The climate is changing remarkably over time and global temperatures are projected to increase by 1.5-5.9℃ in the 21st century. This phenomenon has affected the productivity of agriculture, especially the quality and quantity of crop yields. Zhao et al. (2017) stated that each degree Celsius increase in temperature would result in a decrease in global yields of wheat, rice, corn, and soybeans by 6.0%, 3.2%, 7.4%, and 3.1%, respectively. Climate change influences soil moisture levels by direct climatic effects, including extreme weather events, increased temperature, altered precipitation patterns, reduced water availability, and raised CO2 concentration. Kidron & Kronenfeld (2015) found that an increase in temperature of 1℃ contributes to the rising of soil water evaporation rate by 10%. Prolonged evaporation leads to depletion of water sources for plants, as well as desertification. Hatfield & Prueger (2015) also mentioned that warm temperatures accelerate wheat senescence, thereby reducing plant capability to produce grains. Moreover, an extreme change in the precipitation (rainfall) has aberrated the cropland qualities. Many developing countries had to expand 9% of their croplands due to both soil erosion and dry anomalies that are resulted from increased and decreased intensity of precipitation, respectively (Malhi et al., 2021).
Soil moisture plays a role in agricultural plant growth. The amount of soil water influences plant photosynthesis rates. A low soil water content results in increased leaf diffusion resistance due to the decrease of leaf water potential and increase in stomatal resistance (Chaves et al., 2009). Consequently, CO2 diffusion is hindered and the photosynthesis-regulating genes are downregulated. Furthermore, reduced CO2 uptake and photosynthesis by plants lead to excess light (EL) stress. EL induces the production of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), superoxide (O2-), and singlet oxygen (1O2), which cause degradation of chloroplast and hence are detrimental to plant growth (Li et al., 2009). Chadha et al. (2019) demonstrated that the accumulation of total soluble sugar, phenolic, and chlorophyll content due to drought had stimulated the osmotic stress in plants. Moreover, plants grew to the tallest height (115.14 ± 11.64 cm) in the soil with a water holding capacity (WHC) of 75%. Meanwhile, the shoot height of plants was reduced by ~20% when grown in the soil with a WHC of 25% after 63 days of treatment. This confirms that close monitoring of soil moisture and scheduling of irrigation in agriculture is feasible.
On the other hand, animal manure in the agricultural sector has been reported to be a significant source of greenhouse gases (GHGs), exacerbating climate change. The total annual GHG emissions in the livestock sector reach 7.1 Gigatonnes of CO2 equivalent and animal manure accounts for 65% of that amount (FAO, 2021). The global animal manure from cattle, poultry, and pig livestock counts 120 million tons each year and is also projected to increase to 9-10 billion tons by 2050 (Loyon, 2018; Tadesse et al., 2020). A study in a country in Romania showed that sheep and cattle manure per year respectively produced 61.3 t and 47.2 t CO2. In addition, untreated manure is capable of producing methane gas (CH4), which has a warming power of 28-34 times that of CO2. The same study calculated the CH4 emissions of sheep and cattle manure at 0.83 t and 0.185 t per year, respectively (Vac et al., 2013). Climate change is also caused by an increase in N2O emissions of 17.7% by animal manure (Shakoor et al., 2021). These harmful gases are produced due to the abundant content of organic matter and nitrogen in animal manure (Park et al., 2019).
Based on the previously mentioned agriculture-related issues, an innovative solution should be addressed to manage both soil moisture and animal manure. Microbial fuel cells (MFCs) have attracted the attention of many researchers because of their great potential in providing solutions to energy and environmental problems. MFC is a technology that exploits exoelectrogenic bacteria as catalysts to oxidize organic and inorganic matters to generate electrical energy (Logan et al., 2006). Animal manure processing with MFC reduces the accumulation of organic matter on the land while producing electricity. This essay specifically describes the potential of MFC to reduce livestock carbon emissions and power soil moisture sensors with the generated bioelectricity.
MFC is generally assembled into two chambers, the anode and cathode chambers, that are separated by a proton exchange membrane. In the anode chamber, bacteria anaerobically oxidize various types of organic substrate to produce electrons and protons. Electrons are transferred to the cathode chamber via an external circuit while protons are transported to the cathode chamber via PEM. Meanwhile, in the cathode chamber, protons and electrons are reduced by an oxygen supply to produce water (Ucar et al., 2017). The flow of protons and electrons through these chambers will directly generate electricity. Cao et al. (2019) mentioned various types of bacteria that can be exploited in the MFC, including Archaebacteria (e.g., Haloferax volcanii and Natrialba magadii), Acidobacteria (e.g., Geothrix fermentans), Firmicutes (e.g., Clostridium butyricum and Thermincola sp.), and Proteobacteria (e.g., Acidiphilium cryptum, Rhodoferax ferrireducens, Escherichia coli, Shewanella sp., and Pseudomonas sp.).
Animal manure is a rich source of organic matter and nutrients. The amount of organic matter is usually represented by chemical oxygen demand (COD) and biochemical oxygen demand (BOD). Suresh & Choi (2011) reported that a total of 57,679 mg/L COD and 20,563 mg/L BOD were contained in pig manure from 41 commercial pig farms in South Korea. A total of 4,564 mg/L total nitrogen (TN) was also detected in the manure. Other nutrients found in animal manure include ammoniacal nitrogen, phosphorus, sodium, calcium, and magnesium, which respectively counted 2,875 mg/L, 2,765 mg/L, 493 mg/L, 1,104 mg/L, and 564 mg/L. Meanwhile, 5 grams of poultry and cattle manure each contain 37 mg/L and 1.20 mg/L dissolved organic carbon (DOC). It was also found that a total of 11.8 mg/L and 0.63 mg/L total dissolved nitrogen content were available in the poultry and cattle manure, respectively (Park et al., 2019). The abundance of both the organic and inorganic components of animal manure makes it a suitable feedstock for MFC.
