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Article

Evaluating Potentials of Activated Carbon, Inoculum Diversity, and Total Solids Content for Improved Digestate Quality in Anaerobic Food Waste Treatment

by
Julius G. Akinbomi
1,
Regina J. Patinvoh
1,
Omotoyosi S. Atunrase
1,
Benjamin C. Onyenuwe
1,
Chibuike N. Emereonye
1,
Joshua F. Ajeigbe
1 and
Mohammad J. Taherzadeh
2,*
1
Department of Chemical Engineering, Faculty of Engineering, Lagos State University Ojo, Lagos 100268, Nigeria
2
Swedish Centre for Resource Recovery, University of Borås, 50190 Borås, Sweden
*
Author to whom correspondence should be addressed.
Processes 2025, 13(2), 382; https://doi.org/10.3390/pr13020382
Submission received: 31 December 2024 / Revised: 24 January 2025 / Accepted: 27 January 2025 / Published: 30 January 2025
(This article belongs to the Special Issue Fermentation and Bioprocess Engineering Processes)
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Abstract

:
The potential presence of toxic compounds in the digestate obtained from the anaerobic digestion of biodegradable waste restricts its application as a biofertilizer for soil conditioning and plant growth enhancement. The aim of this study was to assess digestate quality in terms of plant nutrient composition by evaluating the effects of activated carbon supplementation, inoculum source, and total solids content in the anaerobic digestion medium. The anaerobic digestion of food waste was conducted over a 60-day period at 25 °C in a 2.5 L bioreactor. The results demonstrated that inoculum diversity significantly impacted the digestate composition, particularly the zinc nutrient, with a p-value of 0.0054. This suggests that microbial diversity influences the valorization of organic waste into biofertilizer. However, the effects of inoculum diversity on other nutrients, aside from zinc, were not significant due to substantial interaction effects. Furthermore, assessing the impact of activated carbon supplementation proved challenging, as it was analyzed as part of a subset of the other two factors. The results of the digestate composition analysis indicated that activated carbon supplementation exhibited some influence on nutrient composition, necessitating further research to elucidate its significance. The findings of this study may contribute to enhancing the quality of digestate as a biofertilizer.

