E-ISSN: 2814 – 1822; P-ISSN: 2616 – 0668
ORIGINAL RESEARCH ARTICLE
*1Musa, B., 1Ado, S.A., 1Abdulgafar, R., 1Madika, A. and 2Kruse, A.
1Department of Microbiology, Ahmadu Bello University, 810107 Samaru, Zaria, Nigeria
2Department of Conversion Technologies of Biobased Resources, University of Hohenheim, Garbenstraße 9, 70599 Stuttgart, Germany
*Correspondence: bishirmusa73@gmail.com
Because carbon-based additives are very adaptable to large-scale deployment and have minimal running costs, they are a suitable strategy to increase biogas yield. These Carbonaceous additives have been shown to have a positive effect on biogas generation with beneficial effects in the anaerobic digestion (AD) process as explained by the mechanism of direct interspecies electron transfer (DIET), the utilization of which is linked to a variety of additional mechanisms. This study investigated the effect of activated and non-activated carbons on biogas production from municipal organic wastes. In this study, a set of three (3) bio-digesters was used to process organic municipal wastes (food wastes) supplemented with activated carbon (AC) and non-activated carbon. In comparison to the control set-up without the carbonaceous additive, the results demonstrated a direct link between the activated carbon and the non-activated carbon. The biogas yield and rate of anaerobic digestion (measured based on the biogas yield per gram of the substrate per day; results not shown) are significantly increased when 5 – 10 gL-1 of activated or non-activated carbon is used. During biogas production, the bio-digester with activated carbon displayed more encouraging outcomes. During the 14-day retention period, the total Biogas produced by the set-up with activated carbon was the highest (12 870 mL) and most flammable (+++), followed by the non-activated carbon set-up, which produced 11, 250 mL of moderately flammable (++) Biogas. The lowest (9, 755 mL) and least flammable (+) biogas yield were, however obtained from the control set-up having no carbon additive. The activated carbon was shown to significantly improve biogas yield and its quality (flammability) due to its high surface area and porosity, high chemical stability, electrical conductivity, effective biofilm formation as well as its ability to remove harmful substances (micro-pollutants), which collectively improved the performance of the methanogens, thereby accelerating microbial methanogenesis. This study, therefore, revealed that carbonaceous additives supplementation enhances biogas production and, ultimately the overall biogas quality.
Keywords: Anaerobic digestion, Biogas, activated carbon, electron transfer, methanogenesis
One of the most commonly used essential gases, methane, is a
greenhouse gas that is lackadaisical to the earth's climate. Anaerobic
digestion (AD), a process that produces biomethane from biomass
decomposition, is thought to be carbon neutral. (Xiao
et al., 2022). Anaerobic digestion processes require four
steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. These
steps involve several types of microbes, including hydrolytic bacteria,
acid-producing bacteria, acetogenic bacteria, and methanogens (Evans et al. 2019). These bacteria break down
biomass made of macromolecular organic matter into smaller molecules
like methane, hydrogen, acetate, and carbon dioxide through a sequence
of processes. A variety of anaerobic digestion enhancement techniques,
including biogas upgrading, operating condition tuning, and two-stage
anaerobic digestion, have been developed (Wang et
al., 2014). These tactics do not, however, see widespread
application because of their laborious parameter adjustment processes,
excess energy consumption, and capital cost.
Methanogens, mostly from the Archaeal phylum Euryarchaeota, carry out
the final step of anaerobic digestion to create biomethane, a process
known as methanogenesis (Liu and Whitman, 2008). The
use of anaerobic digestion for the treatment of various organic wastes
has received increased attention in recent years. A number of studies
have been conducted to improve the anaerobic digestion performance and
energy efficiency of Biogas producing technologies in order to meet the
global demand for a clean and dependable energy source (Rasapoor et al., 2020)
Given the nature of organic waste, various techniques have been used to increase the waste materials' digestibility. These techniques include co-digestion, pre-treatments, and the use of carbonaceous additives to accelerate microbial activity and lower the concentration of some inhibitory byproducts (Romero-Güiza, 2016). The good impact that carbonaceous additions have on biogas generation, their widespread accessibility, and their inexpensive implementation costs have all demonstrated their effectiveness (Zhang et al., 2018). Char, a byproduct of gasifying woody biomass, can be used to create inexpensive activated carbon (AC) by steam activation, as reported by Maneerung et al. (2016).