MFC is capable of converting the previously mentioned organic components in animal manure into electrical energy by consuming their carbons. Kim et al., (2008) succeeded in generating electricity with a maximum power density of 228 mW/m2 (~0.4 V) when treating swine manure with MFC for 260 hours. Approximately 84% of organic compounds and >99% of odorous chemicals were removed from the manure. Research involving 1000 mL of chicken manure for MFC feed produced electricity with a maximum power of 278 mW/m2 and a current density of 683 mA/m2 with an effective BOD and COD removal efficiency of 82% and 82%, respectively (Jaeel & Farhan, 2015). Yokoyama et al. (2006) utilized cow manure to generate 14.8 mV (0.34 mW/m2) electricity per 1 gram of COD/L slurry. They also obtained an 84% of BOD removal efficiency and recovered essential fertilizer nutrients, such as nitrogen (84%), phosphorus (70%), and potassium (91%). A scaled-up MFC fed with 100 L swine wastewater was able to discharge 530 mg/L COD per day while also producing an average and maximum power of 0.6 Wh/m3 and 2.2 Wh/m3, respectively (Goto & Yoshida, 2019).
The utilization of MFC hinders the accumulation of animal manure on the land, which subsequently reduces GHG emissions to the atmosphere. Wang et al. (2019) recorded that the MFC was able to deplete the emissions of CO2, CH4, and N2O by 9.7–15.6%, 17.9-36.9%, and 7.2-38.7%, respectively. Altogether, the application of MFC to convert organic matter into electricity significantly reduced the total GHG emissions by 5.9–32.4% CO2 equivalents. The significance of GHG depletion is shown by the decrease in COD, where higher COD removal efficiency indicates more GHG emission reduction to the environment. MFC is also a powerful technology to discharge N2O emissions by animal manure. Samrat et al. (2018) cultivated seawater bacterial consortium into both the anode and cathode chambers of MFC to perform denitrification of nitrogen-rich synthetic water. They obtained a 99.8% efficiency of NO2– and NO3– conversion into N2 gases. Wang et al. (2019) also demonstrated the significance of MFC in N2O reduction by removing 84.3–91.4% of the total nitrogen (TN). The reduction of nitrogenous compounds into their gas forms, which can be utilized in biofuel production, is facilitated by the electrons in the cathode chamber. The suppression of GHG emissions will hinder problems with soil moisture content due to climate change.
MFC does not only advantage the environment but also the electrical energy sector. Up to date, the electricity generated from MFC is directly applied to several low-power devices, such as lights and digital clocks (Rahimnejad et al., 2015). Besides, the electricity can also be harvested and stored into a capacitor for further use. In the laboratory setting, Dewan et al. (2010) reported that the time needed to charge a 3-F capacitor from 0-500 mV was 108 minutes with an optimum charging potential of 300 mV. On the other hand, the charging time is significantly reduced when implemented to MFC fed with organic matter-rich sediment. The time required for charging a 3-F capacitor from 0-500 mV was only 5 minutes and its optimal charging potential was 500 mV. This corroborates that a large amount of organic matter that is contained in animal manure provides a great potential for electricity production with MFC.
The electricity generated by the MFC and stored in a capacitor can be used to power a wireless soil moisture sensor. Currently, wireless sensors are being developed to ease the workload of farmers in monitoring volumetric soil moisture content. Soil moisture is a very important indicator to determine irrigation intensity and, in turn, affects crop yield efficiency (Yu et al., 2021). Technological advances have also resulted in the internet of things (IoT)-based soil moisture sensors, such as Arduino Uno, NodeMCU & ESP32, which can provide signals that command automatic irrigation systems (Nahian et al., 2021). Various electrical power inputs are required to operate the sensor. For instance, the operating voltage of Arduino Uno sensors is 5 volts and its optimal electrical supply ranges from 7-12 volts. Meanwhile, both NodeMCU and ESP32 require a power supply and operating voltage of 600 mV and 3.3-5 volts, respectively. Dewan et al. (2010) successfully demonstrated the application of electricity to power wireless sensors and they reported that the storage of electrical energy in capacitors can be used to accomplish the needs of other high-power devices.
In conclusion, processing animal manure with MFC provides multiple benefits in the agricultural sector. The utilization of organic matter and nutrients in animal manure as feedstock for MFC results in reduced GHG emissions and bioelectricity production. The high efficiency of BOD, COD, and nitrogen removal from the manure hinders CO2, CH4, and N2O gases to be released into the atmosphere. Therefore, the influence of climate on soil moisture can be inhibited, and successively the quality and quantity of crop yields can be maintained. The electricity harvested from the MFC can be accumulated in capacitors and used to power current wireless soil moisture sensors, which helps in proper irrigation scheduling. The advantages offered indicate that MFC-based animal manure treatment is feasible to be developed to overcome the agricultural problems mentioned earlier in this essay. Further research and improvement of MFC technology to achieve maximum carbon removal and electricity production by considering its construction, electrode material, bacterial community, and operating conditions should be continued.
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