Additives Improving the Efficiency of Biogas Production as an Alternative Energy Source—A Review

Abstract

Additives for anaerobic digestion (AD) can play a significant role in optimizing the process by increasing biogas production, stabilizing the system, and improving digestate quality. The role of additives largely boils down to, among others, enhancing direct interspecies electron transfer (DIET) between microbial communities, resulting in improved syntrophic interactions, adsorption of toxic substances that may inhibit microbial activity, improving microbial activity, and increasing process stability and accelerating the decomposition of complex organic materials, thereby increasing the rate of hydrolysis. Through the aforementioned action, additives can significantly affect AD performance. The function of these materials varies, from enhancing microbial activity to maintaining optimal conditions and protecting the system from inhibitors. The choice of additives should be carefully tailored to the specific needs and conditions of the digester to maximize benefits and ensure sustainability. In light of these considerations, this paper characterizes the most commonly used additives and their combinations based on a comprehensive review of recent scientific publications, including a report on the results of conducted studies. The publication features chapters that describe carbon-based conductive materials, metal oxide nanomaterials, trace metal, and biological additives, including enzymes and microorganisms. It concludes with the chapters summarising reports on various additives and discussing their functional properties, as well as advantages and disadvantages. The presented review is a substantive and concise analysis of the latest knowledge on additives for the AD process. The application of additives in AD is characterized by great potential; hence, the subject matter is very current and future-oriented.

1. Introduction

Anaerobic digestion (AD), as an environmentally friendly method of waste management and a source of renewable energy in the form of biogas, is a highly recommended solution in times of ongoing geopolitical changes and energy crises [1]. AD is a complex, multistep biotechnological process, and its efficiency is highly dependent on sensitive microorganisms [2]. The performance of these microorganisms is influenced by factors such as substrate type, process environment conditions (e.g., pH and temperature), bioreactor design, operational modes, and access to the medium. Various challenges can adversely affect the process, including inefficient biodegradation, acidification, lag phases, foam formation, complex rheology, and high apparent viscosity [3]. Foaming is the most common operational problem in AD. Hydrophobic filamentous microorganisms are usually considered to be a major cause of this process. However, little is known about the identity of foam-stabilizing microorganisms in AD systems and control measures are poorly developed [4]. In general, the causes of foam formation in an anaerobic digester of a wastewater treatment plant (WWTP) differ from foaming causes in biogas plants that utilize renewables and biogenic wastes. The reason is the different modes of operation, mainly with regard to the concentration and character of substrates, but also the shape of the digesters and the mixing devices [5].

Several methods are employed to enhance AD, such as substrate pretreatment, co-digestion, and modifications to reactor configurations and operating conditions. One promising approach to improving AD kinetics and effectiveness is the use of additives in anaerobic reactors. Additive technology, compared to other solutions, is an effective, environmentally friendly, and relatively cheap method. Materials engineering and knowledge about the possibilities of managing waste materials create new perspectives. The additives can be broadly categorized into inorganic additives (e.g., carbon-based materials, macro- and micro-elements, nanoparticles, and nanomaterials) and biological additives (e.g., microorganisms and enzymes) [6]. While existing scientific research on AD additives is both encouraging and revealing, many issues remain unresolved. The published results offer a solid foundation and inspiration for further research into new materials, additive systems with defined effects, and advanced analyses to clarify key phenomena and the relationships between additive properties and their functions [7]. However, there is a lack of comprehensive understanding of the different types of additives; in particular, the dosages, functions, and effects of each supplement should be discussed.

