Evaluation of Pig Dung for Electricity Generation Using Microbial Fuel Cell

Pig dung was evaluated for electric current and voltage generation using microbial fuel cell (MFC). Pig dung was collected from three different animal farms; FUTA, Air Force and Apatapiti Extension. Isolation and identification of microorganisms from pig dung was carried out before and after electric current and voltage generation using conventional techniques. Physicochemical composition were determined using standard methods. Microbial fuel cells (MFCs) chambers were fabricated. The circuit was completed with electrodes and flexible wires for electron transfer from the anode to the cathode. Pig dung was used as the anolyte while water was used in the cathode as the electron acceptor. Current and voltage were measured in the morning, afternoon and evening for 40 days using the digital multimeter. The result revealed sixteen microorganisms: Enterobacter cloacae, Escherichia coli, Shigella sp, Citrobacter gillenii, Klebsiella singaporensis, Paenibacillus septentrionalis, Bacillus circulans, Salmonella spp, Enterobacter asburiae, Yersinia intermedia, Yersinia enterocolitica, Fusarium sp, Aspergillus flavus, Aspergillus funmigatus, Aspergillus niger, and Penicillium chrysogenum. The highest bacterial and fungal population; 2.71 x 10 cfu/g and 1.47 x 10 sfu/g were observed from Air Force and FUTA pig dung respectively before current and voltage generation. The highest bacterial and fungal population; 1.35 x 10 cfu/g and 1.60 x 10 sfu/g were observed from Apatapiti Extension and FUTA pig dung respectively after current and voltage generation. The highest current and voltage; 0.319 ± 0.00 mA and 572.333 ± 3.84 mV were generated from Apatapiti Extension and Air-Force pig dung. Sterilized pig dung (control) generated a low voltage and current affirms the important role of microorganisms in voltage and current generation. In conclusion, pig dung can be used to generate electrical current and voltage owing to microbial activities present in the pig dung. A nuisance causing waste such as pig dung can serve as a renewable source of energy for electricity generation, this will simultaneously help to resolve the problems of environmental toxics that oozes from its disposal and ultimately serve as a way of mitigating global warming in the world.


Introduction
Energy is a central factor in the advancement and development of the socioeconomic sector of any nation and uninterrupted power supply is a crucial issue for all countries today (Oyedepo, 2012). Globally, the use of fossil fuel (coal, diesel fuel, gasoline, oil, and natural gas) for electricity supply, heating, transportation, and industry has recently increased with the resultant effect as global energy crisis (Franks and Nevin, 2010). New methods of electricity generation from carbon-free renewable energy sources is an important research area for alternative power systems (Fan and Xue, 2016). Studies conducted on a wide range of energy solutions revealed that no solution appears to replace fossil fuels in totality. Owing to this, various alternatives needs to be sourced for in mitigating this major challenge knowing that reliance on fossil fuels is unsustainable due to pollution and finite supplies (Franks and Nevin, 2010). Microbial fuel cell (MFC) has emerged as a technology capable of meeting the dual goals of waste management and energy production in a sustainable way thereby resolving environmental and energy crisis (Fan and Xue, 2016). MFC is drawing wide attention due to its projected use to recover energy in the form of electricity (Cao et al., 2019). The need for more animal protein to sustain an exponentially increasing population has led to a noticeable increase in livestock production over the years with various scales of animal farms been set up. This has invariably resulted to the colossal increase in animal dung consequently posing waste management as a challenge to be resolved (Iregbu et al., 2014). The use of pig dung in a microbial fuel cell as an additional alternative for electricity generation will advantageously lead to a drastic reduction in its volume in the environment, minimize the problems associated with its disposal such as environmental pollution, and create wealth and employment (Onuorah et al., 2017). Present global energy crises are the results of growing demand for fossil fuels (particularly oil, coal and gas) resulting into harmful environmental effect and inadequate electricity supply. There is therefore a need for evaluating newer sources of energy in meeting this growing demand knowing that future economic growth crucially hinge on this and new methods of electricity generation from renewable resources without a net Green House Gases (GHG) emission are much desired (Cao et. al., 2019).

