Utilization of Enhanced Asparagus Waste with Sucrose in Microbial Fuel Cells for Energy Production

Abstract

The rapid increase in agricultural waste in recent years has led to significant losses and challenges for agro-industrial companies. At the same time, the growing demand for energy to support daily human activities has prompted these companies to seek new and sustainable methods for generating electric energy, which is crucial. Sucrose extracted from fruit waste can act as a carbon source for microbial fuel cells (MFCs), as bacteria metabolize sucrose to generate electrons, producing electric current. This research aims to evaluate the potential of sucrose as an additive to enhance the use of asparagus waste as fuel in single-chamber MFCs. The samples were obtained from CUC SAC in Trujillo, Peru. This study utilized MFCs with varying sucrose concentrations: 0% (Target), 5%, 10%, and 15%. It was observed that the MFCs with 15% sucrose and 0% sucrose (Target) produced the highest electric current (5.532 mA and 3.525 mA, respectively) and voltage (1.729 V and 1.034 V) on the eighth day of operation, both operating at slightly acidic pH levels. The MFC with 15% sucrose exhibited an oxidation-reduction potential of 3.525 mA, an electrical conductivity of 294.027 mS/cm, and a reduced chemical oxygen demand of 83.14%. Additionally, the MFC-15% demonstrated the lowest internal resistance (128.749 ± 12.541 Ω) with a power density of 20.196 mW/cm² and a current density of 5.574 A/cm². Moreover, the microbial fuel cells with different sucrose concentrations were connected in series, achieving a combined voltage of 4.56 V, showcasing their capacity to generate bioelectricity. This process effectively converts plant waste into electrical energy, reducing reliance on fossil fuels, and mitigating methane emissions from the traditional anaerobic decomposition of such waste.

1. Introduction

Agricultural and food waste face numerous challenges in the 21st century, primarily because this type of waste occupies more space in designated waste treatment areas and landfills [1]. Agro-industry waste refers to byproducts generated during agricultural production and food processing. This waste can be solid, liquid, or gaseous, and if not managed properly, it can lead to significant environmental issues. However, by adopting a sustainable approach, this waste can be transformed into a valuable resource with potential applications in energy production, fertilizers, and bioproducts [2]. According to the Food and Agriculture Organization (FAO), an estimated 1.3 billion tons of food waste are produced annually, resulting in an economic loss of approximately $490 billion every year [3]. The agro-industry waste still contains high concentrations of nutrients and biomass, making it valuable as a raw material for other industries [4,5]. Conversely, the energy sector urgently needs significant changes in the 21st century [6], such as decarbonization of the energy system. Since energy consumption skyrocketed from 155.22 EJ in 1965 to 55.63 EJ in 2020, it is estimated that natural gas, oil, and coal reserves will last approximately 50 years [7,8]. With fossil fuels accounting for 85% of primary consumption worldwide and the price of kilowatt per hour increasing globally in the last decade, scientists are in a race to find new ways to generate sustainable and environmentally friendly electricity [9,10]. Agricultural and food waste is crucial in microbial fuel cells (MFCs). These wastes, including leftover fruits, vegetables, grains, and other agricultural products, are rich in organic matter and can be used as a substrate for bacteria in MFCs [11]. Agricultural and food waste provides essential nutrients that bacteria need to grow and metabolize [8]. By breaking down the organic matter in these wastes, bacteria produce electrons that can be captured and used to generate electricity [11].