Studies on the supplementation of the AD process with inorganic, organic, and biological additives have shown positive results in enhancing the efficiency of digesters. Among the notable additives are carbon-based conductive materials, such as granular activated carbon (GAC), powdered activated carbon (PAC), and multi-wall or single-wall carbon nanotubes (CNTs) [8,9]. These materials are characterized by high electron conductivity, a large specific surface area that facilitates microorganism adhesion and the adsorption of potentially toxic chemicals, and high corrosion resistance [10]. Additives with high conductivity have been found to stimulate direct interspecies electron transfer (DIET), a concept initially proposed by Kato et al. [11]. The authors demonstrated that DIET could be a new form of syntrophic association, where electrons are transferred directly from electron donors to electron acceptors, involving fewer biological enzymatic reactions [12,13]. It was confirmed that DIET had great potential for shortening the start-up period, promoting organic degradation and enhancing methane production in the AD process. In addition to carbon-based conductive additives, iron-based conductive materials have also been tested for stimulating DIET and methane formation. These include magnetite (Fe₃O₄), hematite (Fe₂O₃) and stainless steel [14,15]. Although these oxides differ in their chemical composition and electrical conductivity (magnetite has 106 times higher conductivity than hematite [16]), their DIET stimulation, in terms of lag time and methane generation rate, is similar. It is also worth noting that inorganic carbon- or iron-based conductive materials can be replaced by conductive polymers, e.g., polyaniline (PANI) nanorods [17]. In addition to the previously mentioned chemical additive applications, research also focuses on the impact of supplementing with enzymes and microorganisms. These biological additives are often considered a form of pre-treatment for difficult-to-biodegrade substrates, typically performed outside the bioreactor [18,19]. The direct application of biological additives into the digester has been studied and utilized less frequently due to the challenging environmental conditions. The goal of inoculating with biological additives is to increase the diversity of microorganisms to enhance hydrolytic and/or methanogenic activity.

Given the growing trend of using additives in anaerobic digestion and the development and study of functional additive systems with various origins and properties, this work provides a comprehensive characterization of these additives based on the latest research reports. Combining functional additives in AD requires a strategic approach to maximize their benefits and ensure the stability and efficiency of the digestion process. Thus, creating up-to-date literature reviews is valuable as it inspires scientists and practitioners to design effective solutions and technologies.

2. Carbon-Based Conductive Materials

Enhancing the efficiency of anaerobic digestion (AD) through the use of conductive materials enables increased profitability of operations and optimization of feedstock management to maximize its utilization. Carbon-based conductive materials are becoming increasingly important. Notable examples include granular activated carbon (GAC), powdered activated carbon (PAC), biochar, carbon nanotubes (CNTs), as well as graphite and graphene. These materials vary in their chemical and physical properties, which lead to different process outcomes.

2.1. GAC and PAC

Granular activated carbon (GAC) is a conductive material derived from the thermal processing of carbon-rich feedstocks (such as coconut shells and coal). During thermal processing, the material’s activity is increased by reducing the oxygen level in the furnace (higher oxygen levels reduce GAC activity) [20]. GAC features a porous structure and a large adsorption surface area, which is crucial for the AD process. GAC performs several functions, including enhancing DIET, promoting the growth of Firmicutes microorganisms in AD, and reducing the lag phase in the process.

In the study by Zhang et al. [21], GAC improved the fermentation of wastewater at 20 °C and also induced an increase in Geobacter bacteria from 9% to 12%. Additionally, there was a 300% increase in biomass in the system. It was found that DIET and physiological changes in organisms led to favorable changes in the chemical oxygen demand (COD) growth rate to 2000 mg COD/L/day, compared to 500 mg COD/L/day without GAC. It was observed that this change resulted from interspecies interactions. Another study [10] noted that GAC can also be beneficial under low-temperature conditions. A method for wastewater treatment using anaerobic digestion at temperatures of 15 °C, 25 °C, and 35 °C was proposed. It was found that the addition of GAC (6 g/L) reduced the lag phase from 15.1 days to 10.6 days (a 29.8% reduction) and increased methane production from 6.4 mL/day to 7.9 mL/day. Thus, GAC supplementation can significantly reduce the sludge decomposition time and enhance methane production. Furthermore, modifying GAC with magnetite has been shown to reduce propionate. A study [9] found that doping GAC with magnetite increased propionate’s syntrophic degradation, a compound that is difficult for microorganisms to break down. The highest methane production (220 mL/g) and COD of 70% were achieved in the bioreactor containing the modified GAC. In another study [22], thermally treated sludge and untreated hydrolysate were examined. It was observed that under three temperature conditions: psychrophilic (16–24 °C), mesophilic (35 °C), and thermophilic (55 °C), the addition of GAC (10 g/L) resulted in a decrease in CH₄ yield (ranging from 6.5% to 36.9%), as well as a reduction in the length of the methanogenic lag phase (19.3% to 30.6%), which accelerated the process and improved efficiency. It was noted that the acceleration of the process was associated with a reduction in phenolic compounds, which contributed to the overall enhancement of the process. Ultimately, the methane yield decreased (by 5.9%–8.1%) after the addition of GAC. It has been suggested that the pre-treatment of the sludge be combined with thermal processing to counteract the reduced methane yield in the AD process.