Making the Salt Bridge (Proton Exchange Membrane):
NaCl was used as an electrolyte for proton exchange in the salt bridge according to the work of (Liu et al., 2005;Kumar et al., 2012). Salt bridge was repared by dissolving 20 g of agar-agar powder into 1000 ml of distilled water containing 75.5 g NaCl which was boiled for about 3 minutes (Akujobi et al., 2017). While the solution was still warm, it was poured into the PVC pipes covered at one end with sterile nylon firmly held by rubber band until it was solidified. Filling the Anode Chamber of the MFC The constructed MFCs chambers were sterilized with ethanol (85%) and irradiated with UV light for 15 minutes in an inoculating chamber to eliminate possible cross-contamination in the cause of its construction. The anode chamber was aseptically filled with 50% of the substrate which implies 600 g of pig dung with 600 ml of sterilized distilled water (Kumar et al., 2012). One thousand two hundred (1200) ml of H2O was used as electron acceptor in the cathode chamber.

Testing the Microbial Fuel Cell:
The anode chambers were tightly closed and sealed so as to maintain an anaerobic condition throughout the experiment while the cathode chamber was left open for aerobic oxidation. The external circuits connection were completed with the 1.5mm flexible copper wires of length 0.4m fixed through the drilled holes on the cover lid into each chamber and sealed with epoxy resin to avoid corrosion of copper wire (Kim et al., 2002;Zou et al., 2007). The bare end of the copper wires was coiled around the electrodes in the respective chambers to facilitate the passage of electrons from the microbial breakdown of the substrate through the circuit. Voltage and current across each cell were recorded with the digital multimeter (SUOER SD 9205A). The observation was determined for 40 days at 6 hours interval per day, starting from 8 am to 8 pm classified as morning, afternoon and evening session.

Making and Testing of the Control MFC:
Six hundred gram of pig dung was weighed and sterilized at 121°C for 15 minutes using the autoclave so as to inhibit all the microorganism without denaturing its mineral and nutritional components. The pig dung was aseptically transferred into the anode container and tightly covered to prevent the penetration of oxygen 600 ml of sterile water was used to dissolve the pig dung. The cathode was filled with distilled water. The same mode of MFC construction as above was used to construct the control MFC.

Statistical Analysis
All numerical data obtained were subjected to Analysis of Variance (ANOVA) and Duncan's New Multiple Range Test using Statistical Packages for the Social Sciences (SPSS) 22.0 version. Data are presented as mean ± standard error (SE). For all tests, the significance was determined at the level of at P≤0.05.

Total Aerobic Bacterial Load of Pig Dung (before and after electric current and voltage generation)
Bacterial load under aerobic condition is represented in Table 1 indicated a significant difference (p≤0.05) in total viable bacterial counts of the three pig dung samples. Initial isolation of pig dung showed a range of 1.73 x 10 5 cfu/g to 2.71 x 10 5 cfu/g, there was no significant (p≤0.05) difference between Air Force and Apatapiti Extension pig dung but both were significantly (p≤0.05) higher than FUTA pig dung. The bacterial load of pig dung from MFC after current and voltage generation showed a range of 6.0 x 10 4 cfu/g to 1.13 x 10 5 cfu/g with FUTA pig dung having the highest load significantly (p≤0.05). All the samples recorded the overall highest bacterial load from initial isolation.

Total Aerobic Fungal Load of Pig Dung (before and after electric current and voltage generation)
Fungal load under the aerobic condition is represented in Table 2 indicated a significant difference (p≤0.05) in total viable fungal counts of the three pig dung samples. The fungal load from the initial isolation showed a range of 4.00 x 10 3 cfu/g to 2.93 x 10 4 cfu/g, there was no significant (p≤0.05) difference among the pig dung samples. The fungal load of the pig dung from MFC after current and voltage generation showed a range of 0.00 X 10 3 cfu/g to 1.20 X 10 4 cfu/g with Apatapiti Extension pig dung having the highest load significantly (p≤0.05).

