Introduction
Two of the most pressing environmental and economical issues in today’s society are disposal of waste and using creating energy in a way that is sustainable for our future. Of the forms of waste in which we must dispose, one of the most precarious forms comes from wastewater treatment. Wastewater sludge contains varied amounts of organic chemicals, toxic metals, chemical irritants, and pathogens which, if disposed of improperly, can become a major threat to the environment.
Wastewater sludge is commonly used as fertilizer for agriculture and silviculture, but especially with a growing population, this fertilizer use is not a sustainable way to dispose of the necessary amount of sludge. Additionally, fossil fuels are our planet’s main source of energy, and are nonrenewable resources that are especially poor forms of energy for the environment. With environmental impacts such as climate change resulting from these fossil fuels as well as a depletion in available fossil fuels, there is currently a major societal push towards finding a source of energy which is both environmentally friendly and economically sound. Electricity generation using microbial fuel cells (MFCs) may be the solution to these conflicting issues.
MFCs work by using bonds in organic compounds to create electrical energy through the catalytic reactions created by microorganisms under anaerobic reactions. Along with creating energy, these MFCs can be used in wastewater treatment systems to break down organic matter. However the effectiveness of these cells is limited due to low density in power potential as well as a high maintenance cost (Du et. al, 2007). This report will break down factors affecting energy production from MFCs, analyze their environmental and economic costs and benefits, and ultimately predict the feasibility of using MFCs both today, and in the future as technology progresses.
Background
Electrical generation in a MFC begins with introducing substrate into an anaerobic chamber that will act as the anode of the fuel cell. Microbes present in this anaerobic chamber will oxidize the substrate, thus releasing proton, electrons, and carbon dioxide (Du et. al, 2007). The cathode of the fuel cell is separated from the anode by a Proton Exchange Membrane, allowing the protons to freely cross into the cathode region (Du et. al, 2007). This separation of the excess protons and electrons creates a charge imbalance in the fuel cell. The excess electrons in the cathode region give it a negative charge, and the extra protons charge the anode positively. An external circuit connects the anode and the cathode, giving the electrons a path to balance the charge. When the electrons pass through the resistance of the circuit a current is generated, creating power that can then be stored or used (Du et. al, 2007).
This process is only possible in the absence of oxygen. Requiring that the anode be strictly anaerobic (Du et. al, 2007). It can be difficult to remove the oxygen from the influent, but the return on this work far exceeds the cost. Aerobically converting the biomass yields carbon dioxide and water, which are difficult to extract energy from (Pham et. al, 2006). When this process is carried out anaerobically, energy can be generated through either combustion or fuel cell conversion, which capture 35% and 90% of potential energy respectively.
When calculating greenhouse gas emissions, carbon dioxide emitted during the substrate oxidation that occurs in the anaerobic chamber of a MFC is ignored (Du et. al, 2007). The reason that this carbon dioxide is not considered is because that it is part of the natural carbon cycle. Meaning that this carbon dioxide was absorbed from the atmosphere by plants, which were either directly consumed by humans, or by animals the were eventually consumed by humans. Through the digestion process it was converted into waste, and then entered the wastewater system. The carbon dioxide will then be oxidized by the plant, and enter into the atmosphere. Ready to begin the cycle again. The net carbon dioxide lost or gained throughout this cycle is considered a relatively negligible amount compared to the amount of carbon dioxide that is being produced by fossil fuels (Graven, 2016). That carbon dioxide have been stored in solid form for millions of years beneath the Earth’s surface, making its introduction into atmosphere a destabilizing process.