Powdered activated carbon (PAC) is another form of activated carbon. Both PAC and GAC can promote the syntrophic metabolism of alcohols and volatile fatty acids (VFAs). However, PAC generally enhances process efficiency more effectively. The more abundant structure of mesopores and micropores in PAC provides better accessibility for microorganisms of the genus Methanosarcina in the system [23]. PAC is also more effective in accelerating the start of methanogenesis compared to GAC [24]. Literature data suggest that researchers have focused more on the granular form of activated carbon. The effects of GAC and PAC are summarized in

Table 1. Dosage and effects of GAC and PAC in the AD process.

Type of Additive	Dosage (g/L)	Type of Substrate	Effect	References
GAC	50.0	dog food	enables the process to proceed at high OLR values; increase in methane yield by 5%	[25]
GAC	6.0	acetic acid and ethanol	increase in methane yield by 31%; rate increased 72%	[26]
GAC	–	rapeseed oil	increase in methane yield after 21 days up to 10.5% with comparison control group	[27]
GAC	33.3	black water (urine and feces)	increase in methane yield by 57%; methane production from anaerobic degradation of black water was improved up to 18.6% by GAC	[28]
PAC	5.0 10.0	food waste, vegetable waste	higher yield provided by PAC than GAC, PAC degrades VFA more efficiently than GAC; when dosing 5 g/L and 10 g/L, the cumulative methane yield is higher by 22% and 10.9%, respectively	[24]
PAC	0.125–1	pre-treated activated sludge subjected to thermal hydrolysis	stimulates hydrolysis activity, increases methanogenic activity, and speeds up VS removal; when dosing 1 g/L methane production is 134% higher than control sample	[29]
PAC	15.0	poultry blood	enhances syntrophic metabolism; methane production is 216 mL CH4/g VS	[30]
PAC	15.0	organic fraction of municipal solid waste	reduction of inhibitor content (FAN, VFA); 17% higher methane yield than control	[31]

, demonstrating that both materials can enhance AD efficiency.

2.2. Biochar

Biochar is a material obtained from a process waste feedstock, unlike activated carbon. Biochar is produced through the thermochemical conversion of biomass in the absence of air. The production of biochar is environmentally sustainable. Feedstock for biochar production can include vegetable waste, garden waste, and agricultural residues. Additionally, animal and household waste can also serve as feedstock [32]. The main difference between biochar and activated carbon, besides the source of feedstock, is its lower porosity. According to researchers [26], the conductivity of GAC is approximately 1000 times greater than that of biochar, which is significant for its impact on DIET. Furthermore, the production temperature of biochar is lower: activated carbon is produced at temperatures between 600 °C and 900 °C, while biochar is produced at temperatures below 600 °C. The lower production temperature and use of waste feedstock make biochar a more environmentally friendly option [33].

Biochar is an additive that supports the growth of microorganisms. According to researchers [34], biochar can enrich the microbial community in AD processes in oil. In mesophilic systems, methane production increased by 32.5% with granular biochar. In thermophilic systems, a 13.3% increase was noted in digesters enriched with powdered biochar. Additionally, biochar was found to alleviate the accumulation of propionic acid, which positively impacts the AD process. There are also modifications of biochar that affect ammonia reduction. Research [35] on animal manure treated with biochar activated by acid reduced NH₃ emissions by 37% to 51% over a month of storage. In contrast, non-activated biochar did not reduce emissions. Another study [36] compared biochar obtained through pyrolysis and hydrothermal carbonization. It was found that biochar accelerates methane fermentation by mitigating mild ammonia concentrations (2.1 TAN kg⁻¹; TAN—total ammonium nitrogen) for pyrolyzed biochar. Biochar produced via hydrothermal carbonization increased methane yield by 32%. Additionally, biochar addition helped alleviate the effects of ammonia at concentrations of 2450 mg/mL, stimulating process efficiency and reactor stability, particularly at a high OLR (6 g/VS L^−d; VS—volatile solids, OLR—organic loading rate) [37].

Table 2. Dosage and effects of biochar in the AD process.