One of the most promising technologies is microbial fuel cell (MFC) systems, which can efficiently, economically, and sustainably address organic waste treatment and electric power generation [12]. MFCs are primarily composed of the anode chamber and the cathode chamber, which are connected by an external circuit on the outside and joined by a proton exchange membrane on the inside [13]. The literature has reported that electricity generation is due to the transfer of electrons captured in the anode chamber by its electrode and then transferred to the cathode chamber via the external circuit [14]. Electrons are produced from the substrate used in microbial fuel cells as fuel [15]. For instance, Ahmad A. (2024) investigated microbial fuel cells using wastewater as a substrate, generating peaks of 0.154 V and 1.450 mW/m² on the fifteenth day, with an internal resistance of 724 Ω. These results demonstrate the potential of MFCs in wastewater treatment and energy generation [16]. Din et al. (2024) used potato waste as a substrate in their microbial fuel cells, showing a maximum voltage of 1.12 V and an electric current of 12.45 mA, using metallic materials as electrodes. This study highlights the versatility of MFCs in utilizing different waste materials for energy production [17]. Yaakop et al. (2023) used domestic waste as fuel in their MFCs, which showed a maximum voltage of 0.110 V with a power density of 0.1047 mW/m² and a current density of 21.84 mA/m² on the twelfth day, with an internal resistance of 117 Ω. Their findings underscore the potential of MFCs in converting domestic waste into valuable energy [18]. Current microbial fuel cell (MFC) technologies face several critical limitations that this document seeks to address [13,15]. Notably, the inefficient electron transfer within the cells leads to high internal resistance values, adversely affecting electrical performance [17]. Additionally, essential components of MFCs, such as proton exchange membranes and electrodes, are often expensive and not always designed for sustainable or cost-effective applications [14,16]. Furthermore, substrates like fruit and vegetable waste exhibit chemical variability, which can result in inconsistencies in electricity generation [18].

Peru’s significant diversity and high volume of fruit production result in substantial fruit waste, constituting an integral part of agro-industrial and municipal waste [19]. This waste includes byproducts generated during various stages such as cultivation, harvesting, processing, and consumption of fruits. Industries related to juice production, canning, and export generate large quantities of byproducts, including peels, pulp, seeds, bagasse, and fibers [20]. In the last decade, asparagus production in Peru has increased substantially. Peru is ranked as the second-largest producer of asparagus globally, with exports primarily to the United States, Spain, and the United Kingdom [21]. The high demand for asparagus is due to its rich folic acid and beta-carotene content, which are vital for preventing heart disease and high blood pressure. It also contains vitamins E, C, and K, magnesium, and potassium [22]. Its high nutritional content has increased consumption, generating significant economic and social benefits for Peru. However, a challenge has arisen: the management of waste generated during the export process and how this waste can be repurposed [23]. In monetary terms, data from 2020 indicates that the average cost of electricity in Peru was approximately 0.13 USD per kilowatt-hour (kWh). However, electricity prices vary depending on several factors, including geographic location, the type of user (residential, commercial, or industrial), and the specific electricity distribution company [20,23]. Electricity rates are regulated and supervised by the Energy and Mining Investment Supervisory Agency (Osinergmin) [21]. As previously shown, microbial fuel cells have been operated with different substrates. Furthermore, it has been shown that the electricity values depend on several factors, including the concentration of carbon sources within the waste used. The use of asparagus as a substrate with varying concentrations of sucrose as an enriched carbon source has not been found in the literature.

The primary objective of this research is to investigate the influence of saccharose in asparagus waste used as fuel in single-chamber microbial fuel cells over 14 days. We will monitor the power density, current density, internal resistance, pH, voltage, electrical conductivity, and electricity of the microbial fuel cells with varying concentrations of saccharose (0%, 5%, 10%, and 15%). This study has the potential to introduce a new method for enhancing electric energy generation in an economically and environmentally sustainable manner, offering promising opportunities to improve the efficiency of microbial fuel cells and make them more attractive for private investment. Energy production and byproducts can generate additional income, especially in rural or agro-industrial areas. Accessible materials, such as activated carbon electrodes or conductive textiles, can reduce manufacturing costs.

2. Materials and Methods

• Manufacturing of the MFC Operationalization

The vessels used as single-chamber microbial fuel cells were manufactured (Ballard Power Systems, Toronto, ON, Canada) from 1000 mL (Boron-silicon, Boron 3.3) materials. The MFCs used had a carbon anode electrode with an area of 45 cm² and a zinc cathode electrode of 12.27 cm², which were joined on the outside with a resistance of 100 Ω and on the inside with Nafion^TM N-117 membrane (0.180 mm thickness, Fisher Scientific, Madrid, España), as shown in Figure 1.

b.