1. Introduction

The global food production rate is approaching a critical juncture where it will no longer be able to sustain the exponential population growth. This situation presents a significant risk of widespread hunger and malnutrition, affecting a large portion of the global population. According to the United Nations’ 2030 Agenda for Sustainable Development Goals (SDGs), approximately 821 million people worldwide were chronically undernourished in 2017, while 148 million children were malnourished in 2022 [1,2,3,4]. By 2030, it is projected that around 600 million people globally will face acute hunger [1,2,3,4]. This alarming statistic can be directly attributed to the adverse effects of environmental degradation, drought, and biodiversity loss. The mismanagement of generated waste and the excessive use of inorganic or synthetic fertilizers can lead to environmental degradation, drought, and biodiversity loss, which negatively impact the social, economic, and environmental sustainability of a country. Burning or landfilling waste is a common yet environmentally harmful method that many developing countries use for waste disposal. Additionally, farmers heavily rely on synthetic or inorganic fertilizers for soil conditioning to enhance plant growth and productivity. While the use of synthetic fertilizers can increase food production, excessive application is associated with adverse effects such as soil and crop degradation, human health risks, groundwater contamination from leached nutrients, and increased crop susceptibility to diseases. Clearly, burning and landfilling waste, along with the excessive use of synthetic fertilizers, are unsustainable practices that hinder the achievement of sustainable development in any country [5].
One sustainable technique for addressing the challenges mentioned above is the adoption of anaerobic digestion (AD) technology [6,7]. AD is a process that breaks down biodegradable or organic waste in the absence of oxygen through microbial activity. This process produces biogas and digestate, which can be used for energy and soil conditioning, respectively. Anaerobic digestion technology can effectively serve as a waste management solution, converting waste into valuable resources such as biogas (an energy source) and digestate (as a biofertilizer). Anaerobic digestate is the liquid residue from the anaerobic digestion of organic materials. It contains undigested feedstock, microbial biomass, and their metabolites. Organic materials, such as food waste, contain nutrients (both macro- and micronutrients) that are released and stored as digestate components during the digestion process. Therefore, applying digestate as a biofertilizer offers several benefits, including soil amendments and improved crop productivity, without the associated health risks. Utilizing digestate as a biofertilizer also conserves resources that would otherwise be used in the production of inorganic chemical fertilizers. Unlike inorganic fertilizers, digestate is an organic alternative that enhances soil organic matter and microbial content, improves soil bulk density and fertility, and increases the nutrient and water absorption capacity of plants. Additionally, applying digestate for soil conditioning supports carbon sequestration, contributing to the mitigation of global warming.
The nutrient composition of soil plays a crucial role in the effectiveness of soil fertilization and crop productivity. Plants require a total of eighteen essential elements to support their growth, development, and overall health. These elements include both non-mineral nutrients, such as carbon, hydrogen, and oxygen, which plants acquire directly from atmospheric carbon dioxide and water, and various mineral nutrients absorbed from soil. These essential elements are categorized based on the quantities needed by plants: primary macronutrients, secondary or major nutrients, and micronutrients. The primary macronutrients—nitrogen (N), phosphorus (P), and potassium (K)—are required in the largest amounts and are fundamental to several key physiological processes. Nitrogen is vital for protein synthesis, chlorophyll formation, and overall plant growth; phosphorus is essential for energy transfer via ATP (adenosine triphosphate), root development, and photosynthesis; and potassium is necessary for regulating various metabolic processes, including carbohydrate and starch synthesis, enzyme activation, and maintaining water balance within plant cells [8].
Secondary or major nutrients include calcium (Ca), magnesium (Mg), and sulfur (S). Although these elements are required in smaller quantities than primary macronutrients, they are equally important. Calcium contributes to soil improvement by facilitating soil aggregation and enhancing nutrient uptake and cell wall development within the plant. Magnesium is a central component of the chlorophyll molecule, which is essential for photosynthesis, and also acts as a cofactor in numerous enzymatic reactions. Sulfur is involved in the synthesis of amino acids, proteins, and vitamins and plays a role in enzyme function and chlorophyll formation. Micronutrients include elements such as boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), cobalt (Co), nickel (Ni), and zinc (Zn), which are required only in trace amounts but are indispensable for various biochemical and physiological functions. For example, iron is crucial for chlorophyll synthesis and acts as a catalyst in several enzymatic reactions; zinc plays a role in protein synthesis, growth regulation, and hormone production; and manganese is involved in photosynthesis, nitrogen metabolism, and the formation of essential enzymes. These nutrients are absorbed from soil through the plant’s root system, and for optimal plant growth, it is essential that all macronutrients are present in adequate quantities and balanced proportions. Sufficient nitrogen levels, for example, ensure robust vegetative growth; adequate