MICROBIAL FUEL CELLS FOR SUSTAINABLE FOOD WASTE DISPOSAL

Transcription

1 MICROBIAL FUEL CELLS FOR SUSTAINABLE FOOD WASTE DISPOSAL 1.0 Problem Statement The disposal of municipal solid wastes is one of the most serious problems facing the 21st century. Waste generation is on the increasing trend due to exponentially increasing population growth, which renders waste management as a significant problem. The solid waste generation (lb/capita/day) has been on increasing trend since 1980, with the current value reaching as high as 1 lb. per capita/day. i New York State alone generates 8..4 million tons of municipal solid waste per year and are rapidly approaching their limit for the built and permitted capacity of new landfills. On the Campus, food waste has become an increasingly large issue. In the Our dining hall on campus alone, an estimated 4 to 10% of food prepared ends up in the trash. While Our dining hall is making efforts on select campuses s across the country to reduce this waste, we remain unaffected. With Our dining hall serving over 3300 students, food waste generated on campus is around 3300 pounds per day. Over the course of a school year, this means we are throwing away over 330 tons of food. As such, it is essential that this food waste be reduced. ii To bring the campus waste treatment up to modern standards, we propose an investigation into a series of biological treatment processes. These processes which, when combined; could drastically contribute to our waste disposal problem. First, the biological treatment processes of bio- hydrogen fermentation, in dark and acidic conditions, can process complex organic 1

2 substrates into a simpler, more homogenous mixture of short-chain fatty acids, such as acetic acid and butyric acid. This can be followed by a digestion in a microbial fuel cell, which further coverts the remaining chemical energy into usable electrical current while simultaneously treating water. Although the main scope of this work focuses around effective treatment methods, i.e. the removal of organic carbon, these processes yields exciting potentials. Further investigation into this process could help create a self sufficient campus 2.0 Project Summary/Background Solid waste and high organic wastewater stand to be a larger source usable energy, but this chemical energy can only be used efficiently in few systems. This problem is due to the sophisticated form chemical energy can take. Many of these complex starches, where mass amounts of organic energy are bound, can be easily de-branched, or hydrolyzed, by the enzymes present during dark fermentation. Dark, Sucrose (g COD /L) 10 Sucrose Butyrate Acetate Propionate Organic Acids [m g COD/L] low ph, fermentation utilizes particular processes which digest complex carbon molecules into simpler volatile fatty acids (VFAs) such as butyrate, acetate, and propionate. The production of these short chain fatty acids drastically lowers the ph of the waste product iii. This waste product can then be reprocessed in a Microbial fuel cell. Microbial Fuel Cells (MFCs) utilizes an evolutionary process that has been developing for millions of years. The microbes on the anode surface, in the anode compartment of a fuel cell, uses cellular respiratory pathways to convert the chemical energy stored in organic waste into electrical energy. Under anaerobic conditions, the sugar substrates are converted into carbon dioxide, protons, and electrons from anaerobic 2

3 Theses electrons, produced at the anode, flow through the external load before being consumed at the cathode. The potential difference between the anode and the cathode, together with the flow of electrons results in the generation of electrical power iv. These fuel cells utilize an electrical potential difference to create an electrochemical "snorkel which bacteria can use to breathe and subsequently break down organic matter at an increased rate under anaerobic conditions. 3.0 Relationship to Sustainability Microbial fuel cells are becoming increasingly more popular in the research world. More and more academics as well as industrial professionals are looking to microbial fuel cell to solve one of this generations most growing concerns, the availability of water. This team focus on sustainability stems from the acknowledgment that there is a finite amount of resources available at this present time. The sustainability movement has been enlightening, in that it helps us preserve and reuse what we already have, instead scavenging for more. Currently average food supplier simply disposes spends nearly $60,000 to dispose 365 ton of organic waste generated on an annual basis. Due to the limited capacity of solid waste disposal facilities, often large quantities of this food waste is ground up and flushed down the drain. This water, full of privately owned organic garbage, is often pumped into municipally owned waste water treatment plants. This overload of carbon rich water can place a serious burden on our outdated water infrastructure as well as costing taxpayers to dispose inefficiently of energy rich waste. These water treatment systems consume 112 KW per million gallons treated per day to remove organics in wastewater. This cost is often due to the secondary stage of wastewater treatment which 3

4 is an extremely energy-intensive process. By harnessing alternative digestion methods, savings in electrical costs may be seen without requiring a complete overhaul of waste treatment processes. 4.0 Materials and Methods: 4.1 Laboratory Scale Fuel Cell Design: To design a large scale microbial treatment process we first idealized the problem by cultivation in laboratory scale test MFCs. These MFCs set up to evaluate the feasibility of fermented food waste as the electron donor in MFCs, These MFCs were operated in two repeatable cycles during a span of 672 h in a fed-batch. The test and control MFCs used in this study were based on two-compartment MFC kits obtained from a local buisness. The anode and cathode chambers had a total volume of 225 ml each. 4.2 Research and Development: Solid Waste Processing: Two-compartment MFCs were evaluated under batch-fed mode using Laminaria saccharina as a model for solid waste as the electron donor. This microbial fuel cell cultivated mixed microbial consortia as the biocatalyst, in the anode compartment. The Laminaria-based MFCs (LBMs) were studied with three different pretreatment conditions for the L. saccharina: (i) autoclaving (Auto), (ii) microwave irradiation (Micro), and (iii) no treatment (Control). v 4.3 Research and Development: Bio-Hydrogen Fermentation Process: Glucose was used in this study to model food waste. Glucose was fermented in anaerobic jars using a chemical oxygen demand of g/ L concentration. Anaerobic inoculums were obtained from our Sewer District's Anaerobic Digester. These inoculums were spread into a 1 cm thick paste and then cooked until internal temperature reached 53.9 C in order to induce spore formation of the desired bacteria. After heat treatment, the anaerobic inoculums were added to the glucose solution at a concentration of 10 g/l. The solution was contained within 500ml brown bottles which contained 20g of activated 4