Obtaining samples used as a substrate.

The asparagus waste was collected from CUC SAC, Trujillo, La Libertad, Peru. In total, 6 kg of waste was collected, which, once washed, was left to dry at 25 °C for 24 hours until finally, with an extractor (Oster—FPSTJE317R, Newell Brands, Atlanta, GA, USA), a liquid solution of 3500 ml was obtained; this solution was used as a substrate in the MFC. While saccharose was included in the waste from a Sterile Stock solution at 50% of saccharose, 100 mL of 3 working solutions with different concentrations (0/ Target, 5, 10, and 15%) were prepared. Subsequently, 80 mL of each working solution was added to the inside of the MFC and homogenized with the white asparagus residue, with the final volume of the working residue in the MFC being 800 mL.

c.

Obtaining Physicochemical Parameters of the MFCs

A Prasek premium brand multimeter (PR-85, Gainesville, FL, USA) was used to measure the voltage and electric current to obtain the monitored values of the single-chamber microbial fuel cells. For the observation of the values of the oxidation-reduction power (ORP), pH, electrical conductivity, and chemical oxygen demand, the values were observed using the BioMars brand multiparameter equipment (HI98194, Hanna, MI, USA) and the measurement of the internal resistance of the MFC was completed using an energy sensor (Vernier— ±30 V and ±1000 mA). For the power density (PD) and current density (CD) values, the method performed by Segundo et al. (2024) was used with different internal resistance values (0.2 ± 0.1, 5 ± 0.3, 30 ± 2.2, 60 ± 5.5, 110 ± 10.5, 250 ± 13, 360 ± 23.1, 530 ± 40.5, 750 ± 50.2, and 1000± 70.5 Ω) where the power density was found by the formula PD = V*I/A and the current density (CD) = I/A [24]. The schematization of the research is shown in Figure 2.

3. Result and Analysis

The pH values obtained from monitoring microbial fuel cells are shown in Figure 3a, where the MFC values increase slightly with a tendency to reach neutral levels. However, they remained in the somewhat acidic range until the end of monitoring. The optimal operating values for MFCs with 0%, 5%, 10%, and 15% sucrose were 4.87, 4.99, 5.13, and 5.29, respectively. The literature has observed that pH influences oxidation and reduction reactions in the electrodes, as the chemical species involved are in different ionization states depending on the pH [25,26]. Additionally, in an acidic medium (as in this case), protons are abundant and can facilitate specific reduction reactions, such as the reduction in oxygen in MFCs [27]. This knowledge can be applied in the design and optimization of MFCs. Recently, Aliyu et al. (2024) used a mixture of fruit waste (banana, lemon, orange, watermelon, and pawpaw) as a substrate in their research, which showed an optimal pH of 3.45 ± 0.62 and a peak voltage of 0.650 V [28]. Similarly, Kumar et al. (2024) used banana waste as a fuel source for their MFCs, demonstrating an optimal operating pH of 5.95. They noted that compounds containing glucose and fructose (such as sucrose) influence pH values due to the biochemical processes that occur when the waste decomposes and interacts with the microbes in the substrates used in the MFCs [29]. Agricultural and food wastes contain various chemical compounds, including organic acids, sugars, and proteins, which can affect the pH of the medium in the MFC. The decomposition of these compounds by bacteria can release ions that modify the pH [27,29]. The oxidation-reduction potential (ORP) values shown in Figure 3b were obtained from monitoring. It was observed that the 15% MFC had a higher ORP (405.887 mV) compared to the 0%, 5%, and 10% MFCs, which had values of 297.707, 336.767, and 381.0992 mV, respectively. The literature indicates that sucrose, a polysaccharide composed of fructose and glucose, serves as an energy source for the microbes in the substrate. The microbes metabolize sucrose primarily through glycolysis, causing them to oxidize glucose and reduce NAD+ to NADH, introducing a redox equilibrium [30,31,32]. For example, Córdova et al. (2020) used sucrose as a substrate in MFCs, reducing sucrose from 1 to 0.85 g/L and generating 539 ± 22 mV. They also mentioned that sucrose is hydrolyzed into simpler sugars used in electron transport chains after ATP synthesis occurs [33]. This research provides valuable insights for applying MFCs in bioenergy production. Agricultural and food waste offers a rich source of organic matter that bacteria can metabolize. This decomposition of organic matter is essential for producing electrons and protons, which are crucial for redox reactions [31].