phosphorus supports strong root systems and efficient energy transfer; and sufficient potassium enhances the plant’s ability to synthesize carbohydrates and starches, which are critical for energy storage and structural integrity [9]. While secondary nutrients are needed in smaller amounts, they still play a significant role in plant health by facilitating processes such as soil acidification (calcium), photosynthetic efficiency (magnesium), and protein synthesis (sulfur). Micronutrients, though required only in trace amounts [10], are vital for specific enzymatic activities and overall plant metabolism. Given the complexity and interdependence of these nutrients, it is crucial that any fertilizer used—whether synthetic or organic, including digestate—contains the appropriate concentrations of these essential elements. This ensures that plants receive a balanced supply of nutrients necessary for healthy growth, optimal agricultural productivity, and soil health maintenance. Effective nutrient management through balanced fertilization is fundamental to sustainable farming practices and helps prevent nutrient deficiencies or toxicities that can negatively affect plant health and crop yields.
Meanwhile, the inorganic nutrients (such as nitrogen, phosphorus, and potassium) present in the digestate after the anaerobic digestion process are often the same as those found in the feedstock used during digestion, since microorganisms primarily degrade the organic or carbon component of the feedstock. As a result, the quality of digestate as a biofertilizer depends on its composition, which in turn is influenced by both the feedstock composition and the conditions of the anaerobic digestion process. However, it is important to note that digestate may contain toxic compounds, such as heavy metals (e.g., palladium, cadmium, chromium, nickel, copper, and zinc) and organic contaminants (e.g., monocyclic aromatics, polychlorinated dibenzodioxins, polychlorinated dibenzofurans, phthalic acid esters, organochlorinated pesticides, chlorobenzenes, amines, nitrosamines, and phenols). Ideally, digestate should be free from toxic substances that could lead to soil degradation or impede crop growth. Therefore, it is crucial to minimize the release of toxic substances from both the feedstock and the anaerobic digestion process. As a result, various techniques have been employed in previous studies to enhance digestate quality, including the use of specialized digesters [11], nitrification processes [12,13], struvite precipitation [14] to reduce high levels of ammonium ions (NH4+), and dilution or biochar methods to mitigate sodium chloride and NH4+ toxicity [15,16,17] in digestate products.
The activated carbon supplementation of the digestion media or vermicomposting of the digestate prior to its application as a biofertilizer can effectively remove toxic compounds from the digestate, thereby ensuring that humans and animals are not exposed to contaminated plants and water. Activated carbon has been extensively studied for its ability to adsorb inhibitory compounds. It has also been shown that activated carbon supplementation enhances the stability of anaerobic digestion operations, increases methane production, and improves the removal of color from digestates produced during the digestion process [18,19,20,21]. Additionally, the solid content, or total solids, of the biodegradable substrate is a crucial factor that influences anaerobic digestion. The total solids content in biodegradable wastes can range from low to high, depending on whether it is less than or equal to 10% or greater than or equal to 20%, respectively [22]. While low-solid anaerobic digestion is commonly practiced, high-solid anaerobic digestion is becoming more popular due to its associated benefits, such as water conservation and a smaller digester footprint. However, high-solid AD processes can affect stability and performance if not properly managed. The success of the AD process depends on how well various influencing factors are managed, including temperature, pressure, pH, inhibitors, organic loading rate, retention time, and others. Furthermore, the importance of inoculum in the anaerobic digestion process cannot be overstated, as it provides the microorganisms necessary for improving substrate biodegradability and converting it into products like digestate [23,24].
Limited research has been conducted on the influence of inoculum diversity, digestion type (high- or low-solid digestion), and activated carbon supplementation on the reduction in toxicity (specifically high heavy metal content) in digestate products. Therefore, the primary objectives of this study were to assess the feasibility of using either low- or high-solid anaerobic digestion processes for the valorization of food waste into biofertilizer, investigate the impact of activated carbon supplementation, and examine the effects of various inoculum sources on the digestion process, with a specific focus on improving the quality of the digestate produced. This study investigated how variations in feedstock total solids, inoculum source, and activated carbon supplementation affect digestate quality, particularly in terms of reducing heavy metal content. It is hypothesized that activated carbon, due to its porous structure and capacity to adsorb substances, can help mitigate the concentration of toxic compounds in digestate. Typically, farmers are reluctant to use digestate as a biofertilizer due to concerns that it may contain toxic compounds, such as heavy metals and certain organic substances, which could negatively impact crop growth. The presence of these toxic compounds presents a challenge for the widespread use of digestate as a biofertilizer. Therefore, the findings from this study are expected to help develop standard techniques for minimizing digestate toxicity, thereby enhancing its commercial application as a biofertilizer for soil conditioning and crop growth improvement.