According to Caizan-Jüanarena et al. (2020), raw materials like coal, wood, and coconut shells are used to make activated carbon (AC). Granular activated carbon (GAC) and powdered activated carbon are two categories of activated carbon based on particle size. Compared to powdered activated carbon, GAC has a higher particle size and a smaller exterior surface. Because of its exceptional adsorbing capability, great mechanical strength, and superior chemical stability, activated carbon is effective in anaerobic digestion during the production of Biogas. The use of GAC is both environmentally and financially viable.
In conclusion, due to its high conductivity, activated carbon increased the synthesis of biomethane through direct interspecies electron transfer (DIET) associated with CO2 reduction (Yang et al., 2020). The high conductivity of activated carbon, as demonstrated by microbial abundance analysis, has led to a rise in DIET, which has shown promising improvement in substrate decomposition and biomethane production. In addition to its conductive properties, activated carbon's porous nature makes it a great adsorbent for enriching substrates and perhaps removing hazardous substances, which promotes the creation of biofilms. It plays a crucial function in improving anaerobic digestion by acting as a capacitor to receive or release electrons. (Xiao et al., 2022). It was predicted that augmentation with AC could enhance the anaerobic digestion of food waste (FW) for higher methane yield, stable operation process, and effective color removal of the liquid phase of AD digestate (Zhang et al., 2018). Recently, AC has been successfully used in anaerobic digestion as an additive to enhance process efficiency in wastewater treatment (Monser and Adhoum, 2002; Malik, 2004; Skouteris et al., 2015). In order to treat food wastes and produce biomethane, this study focused on assessing the effect of adding activated and non-activated carbons to anaerobic digesters during biogas production.
The source of municipal organic wastes (food wastes) was Samaru, Zaria, Kaduna State, Nigeria, which is situated at 11.0855°N, 7.7199°E.
Start-up culture, consisting of fresh rumen content was obtained from Zaria abattoir situated in Zangon-Shanu, Samaru, Zaria. Using hand gloves, the sample was taken from a newly slaughtered cattle rumen and placed in an airtight glass bottle (Container). It was then transported immediately to the Department of Microbiology for subsequent processing.
Three (3) digesters, each with a capacity of five (5) liters, were constructed in a modified procedure of Atta et al. (2021) to facilitate the digestion of a substrate for the production of Biogas. Using a nail, a hole was bored into the cover/lid of every gallon to accommodate the flexible collection tubes. To allow the created Biogas to move from the digester to the biogas-collecting vessel, an aperture was drilled. To prevent any infiltration into the anaerobic digester and biogas escape, the tubes were securely inserted into the entrance and sealed with "A & B" adhesive gum. Prior to loading the substrate, the digesters were completely cleaned to get rid of any substances that would restrict microbial development and the production of Biogas. The substrate was also made to be firmly anoxic to help with the anaerobic fermentation process.
Plate I: Experimental Set-up for the Anaerobic Digestion of Municipal Organic Wastes
A solution of 1 % w/v potassium hydroxide (KOH) was prepared by dissolving 1 g of KOH in 99 mL of water. A total volume of 6000 mL of the solution was prepared, where 60 g of KOH was dissolved in 5940 mL of water and then used for purification during gas collection in the collection jar (1000 mL measuring cylinder).