Figure 3c shows the electrical conductivity values, where a successive increase in all the MFCs with different percentages of sucrose is observed. The MFC-15% exhibited the highest peak electrical conductivity value (294.027 mS/cm) on the eighth day, while the MFC-0% showed the lowest value (205.479 mS/cm) on the seventh day. The high values obtained in this research can be explained by the results of electrical conductivity, as this factor, according to the literature, is responsible for indexing the efficiency and speed of the ions within the MFCs, influencing the capacity of the microbes to generate a stable electric current [34]. Previous reports indicate that plant waste materials, such as fruit peels, leaf residues, and sugarcane bagasse, typically exhibit low natural electrical conductivity. This low conductivity is primarily due to their composition, mainly of non-conductive organic compounds like cellulose, hemicellulose, and lignin. In contrast, waste materials with higher concentrations of soluble salts, such as potassium, sodium, calcium, and magnesium, tend to have higher electrical conductivity [18,19,35]. Pineapple waste used as a substrate in MFCs has been reported to show electrical conductivity values of 69.47 ± 0.91 mS/cm and a power density of 0.967 ± 0.059 W/cm² [36]. The chemical oxygen demand (COD) values reported by the MFCs are shown in Figure 3d, where an apparent decrease in COD values is demonstrated from the beginning to the end. The MFCs with 0%, 5%, 10%, and 15% sucrose showed reductions of 83.14%, 83.95%, 86.67%, and 87.82%, respectively. The dependence of COD on electricity generation in MFCs is significant because the microbes present in the substrates use organic matter as a fuel source for their metabolism (which is the process by which electricity is generated). The high initial COD values in all cells indicate much organic matter available for microbial use. The COD values decreased as time passed, leaving no usable source for the microorganisms [37]. The literature has reported that residues rich in sugars, starches, or proteins have a higher chemical demand for biodegradable oxygen than those containing lignin and cellulose, which are more recalcitrant. Additionally, pH values and nutrient concentration also affect the degradation rate and the efficiency of COD conversion to energy [38].

The voltage values reported by the MFCs were initially very low, at approximately 0.06 V for all cells. However, these values successively increased until the eighth day, when they showed their maximum values of 1.034 V, 1.218 V, 1.561 V, and 1.729 V for the MFCs with 0%, 5%, 10%, and 15% sucrose, respectively. After reaching these peaks, the voltage values decreased until the last monitoring day, as seen in Figure 4a. The voltage values are highly related to the redox reactions between the electrodes. Changes can influence these reactions in the type of substrate, microorganisms, and other factors affecting the cell’s efficiency in converting energy [39]. Akinwumi et al. (2024) used pineapple and mango waste as substrates in their microbial fuel cells, generating values greater than 0.6 V, noting that the sugar content in these fruit wastes helped improve the voltage values [38]. Similarly, Yaqoob et al. (2024) used dragon fruit waste as fuel in MFCs, achieving maximum voltage values of 0.165 V. The behavior observed is similar to our research, showing a decrease in COD and a reduction in voltage values [40].

