Microbial Fuel Cells (MFC): a Game-Changer in Renewable Energy Development

Problem identification

World population and welfare are increasingly rapidly, leading to a high demand for energy, specifically low-cost electricity. However, currently about 1.3 billion of global populations living in remote areas of developing world still suffer from chronic energy poverty, while over 2 billion lack of clean water, of which 400 million are children and elderly group. Proper water treatment is required to prevent them from illness/ death. Nevertheless, they do not have the economic means to afford it. Practical and novel solutions are required to address energy scarcity and water pollution problems.

Idea/Concept

One man’s trash is another man’s treasure. Landfill leachate that contains organic pollutants with less economic value is a trove if we know how to harvest it effectively for generating electricity on site using bacteria, the planet’s tiniest inhabitants.

MFC that simultaneously addresses the two issues — wastewater treatment and sustainable energy production — fits the United Nations (UN)’s mission in developing renewable, sustainable, green, and clean energy. Producing electricity using MFC would add economic value to unused wastewater and represent an attempt to fight against global water pollution. Our interests in MFC stem from its ability to operate at all weather conditions and pressure using inexpensive Geobacter sulphurreducens that could form biofilms onto electrode’s surface.

If not generating electricity on site, MFC may partly reverse the process to produce hydrogen gas. When applying a certain voltage to the bacteria to complement the voltage supplied by the biodegradation of organic pollutants, this will lead to the electrolysis of water, thus generating hydrogen (H₂) gas, a renewable fuel that can be commercially sold in the market. This energy production in the recovered hydrogen gas may help industries offset their treatment costs of wastewater. This energy is preferable to landfill gas (LFG) that not only poses odorous nuisance and safety hazard, but also contains CH₄, one of the emission sources that contribute to climate change. Using H₂ as a fuel would benefit society in the long-term, as it not only potentially replaces fossils fuel-derived energy, but also reduce environmental impacts, protecting the environment. Unlike fossil fuels, utilizing H₂ gas as a fuel does not emit CO₂ into the atmosphere.

Unlike activated sludge, MFC is practical for leachate treatment. As the leachate has high concentration of organic pollutants, MFC generates high energy potentials. Organic matter in the leachate has chemical energy that can fuel MFC when its bioreactor releases electricity in electron flows from anode to cathode in an external circuit, generating power as a by-product.

Anode: C₆H₁₂O₆ + 6 H₂O --> 6 CO₂ + 24 H⁺ + 24 e^-

Cathode: 6 O₂ + 24 H⁺ + 24 ---> 12 H₂ O +

Total: C₆H₁₂O₆ + 6 O₂ ---> 6 CO₂ + 6 H₂O

The process is time-efficient, cost-effective, and eco-friendly. The only emissions from the fuel cell itself are water and waste heat, which can be captured and put to use.

The electron exchange takes place between the anode and the cathode chambers respectively. In the anode chamber, oxygen-starved organic material of the leachate is oxidized by the bacteria. This oxidation process releases protons and electrons, from which the MFC extracts electrical power.

During the reactions, the bacteria get food from waste for survival, while we get electricity for energy, representing a mutual symbiosis between human and micro-organisms in the nature for environmental protection. Furthermore, harnessing bacterial metabolism for producing energy that can be further commercialized will generate revenues for wastewater operators. The revenue not only defrays the cost of MFC’s operations, but also provides an incentive and means to keep on improving its prototype design and operations. The electricity generated in the MFC’s operations may supply the needs of power, while the value of the energy produced by the product may create job opportunities for local community.

As wastewater needs to be treated, more wastewater operators may consider the MFC to generate treated effluents that can meet the limit requirements set by local environmental legislation. The applications of MFC for building self-powered municipal water treatment system may represent a breakthrough, as it opens the door to future, where a WTP not only power itself in its operation, but also sells electricity it generates. This not only revolutionizes the existing water technologies, the way that wastewater has been treated universally, thus substituting the most widely used activated sludge treatment for water purification. Eventually, MFC provides a practical solution that directly addresses the need of our global society for clean water and low cost energy.

Unlike natural gas that takes millions of years to develop, electricity can be generated on site by MFC for a shorter period of time with less or zero emissions. This provides not only net GHG (greenhouse gases) savings, but also an improved productivity with respect to cost, time, and manpower. So far there is none of energy-producing technology that offers benefits as does MFC. The clean energy the MFC generated represents an opportunity for us to live in sustainable ways.

The biofilms not only facilitate electron flow to electrodes, but also release c-type cytochrome (OmcZ) that accumulates at biofilm-electrode interface to promote rapid electron transfer to electrodes. The presence of the bacteria in MFC eliminates the need to isolate costly individual enzymes, as bacteria provide inexpensive substrates for facilitating MFC operations. The process may occur in small bioreactors, where the microbes may be retained in stable conditions for a period of time as biofilms, thus saving their operational and maintenance (O&M) costs.

Landfill leachate richly contains a variety of organic matters. Such organic compounds have tremendous chemical energy, which bacteria can release in the form of electron flows. By exploiting the electron-transfer ability of the G. sulphurreducens, MFC could produce electricity directly without combustion of the organic contaminants within the wastewater, while allowing industrial users to treat it without consuming conventional energy resources. This enables users to meet regulatory obligations at lower costs — as the amount of power generated during MFC operations halve the electricity required for aerating activated sludge— but with the same capability of removing organic matter as efficiently as do conventional water treatment plants.