Microbial fuel cells: food waste as a sugar source

Summary

The recent threat of climate change and rising demands for electricity globally has prompted research into Microbial fuel cells (MFCs), a potential source of renewable energy. Another troubling global issue is the large amounts of food waste. Use of food waste as substrates in MFCs could convert the food waste into clean energy, tackling both global problems. Hence, this research aims to determine if food waste is a viable substitute for currently used substrates (glucose) in MFCs by replacing glucose with banana peels and sugarcane bagasse (inner and outer layers). Our hypothesis was that the types of food waste used would have a comparable voltage performance to that of glucose. For each substrate, MFC set-ups were run and the Benedict’s Test (quantitative) and Iodine Test were performed. Banana peel was found to be the best substrate among the three food wastes with the stablest voltage performance, highest peak and highest average voltage performance. This was followed by the outer layer of sugarcane and lastly, the inner layer of sugarcane which also had the lowest concentration of reducing sugar. Thus, it was concluded that the substrate’s concentration of reducing sugar may affect voltage performance of the MFC. Furthermore, banana peels and the outer layer of sugarcane were found to be viable alternatives for glucose due to a higher voltage performance. Further research on the components of the substrates and the effects they have on the MFC performance can be done. 

 

Question / Proposal

This research aims to find out if food waste is a viable substitute for currently used substrates in MFCs by experimenting using banana peels and the inner and outer layers of juiced sugarcane pulp as the substrate for MFCs.

Our hypothesis for this research is that food waste is a viable alternative for currently used substrates. Our expected outcome is that all the substrates used (banana peels, inner layer and outer layer of sugarcane) will have a comparable voltage performance (equal or better) to that of currently used substrates (glucose), as previous research shows that other types of food waste (kitchen and bamboo waste), when used as a substrate in MFCs, have been able to generate relatively high voltage performances. [7] Additionally, banana peels and sugarcane bagasse are known to contain relatively high levels of sugar, which will provide an ample food source for the yeast. 

We also expected MFCs using the inner layer of the sugarcane (pith) as the substrate to have the highest voltage performance. This is because the pith of the sugarcane is known to have an abundance of sucrose which the yeast can also feed on.

Research

The idea of microbial fuel cells (MFCs) was initiated by M.C. Potter in the early twentieth century, and since then MFCs have started to gain popularity due to its potential as a renewable source of energy. In the past decades, significant improvements were made to the design that increased its efficiency, making it a more viable alternative to fossil fuels.

The typical MFC used in experimentation is dual-chambered, with a proton-exchange membrane separating the anode and cathode chamber. A biocatalyst will oxidise a substrate in the anaerobic anode chamber, generating electrons and protons separately. A mediator will then transfer the electron to the anode electrode, before it passes through an external circuit before entering the cathode chamber. The flow of electrons generates an electrical current. Meanwhile, the proton will pass into the cathode chamber through the proton-exchange membrane to rejoin with the electron.[11]

Figure 1. Dual-chambered MFC setup

Throughout the years, the focus of research in MFCs has shifted from that of optimising MFC performance [14] to that of finding sustainable, low-cost materials that can enable it to be used in large-scale, real-world operations. There has also been research into the different kinds of substrates that the biocatalytic microorganisms can feed on, comparing the efficiency of each substrate.

Common substrates include the pure forms of carbohydrates, mainly glucose, and acetate. As MFCs are able to treat wastewater while generating clean electricity, many forms of wastewater sludge are used as well. Such substrates have shown to be able to produce satisfactory MFC performances.[10] However, although there has been extensive research into the use of the aforementioned substrates in MFCs, there is limited research into food waste as an alternate sugar source in MFCs.

The constantly increasing amounts of food waste is a huge contributing factor to climate change and is a problem faced by several countries, including the USA and Singapore. Approximately one third -- 1.3 billion tonnes -- of food produced globally for human consumption annually ends up as food waste. [3] When such food waste are disposed of in landfills, the waste gets digested anaerobically by bacteria, increasing levels of methane, a known greenhouse gas that is more potent than carbon dioxide.

However, in actuality, food waste has a lot of potential to be used as substrates in MFCs. [6] Food waste contains many carbohydrates that the microorganism can feed on, and is a sustainable and abundant resource. As fruit and vegetables account for the majority of food wastage rates [3], and lignocellulosic biomass has proven to be effective substrates used in MFCs, [10] we decided to use banana peels and sugarcane bagasse. This allows us to recycle common types of food waste and divert the food waste from going to the landfill, as well as provide clean electricity, making MFCs an even more environmentally friendly and viable source of energy.