5 carbon. This solution was filled with 450ml of media utilizing a remaining headspace of 50ml to maintain an expectable pressure throughout the experiment. This headspace in the bottle was vented once every three days by aggregating the bottle then using a needle to quickly release the pressure from the off gas of the fermentation byproducts. This fermentation bottle was left to ferment for a six day period; 4.3 Research and Development: Low ph Microbial Fuel Cells: Two-compartment MFCs were evaluated under batch-fed mode using effluent from the dark, low ph, bio-hydrogen fermentation. This study was conducted similarly to that of the Laminaria but MFCs were used to evaluate the operation of these biological fuel cells under novel conditions. 4.4 Scaled Reactor Design: The larger scale treatment process requires many interdisciplinary skill sets. Validation of the treatment process allows for the further investigation of larger scale modules. Electrode materials as well as general reactor design were evaluated based on scalability, ease of manufacture, and cost. LpH fermentation requires a robust biofilm on the anode surface, therefore investigation into the establishment of an anodophilic biofilm is crucial to scale up of this process. To tackle these problems a design of an ideal reactor system was developed. This reactor design was a very common tubular design with modified electrode materials. To create physical model of scaled reactor, a 5

6 contractor was able to provide schematics on one of their original design. From this design, utilizing major design tweaks we were able to conceptualize a larger scale module reactor. 4.5 Electrical Connections: To harness energy from food waste, we will require a specialized equipment to get the largest amount of usable current while not over taxing the bacteria. These scaled reactors will require a harvesting technique that uses an ultra-low-voltage DC-DC converter. The advantages of this chip is that they are relatively low power consumption and have low input values. 5.0 Results, Evaluation and Demonstration 5.1 Vegetable Kelp Food Waste Based Microbial Fuel Cells: Investigation into complex solid organic wastes streams led to the acknowledgement of pretreatment prior to substrate utilization by microbial fuel cells. Complex substrates can often be detrimental to microbial fuel cells due to the amount of enzymes needed. In this study we were able to effectively increase the amount of total organics removed by our microbial fuel cells by pretreatment in an autoclave. This demonstrates a significant advancement in the ability of fuel cells to degrade complex solid wastes; however, autoclaves require a significant amount of energy for preprocessing. 6

7 5.2 LpH Microbial Fuel Cells: Alternately to using an autoclave for preprocessing, the development of a biological treatment process to efficiently homogenize complex waste streams presented a novel strategy to extract energy while simultaneously treating water. To overcome the same hurdles as autoclaving preprocessing, LpH microbial fuel cells aim to overcome the inherent problem of requiring an efficiently buffered system. Based on the tables presented, mixed culture microbial fuel cells showed to be an effective treatment method. Even more impressive is the realization to the magnitude in which buffers may have inhibited further waste removal due to the affinity for the fermentation bacteria for low ph conditions. Investigation into the usefulness of the electrochemical gradient in the removal of organic matter was simultaneously assed. The conditions in a microbial fuel cell are similar to that of a fermentation reactor, however; a microbial fuel cell allows direct solid phase electron transfer to an anode. Based on these results, microbial fuel cells enhance the ability of anaerobic bacteria to degrade organic carbon waste streams even in low ph conditions. 7

8 5.3 Energy Harvesting: This design was successfully able to boost the voltage from mili volts to volts on an order of magnitude of 30 times. While it was not boosted to the goal of 3.3 volts the current into the chip was below the minimum requirements. Losses were also not accounted for due to the fact that the chip should be fabricated directly to a board. Using a breadboard for this purpose of low voltage and current had applications which resulted in a large impact on the overall output. 5.4 Interpretations and Implications for Scale up: The laboratory treatment trials have established LpH microbial fuel cells as effective treatment method for high strength organic waste. This process can be adapted to the lamanaria microbial fuel cells for the development of a large scale module reactor which will efficiently prove larger scale organic removal. Already we have begun conversations with local innovators and begun design into a custom reactor which fits our needs. These large modules will be linked and will be able to create a series which could be used to remove large volumes of food waste while generating electricity. 6.0 Conclusion Demonstrated above is a processing method which could easily be adopted to fit a larger scale implemented on campus, or in the immediate area. We have been fortunate to be exposed to people who share our interests in bringing sustainability to our campus. The need for our team to fabricate a reactor is unnecessary when local innovators have already taken on this call. i United States. Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and FIgures for 2008., Print. ii iii Fermentation systems towards hydrogen production, Venkata ramana gadamhshetty iv General Principles of MFCs. (2008, December 9). Retrieved October 12, 2011, from Microbial Fuel Cells: v Gad.Gadhamshetty, et al. "Evaluation of Laminaria based Microbial Fuel Cells for Electricity Production." Bioresource Technology (2012). 8