Dose of Biochar (g/L)	Type of Substrate	Effect	References
10	glucose	reduced downtime and faster fermentation start; when dosing 4 g/L of glucose, a maximum methane production yield increase of 86.6% was achieved	[38]
10	food waste	increased average methane yield by 14%; methane yield is 18% higher than control samples	[39]
–	waste water	improved methane yield and enhanced degradation of protein substances; 28% higher methane yield	[40]
20	volatile fatty acids	reduced downtime and faster methane production start; the maximum methane production rate was 14.5–30.2% higher than control samples	[41]

shows selected system parameters and effects of biochar.

2.3. SWNTs and MWNTs

Conductive nanomaterials have been the subject of numerous studies aimed at enhancing the performance of the AD process. One such material is carbon nanotubes, which form spatial structures resembling cylindrical networks of hexagonal carbon rings. They represent an allotrope of carbon, similar to diamond and fullerene. Carbon nanotubes are categorized into single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). SWNT consists of a single layer of carbon atoms, while MWNTs are composed of multiple layers. These structures exhibit various properties that can be useful for AD, including promoting DIET and accelerating both abiotic and biotic reductions of electrophilic compounds, though they can also be toxic to microorganisms [42]. They are known for their high mechanical and thermal strength, as well as their large specific surface area [43]. It has been indicated [44] that carbon nanotubes improve organic loading rate and process stability, reduce ammonia levels, and support DIET. Research [45] on the effect of single-walled carbon nanotubes in wastewater sludge showed that adding 1000 mg/L did not impact the maximum methane yield but accelerated the process and increased electrical conductivity in the sludge, potentially enhancing DIET. Researchers Shen et al. [46], in studies conducted on a UASB (up-flow anaerobic sludge blanket) reactor under thermophilic conditions with high acetate concentrations, found that the addition of 1000 mg/L SWNT had little impact on methanogenesis and acetate concentration but induced syntrophic acetate oxidation (SAO). This led to the growth of Coprothermobacter and Thermoacetogenium, which played key roles in the SAO pathway. Thermoacetogenium bacteria contributed to a potential DIET pathway, ultimately accelerating the process.

Research [47] on MWNTs indicates that they increase methane production by influencing specific trophic levels, such as methanogens and syntrophic bacteria. An experiment with anaerobic sludge and river sediment demonstrated increased methane yields for acetate, butyrate, ethanol, and hydrogen by factors of 2.6, 2.1, 1.2, and 1.1, respectively. Additionally, MWNTs doped with iron achieved significantly higher methane yields compared to those without iron. Another study [48] investigated various concentrations of nanotubes used in methane production from poultry manure under mesophilic conditions, with concentrations ranging from 0.5 to 6.5 g/L. The study found that the addition of 5 g/L of nanotubes had the most significant impact on methane production. Furthermore, nanotube structures influenced the biodegradation of short-chain fatty acids (SCFAs) such as butyrate, propionate, and acetate, resulting in a 15–16% increase in methane production with nanotube concentrations of 5–6.5 g/L. The conclusion drawn was that nanotube addition improves both the kinetics and productivity of the AD process in anaerobic reactors.

2.4. Graphite and Graphene

Graphite is a widespread natural mineral [49] used as feedstock for producing graphene oxide, which is converted by chemical reduction into the thinnest and strongest material, graphene [50]. Graphene is an allotrope of carbon with a two-dimensional structure resembling a honeycomb shape [51]. Like graphite, graphene has a large adsorption surface and high electrical and thermal conductivity properties [49,52].