Method / Testing and Redesign

1 Preparation of food waste substrates

Juiced sugarcane pulp, sourced from hawker stalls, was dried in the oven for around 3 hours at 100oC. The inner layer and outer layer of the sugarcane were then separated. Each layer was cut into smaller pieces and blended into fine powder.
Bananas were acquired from a local supermarket. Banana peels were dried in the oven for 2 hours at 100oC, before being separated and cut into smaller pieces. The pieces were then blended into fine powder.

2 Assembling of microbial fuel cells

  

Fig 2. Diagrams of MFC setups (with a voltmeter and with both resistor and voltmeter)

Fig. 2 shows the MFC setups used during experimentation. Dual-chambered yeast MFC setups were assembled using a Nafion Proton Exchange Membrane (PEM) between the anode and cathode chamber, with each chamber containing an electrode cut from carbon fibre electrode tissue. All MFCs used selected food waste (0.2g of food waste added to 4.25ml of buffer) or 0.5% (D+) Glucose solution as the substrate, Methylene Blue (added to buffer) as the mediator and Potassium Permanganate (added to buffer) as the catholyte in a 17: 10: 32 ratio by volume. Buffer used was 0.1M Phosphate buffer (pH 7.2).

3 Testing of microbial fuel cells

For each of the food waste substrates (inner layer of sugarcane, outer layer of sugarcane and banana peel), three MFCs were constructed; two setups with a voltmeter and without a resistor, and one setup with a resistor of 100 ohms and a voltmeter connected in parallel. The setup with the resistor allows for calculation of the amount of current generated by the MFC, while the setups without showed the voltage generated (recorded by the voltmeter) during experimentation. The MFC setup using (D+) Glucose solution was with a voltmeter and without a resistor. (Fig 2.) Each MFC was run for a duration of 48 hours during which it was left undisturbed at room temperature, while a data logger collected data samples every minute.

4 Food tests

Benedict's test was performed on all 3 substrates. Each substrate was added to buffer in the amount to be used during MFC experimentation. The Benedict's test was also performed as a quantitative test by testing glucose solutions of 1%, 0.1%, 0.05%, 0.01% and 0.005% concentrations and comparing the colour change with that of the substrates researched on. Iodine test was also performed on all 3 substrates.

All experimentation was conducted in the school's laboratory. Gloves were used when handling harmful chemicals and all chemical waste was disposed of in the proper bin. 

Results

1 Microbial fuel cell setups results

Fig. 3 shows that all substrates used had an increasing voltage output in the first 100 minutes before reaching a power peak. After a period of time (varied for each substrate) the voltage output started to decrease from the voltage peak until the end of experimentation.

The highest voltage peak of all MFC setups was 6V, the highest voltage that the data logger used could measure. This voltage peak was observed in both MFC setups using the outer layer sugarcane (OLMFC1 and OLMFC2) as well as  the first setup of the MFCs using banana peels (BPMFC1).The peak for the MFC using D+ glucose (GLMFC1) was 5.94888V. The MFCs using banana peels also attained the highest overall average (across both setups) of 4.82508V. This was followed by outer layer sugarcane with an overall average of 4.22395V, then  (D+) Glucose (4.125874V) and lastly inner layer sugarcane (3.20083V). As banana peels and the outer layer of the sugarcane have shown comparable voltage performances to glucose, they are viable alternatives for glucose (a commonly used substrate). 

Fig 4. shows that with the added resistor, all MFCs had unstable voltage performances. MFCs using banana peel still had the highest average voltage output of 0.24601V followed by outer layer (0.01264V) and inner layer of sugarcane (0.00243V). 

Banana peel MFCs also had the best voltage performance, as it had the highest and most stable average voltage output. The overall data suggested that banana peel is the best substrate to be used out of the three researched as it had the best voltage performance (highest and most stable average voltage performance across all setups). 


2 Food test results

All three substrates were shown to contain no starch. All three substrates were also shown to have concentrations of reducing sugar between 0.1% and 0.5%. The inner layer of the sugarcane was observed to have the lowest concentration of reducing sugar amongst the 3 food substrates.

3 Discussion

The MFC setups using the outer layer and banana waste had better voltage performances than that of the inner layer, which contradicts the hypothesis. After referring to the food test results, it could be concluded that the differing performances may be due to the composition of the food waste. Banana peels contains ethylene, a natural plant hormone released as the fruit ripens, which breaks down complex sugars and pectin in the peel. [8] This causes a higher reducing sugar concentration. The outer layer of the sugarcane has the second highest concentration as it contains many non-starch polysaccharides, [2] and also has majority of the vascular bundles. In contrast, inner layer of the sugarcane consists of an abundance of sucrose-storing parenchyma cells, [13] and since sucrose is not a reducing sugar, it has a low reducing sugar concentration.