Total Anaerobic Bacterial and Fungal Load of Pig Dung (after electric current and voltage generation)
Bacterial and fungal load of the pig dung after current and voltage generation incubated under the anaerobic condition shown in Table 3 indicated there was significant difference (p≤0.05) in total viable bacterial and fungal counts across the samples. The anaerobic bacterial load of the pig dung from the MFCs showed a range of 1.50 x 10 4 cfu/g to 4.00 x 10 4 cfu/g. FUTA pig dung had significantly (p≤0.05) the highest anaerobic bacteria load while Air Force pig dung had the lowest from all the MFCs. The anaerobic fungal load from the MFCs showed a range of 0.00 X 10 3 sfu/g to 1.20 X 10 4 sfu/g with Apatapiti Extension pig dung having significantly (p≤0.05) the highest anaerobic fungal load among all the MFCs while FUTA pig dung had no growth. Total Coliform Load of Pig Dung (before and after electric current and voltage generation) Coliform load of the pig dung before and after current and voltage generation shown in Table 4 indicated there was a significant difference (p≤0.05) in total coliform counts across the samples. The coliform count of the pig dung from initial isolation showed a range of 3.97 x 10 4 cfu/g to 5.97 x 10 4 cfu/g with no significant (p≤0.05) difference among the pig dung samples. The total coliform count from the MFC after current and voltage generation showed a range of 4.67 x 10 3 cfu/g to 8.67 x 10 3 cfu/g with no significant (p≤0.05) difference among pig dung samples. All samples have the highest bacteria load from initial isolation. Table 1: Total aerobic bacteria load of pig dung before and after current and voltage generation PIG DUNG Initial (Cfu/g) Final (Cfu/g) FUTA 1.73 x 10 5 ± 25.44 a 1.13 x 10 5 ± 2.89 c Air Force 2.71 x 10 5 ± 25.10 b 6.00 x 10 4 ± 3.46 a Apatapiti Extension 2.67 x 10 5 ± 7.06 b 9.20 x 10 4 ± 1.15 b Values are means ± Standard error of pig dung. Values in the same column carrying the same superscript are not significantly different at (p≤0.05) using Duncan's New Multiple Range Test Table 2: Total aerobic fungal load of pig dung before and after current and voltage generation Pig Dung Initial (Sfu/g x 10 3 ) Final (Sfu/g x 10 3 ) FUTA 1.47 x 10 4 ± 0.88 a 0.00 x 10 3 ± 0.00 a Air Force 2.93 x 10 4 ± 13.78 a 9.00 x 10 3 ± 0.58 b Apatapiti Extension 4.00 x 10 3 ± 2.00 a 1.20 x 10 4 ± 1.15 c Values are means ± Standard error of pig dung. Values in the same column carrying the same superscript are not significantly different at (p≤0.05) using Duncan's New Multiple Range Test. KEY: Sfu/g-Spore forming unit per gram. .00 x 10 4 ± 2.89 c 0.00 x 10 3 ± 0.00 a Air Force 1.50 x 10 4 ± 3.46 a 9.00 x 10 3 ± 0.58 b Apatapiti Extension 2.20 x 10 4 ± 1.15 b 1.20 x 10 4 ± 1.15 c Values are means ± Standard error of pig dung. Values in the same column carrying the same superscript are not significantly different at (p≤0.05) using Duncan's New Multiple Range Test. KEY: Cfu/g-Colony-forming unit per gram. Sfu/g-Spore forming unit per gram. Table 4: Total coliform load of pig dung before and after current and voltage generation Pig Dung Initial (Cfu/g x 10 3 ) Final (Cfu/g x 10 3 ) FUTA 5.97 x 10 4 ± 3.67 a 8.67 x 10 3 ± 1.67 a Air Force 3.97 x 10 4 ± 9.67 a 5.67 x 10 3 ± 1.33 a Apatapiti Extension 5.93 x 10 4 ± 3.33 a 4.67 x 10 3 ± 2.33 a Values are means ± Standard error of pig dung. Values in the same column carrying the same superscript are not significantly different at (p≤0.05) using Duncan's New Multiple Range Test. KEY: Cfu/g-Colony-forming unit per gram.

Biochemical characterization of bacterial isolates (before and after electric current and voltage generation)
The biochemical characterization of the bacteria isolated before and after current and voltage generation is represented in Tables 9 and 10. The result revealed seven bacterial isolates were identified before current and voltage generation, these include; Enterobacter cloacae, Escherichia coli, Shigella sp, Citrobacter gillenii, Bacillus circulans, Salmonella spp and Yersinia intermedia while five were identified after current and voltage generation, these include; Klebsiella singaporensis, Paenibacillus septentrionalis, Bacillus circulans, Enterobacter asburiae, and Yersinia enterocolitica.