Analyses by Muratcobanoglu et al. [53] and Wang et al. [54] showed that the addition of graphite and graphene stimulated the growth of hydrolytic bacteria (e.g., Curvibacter sp.—decomposition of aromatic compounds, Actinobaculum sp.—decomposition of sugars), thus leading to the acceleration of organic matter degradation and decomposition of the feedstock. Additionally, Muratcobanoglu et al. [53] reported an increase in Aminiphilus sp. bacteria, which ferment amino acids and fatty acids, thus minimizing the risk of acidification and inhibition of the fermentation process [55]. Moreover, the results of many studies indicate that the addition of graphite and graphene creates ideal conditions for the proliferation of many groups of bacteria capable of direct interspecies electron transfer (DIET) (e.g., Methanosaeta sp., Methanobacterium sp., Methanosarcina sp.), which, through the mechanism of DIET, increase the production of biogas and biomethane [56,57,58]. According to Wang et al. [54], improving the direct transfer of electrons and energy between microorganisms is possible because the additives show the ability to function as electron conduits. Graphite and graphene stimulate bacteria to aggregate and also allow cells to settle on their own surface, thus reducing the physical distance between microorganisms and accelerating electron and energy transfer [55,59]. Additionally, Wu et al. [55] suggest that graphite can serve as a supporting material for biomass immobilization, initiating methanogenesis and shortening the lag time of an anaerobic fermentation system. Furthermore, analyses by Muratcobanoglu et al. [53] show that bacteria present in reactors with graphite addition were surrounded with extracellular polymer substance (EPS), which could also improve interspecies mass transfer. Tian et al. [60] showed that the dense structure formed by bonded EPS and graphene acts as a barrier for microorganisms and protects them from adverse environmental factors (e.g., too low temperature). Lu et al. [58] found that graphite improved biomethane production by 42.4% in methane fermentation under high H₂ partial pressure. In turn, Wu et al. [61] reported that adding graphene can help stabilize the fermentation process after acidic shock, increasing biomethane production by 11%. Additionally, Lu et al. [59] and Tian et al. [60] showed that adding graphite and graphene positively affects the activity of acidogenesis and methanogenesis enzymes such as acetate kinase and coenzyme F₄₂₀.

Moreover, the above-mentioned authors found that the discussed additives promote biogas and biomethane production by supporting the acetoclastic pathway [53,54,55,56,57,58,59,60]. The effects of graphite and graphene are summarised in

Table 3. Yield of biogas and biomethane in anaerobic digestion using graphite and graphene.

Type of Additive	Type of Substrate	Yield of Biogas/Biomethane	References
graphite	food waste and cow manure	increase in biogas production by 19.57% increase in methane production by 67%	[53]
graphite	sewage sludge and food waste	increase in methane production by 27.34%	[54]
graphite	waste-activated sludge	increase in biogas production by 12.5%	[55]
graphite	waste-activated sludge	increase in methane production by 38.3%	[58]
graphene	sewage sludge and food waste	increase in methane production by 36.09%	[54]
graphene	ethanol	increase in methane production by 25%	[58]
graphene	wastewater sludge	increase in methane production by 51.4%	[59]
graphene	grass silage, cattle manure, seaweed, and food waste	increase in methane production by 11%	[60]

, demonstrating that both materials can enhance AD efficiency.

Figure 1 shows a diagram related to the most important functions of the above-described carbon-based conductive materials.

2.5. DIET Mechanism

Studies have shown that conductive carbon materials significantly accelerate and improve the efficiency of anaerobic digestion by intensifying direct interspecies electron transfer (DIET) [62]. DIET promotes the acetogenic methanogens metabolic pathway, one indicator of which is an increased concentration of acetic acid and a decreased concentration of propionic acid, with CO₂ and CH₄ being the final products, as described by the following equation [63]:

DIET pathway: CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O

(1)

In DIET communities, electrons released from exoelectrogenic microorganisms are directly transferred to electron-capturing microorganisms via membrane-associated cytochromes and conductive pili that form electrical connections between syntrophic partners. The mechanism can also be based on the abiotic conductivity of materials within methanogens. Below (see Figure 2) is a diagram illustrating the DIET mechanism [64].

The DIET mechanism through conductive materials varies depending on the type of conductive material used. Conductive materials typically have a large specific surface area to which microorganisms can attach, grow, and transfer electrons on their surface.

Regarding the action of carbon materials [12], it was found that active carbon promotes DIET due to its higher conductivity compared to methanogenic aggregates. However, the promotional effect of biochar is nearly the same as that of active carbon, even though biochar’s conductivity is only one-thousandth that of activated carbon [65,66]. This suggests that any conductivity above a certain threshold is sufficient to trigger the DIET methanogenic pathway [67].

Researchers are striving to discover various materials that mediate electron transfer involved in methanogenesis. Modern spectroscopy techniques provide crucial information on the composition and structural organization of biofilms, which can be useful in elucidating the additional functions of this unique layer of microbial cells. However, there are many challenges, such as the unclear biological mechanisms, influences of non-DIET mechanisms, limitations of organic matters syntrophically oxidized by way of DIET, and problems with the practical application of DIET mediated by conductive materials [68]. When discussing the ambiguities surrounding the mechanism, the electron transfer biological pathway between microorganisms is considered not fully explained. It has not been discovered how electron-donating bacteria transfer electrons outside the cell. Geobacter is the most commonly used microbial model in studies of the mechanism because of its fully sequenced genome and advanced gene engineering methods.

