chamber holds a small graphite electrode, one drawing electrical charge and the other emitting it, connected above by a wire that "closes the loop" of the circuit and allows the electrical current to flow and be measured.

As the microbes in the negative chamber digest the cellulose, electrons are released and protons are created, which move into the positive (cathode) chamber from the negative (anode) chamber through a porous micro-filter membrane. The electrical current is created by this movement of protons, along with the movement of electrons across the resistor and wire. The team's rumen fuel cells consistently produced about 600 millivolts, whilst the manure fuel cells produced between 300 and 400 millivolts in trials that lasted over thirty days.

Real world applications

With the developing world in mind, Ohio State University has applied for funding to research larger fuel cells - about the size of small table - which will have the capacity to charge 12 volt batteries. On a farm or village in a developing country, this could provide power for a multitude of applications such as water pumps, lights and motors.

Using basic and readily available components, the scientists plan to create fuel cells which are as simple and cost effective as possible. The membrane between the two chambers may not be needed, and aerated water is a viable alternative potassium ferricyanide. In terms of the electrodes Christy says, "Graphite gasket material is the cheapest material that does the job, and it is certainly plentiful." Extra cellulose fuel is always available in the form of crop residue left behind after harvest, and manure is plentiful. Rumen contents, which generally are discarded, are available each time ruminants (sheep, goats, llamas, camels and cattle) are slaughtered. Furthermore, a renewal of rumen fluid or manure will often not be required, as "the microbes in the fuel cell will multiply," says Christy.

According to Hamid Rismani-Yazdi, a PhD student working on the project, the research has proved that "Rumen fluid and manure can be added to the range of materials that can be used as an accessible fuel in microbial fuel cells." This technology is also appropriate for developing countries as the warm climatic temperatures promote high levels of microbial activity resulting in a quick charging time of the battery. The optimum temperature for the microbes is 39 degrees Celsius, the same as that found in an animal's rumen.

Written by Treena Hein