As most landfill operators are probably aware, reducing the nitrogen in leachate can be the most expensive and troublesome component of on-site leachate treatment. The reason for this is that concentrations of ammonium (a form of nitrogen) in leachate are typically high (e.g., 500 to 2,500 parts per million), and the concentrations persist at these levels for many years after a landfill has closed and levels of other pollutants, such as chemical oxygen demand (COD) and biological oxygen demand (BOD), have long since dropped. However, it's possible that in another decade or so, landfill owners across the country will be able to use microbial fuel cells (MFCs) to easily and inexpensively reduce COD/BOD and remove nitrogen from leachate while generating, of all things, electricity.
An MFC harnesses the metabolism of microbes to reduce pollutants. Normally microbes excrete waste materials, much like humans do, but when placed in an MFC, a microbe's metabolism is “short-circuited” to generate electricity as a by-product.
Essentially, a microbial fuel cell can be thought of as a battery that is powered by microorganisms. As microbes “eat” pollutants and other organic materials, they attach themselves to an electrical contact called an anode (which is scientific speak for a positive battery terminal). This creates a flow of electricity through a wire to another electrical contact called a cathode, or the negative battery terminal. Besides electrical power, the by-products of an MFC are carbon dioxide and water.
A significant advantage of the MFC is that it requires no energy input, yet can reduce pollutants such as BOD/COD and nitrogen, which typically are treated in leachate with costly injections of air. The ability of a microbe to generate electricity was first observed in 1911 by M.C. Potter; however, it was not until the 1990s that significant research on MFC technology was done. During the last decade, most MFC research has focused on developing practical applications for MFCs. Currently, MFC technology is still in the development phase. Thus, the batteries are relatively small in size — not much larger than a one- or two-liter soda bottle. However, the technology has advanced such that now substantial efforts are underway to scale up the technology.
Previously, most MFC research was performed using pure substrates, such as acetate or vinegar, but MFC development recently has advanced to accommodate “real world” substrates like domestic wastewater. To better understand how MFCs could be used in the solid waste industry, the Raleigh, N.C.-based Environmental Research and Education Foundation (EREF) provided a grant to Dr. Jenna Jambeck and her team at the University of New Hampshire to conduct research on this emerging technology (Jambeck is now at the University of Georgia). The goals of the research were to:
evaluate how leachate characteristics change when used as a substrate for an MFC.
determine how efficient an MFC is at producing electricity using leachate.
validate that the species of microbes required for MFC operation already are present in leachate.
create a larger-scale MFC to evaluate the applicability of this technology in the field.
A unique component of this study was the use of leachate as a substrate for MFCs. While pure substrates are suitable for laboratory evaluation, the use of leachate represents a more realistic application for an MFC in the real world. Consequently, leachate was found to be well-suited to use in a microbial fuel cell because of its relatively high amount of organics, conductivity and buffering capacity, and the minimal presence of solids.
By contrast, other types of wastewaters and effluents do not share these unique traits, which can limit their utility as substrates for MFCs. It should be noted that a special group of microorganisms, called exoelectrogens, are necessary for MFC operation, and, in many cases, these microbes must be added to a substrate. However, the study found that landfill leachate already has a ready population of exoelectrogens needed for MFC operation. No outside source of inoculation is necessary, which reduces management complexity and cost compared to other substrates.
From a leachate treatment perspective, the removal of major constituents such as BOD and ammonia suggest that this technology could be a viable option for leachate treatment or pre-treatment. The study found that MFCs were able to reduce BOD and ammonium in leachate, on average, by 60 percent each. An MFC system could be utilized as a pre-treatment for recirculation or to reduce energy use from further treatment. It also could be used as a standalone treatment process. For landfills in the post-closure monitoring period of operation, an MFC could be an option for a low operation and maintenance system to treat leachate to levels acceptable for direct discharge into surface water or groundwater.
In this research, a roughly one-liter MFC produced an electricity output of about 500 millivolts. This is enough electricity to operate an LED light, calculator or small electronic toy. With this knowledge, the obvious thought would be to scale up the MFC by simply retrofitting an existing leachate holding tank with an anode, cathode and membrane to make a single, large MFC.
However, research shows that MFCs cannot just be scaled up to increase output linearly due to issues related to internal resistance and material conductivity. The solution to this would be to connect many small cells in a series or design a large MFC to operate in a slow-moving continuous mode (this study's research was in batch mode) to treat the organic matter and capture more electricity. For example, in the case of the landfill that produced 100,000 gallons of leachate daily, if new MFC designs were created that enabled amounts and rates of electricity to be captured that were comparable to those measured in this study, then up to 2,000 kW-hr per month could be produced. This is enough electricity to power about two average American homes or most of the non-industrial electrical needs (e.g. scale house, office) at a landfill. While it remains to be seen if this kind of design and electricity output is truly possible when scaled up, research and design improvements have increased power production in MFCs by six orders of magnitude in only the past 10 years.
The most promising aspect of MFCs is that they hold the potential of treating leachate while reducing energy costs. The EREF-funded University of New Hampshire study is the first to apply MFC technology to leachate in this manner and shows that MFCs may be a viable leachate treatment option. A key finding is that significant reductions in COD/BOD and ammonium can occur.
Although this technology is still in the research and development phase, it has been suggested that more efficient electricity production is possible as design configurations are improved. Part of these improvements will be to develop less expensive cathode materials and operate cells in series. If the technology can be implemented at field scale, which could happen in another five to 10 years, cost savings would be realized on two fronts: (1) via a reduced nitrogen load discharged to publicly owned treatment works and (2) by offsetting energy costs for on-site treatment. Copies of the study's findings can be found at the EREF Web site (www.erefdn.org/index.php/grants/fundedresearch).
Dr. Bryan Staley is the vice president of environmental research at the Environmental Research and Education Foundation. Dr. Jenna Jambeck is an assistant professor in the University of Georgia's Department of Engineering.