3. Metal Oxide Nanomaterial

Analyses by Baniamerian et al. [69] show that metal oxide nanoparticles (NPs) are the second most commonly used type of nanomaterial in methane fermentation processes (metallic nanoparticles—38%, metal oxide nanomaterials—35%, carbon-based nanomaterials—18%, nanocomposite materials—9%). The relatively broad application of these metal oxides not only in the energy sector but also in many other industries is due to their unique physical, chemical, mechanical, and biological properties [70,71]. The advantages of metal oxide nanoparticles include small particle size, large surface area and density, stability, high tensile strength, and resistance to degradation by chemicals or microorganisms. They also exhibit high-temperature superconductivity, low cost, ease of use, and recovery post-fermentation. Additionally, these nanoparticles are non-toxic and available in various forms [69,72,73].

Years of research have shown that metal oxide nanoparticles used as a biological additive in methane fermentation increase the efficiency and effectiveness of the process, leading to the production of more biogas and biomethane (see Table 4). Nanoparticles stimulate energy production by shortening the lag phase [74,75], which is the adaptation period of microorganisms to new environmental conditions. According to Arya et al. [76] and Chen et al. [77], metal oxide nanoparticles can promote the synthesis of enzymes and cellular components essential for the proper growth and development of microorganisms, improving their ability to acclimate in the bioreactor and serving as a source of essential nutrients. Additionally, nano-TiO₂ has demonstrated the ability to reduce hydrogen sulfide content in the fermentation environment by forming irreversible bonds with sulfur [78]. Metal oxide nanoparticles enrich the microbial community in the digester, leading to the growth and development of methanogenic bacteria (e.g., Methanobacterium sp., Methanobrevibacter sp., Methanothrix sp.) and hydrolytic microorganisms capable of degrading specific feedstock components (e.g., Chloroflexi sp., Bacteroidetes sp.—polysaccharide decomposition, Lysinibacillus sp., Ruminofilibacter sp.—cellulose decomposition). Nano-Al₂O₃ and nano-Fe₂O₃ also promote the action of coenzyme F₄₂₀, which is indicative of methanogenic bacterial activity and is a key element in methane formation [75]. Furthermore, nanoparticles increase biogas and biomethane production by stimulating methanogen activity through inherent indirect electron transfer (IET) and previously discussed DIET between methanogens and fermentative bacteria [14,74,79], evidenced by the high efficiency of organic matter degradation [80]. Wang et al. [81] found that nanoparticles promote DIET in methanogenesis pathways using acetate and H₂/CO₂ and reduce the risk of reactor acidification. Li et al. [82] reported that nano-Fe₃O₄ (magnetite) stimulated DIET with increased acetic acid concentration and decreased propionic acid concentration. The stimulating effect of iron-based materials differs from that of conductive carbon materials, likely due to their size and structural differences. Magnetite nanoparticles are generally 20–50 nm in size, which is smaller than microbial cells, while conductive carbon materials are significantly larger and provide a substantial surface area for microbial attachment.

Properly selecting a metal oxide nanoparticle is crucial for the success of the methane fermentation process [70], as some metal ions exhibit strong toxic properties (see Table 4). Nanoparticles such as ZnO and CuO increase the amount of reactive oxygen species (ROS), heightening the risk of oxidative stress [83]. This stress can damage cell membranes, cause leakage of cytoplasmic contents, inactivate enzymes, and disrupt DNA structure, thereby limiting the growth of methanogenic bacteria (e.g., Methanothrix sp.). This can lead to microorganisms’ apoptosis and bioenergy production inhibition [75,78]. Analyses conducted by Chen et al. [77] indicated that some metal oxides negatively affect the methane fermentation process during the hydrolysis stage of the feedstock. This is evidenced by a decrease in the degradation rate of soluble proteins and polysaccharides compared to the control sample. The authors found that zinc and copper cations inhibit the activity of protease and cellulase, enzymes responsible for the breakdown of proteins and cellulose present in the substrates. A low amount of soluble proteins and polysaccharides can slow the conversion of butyric acid to acetic acid and, subsequently, acetic acid to methane. Furthermore, high concentrations of volatile fatty acids negatively impact the functioning of methanogenic bacteria [84,85].