We can infer that the concentration of reducing sugar found in the substrates influenced the voltage performances of the MFCs, with a higher amount of reducing sugar leading to a better voltage performance.

Conclusion

MFC setups were run using three different types of food waste, banana peels and the inner layer (pith) and outer layer (rind) of sugarcane as substrates. Out of the three food types used in experimentation, Banana Peel was shown to be the best substrate for MFCs as it has the highest peak, highest average voltage output in voltage performance (with and without the added resistor) and the most stable performance. An increase in concentration of reducing sugar in the substrate may lead to an increase in voltage performance of the MFC. In addition, banana peel and outer layer of sugarcane were shown to be viable alternatives to currently used substrates in MFCs (glucose) due to comparable voltage performances in comparison to the results of the MFC using glucose as a substrate.

To increase the reliability of our results, we could repeat experimentation several times to ensure consistency and to eliminate any anomalies. The duration of our experiments could also have been extended, till the voltage dropped to zero, to have a better understanding of the substrate's performance in MFCs. Deeper analysis of the food waste used could have been carried out too, to find out if any biomolecules or chemicals present could have affected the voltage performance. 

Despite these limitations, our project was successful in finding viable alternatives to glucose in MFCs, namely banana peel and outer layer of sugarcane. Our findings show that food waste can be used as a potential substrate in MFCs, capable of generating relatively high voltage performances. We also found a possible factor (reducing sugar concentration of substrate) that could increase the efficiency of MFCs, contributing to research being done to optimise MFC performance. This takes us a step closer to making MFCs a more viable alternate energy source for fossil fuels in large-scale operations, helping us fight climate change.

Future work regarding research into the use of other types of food waste as substrate in MFCs (eg. citrus peels), as well as to investigate the effect of different percentages of fibre and complex sugars in the food waste on MFC performance can be done. Additionally, future work concerning the processing of food waste to facilitate the breakdown of complex sugars such as cellulose in fibres to simple sugars (for example, through the addition of enzymes) before using as a substrate for the yeast can be carried out. Further investigation into why the outer layer (rind) of the sugarcane had such a high concentration of reducing sugar can also be done.

About me

Jenevieve Ho

My hobbies include playing the guitar and reading just about anything.

My teachers introduced the joy of science and research to me, showing us interesting science videos and conducting cool experiments, and this sparked my interest. It has also encouraged me to always be ready to learn new things and have an inquisitive attitude.

Alexander Humboldt’s determination and passion to gain a deeper understanding of the world around us is one of my biggest inspirations.

I plan to study and work in a biology-related area, whether it’s zoology or medicine. Winning the fair would allow me to pursue my interest in science and provide once-in-a-lifetime opportunities to experience what it’s like to work in a specialised company in the area of science I love.

Emma Tan

In my leisure, I enjoy dancing and reading. 

I grew up reading science magazines and doing fun practicals during science lessons. Science fascinated me because it allows us to understand the inner workings of our world. My teachers kindled this interest in science, and introduced me to research, which taught me to analyse and question everything happening around me.

Ernest McCulloch and James Till inspire me with their inquisitivity, creativity and how they sought to understand every detail of their research.

I plan to study and pursue a career in medicine. Winning this science fair would encourage me in my pursuit of science and  provide opportunities for unique, eye-opening experiences that would allow me to further explore my field of interest— biology.

Health & Safety

Guidelines

PART VIII

SCHOOL WORKSHOPS AND SCIENCE LABORATORIES

Safety precautions

41.—(1)  The supervisor shall ensure that all necessary safety precautions are adopted in school workshops and school science laboratories and shall modify or extend such precautions as the Director-General may require.

(2)  The principal shall ensure that no instruction is given in the use of tools or the operation of machines or in science experiments except by a teacher competent to give the type of instruction concerned.

Plan of layout

42.  Whenever it is desired to install any machinery or machine tools in a school workshop, the supervisor shall submit to the Director-General a plan of the proposed layout of the workshop.

Consent for installation

43.  No machinery or machine tools shall be installed in a school workshop except with the written consent of the Director-General.

Remote control switches

44.  In any school workshop in which machinery is installed there shall also be installed remote control switches whereby the teacher may stop all machines.

Maintenance and suitability

45.  All machinery, machine tools, hand tools and other equipment in a school workshop or science laboratory shall be suitable for the course intended and shall be maintained in proper working order.

Windows of workshops

46.  Every school workshop in which power machinery is installed shall contain windows on opposite sides of the workshop and the total area of the windows shall not be less than one-eighth of the floor space of the workshop.