2. Materials and Methods

2.1. Materials

The food waste mixture, consisting of vegetable leftovers, rice, and cassava flour, was the main feedstock used for anaerobic digestion. The food waste was collected from various local restaurants around Lagos State University (Epe, Lagos, Nigeria), while the chicken dung used as inoculum was obtained from a local poultry farm. After collection, the food waste was blended and stored in a refrigerator at 4 °C until it was used. Before samples were taken from the food waste and inoculum for experimentation, they were thoroughly mixed to ensure homogeneity. The materials and equipment used in this experiment included a 2.5 L reactor, a 10 L capacity container for initial digestion, a furnace, pressure gauges, gas pipes, hose connectors, gas valves, a blender, measuring bottles, a pH meter, 1 L measuring cylinders, beakers, an oven, desiccators, a beehive shelf, a retort stand, a mixing container, a weighing balance, a trough for holding carbon dioxide (CO2) solution for gas absorption, Topgit gum, sodium hydroxide (NaOH), and hydrochloric acid (HCl) reagents (Figure 1).

2.2. Experimental Methods

The experimental procedure involved the anaerobic digestion of food waste at 25 °C in a 2.5 L reactor with an active volume of 1.5 L. Three experimental setups—blank, control, and activated carbon supplementation—were studied for comparative analysis. The blank setup contained a fixed mixture of inoculum and water only; the control setup had a fixed mixture of inoculum, food waste, and water; and the activated carbon supplementation setup [25] included a fixed mixture of inoculum, food waste, activated carbon, and water (Table 1). This study investigated the effects of three factors: total solids, inoculum source, and activated carbon supplementation. Three types of inoculums were used during this experiment: chicken dung, partially digested food waste, and a mixture of chicken dung and partially digested food waste. The partially digested food waste was prepared by placing the waste in a covered 10 L plastic container and allowing it to undergo anaerobic digestion for 30 days.
The activated carbon used in this experiment was obtained by chemically activating the carbon black residue from rubber tire pyrolysis. The carbon black produced during the pyrolysis of discarded rubber tires was ground into pellets and subjected to chemical activation using a 2 M potassium hydroxide (KOH) solution. Approximately 200 g of the ground carbon was impregnated with 200 g of 2 M KOH in a 1000 mL measuring cylinder at room temperature for 24 h. After this period, the impregnated material was packed into plastic containers. The digestion medium was supplemented with activated carbon at a mass concentration of 15 g/dm3. The amount of activated carbon used was based on the optimal value used in a research study by Zhang et al. [25]. The surface area of the activated carbon was determined using the Brunauer–Emmett–Teller (BET) equation to calculate the nitrogen quantity required to form a monolayer on the activated carbon surface. Initial contaminants, including water and carbon dioxide (CO₂), on the activated carbon surface were removed by pretreating the carbon with heat and vacuum. After pretreatment, carbon was cooled to cryogenic temperatures under vacuum by dosing it with liquid nitrogen in controlled increments while allowing the pressure to equilibrate [26]. The pore volume and pore size distribution were determined by gradually increasing the gas pressure until all the pores on the activated carbon were filled with liquid. The gas pressure was then reduced to evaporate the condensed gas from the activated carbon. Pore volume and pore size were calculated using methods such as BJH (Barrett–Joyner–Halenda) or DFT (Density Functional Theory) to evaluate the adsorption and desorption isotherms [26].
The components of the different experimental setups were thoroughly mixed and poured into their respective 2.5 L reactors. The total solids in the reactor volume for each experimental setup were approximately 15% for the low-solid anaerobic digestion process and 24% for the high-solid anaerobic digestion process. The pH of each reactor was adjusted to 7.0 using 2 M sodium hydroxide (NaOH) and hydrochloric acid (HCl) solutions. A specialized instrument with three spouts—one for the inlet feed hose, one for the pressure gauge, and one for the gas hose—was used to seal each reactor and ensure air-tight connections without any leakage. The gas hose was directed into a 1000 mL measuring cylinder containing distilled water, which was supported by a beehive shelf placed over the cylinder’s mouth. The measuring cylinder was carefully inverted and placed in a trough containing a molar NaOH solution. The anaerobic digestion of food waste was conducted as a semi-continuous process for 60 days at 25 °C in a 2.5 L plastic reactor vessel with an active volume of 1.5 L. The experimental assays, which were triplicated for each observation, were allowed to settle for three days before gas production was measured. At specific intervals, gas valves were opened to allow biogas buildup in the digester to escape through the hose into the cylinder. The biogas generated in the reactor passed through the beehive shelf into the cylinder, displacing a corresponding amount of water. The amount of water displaced in the cylinder was equal to the volume of methane gas produced. Since the primary focus of this study was on digestate production, biogas was neither measured nor stored.

2.3. Analytical Methods

The procedure for analyzing the experimental samples was conducted in two phases. The first phase involved the collection of samples of food waste (substrate), inoculum, and carbon black in sample bottles for Brunauer–Emmett–Teller (BET) analysis, Scanning Electron Microscopy–Energy-Dispersive X-ray (SEM-EDX; Thermo Fischer, Waltham, MA, USA) analysis, and proximate analysis, prior to the commencement of the anaerobic digestion process. BET analysis, using a BET surface area analyzer (Quantachrome NOvaWin 1994–2013, version 11.03, UK), was performed to determine the specific surface area, pore size, and pore volume of the activated carbon used [27]. SEM-EDX analysis was employed to assess the elemental composition of the activated carbon before its supplementation in this experiment [28]. Proximate analysis was conducted to determine the percentage concentration of moisture, ash, ether extract (EE), crude fiber (CF), crude protein (CP), cellulose, hemicellulose, lignin, and total solids content in the inoculum and food waste [29].
The second phase involved the collection of digestate products from the various experimental setups for plant nutrient composition analysis. Analyses for BET, SEM-EDX, and proximate compositions were carried out at the National Animal Production Research Institute, Ahmadu Bello University (Zaria, Nigeria), while plant nutrient composition analysis was performed at Amalab Laboratory Services Limited (Ibadan, Oyo State, Nigeria).

2.4. Statistical Analyses

A two-way ANOVA (analysis of variance) was performed using Excel software (Microsoft® Excel® 2016 MSO, version 2501 Build 16.0.18429.20044) to analyze the results obtained from the experimental work. For the activated carbon factor, there were two levels: with activated carbon (1) and without activated carbon (0). For the total solids content factor, there were two levels: high solid content and low solid content. For the inoculum diversity factor, there were two types of inocula based on the total solids content. For high solid content, the inoculum types were either chicken dung or a mixture of chicken dung and partially digested food waste. For low solid content, the inoculum types were either a mixture of chicken dung and partially digested food waste or just partially digested food waste.
The two-way ANOVA was used to determine whether total solids content or inoculum diversity influenced differences in digestate nutrient composition and to identify any interaction effects between the two factors. Total solids content and inoculum diversity were the two main factors analyzed, while the third factor (activated carbon supplementation) was treated as a subset of the other two factors. A pre-determined alpha (α)-level of 0.05 was selected as the criterion for determining whether the effect of each factor was significant. If the p-value was less than or equal to the alpha (α)-level of 0.05, the effect of the factor was considered significant, indicating that the means for that factor were significantly different. Conversely, if the p-value was greater than 0.05, the effect of the factor was deemed not significant. Additionally, if the interaction effect was significant (p < 0.05), this suggested that the effects of each factor were different at varying levels of the other factor, meaning the individual effects were not independent.