The municipal organic waste (food waste) was sorted out by removing the non-degradable wastes and shredded to a size below 3 mm to increase its surface area for easy digestion. A total of 1000 g of the substrate (food waste) was then transferred into a set of three (3) digesters of 5 litres capacity each, and 1000 mL of water was added to obtain a slurry of just 1:1(substrate: water) ratio; compared to various rations of municipal solid waste to water dilution which was reported by Haftu et al. (2018). A total of 19 g of activated carbon was added to one of the bio-digesters, and also 19 g of non-activated carbon was added to another digester using 5 - 10 gL-1 in accordance with Elvira et al. (2020), and the control set-up was without carbonaceous additive. The pH of the slurries was determined using a pH meter, and the temperature was kept ambient (Room temperature). An estimated 191.92 g of rumen content collected from freshly slaughtered cattle was incorporated into each of the bio-digesters, which serves as inoculum (start-up culture) for methanogens, and this was followed by occasional agitation or shaking to mix the slurry properly (Musa and Raji., 2016).
To avoid gas leaks, the digesters were sealed with "A & B" adhesive gum and stoppers. Rubber tubing was then used to link the digesters to a gas collecting jar (a measuring cylinder with a 1000 mL capacity) that was inverted over a 1 %w/v KOH solution. By using "upward delivery and downward displacement" of KOH solution, the gas was collected (Atta et al., 2021). The alkaline KOH solution aids in the dissolution of acidic gases, which are regarded as contaminants in Biogas and include CO2 and H2S. Different techniques are used to absorb hydrogen sulfide (H2S) and carbon dioxide (CO2) from Biogas. The most popular and practical technique is to pass the Biogas through alkaline solutions, such as calcium, potassium, and sodium hydroxides (Muntaha et al., 2022). In this study, 1 %w/v KOH solution was used.
2KOH + CO2K2CO3 + H2O
Each experiment lasted for a hydraulic retention time (HRT) of 14 days. The production of gas was monitored using the volume displacement method every 24 hours.
The following factors were taken into account when evaluating the performance of the experimental substrates: daily yield of gas, total volume of gas produced throughout the study's 14-day (HRT) period (measured by the method of upward biogas delivery and downward displacement of the KOH solution in the 1 L measuring cylinder), and time record of gas production, including when it begins, peaks, and ends (Musa and Raji, 2016).
The degree of flammability of the Biogas produced was used to determine its quality. In order to do this, a match was struck, and the flame was passed over the measuring cylinder's (or gas collection jar's) nozzle to collect gas and record its degree of flammability (Atta et al., 2021).
The results of the effect of activated carbon on biogas production from municipal organic wastes are presented in Figure 1 and Table 1.
Table 1 shows the assessment of Biogas produced from the digestion of food wastes (municipal organic wastes) under anaerobic conditions for a hydraulic retention time (HRT) of 14 days. The control set-up had an initial pH of 6.8, the set-up with non-activated carbon had 6.8, and the set-up with activated carbon had 7.2 (Table 1). Gas production started on the 1st day for all set-ups (control, non-activated and activated carbon). The day of peak production for the control set-up was the 9th day, the 1st day for the non-activated carbon set-up, and the 4th day for the activated carbon set-up. No cessation in the production for the entire Hydraulic Retention Time was observed in the control set-up, non-activated carbon set-up, and activated carbon set-up.
Flammability started on the 11th day for the control set-up, set-up with non-activated carbon started on the 9th day and activated carbon set-up started on the 8th day. The control set-up was less flammable, the non-activated carbon set-up was moderately flammable, activated carbon set-up was highly flammable (Table 1).
The control set-up had a total biogas production of 9,755 mL, the set-up containing non-activated carbon had 11,250 mL, and that containing activated carbon had 12,870 mL, as shown in Table 1
Figure 1 shows an illustration of the volume of Biogas produced by the control set-up, non-activated carbon set-up, and activated carbon set-up during the retention period of 14 d digestion of the municipal organic waste anaerobically.