Microscopic characterization of fungal isolates (before and after electric current and voltage generation)
The microscopic characterization of fungal isolates is represented in     It is characterized by thallus which varies in colour, hyaline septate hyphae, produces both macro-and microconidia from slender phialides. Microconidia are hyaline, two to several-celled, fusiform to sickle-shaped, mostly with an elongated apical cell and pedicellate basal cell. Microconidia are one or two-celled, hyaline, smaller than macroconidia, fusiform to ovoid.
Fusarium sp 2 Conidiophores stipes are hyaline and coarsely roughened, often more noticeable near the vesicle, conidial heads globose to subglobose, pale green and conspicuously echinulate.

Aspergillus flavus
3 Thin, spreading colonies velvety and grass-green to bluish, green with pale to bright yellow sometimes vinaceous reverse and yellow exudate conidiophores two-to three-stage branched conidia smooth-walled odour often fruity conidia are tri-nucleate.

Penicillium chrysogenum 4
Colonies are typically black, conodiophores arising from long, broad thick-walled, sosmetimes branched foot cells. It has tall conidiophores and conidia are large with radiating heads.

5
Conidiophores hyaline or slightly coloured, short green, particularly in the upper part, smooth-walled, conidial heads radiate to nearly globose.

Physicochemical Composition of Pig Dung
Physicochemical composition of pig dung from different sites are represented in 13 ± 0.67) µS/cm significantly (p≤0.05). The total organic matter ranged from 1.98 % to 3.19 %. Apatapiti Extension pig dung had the lowest value of (1.98 ± 0.01) % while Air Force pig dung had the highest value of (3.19 ± 0.01) % significantly (p≤0.05).The total nitrogen ranged from 0.5800 % to 1.0300 %. FUTA pig dung had the lowest value of (0.58 ± 0.01) % while Apatapiti Extension pig dung had the highest value of (1.03 ± 0.01) % significantly (p≤0.05). The sodium composition ranged from 17.40 mg/kg to 20.10 mg/kg with no significant (p≤0.05) difference between FUTA and Apatapiti Extension pig dung having values (18.50 ± 0.36) mg/kg and (17.40 ± 0.26) mg/kg respectively while Air Force pig dung had the highest value of (20.10 ± 0.32) mg/kg significantly (p≤0.05). The potassium composition ranged from 35.90 mg/kg to 42.30 mg/kg. Apatapiti Extension pig dung had the lowest value of (35.90 ± 0.31) mg/kg while Air Force pig dung had the highest value of (42.30 ± 0.12) mg/kg significantly (p≤0.05). The calcium composition ranged from 7.93 mg/kg to 13.00 mg/kg. Air Force pig dung had the lowest value of (7.93 ± 0.09) mg/kg while Apatapiti Extension pig dung had the highest value of (13.00 ± 0.42) mg/kg significantly (p≤0.05). The magnesium composition ranged from 4.27 mg/kg to 4.89 mg/kg. FUTA pig dung had the lowest value of (4.27 ± 0.01) mg/kg while Air Force pig dung had the highest value of (4.89 ± 0.03) mg/kg significantly (p≤0.05). The copper composition ranged from 0.14 mg/kg to 0.33 mg/kg with no significant (p≤0.05) difference between the copper composition of Air Force and Apatapiti Extension pig dung having values (0.17 ± 0.01) mg/kg and (0.14 ± 0.01) mg/kg respectively while FUTA pig dung had a higher value of (0.33 ± 0.01) mg/kg significantly (p≤0.05). The Zinc composition ranged from 0.43 mg/kg to 0.63 mg/kg. Air Force pig dung had the lowest value of (0.43 ± 0.01) mg/kg while FUTA pig dung had the highest value of (0.63 ± 0.01) mg/kg significantly (p≤0.05).  The voltage generated from FUTA pig dung is represented in Figure 1. There was a significant difference (p≤0.05) in the voltage generated across the three sessions of the day. FUTA pig dung (morning session) generated significantly (p≤0.05) its lowest voltage on day 3 with the value of 43.0000 ± 0.57735 mV while its highest voltage was generated on days 17 and 29 with values 500.0000 ± 0.57735 mV and 496.0000 ± 0.57735 mV respectively. FUTA pig dung (afternoon session) generated significantly (p≤0.05) its lowest voltage on day 3 with the value of 87.0000 ± 0.57735 mV while its highest voltage was generated on days 17 and 29 with values 501.0000 ± 0.57735 mV and 497.6667 ± 0.88192 mV respectively. FUTA pig dung (evening session) generated significantly (p≤0.05) its lowest voltage on day 1 with the value of 21.6667 ± 0.57735 mV while its highest voltage was generated on day 17 with the value of 497.6667 ± 0.57735 mV. Current Generated from FUTA Pig Dung The current generated from FUTA pig dung is represented in Figure 2. There was a significant difference (p≤0.05) in the current generated across the three sessions of the day. FUTA pig dung (morning session) generated significantly (p≤0.05) its lowest current on days 2 and 4 with the value of 0.0163 ± 0.00088 mA and 0.0177 ± 0.00088 mA respectively while its highest current was generated on days 26 and 28 with values 0.1817 ± 0.00088 mA and 0.1813 ± 0.00578 mA respectively. FUTA pig dung (afternoon session) generated significantly (p≤0.05) its lowest current on days 3 and 6 with values 0.0157 ± 0.00285 mA and 0.0173 ± 0.00120 mA respectively while its highest current was generated on days 26 and 28 with values 0.1880 ± 0.00058 mA and 0.1877 ± 0.00120 mA respectively. FUTA pig dung (evening session) generated significantly (p≤0.05) its lowest current on days 7 and 8 with values of 0.0220 ± 0.00058 mA and 0.0220 ± 0.00058 mA respectively while its highest current was generated on day 9 with the value of 0.3130 ± 0.00153 mA.