Table 4. Yield of biogas and biomethane in anaerobic digestion using metal oxide nanomaterials.

Type of Additive	Type of Substrate	Yield of Biogas/Biomethane	References
Al2O3	animal fat	increase in biogas production by 285%	[75]
Al2O3	sewage sludge	increase in biogas production by 23.4%	[77]
Al2O3	waste-activated sludge	increase in methane production by 14.8%	[80]
Fe2O3	animal fat	increase in biogas production by 45.87%	[75]
Fe2O3	granular sludge	increase in methane production by 38%	[8]
Fe2O3	waste activated sludge	increase in methane production by 117%	[69]
Fe3O4	corn straw and sewage sludge	increase in methane production by 60.47%	[82]
Fe3O4	waste sludge	increase in methane yield by 58.7%	[86]
Fe3O4	wastewater sludge	increase in biogas production by 96% increase in methane production by 144%	[79]
Fe3O4	wastewater sludge	increase in biogas production by 107% increase in methane production by 153%	[79]
Fe3O4	municipal solid waste	increase in methane yield by 72.09%	[87]
TiO2	fresh dairy cattle manure	increase in methane yield by 121%	[78]
TiO2	anaerobic sludge	increase in methane yield by 14.9%	[80]
CeO2	waste-activated sludge	increase in methane production by 9.2%	[88]
MnO2	seed sludge	decrease in methane production by 93%	[83]
MgO	waste activated sludge	decrease in methane production by 99%	[69]
CoO	sewage sludge	decrease in biogas production by 60%	[80]
CeO2	sludge	decrease in biogas production by 35%	[69]
CeO2	cellulose	decrease in biogas production by 100%	[69]
CuO	cattle manure	decrease in biogas production by 96%	[69]
CuO	sewage sludge	decrease in biogas production by 17.3%	[77]
ZnO	animal fat	decrease in biogas production by 17%	[75]
ZnO	municipal solid waste	decrease in biogas production by 15%	[89]
ZnO	waste-activated sludge	decrease in methane production by 50%	[84]
ZnO	sewage sludge	decrease in biogas production by 90.2%	[77]

Table 4. Yield of biogas and biomethane in anaerobic digestion using metal oxide nanomaterials.

Type of Additive	Type of Substrate	Yield of Biogas/Biomethane	References
Al2O3	animal fat	increase in biogas production by 285%	[75]
Al2O3	sewage sludge	increase in biogas production by 23.4%	[77]
Al2O3	waste-activated sludge	increase in methane production by 14.8%	[80]
Fe2O3	animal fat	increase in biogas production by 45.87%	[75]
Fe2O3	granular sludge	increase in methane production by 38%	[8]
Fe2O3	waste activated sludge	increase in methane production by 117%	[69]
Fe3O4	corn straw and sewage sludge	increase in methane production by 60.47%	[82]
Fe3O4	waste sludge	increase in methane yield by 58.7%	[86]
Fe3O4	wastewater sludge	increase in biogas production by 96% increase in methane production by 144%	[79]
Fe3O4	wastewater sludge	increase in biogas production by 107% increase in methane production by 153%	[79]
Fe3O4	municipal solid waste	increase in methane yield by 72.09%	[87]
TiO2	fresh dairy cattle manure	increase in methane yield by 121%	[78]
TiO2	anaerobic sludge	increase in methane yield by 14.9%	[80]
CeO2	waste-activated sludge	increase in methane production by 9.2%	[88]
MnO2	seed sludge	decrease in methane production by 93%	[83]
MgO	waste activated sludge	decrease in methane production by 99%	[69]
CoO	sewage sludge	decrease in biogas production by 60%	[80]
CeO2	sludge	decrease in biogas production by 35%	[69]
CeO2	cellulose	decrease in biogas production by 100%	[69]
CuO	cattle manure	decrease in biogas production by 96%	[69]
CuO	sewage sludge	decrease in biogas production by 17.3%	[77]
ZnO	animal fat	decrease in biogas production by 17%	[75]
ZnO	municipal solid waste	decrease in biogas production by 15%	[89]
ZnO	waste-activated sludge	decrease in methane production by 50%	[84]
ZnO	sewage sludge	decrease in biogas production by 90.2%	[77]