3. Results and Discussion

The primary objective of this study was to determine the influence of total solids content, inoculum source, and activated carbon supplementation on the quality of digestate products, with a specific focus on the composition of plant nutrients.

3.1. Effects of Total Solids on Digestate Composition

The effects of total solids content, inoculum source, and activated carbon supplementation on digestate quality in relation to plant nutrient compositions were evaluated through a semi-continuous anaerobic digestion process of food waste over a 60-day period at 25 °C in a 2.5 L plastic reactor vessel with an active volume of 1.5 L. The analysis used to determine the percentage concentration of moisture, ash, ether extract, crude fiber, crude protein, cellulose, hemicellulose, lignin, and total solids content of the inoculum and food waste is presented in Figure 2. The results showed that food waste comprised both organic and inorganic components. The organic component consisted of the digestible portion (volatile solids: 29.70% and ether extract or crude lipids: 22.18%), while the non-digestible portion included crude fiber (2.70%), cellulose (2.23%), hemicellulose (1.66%), and lignin (1.30%). The inorganic component included crude protein (total nitrogen: 4.25%) and ash (1.13%). The total solids content in the food waste was 31.38%, indicating a substantial amount of substrate available for conversion into valuable resources, such as digestate (biofertilizer). The digestate products included both digested and undigested feedstock components, as well as microbial biomass produced during the anaerobic digestion process.
Table 2 presents the nutrient compositions of the digestate products obtained from the anaerobic digestion of food waste. To assess the impact of low and high solid contents on the feedstock, a two-way ANOVA was employed to determine the significance of these factors in digestate nutrient composition (Table 3). Using a pre-determined α-level of 0.05 as the criterion for statistical significance, it is evident from Table 3 that the interaction effects of the factors on digestate nutrient composition are significant for all nutrients except for zinc, which showed an interaction effect with a p-value of 0.295415. This p-value exceeds the pre-determined α-level of 0.05, indicating that the interaction effects are not statistically significant. As a result, the effects of each factor are considered relevant. However, the results in Table 3 indicate that the effect of total solids content on the digestate zinc composition was not significant, as the p-value of 0.951393 is greater than the pre-determined α-level of 0.05.

3.2. Effects of Activated Carbon Supplementation on Digestate Composition

Activated carbon was added to the anaerobic digestion media to mitigate inhibitory compounds that could impair process performance and affect the quality of the digestate products. The activated carbon used had a surface area, pore volume, and pore size of 627.50 m2/g, 0.221 cm3/g, and 65.38 Å, respectively. The elemental composition analysis of the activated carbon showed that it contained aluminum (33.01%), carbon (15.73%), silicon (12.99%), sulfur (12.67%), potassium (9.77%), calcium (3.62%), silver (2.82%), chlorine (1.98%), sodium (1.93%), oxygen (1.52%), magnesium (1.50%), phosphorus (1.46%), and titanium (1.01%). The presence of other elements alongside carbon may have originated from pyrolyzed rubber tires and the activating agent (KOH) used in the production of activated carbon. The digestion medium in the reactor’s working volume was supplemented with 15 g/dm3 of activated carbon, based on the optimal amount used in a study by Zhang et al. [22].
The effects of activated carbon supplementation on digestate nutrient composition were difficult to assess directly, as total solids content and inoculum diversity were the two main factors analyzed, with activated carbon supplementation as a subset of these factors. However, the results in Table 2 indicate that, in the high-solid anaerobic digestion process, the augmented media (with activated carbon) had significantly higher concentrations of nutrients, including nitrogen, phosphorus, potassium, calcium, iron, copper, and zinc. Conversely, in the low-solid anaerobic digestion process, only potassium, iron, and zinc had higher concentrations in the augmented media compared to the non-augmented media.
The application of activated carbon supplementation to digestate composition may have a positive impact on the anaerobic digestion process. This could lead to improvements in the process and a more effective management of the factors that influence it. Activated carbon supplementation has the potential to enhance the quality of digestate as a biofertilizer due to its high surface area, which enables it to adsorb toxic compounds. Additionally, it facilitates the microbial degradation of food waste during digestion, improves soil water retention, and prevents nutrient leaching during soil fertilization [30].