Voltage Generated from Apatapiti Extension Pig Dung
The voltage generated from Apatapiti Extension pig dung is represented in Figure 3. There was a significant difference (p≤0.05) in the voltage generated across the three sessions of the day. Apatapiti Extension pig dung (morning session) generated significantly (p≤0.05) its lowest voltage on day 9 with the value of 182.0000 ± 1.15470 mV while its highest voltage was generated on day 17 with a value of 523.0000 ± 0.57735 mV. Apatapiti Extension pig dung (afternoon session) generated significantly (p≤0.05) its lowest voltage on day 9 with the value of 185.6667 ± 1.45297 mV while its highest voltage was generated on day 17 with a value of 527.3333 ± 0.57735 mV. Apatapiti Extension pig dung (evening session) generated significantly (p≤0.05) its lowest voltage on day 4 with the value of 114.3333 ± 2.33333 mV while its highest voltage was generated on day 17 with the value of 522.3333 ± 1.20185 mV.

Current Generated from Apatapiti Extension Pig Dung
The current generated from Apatapiti Extension pig dung is represented in Figure 4. There was a significant difference (p≤0.05) in the voltage generated across the three sessions of the day. Apatapiti Extension pig dung (morning session) generated significantly (p≤0.05) its lowest current on day 6 with a value of 0.0520 ± 0.001155 mA while its highest current was generated on day 26 with a value of 0.3190 ± 0.00058 mA. Apatapiti Extension pig dung (afternoon session) generated significantly (p≤0.05) its lowest current on days 2 and 3 with the value of 0.0570 ± 0.000577 mA and 0.0607 ± 0.000882 while its highest current generation was on day 26 with a value of Journal of Energy Technologies and Policy www.iiste.org ISSN 2224-3232 (Paper) ISSN 2225-0573 (Online) Vol.10, No.1, 2020 0.3123 ± 0.006173 mA. Apatapiti Extension pig dung (evening session) generated significantly (p≤0.05) its lowest current on day 2 with a value of 0.0530 ± 0.001528 mA while its highest current was generated on day 26 with the value of 0.3140 ± 0.00058 mA.