The values found from the monitoring of the MFCs are shown in Figure 4b. It was observed that the MFC with 15% sucrose obtained the highest current value in the electric field on the eighth day (5.532 mA), compared to the MFC with 0% sucrose, which obtained 3.525 mA on the same day, representing 36.28% less. Meanwhile, the MFCs with 5% and 10% sucrose obtained 4.093 mA and 4.749 mA, respectively. The high electrical values for the MFCs with different percentages of sucrose are attributed to the microbes in the anodic substrate breaking down sucrose into glucose and fructose through their metabolism during fermentation [33,41]. It has been reported that sucrose, a rich carbon source, can be easily decomposed by several bacteria, generating the electrons captured by the anodic electrode and flowing through the external circuit to produce electricity. An MFC containing more sucrose will create increased electrical energy [42,43]. Tariq et al. (2021) used sucrose as a substrate in their microbial fuel cells, achieving current and voltage peaks of 0.061 V and 0.61 mA, with the substrate operating at 35 °C [44]. Similarly, Bose et al. (2023) used wastewater enriched with sucrose to generate 870 ± 20 mV, noting that carbon-based electrodes help facilitate electron capture in energy generation [45]. In MFCs, as bacteria break down organic matter in waste, they produce electrons as byproducts of their metabolism. These electrons are captured by the anode of the MFC and are used to generate an electric current [33,44].

The internal resistance values of the MFCs are shown in Figure 5. The internal resistance was calculated through the polarization curve, where the voltage and current values were placed on the “x” and “y” axes, with the slope of the linear fit representing the MFC resistance. The internal resistances found in the MFCs with 0%, 5%, 10%, and 15% sucrose were 187.457 ± 26.548 Ω, 154.897 ± 15.742 Ω, 136.870 ± 18.529 Ω, and 128.749 ± 12.541 Ω, respectively. The literature has reported that good biofilm formation influences the values of internal resistances in MFCs, and sucrose can affect the stability of the biofilm, which impacts the ability to transfer electrons between microbes and electrodes, generating variations in resistance [46,47]. Verma et al. (2023) used cellulosic waste in their microbial fuel cells, generating an internal resistance of 370 ± 0.10 Ω, noting that the use of metallic materials in the cells helps reduce resistance values due to their conductive nature [48]. Aleid et al. (2023) used fruit waste in their microbial fuel cells, showing an internal resistance of 734.0 Ω, and mentioned that the internal resistance of composite cells with high glucose content depends on several factors, including sucrose concentration [49]. The power density values of the MFCs with different percentages of sucrose (0%, 5%, 10%, and 15%) are shown in Figure 6, where it is observed that the maximum power density values were 14.874, 17.097, 18.133, and 20.196 mW/cm², and a current density of 3.924, 5.117, 5.009, and 5.574 A/cm² for the MFCs with 0%, 5%, 10%, and 15% sucrose, respectively. According to reports, sucrose, being a disaccharide, is easily broken down by bacteria into monosaccharides, which can be readily used in the metabolism of microbes in the substrates, leading to a greater number of electrons being produced and captured by the electrodes [50,51]. Having a high sucrose concentration would improve electron production and, therefore, power density [52]. García et al. (2023) used sucrose as a carbon source for their Saccharomyces cerevisiae in their MFCs, reporting 63 mW/cm² with a sucrose concentration of 10%, noting that higher concentrations did not show any improvement for that microorganism [53]. Similarly, Bhattacharya et al. (2023) used sucrose and glucose as substrates for their MFCs, generating maximum voltage values of 500 ± 15 mV and 75 mW/m², and mentioned that high concentrations of sucrose inhibit microbial growth and cause an imbalance in the metabolic process [54]. The asparagus waste placed in the microbial fuel cells with different percentages of sucrose (0%, 5%, 10%, and 15%) was successfully connected in series, generating maximum values of 4.56 V, which is high compared to those reported with other substrates in the literature. This voltage was enough to turn on a red LED light, as shown in Figure 7. Utilizing fruit waste to generate energy can significantly reduce the amount of organic waste sent to landfills, supporting recycling efforts and the circular economy [55,56]. These cells can produce electricity cleanly without emitting polluting gases or generating toxic waste [57].

Additionally, fruit waste is abundant, inexpensive, and readily available, making it an attractive choice for small-scale energy generation, particularly in regions rich in fruit production [58]. Using vegetable waste in microbial fuel cells offers a sustainable solution for managing waste and generating energy. While there are still technical and economic challenges to overcome, implementing this technology has significant potential to support the circular economy, mitigate climate change, and enhance energy access, particularly in rural and agro-industrial communities. With investments in research, supportive policies, and education, microbial fuel cells can become a vital technology for a more sustainable future.