3.3. Effects of Inoculum Source on Digestate Composition

The significance of inoculum activity in anaerobic processes cannot be overstated, as it acts as a catalyst to enhance anaerobic digestion. Inoculum from diverse sources may exhibit varying effects during the digestion process due to species differences. Therefore, investigating the impact of inoculum source on digestate composition is essential. The findings from this study, as shown in Table 3, indicate that the effects of inoculum diversity on digestate nutrient composition are significant only for the digestate zinc composition. Notably, the interaction effects of the factors involved were not significant. The results show that the effects of inoculum diversity on digestate zinc composition were significant, with a p-value of 0.005408.
A further enhancement in digestate quality, as demonstrated in this study, will make digestate suitable for use as an organic fertilizer, as it contains a substantial concentration of essential elements necessary for plant growth. Additional research findings [30,31,32] also support the potential of digestates from food waste and other organic substances to be used as biofertilizers for soil amendments and crop production improvement. Several studies have elucidated the impact of various factors, such as inoculum source or type [33], substrate solubility [34,35,36,37], and activated carbon supplementation [38], on enhancing the stability of the anaerobic digestion process. This, in turn, leads to improvements in the quality of digestate products. Beyond its utility as a soil fertilizer, the application of digestate can help mitigate the depletion of resources required for the production of inorganic chemical fertilizers, thus promoting sustainability and environmental protection [39,40].

4. Conclusions and Future Work

This study explored the feasibility of utilizing feedstock total solids content, inoculum species diversity, and activated carbon supplementation to enhance the quality of digestate products derived from the anaerobic digestion of food waste. Statistical analysis revealed that using a diverse inoculum species significantly impacted digestate quality, specifically regarding zinc composition. The p-value for the influence of inoculum diversity on zinc composition was 0.005408, indicating substantial effects. In contrast, the influence of total solids on digestate nutrient composition was not significant for zinc composition, as the p-value was 0.951393, which is greater than the pre-determined α-level of 0.05. The effects of activated carbon supplementation were inconclusive, as it was considered as a subset of the other two factors (inoculum diversity and total solids content). However, the digestate nutrient composition table suggested that activated carbon supplementation did have some impact on nutrient compositions, showing higher concentrations of certain nutrients when compared to non-augmented media, especially under both low- and high-solid anaerobic digestion conditions. This suggests that activated carbon supplementation may influence digestate nutrient compositions, warranting further investigation into its significance. Future research will focus on maximizing the effects of activated carbon supplementation and total solids content while exploring other techniques (including the use of vermicompost digestate as a biofertilizer) to enhance digestate quality. This will help make the process more economically, socially, and environmentally sustainable. Such efforts will facilitate the widespread adoption of digestate as a biofertilizer, contributing to environmental, social, and economic well-being.

Author Contributions

Conceptualization—J.G.A., R.J.P. and M.J.T.; methodology—J.G.A., R.J.P., O.S.A., B.C.O. and C.N.E.; formal analysis—J.G.A., R.J.P., O.S.A., B.C.O. and C.N.E.; investigation—J.G.A., R.J.P., O.S.A., B.C.O. and C.N.E.; resources—J.G.A., R.J.P., O.S.A., B.C.O. and C.N.E.; data curation—J.G.A., R.J.P., O.S.A., B.C.O. and C.N.E.; statistical analysis and drawing—J.G.A. and J.F.A.; writing (initial draft)—J.G.A.; writing and editing—J.G.A. and M.J.T.; project administration—J.G.A., R.J.P. and M.J.T.; funding acquisition—J.G.A. and M.J.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Tertiary Education Trust Fund (TETFUND), NIGERIA, under grant number LASU/VC/DSI/RP/23/028 and Swedish Research Council FORMAS under grant number 2021-02458.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to express their gratitude to the University of Borås (Sweden) and Lagos State University (Lagos, Nigeria) for their invaluable support during this research study and in the publication of the results from this study. Adenike Adetayo, Lukman Olabanjo, and George Isiguzo are also acknowledged for their assistance during this research study.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

ADAnaerobic digestion
ANOVAAnalysis of variance
ATPAdenosine triphosphate
BETBrunauer–Emmett–Teller
BJHBarrett–Joyner–Halenda
CaCalcium
CFCrude fiber
CO2Carbon (iv) oxide
CPCrude protein
CuCopper
DFTDensity Functional Theory
EEEther extract
dFDegrees of freedom
FF value from the F-test
F-critF-critical value
FeIron
HClHydrochloric acid
KPotassium
KOHPotassium hydroxide
MgMagnesium
MnManganese
MSMean sum of squares
NaSodium
NaOHSodium hydroxide
NNitrogen
PPhosphorus
R1Experimental assay 1
R2Experimental assay 2 (replicate)
R3Experimental assay 3 (replicate)
SDGsSustainable Development Goals
SEM-EDXScanning Electron Microscopy–Energy-Dispersive X-ray
SS Sum of squares
ZnZinc