Voltage Generated from Air Force Pig Dung
The voltage generated from Air Force pig dung is represented in Figure 5. There was a significant difference (p≤0.05) in the voltage generated across the three sessions of the day. Air Force pig dung (morning session) generated significantly (p≤0.05) its lowest voltage on day 9 with the value of 111.0000 ± 0.57735 mV while its highest voltage was generated on day 13 with a value of 522.0000 ± 0.57735 mV. Air Force pig dung (afternoon session) generated significantly (p≤0.05) its lowest voltage on day 1 with the value of 151.0000 ± 0.57735 mV while its highest voltage was generated on day 12 with a value of 572.3333 ± 3.84418 mV. Air Force pig dung (evening session) generated significantly (p≤0.05) its lowest voltage on day 4 with the value of 161.0000 ± 0.57735 mV while its highest voltage was generated on days 12 and 13 with the value of 524.0000 ± 0.57735 mV and 524.6667 ± 0.3333 mV respectively. Current Generated from Air Force Pig Dung Current generated from Air Force pig dung is represented in Figure 6. The result showed that there was a significant difference (p≤0.05) in the current generated across the three sessions of the day. Air Force pig dung (morning session) generated significantly (p≤0.05) its lowest current on days 2 and 6 with values 0.04200 ± 0.001155 mA and 0.04267 ± 0.00176 mA respectively while its highest current was generated on day 9 with a value of 0.2760 ± 0.001155 mA. Air Force pig dung (afternoon session) generated significantly (p≤0.05) its lowest current on day 2 with a value of 0.03400 ± 0.00115 mA while its highest current was generated on day 8 with a value of 0.14233 ± 0.00145 mA. Air Force pig dung (evening session) generated significantly (p≤0.05) its lowest current on day 2 with a value of 0.03067 ± 0.00033 mA while its highest current was generated on day 9 with the value of 0.25167 ± 0.00202 mA. Voltage Generated from Control MFC The voltage generated from control MFC with KMnO4 and H2O as electron acceptor are shown in Figures 7 respectively. The result showed that there was a significant difference in the (p≤0.05) in the voltage generated daily, the highest voltage was generated on day 1 for both MFC with KMnO4 and H2O as electron acceptor with values 379.00 ± 0.00 mV and 183.33 ± 0.00 mV respectively after which there was a steady decrease to the minimum values on days 8 and 6 respectively.

Current Generated from Control MFC
The current generated from control MFC with KMnO4 and H2O as electron acceptor are shown in Figures 8 respectively. The result showed that there was a significant difference in the (p≤0.05) in the voltage generated daily, the highest current was generated on day 1 for both MFC with KMnO4 and H2O as electron acceptor with values 0.030 ± mA and 0.211 ± 0.00 mA respectively after which there was a steady decrease to the minimum values on days 8 and 6 respectively. Journal of Energy Technologies and Policy www.iiste.org ISSN 2224-3232 (Paper) ISSN 2225-0573 (Online) Vol.10, No.1, 2020

