April 9, 08
Rice Paddies Used as Microbial Fuel Cells to Produce Electricity

Rice paddies cover up to 400,000 square miles of land and are generating about 15% of the world's methane emissions. Scientists think they have discovered a way to reduce the world's methane production and produce electricity at the same time. In a recent Environmental Science & Technology article, a team of scientists describes a method that takes advantage of a process called rhizodeposition to capture some of the energy produced in rice paddies before it is transfered to methane.

Rhizodeposition is a natural process where plants transfer organic material through their roots to the soil. Rhizodeposition releases exudates and root residues, which decompose to produce methane. It is possible to reduce methane emissions by using rhizodeposits, the organic material produced by rhizodeposition, to generate electricity via sediment microbial fuel cells (SMFC). In the author's SMFC design, the anode is submerged in the soil where the plant is rooted and a cathode is placed in the overlying water. Microbially catalyzed oxidation of rhizodeposits delivers electrons to the anode. The electrons then travel through an electrical circuit that contains a power user and then back to the cathode. Upon reaching the cathode, the electrons react with the available oxygen. The scientists believe that this process can produce power from living rice plants.

Over the span of two years, the scientists tested the performance of SMFCs in rice paddies. They discovered that, by using SMFCs, the oxidation of rhizodeposits does produce electrical power in a sustainable way. The highest sustained electrical output was 330 W per hectare of growing area. The SMFCs have a coulometric efficiency of 31 percent and an energetic efficiency of 9 percent.

The scientists did not provide data regarding the level of methane emissions from the plants that were coupled with SMFCs. Theoretically, once the rhizodeposits have been oxidized at the anode, their chemical makeup has changed enough to reduce methane formation.

Regardless of methane reduction, the ability to generate energy from the process of rhizodeposition has many advantages. The method is renewable, nondestructive, and economical. The living plants will continuously deliver organic materials to the SMFC for oxidation at the anode. The construction of SMFCs is also cheap compared to other devices such as photovoltaic cells, which require costly materials. The efficiency of SMFCs needs improvement, but further engineering should be able to upgrade the system. Overall, the researchers think they have effectively demonstrate the potential for sustainable energy production by coupling SMFCs with plants.