Table 1: Biogas Produced during a 14 d Hydraulic Retention Time (HRT) of Anaerobic Digestion of Municipal Organic Wastes (Food Waste).
| Parameters | Control | With Non-activated Carbon | With Activated Carbon |
|---|---|---|---|
| Initial pH | 6.8 | 6.8 | 7.2 |
| Day Production Started | 1st | 1st | 1st |
| Day Flammability Started | 11th | 9th | 8th |
| Biogas Flammability | + | ++ | +++ |
| Day Production Peaked | 9th | 1st | 4th |
| Day Production Ceased | --- | --- | --- |
| Total Biogas Produced (mL) | 9,755 | 11,250 | 12,870 |
Key: +++ = Highly flammable, ++ = Moderately flammable, + = Less flammable, --- = No cessation in the production for the entire Hydraulic Retention Time (HRT).
Figure 1: Biogas Yield from Anaerobic Digestion of Municipal Organic Waste during Retention Period of 14 d.
Plate II: Flammability (Quality) Test for Methane Content of the Control Set-up (without supplementation)
Plate III: Flammability (Quality) Test for Methane Content of the Set-up with Non-activated Carbon Supplementation
Plate IV: Flammability (Quality) Test for Methane Content of the Set-up with Activated Carbon Supplementation
The production of Biogas is generally known to be influenced by various factors at varying degrees. In this study, the initial pH (7.0 ± 0.2) was fixed, and the performance of the digested substrates for biogas production was therefore solely dependent on the nature of the substrate’s treatment with (activated or non-activated) or without carbon additive. In all the experiments, a fixed amount (19 g) of activated and non-activated carbon was separately assessed in bio-digesters alongside a control (with no additive), and the volume of Biogas generated (quantitatively and qualitatively) was recorded. Interestingly, the near-optimal levels of pH observed, especially for the bacteria, might have favored the preliminary stages of hydrolysis, acidogenesis, and acetogenesis during the biogas production. Generally, as the efficiency of these stages increases, the rate and volume of Biogas generated is ultimately expected to also increase (Atta et al., 2021). The work of Xu et al. (2018) lends credence to the findings. They reported that the addition of biochar into the AD process could increase alkalinity and solution pH, which reduces ammonia inhibition and acid stress to the microbial community, thereby enhancing the AD process. The biochar was found to elevate the alkalinity (pH = 6) of AD, promoting better microbial action for quick CH4 production and adaptability to initial loading shock. This is similar to the work of Li et al. (2017) who reported a pH rise to ≥ 6 after biochar addition. Therefore, with the addition of biochar, a continuous AD system can work more efficiently and with an even shorter hydraulic retention time (HRT).
The highest volume (12,870 mL) of Biogas obtained was from the municipal food waste supplemented with activated carbon. The control set-up, having no carbon additive supplementation, produced the lowest biogas yield of 9,755 mL, as expected. On the other hand, the food waste supplemented with non-activated carbon produced a total biogas yield of 11,250 mL, which was 12.6 % lower than the set-up supplemented with activated carbon but 13.3 % higher than the control set-up. This difference might not be unconnected with the high electrical conductivity of the activated carbon added, which reduces internal resistance and increases the conductivity of the slurry by enhancing the electron transfer rate, especially during the methanogenesis of biogas production. The biochar conductivity and its microbial association in AD are, however, highly influenced by the pH, as also reported by Yin et al. (2019). The large pore diameter and high chemical stability of the activated carbon might also contribute to significant biogas yield obtained from the set-up supplemented with activated carbon.
Additionally, micro-pollutants, which are inhibitors of bacterial metabolism, might have been removed by the activated carbon, possibly via sequestration, thus resulting in enhanced bacterial proliferation, improved biofilm formation, and optimal metabolic activities leading to higher biogas yield. This corroborates with the work of Zhang et al. (2017), who also reported an interesting observation that during the biofilm formation, biochar induced the secretion of extracellular polymeric substance (EPS) from microbes, thus facilitating microbial adhesion on the biochar surface. This presents a simple and low-cost solution using biochar to avoid rapid sludge granulation and minimize the loss of methanogens in the anaerobic digesters. Sun et al. (2016) confirmed the enriched microbial abundance in the presence of biochar carriers.