Discussion
In this study, the potential of pig dung from FUTA, Air Force and Apatapiti Extension to produce current and voltage using a microbial fuel cell was evaluated. The microbial population obtained in this study under aerobic and anaerobic condition showed that pig dung is a natural host for bacteria and fungi, this agrees with the report of (Onuorah et al., 2017). The authors studied the biodeterioration potentials of microorganisms isolated from pig manure and reported the presence of diverse bacteria and fungi which will be useful in remediating oil-polluted environments when applied singly and as consortia. Pig dung from Air Force had the highest bacteria load; this was probably due to its significantly highest organic matter content than the other two samples. Bacteria counts were much higher than the fungi count in all the pig dung samples, the high bacteria load obtained agrees with the report of Onuorah et al. (2017). The authors reported that pig manure contains a teaming population of heterotrophic bacteria. Coliform organisms were present in all the pig dung samples at a population intermediate to bacteria and fungi counts, this is because coliform bacteria are key faecal indicator microorganisms (Lim et al., 2018). The level of coliform present implies that the use of pig dung for electricity may pose human and animal health hazard if not properly managed (Trawinska et al., 2015).
The results of the final isolation from the anodic chambers of the MFCs after 40 days of current and voltage generation showed that all the microbial fuel cells maintained a moderate population of bacteria, fungi and coliforms throughout the experiment, this implies that these microorganisms are indeed responsible for the continuous generation of the electric voltage and current ditto to their presence before and after current and voltage generation; this agrees with the findings of (Kumar et al., 2012). The authors studied electricity generation from cow dung using microbial fuel cell and established that the bacteria present in it could engage the anode in their metabolism, resulting in bioelectricity generation. Microorganisms present before and after the electric energy period was an indicator of their decomposing ability and ability to tolerate little or no oxygen (i.e. live in an anaerobic condition) which conferred on them the surviving ability during the period, this conforms to the findings of (Lim et al., 2018). The authors stated that the various microorganisms that are present in pig slurry play a vital role in the decomposition of manure, these organisms live in a partially anaerobic environment and therefore utilize a variety of potential substrates, including proteins and non-starch polysaccharides. Microorganisms interact with the pig dung in the MFC as their source of nutrients and energy for growth which effectuated the microbial decomposition of the pig dung and simultaneous release of electrons, protons and CO2 under anaerobic oxidation; this is the mechanism of operation in a typical microbial fuel cell and corroborates with the description of (Sharma and Bulchandani, 2012). The authors reported that microbial fuel cell is a device that converts chemical energy from the bio-convertible organic substrate, directly into electrical energy through the metabolic activities of microorganisms. However, in this study, higher microbial load was recorded for cultivation under aerobic condition compared to cultivation under anaerobic condition throughout the study, this observation is in agreement with that of (Pa´dua Ferreira et al., 2011). The authors reported higher bacterial counts observed in aerobic condition in comparison with anaerobic conditions.
There was a relative decline in the bacterial and coliform load of all MFCs after current and voltage generation under both aerobic and anaerobic condition in comparison to the initial load obtained from all the pig dung samples. This was possible because it was expected that after 40 days of electric current and voltage generation under anaerobic condition, nutrients becomes less available, waste products released from the microorganisms increase, secondary metabolites are released, and the number of dying cells continues to rise; this is a typical description of the death phase of microbial growth curve. In the death phase, the number of living cells decreases exponentially and population growth experiences a sharp decline, adaptation and succession takes place; as dying cells lyse or break open, they spill their contents into the environment making these nutrients available to other bacteria. This helps spore-producing bacteria to survive long enough for spore production. Spores are able to survive the harsh conditions of the death phase and become growing bacteria when placed in an environment that supports life (Nina et al., 2017). This was not the same for the fungi population in the MFCs of the three pig dung samples after current and voltage generation, there was an increase in fungi population in the Apatapiti MFC, this can be attributed to the ability of spores to survive longer in harsh condition according to Nina et al. (2017), no growth was recorded in FUTA MFCs, and there was neither increase nor decrease in the Air Force MFCs compared to the initial fungal load from the pig dung (Paul et al., 2019).
There was a decline in the population of the microbial community between the initial and final isolation. This is possible as a result of microbial adaptation, succession and evolution which are likely to have taken place in the anode chamber of the MFC within the 40 days period of current and voltage generation, this naturally occurs in response to environmental changes dictating the survival, elimination or evolution of the members in the microbial community (Zegeye et al., 2019). All the identified bacteria at the phylum level can be classified into three which include the phylum Firmicute, the phylum Proteobacteria and the phylum Actinobacteria. Bacillus sp belonging to the phylum Firmicutes was the dominating organism in both the initial and final isolation; this agrees with the report of Lim et al. (2018), they studied the effects of temperature and storage time on the evolution of bacterial communities in pig manure, and identified the bacterial community on phylum basis to consist of the phylum Firmicute, the phylum Proteobacteria and the phylum Actinobacteria and as well reported that Firmicutes was the dominant phylum in all samples (ranging from 89.3% to 98.8% of the total isolate identified).