Table 1. Comparison of MFC performance using different substrates.

Substrate	Maximum Voltage (V)	Power Density (mW/cm2)	Current Density (A/cm2)	Internal Resistance (Ω)	Ref.
Asparagus waste with 15% sucrose	1.729	20.196	5.574	128.749	This study
Potato waste	1.12	N/A	12.45	N/A	[17]
Household waste	0.110	0.1047	21.84	117	[18]
Fruit waste (mixture)	0.710	0.53	12.59	88	[59]
Pineapple waste	0.99 ± 0.03	513.99 ± 6.54	6.123	865.845 ± 4.726	[60]
Tomato waste	0.568	0.720	6.762	200	[46]
Mango waste	0.845 ± 0.314	657.958 ± 21.114	4.484	205.056 ± 25	[25]
Banana waste	0.286	41.3	0.286	580.99	[36]

highlights key variations in the performance parameters of microbial fuel cells (MFCs) based on the substrate used. The highest maximum voltage was achieved with asparagus waste enriched with 15% sucrose, reaching 1.729 V and a power density of 20.196 mW/cm². Higher carbon content and available sugars enhance electricity production [36]. In contrast, domestic waste exhibited the lowest reported voltage (0.110 V), indicating lower efficiency than other substrates [18]. Asparagus waste with 15% sucrose also achieved the highest current density (5.574 A/cm²), reflecting a greater electron flow facilitated by the availability of a richer substrate [40]. The lowest internal resistance was recorded with asparagus waste enriched with 15% sucrose (128.749 Ω). Lower internal resistance indicates more efficient electron transfer, optimizing energy generation [36]. Fruit waste (mixture) and mango waste display absent or low values in several parameters, which may suggest inconsistencies in energy generation, possibly due to their less suitable chemical composition [59]. The data indicate that asparagus waste enriched with 15% sucrose is the most effective substrate among those reported, standing out in all key parameters such as voltage, current density, and power density. This validates the use of simple sugars like sucrose to enhance MFC performance.

4. Conclusions

The values obtained in the research show the influence of sucrose on the current values of single-chamber microbial fuel cells with asparagus waste over fourteen days. The obtained electric current and voltage values peaked on the eighth day, reaching 5.532 mA and 1.729 V in the MFC containing 15% sucrose, while the MFC containing 0% sucrose achieved 1.034 V and 3.525 mA on the same day. All MFCs showed slight increases in their pH values, remaining in the slightly acidic range. In contrast, the oxidation-reduction potential values increased from the first day, with the MFC-15% showing the highest value (3.525 mA) with an electrical conductivity of 294.027 mS/cm and a reduction in chemical oxygen demand of 83.14%, all observed on the eighth day. The internal resistance of the MFC-15% was 128.749 ± 12.541 Ω, and the MFC-0% was 187.457 ± 26.548 Ω, with an apparent decrease in internal resistance as sucrose concentration increased. The power density values observed were 14.874, 17.097, 18.133, and 20.196 mW/cm², corresponding to current densities of 3.924, 5.117, 5.009, and 5.574 A/cm² for the MFCs with 0%, 5%, 10%, and 15% sucrose, respectively. Finally, the microbial fuel cells were connected in series, generating peaks of 4.56 V, which is sufficient to light an LED bulb.

For future work, it is recommended that the use of microbes is explored to further enhance the efficiency of microbial fuel cells and to study other natural compounds to improve electron production. The anode electrodes should be studied by increasing their surface area per unit volume to maximize electron capture and electricity production in the cells. Although microbial fuel cells currently offer lower power density compared to other energy generation technologies, and the initial costs for deploying them at scale can be high, particularly if advanced materials or specialized maintenance are needed, they can be implemented in rural communities, providing access to renewable energy and effective waste management solutions. Additionally, they can promote environmental education by demonstrating how waste can be transformed into valuable resources.