Placing of machines

47.  Machines and work-benches shall be in such positions as to ensure that they are adequately lighted.

Storage of poisons and dangerous chemicals

48.  All poisonous substances or dangerous chemicals shall be under the charge of a responsible teacher and, except for such small quantities as are necessary at any given time for practical science experiments, shall be stored in a locked room or cupboard to which no pupil shall have access.

Containers to be marked

49.  All poisonous substances or dangerous chemicals shall be contained in suitable containers clearly marked with the name of the substance and labelled “DANGEROUS” or with some similar designation.

Teacher in laboratory to have unobstructed view of pupils

50.  Furniture and equipment in a science laboratory shall be so arranged that the teacher has an unobstructed view of every pupil.

Location: Sigma Lab, National Junior College, Singapore

Lab Manager: Mr Cezanne Santos Cabulay

Contact: 6466 1144

Bibliography, references, and acknowledgements

Acknowledgements:

We would like to thank our school National Junior College, Singapore for the opportunity to conduct this research project under the Science Research and Training Programme (Junior) as well as our mentor, Mr William Phua for guiding us through this project.

References:

[1] Archibald, J. (1949). Nutrient Composition of Banana Skins. Journal Of Dairy Science, 32(11), 969-971. http://dx.doi.org/10.3168/jds.s0022-0302(49)92146-3  

[2] Han, G., & Wu, Q. (2004). Comparative properties of sugarcane rind and wood strands for structural composite manufacturing. FOREST PRODUCTS JOURNAL, 54(12).

[3] Key facts on food loss and waste you should know! (n.d.). Retrieved from http://www.fao.org/save-food/resources/keyfindings/en/

[4] Kummu, M., de Moel, H., Porkka, M., Siebert, S., Varis, O. and Ward, P. (2012). Lost food, wasted resources: Global food supply chain losses and their impacts on freshwater, cropland, and fertiliser use. Science of The Total Environment, 438, pp.477-489.

[5] Lohar SA, Patil VD, Patil DB. Role of Mediators in Microbial Fuel Cell for Generation of Electricity and Waste Water Treatment [Online]. International Journal of Chemical Sciences and Applications 6: 1–11, 2014. https://bipublication.com/files/IJChs-V6I1-2015-02.pdf.

[6] Lin, C. S., Pfaltzgraff, L. A., Herrero-Davila, L., Mubofu, E. B., Abderrahim, S., Clark, J. H., . . . Luque, R. (2013). Food waste as a valuable resource for the production of chemicals, materials and fuels. Current situation and global perspective [Abstract]. Energy & Environmental Science, 6(2), 426. doi:10.1039/c2ee23440h

[7] Moqsud, M. A., Omine, K., Yasufuku, N., Bushra, Q. S., Hyodo, M., & Nakata, Y. (2014). Bioelectricity from kitchen and bamboo waste in a microbial fuel cell. Waste Management & Research, 32(2), 124-130. doi:10.1177/0734242x13517160

[8] Orwig, J. (2015). People around the world are eating banana peels because they know something that Westerners do not. Business Insider Singapore. Retrieved 4 January 2018, from http://www.businessinsider.sg/benefits-of-eating-banana-peels-2015-9/?r=US&IR=T

[9] Pandey, A., Soccol, C., Nigam, P., & Soccol, V. (2000). Biotechnological potential of agro-industrial residues. I: sugarcane bagasse. Bioresource Technology, 74(1), 69-80. http://dx.doi.org/10.1016/s0960-8524(99)00142-x

[10] Parkash, A. (2016). Microbial Fuel Cells: A Source of Bioenergy. Journal of Microbial & Biochemical Technology, [online] 8(3). Available at: http://dx.doi.org/10.4172/1948-5948.1000293.

[11] Rahimnejad, M., Adhami, A., Darvari, S., Zirepour A., Oh, S. (2015). Microbial fuel cell as new technology for bioelectricity generation: A review. Alexandria Engineering Journal, 54(3), p.p..745-756.

[12] Serrano-Ruiz JC. New microbial technologies for advanced biofuels: toward more sustainable production methods. Illustrated. Apple Academic Press, 2015, 2016.

[13] Siqueira, G., Milagres, A., Carvalho, W., Koch, G., & Ferraz, A. (2011). Topochemical distribution of lignin and hydroxycinnamic acids in sugarcane cell walls and its correlation with the enzymatic hydrolysis of polysaccharides. Biotechnology For Biofuels, 4(1), 7. http://dx.doi.org/10.1186/1754-6834-4-7

[14] Winfield, J., Gajda, I., Greenman, J., & Ieropoulos, I. (2016). A review into the use of ceramics in microbial fuel cells. Bioresource Technology, 215, 296-303. doi:https://doi.org/10.1016/j.biortech.2016.03.135