Figure 1 shows the weekly volume of Biogas produced with the highest yield obtained during the first week of anaerobic digestion of the food wastes for both experimental treatments. However, it can be speculated that this Biogas contained mostly CO2 rather than methane, hence less flammable at this time. The porous nature and large surface area of biochar might have promoted the colonization of bacteria and archaea, resulting in an improved AD performance during the first week of AD, as also reported by Qin et al. (2017) and Martínez et al. (2018). However, the CO2 concentration might have decreased with a corresponding increase in the CH4 as the AD process proceeds. This is because the abundant surface functional groups and good electrical conductivity of the biochar could enhance the methane yield via direct or indirect electron transfer mechanism among anaerobic microbes, as also reported by Chiappero et al. (2020) and Baek et al. (2018).
The flammability test in this study revealed that the Biogas obtained from the digestion of food waste supplemented with activated carbon was most flammable, particularly during the second week. The high degree of flammability observed might be due to high methane content and/or low levels of such incombustible gases as carbon dioxide, oxygen, nitrogen and ammonia as components of the Biogas. This result corroborates the finding of Jatau et al. (2001).
The decrease in the flammability of the Biogas observed in the food waste with non-activated carbon might be due to the absence of desirable properties of the carbon supplement, including but not limited to high electrical conductivity, high porosity, and high surface area. Previous studies also highlighted the importance of biochar in addition to its capability of electron transfer processes between archaea and anaerobic bacteria, which improves methane yield and, ultimately, the biogas flammability. The efficiency of the AD system is primarily dependent upon the syntrophic interactions between methanogens and bacteria, which trade electrons to comply with their energy necessities (Martins et al., 2018). This occurs through multiple routes: DIET via a conductive medium (e.g. magnetite, carbon cloth, biochar) (Zhang et al., 2018), membrane-bound transporter proteins (Martins et al., 2018), electric conductive pili (Barua and Dhar, 2017), and indirect interspecies electron transfer (IIET) through insoluble (humic compounds) as reported by Roden et al. (2010) as well as soluble (acetate, formate, hydrogen) substances which was reported in the work of Schink et al. (2017).
Additionally, the non-activated carbon could not have removed the micro-pollutants such as ammonia in the Biogas, and this might have reduced the flammability since ammonia is non-combustible. One of the prime features offered by biochar to elevate AD functioning is its inhibitor adsorption potency. The surface area of biochar is considered one of the key factors, along with others, in the adsorption of environmental contaminants (Luz et al., 2018). According to Shanmugam et al. (2018) and Cheng et al. (2018), CH4 yield was elevated due to biochar adsorption of volatile fatty acids (VFAs) and acid alleviation. Wang et al. (2018) also observed a direct proportionality between hydrochar surface area and NH4 adsorption. In an investigation on CO2 adsorption by biochars obtained from chickory wood and bagasse, Creamer et al. (2014) realized that it was effectively achieved due to the physical adsorption through a high surface area and N2 groups.
Biogas was produced from municipal organic waste (food waste). The supplementation of activated carbon during biogas production has significantly improved the performance of the anaerobic digestion process. This is indicated in the total amount of Biogas produced, with the set-up containing activated carbon having the highest biogas yield (12,870 mL) within the hydraulic retention period (14 d).
In terms of quality, the Biogas produced in the set-up containing activated carbon was the most flammable (+++) compared to that with non-activated carbon, which was moderately flammable (++), and that of the control set-up, which was least flammable (+). This clearly indicated that there was the highest methane (flammable gas) content in the Biogas produced with activated carbon supplementation.
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