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Processes 13 00382 g001
Figure 1. Experimental setup (a) without internals and (b) with internals.
Figure 1. Experimental setup (a) without internals and (b) with internals.
Processes 13 00382 g001
Processes 13 00382 g002
Figure 2. Compositions of food waste and inoculum used in this study.
Figure 2. Compositions of food waste and inoculum used in this study.
Processes 13 00382 g002
Table 1. Experimental setup measurements.
Table 1. Experimental setup measurements.
Low-Solid Anaerobic DigestionHigh-Solid Anaerobic Digestion
Inoculum DiversityExperimental SetupsInoculum
Mass (g)		Substrate (Food Waste) Mass (g)Mass
Concentration of Activated Carbon (g/dm3)		Total Solids in Reactor (%)Inoculum
Mass (g)		Substrate (Food Waste) Mass (g)MassConcentration of Activated Carbon (g/dm3)Total Solids in Reactor (%)
Chicken DungPartially Digested Food WasteChicken DungPartially Digested Food Waste
(A)
With chicken dung as inoculum		with activated carbon1034.190256.861515.011034.190256.861523.97
without activated carbon1034.190256.86015.011034.190256.86023.97
blank (inoculum and water)1034.1900015.031034.1900023.75
(B)
With chicken dung and partially digested food waste as inoculum		with activated carbon517.09316.20256.861515.00517.09316.20256.861524.12
without activated carbon517.09316.20256.86015.00517.09316.20256.86024.12
blank (inoculum and water)517.09316.200015.03517.09316.200024.12
(C)
With partially digested food waste as inoculum		with activated carbon0632.40256.861514.960632.40256.861524.16
without activated carbon0632.40256.86014.960632.40256.86024.16
blank (inoculum and water)0632.400015.070632.400024.08
Table 2. Triplicated values of digestate nutrient compositions from anaerobic digestion of food wastes at different total solid contents.
Table 2. Triplicated values of digestate nutrient compositions from anaerobic digestion of food wastes at different total solid contents.
S/NInoculum TypeDigestion MediaN (g/kg)P
(g/kg)		K
(g/kg)		Ca (g/kg)Mg (g/kg)Na (g/kg)Mn (g/kg)Fe (g/kg)Cu ((g/kg)Zn (g/kg)
(A) High-Solid Digestion
1With chicken dung and partially digested food waste as inoculumFood wastes with activated carbonR116.9023.8012.9087.0011.107.910.202.920.071.18
R217.2024.1013.1089.209.808.320.412.620.131.20
R316.9024.1013.0090.809.108.070.292.860.101.22
Food wastes without activated carbon (control)R115.7020.8010.8073.007.907.010.322.720.051.01
R216.1021.3011.2075.2011.207.250.272.660.040.99
R316.2020.9011.0076.8010.906.740.312.720.031.00
2With chicken dung as inoculumFood wastes with activated carbonR113.8036.8047.80143.0015.7012.540.645.130.430.73
R214.3037.2048.20145.2016.1012.130.605.310.050.66
R313.9037.0048.00146.8016.2011.330.565.160.120.71
Food wastes without activated carbon (control)R116.9035.9016.80144.0015.105.550.484.530.090.39
R217.3036.3017.30146.1016.205.670.484.610.120.41
R316.8035.8016.90147.9016.706.480.544.660.720.40
(B) Low-Solid Digestion
1With chicken dung and partially digested food waste as inoculumFood wastes with activated carbonR119.709.007.9024.800.0012.100.080.290.001.44
R220.4010.208.2025.202.0011.970.130.310.000.87
R319.9010.807.9025.001.0011.630.090.300.000.97
Food wastes without activated carbon (control)R119.9011.905.7028.700.0012.890.190.190.000.29
R220.2012.306.2029.103.0013.100.010.220.000.29
R319.9011.806.1029.200.0013.010.100.190.000.32
2With partially digested food waste as inoculumFood wastes with activated carbonR114.7018.7034.896.8011.0014.220.393.300.002.65
R215.3019.4035.397.2013.2014.440.393.340.002.74
R315.0018.9034.997.0014.8015.120.423.260.002.71
Food wastes without activated carbon (Control)R117.9026.7021.899.8011.0013.770.383.150.110.38
R218.2027.2022.3102.0012.3014.020.423.070.110.39
R317.9027.1021.998.2012.7013.610.403.080.080.43
R1—experimental assay 1; R2—experimental assay 2 (replicate); R3—experimental assay 3 (replicate). N = nitrogen; P = phosphorus; K = potassium; Ca = calcium; Mg = magnesium; Na = sodium; Mn = manganese; Fe = iron; Cu = copper; Zn = zinc.
Table 3. Significant effects of activated carbon supplementation, inoculum diversity and total solids content on digestate nutrient composition.
Table 3. Significant effects of activated carbon supplementation, inoculum diversity and total solids content on digestate nutrient composition.
1. NITROGEN (N) 6. SODIUM (Na)
Source of variation SSdf MS Fp-valueF-critSource of variation SSdf MS Fp-valueF-crit
Sample22.5734437.5244795.4735920.0087933.238872Sample45.42003315.140014.9023330.0132763.238872
Columns2033.8363677.9453493.16325.7 × 10−163.238872Columns739.53153246.510579.820057.68 × 10−103.238872
Interaction28.3853193.1539242.2942840.0706242.537667Interaction68.1117597.5679732.4505080.0565212.537667
Within21.995161.374688 Within49.41325163.088328
Total2106.7931 Total902.476531
2. PHOSPHORUS (P)7. MANGANESE (Mn)
Sample690.6453230.21546.008494.31 × 10−83.238872Sample1.52018430.5067283.9535050.0275923.238872
Columns3873.41531291.138258.03419.36 × 10−143.238872Columns15.6704335.22347840.75371.02 × 10−73.238872
Interaction638.555970.9505614.179484.81 × 10−62.537667Interaction2.91155390.3235062.5240010.0509662.537667
Within80.06165.00375 Within2.05075160.128172
Total5282.67531 Total22.1529231
3. POTASSIUM (K) 8. IRON (Fe)
Sample934.90343311.63454.3840660.0196453.238872Sample26.999538.99983356.613859.68 × 10−93.238872
Columns2827.8083942.602813.260510.0001323.238872Columns27.2464339.08214257.131629.06 × 10−93.238872
Interaction921.02539102.33611.4396620.251382.537667Interaction22.3299892.48110815.607522.5 × 10−62.537667
Within1137.3351671.08344 Within2.5435160.158969
Total5821.07231 Total79.119431
4. CALCIUM (Ca) 9. COPPER (Cu)
Sample14,282.8534760.95331.00231.33 × 10−143.238872Sample1.26607530.4220253.2516610.0494533.238872
Columns60,075.27320,025.091392.2321.5 × 10−193.238872Columns20.0506336.68354251.496041.92 × 10−83.238872
Interaction14,104.5191567.168108.95641.22 × 10−122.537667Interaction2.821790.3135222.4156580.059382.537667
Within230.1351614.38344 Within2.0766160.129788
Total88,692.7631 Total26.21331
5. MAGNESIUM (Mg) 10. ZINC (Zn)
Sample235.0525378.35083136.41061.32 × 10−113.238872Sample0.17526330.0584210.1127180.9513933.238872
Columns592.6053197.535343.91299.81 × 10−153.238872Columns9.61641333.2054716.184660.0054083.238872
Interaction223.3275924.8141743.202031.53 × 10−92.537667Interaction6.21271390.6903011.3318730.2954152.537667
Within9.19160.574375 Within8.2927160.518294
Total1060.17531 Total24.2970931
Table Legend: SS: sum of squares; df: degrees of freedom; MS: mean sum of squares; F: F value from the F-test; F-crit: F-critical value; Sample: total solids content factor; Columns: inoculum diversity factor.
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Akinbomi, J.G.; Patinvoh, R.J.; Atunrase, O.S.; Onyenuwe, B.C.; Emereonye, C.N.; Ajeigbe, J.F.; Taherzadeh, M.J. Evaluating Potentials of Activated Carbon, Inoculum Diversity, and Total Solids Content for Improved Digestate Quality in Anaerobic Food Waste Treatment. Processes 2025, 13, 382. https://doi.org/10.3390/pr13020382