These microorganisms are facultative anaerobes which can grow under aerobic and anaerobic condition because there was growth from all the microbial fuel cells when cultivated under both aerobic and anaerobic condition; this agrees with the work of Lim et al. (2018) in their evaluation of pig slurry, they stated that microorganisms live in a partially anaerobic environment and therefore utilize a variety of potential substrates, including proteins and non-starch polysaccharides. Stieglmeier et al. (2009) also reported about facultative anaerobes when isolated on anaerobic and aerobic media are capable of growing under both conditions. The pH of Apatapiti Extension pig dung, Air Force pig dung and FUTA pig dung were within the range of slightly neutral to slight alkalinity (7.02 to 7.83). The result obtained is in consonance with the report by Marszałek et al. (2014), who stated that the reaction of typical pig slurry is usually slightly alkaline, they compared the pH of pig slurry from about seven countries which ranged from 7.05 to 8.11. The mineral composition of the pig dung showed variation across the different pig dung samples. The presence of sodium, potassium, calcium, zinc, copper, magnesium and total nitrogen in the pig dung is similar to the findings of Marszałek et al. (2014) and Okoli et al. (2019), they identified the presence of macro-minerals such as potassium, calcium, sodium, nitrogen, magnesium; trace minerals such as copper and zinc in pig dung. The total nitrogen composition in the pig dung was lower than the value recorded by Okoli et al. (2019) but higher than the value reported by Marszałek et al. (2014). All the pig dung samples were considerably high in the potassium content, this is possibly due to high potassium content in the palm kernel cake and forage which constitute the bulk of their feed because it is cheap and readily available (Okoli et al., 2019). These findings conform to the work of Okoli et al. (2019), they reported that pig dung was high in potassium and low in other minerals and the findings of Okoli et al. (2011) reported a high level of potassium in the pig dung in their study area. Udebuani et al. (2012), however reported much higher calcium, magnesium and phosphorus values than the present result. Samples in this study were generally much higher in zinc content than the other micro-elements, this agrees with the findings of Udebuani et al. (2018). They reported a higher zinc value among the micro-minerals but the value in this present study is much higher than the high value recorded in their study.
Presence of the salts of these minerals within the pig dung may be responsible for the high electrical conductivity (EC) in all the samples (Fitzgerald and Racz, 2001). Electrical conductivity was generally high in all the samples. However, pig dung from Air Force had the highest electrical conductivity (1098.00 µS/cm), this can be attributed to its highest values of mineral salts (sodium, potassium, calcium and magnesium) above the other samples. This agrees with the report of Fitzgerald and Racz (2001), they stated that electrical conductivity is an indicator of the total soluble salt concentration, it can be high mainly because of the presence of water-soluble nutrients and the dominant ions that contribute to electrical conductivity of manures are NH4, Na, Ca, Mg, K, Cl, sulphate and bicarbonate.
In the study, the current and voltage generated across the observation period (40 days) from most of the samples were in a sinusoidal manner on graphic representation with irregular falls and rises (indicating increase and decrease at irregular intervals). This disagrees with the steady and gradual increase throughout the observation period (10 days) in the work of Meignanalakshmi et al. (2013) and the progressive increase in the generation period (14 days) reported in the work of Adegunloye and Ojo (2019), when decayed wood was used for current and voltage generation.
Highest voltage and current were generated from the pig dung samples at varying times. Pig dung from Air Force generated the highest voltage (572 mV) across all the MFC, this was recorded in the afternoon session of day 12. This is higher than the maximum voltage (330 mV) obtained from the MFC (with H2O as electron acceptor) for swine waste samples in the work of (Meignanalakshmi et al., 2013). Pig dung from Apatapiti Extension generated the highest current (0.319 mA) across all the MFCs, this was recorded in the evening session of day 20. This is close to the maximum current (0.330) mA obtained from the MFC (with H2O as electron acceptor) for pig waste samples in the work of (Meignanalakshmi et al., 2013).
Generation of low voltage and current from control MFC revealed that chemical and biological factors based on difference of potential between the two chambers might be responsible for this voltage and current generated. Thereafter, the voltage and current rapidly decreased due to absence of microorganisms for biological activities unlike in other MFCs where the presence of microorganisms enabled increase and continuous generation of current and voltage. This is similar to the report of (Akujobi et al., 2017).

Conclusion
The potential of generating electricity from pig dung as an additional alternative electrical energy was evaluated in this study. From the result and observation obtained, pig dung contained a consortia of microbial flora capable of oxidizing its organic matter constituent to produce electrical energy via MFC technology. The current and voltage generated was comparable to those reported for other substrates. Hence, electricity generation using a waste like pig dung via this technology is a promising alternative for electricity generation. We hereby recommend its evaluation at large scale in other to harness this as an alternative and additional sources of electricity.