AMA Style

Akinbomi JG, Patinvoh RJ, Atunrase OS, Onyenuwe BC, Emereonye CN, Ajeigbe JF, Taherzadeh MJ. Evaluating Potentials of Activated Carbon, Inoculum Diversity, and Total Solids Content for Improved Digestate Quality in Anaerobic Food Waste Treatment. Processes. 2025; 13(2):382. https://doi.org/10.3390/pr13020382

Chicago/Turabian Style

Akinbomi, Julius G., Regina J. Patinvoh, Omotoyosi S. Atunrase, Benjamin C. Onyenuwe, Chibuike N. Emereonye, Joshua F. Ajeigbe, and Mohammad J. Taherzadeh. 2025. "Evaluating Potentials of Activated Carbon, Inoculum Diversity, and Total Solids Content for Improved Digestate Quality in Anaerobic Food Waste Treatment" Processes 13, no. 2: 382. https://doi.org/10.3390/pr13020382

APA Style

Akinbomi, J. G., Patinvoh, R. J., Atunrase, O. S., Onyenuwe, B. C., Emereonye, C. N., Ajeigbe, J. F., & Taherzadeh, M. J. (2025). Evaluating Potentials of Activated Carbon, Inoculum Diversity, and Total Solids Content for Improved Digestate Quality in Anaerobic Food Waste Treatment. Processes, 13(2), 382. https://doi.org/10.3390/pr13020382

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