4. Trace Elements

Trace elements (TEs) are crucial additives in the process of anaerobic digestion to enhance the efficiency and stability of biogas production. Trace elements, also known as micronutrients, are required in small amounts but play significant roles in various metabolic pathways of the microorganisms involved in anaerobic digestion. The trace elements must be adequate for supporting the metabolism of microorganisms in order to maintain the effective digestion process. Otherwise, the performance of anaerobic digestion will significantly deteriorate [90]. The primary functions of TEs include (i) supporting the metabolic activity of microorganisms, leading to increased biogas production and higher methane content; (ii) ensuring the proper functioning of enzymes and metabolic pathways, which helps maintain a stable biodegradation process; (iii) reducing inhibitory compounds, preventing the accumulation of inhibitors such as volatile fatty acids (VFAs) and ammonia; and (iv) preventing toxicity caused by excessive accumulation of certain metals, such as copper or zinc [90,91].

Trace metals such as cobalt (Co), nickel (Ni), selenium (Se), iron (Fe), molybdenum (Mo), and tungsten (W) are important cofactors necessary for the enzymes that catalyze methane production. Their addition to AD has stimulated and stabilized the biogas production process [92,93]. Combinations of trace elements can exhibit synergistic effects [94]. For example, the combination of elevated concentrations of Ni and Co can accelerate the initial exponential rate of the process, increase methanogen density, and thereby improve AD efficiency [94,95]. The supplementation of iron also plays a significant role in the AD process. A study on the feasibility of using iron-rich activated sludge as a stabilizing agent for the AD of food waste confirmed the positive impact of Fe addition on AD stability [92]. In another study, Ketheesan et al. [96] illustrated the importance of Fe²⁺ bioavailability in controlling VFA concentration in batch reactors and organic shock loads in submerged anaerobic membrane bioreactor (SAMBR). The authors noted an increase in methane production from acetate and propionate compared to reactors without iron addition.

It is also important to note that different metals have different bio-uptake processes due to different kinetic and equilibrium processes. The presence of one metal can also influence the speciation and, thus, the bioavailability of another metal. For example, high concentrations of iron in the AD digester can promote co-precipitation, adsorption, and ion substitution of Co and Ni on FeS [97]. Therefore, to optimize the concentration of TEs (and maximize methanogenic activity), it is essential to understand how the speciation of TEs affects their bioavailability [98]. Due to the significance and effects of TE bioavailability in the microenvironment, Zhang et al. [99] developed an innovative approach to encapsulate Ni in dry water.

Trace elements play various roles in biochemical transformations and the AD process. Below are the key functions of trace elements in the process:

• Iron (Fe) is the most commonly studied element for the impact of supplementation on anaerobic digestion due to its high requirement based on fundamental knowledge. First and foremost, Park and Novak [100] demonstrated the direct addition of Fe (III) at 1.25% (by weight) into a sewage sludge digestion system to remove the odor-causing byproducts. They observed that the problematic and corrosive hydrogen sulfide (H₂S) which had been reduced by more than 65%. H₂S generation is reduced due to the FeS precipitation. Kegl [101] also proposed a BioModel based on modified Michaelis–Menten kinetics in the study of the activity of, among others, various forms of iron that increase the production of CH₄ and reduce the content of H₂S in the produced biogas. Based on the results obtained, the author concluded that the absence of additives (including iron) results in a substantially lower specific biogas production rate (by around 50%). Moreover, the H₂S content in biogas is significantly higher (by around 80%).
  It should also be mentioned that Fe plays many roles in anaerobic processes, mainly due to its exceptionally high reduction potential. Because of its properties, it plays a special role in energy metabolism. Iron is crucial for the activity of various enzymes, including hydrogenases and ferredoxins, which are involved in electron transfer and hydrogen metabolism [93,102]. This metal is utilized in transporting methanogenic bacteria for converting CO₂ to CH₄ and serves as both an electron acceptor and donor [6]. Therefore, adequate iron levels can enhance biogas production and stabilize the AD process.
• Nickel (Ni) is essential for the function of several enzymes, such as methyl-coenzyme M reductase, which is key in the final step of methanogenesis. Anaerobic bacteria are heavily dependent on nickel, while carbon dioxide and hydrogen are the sole sources of energy. The nickel tetrapyrrole, coenzyme F₄₃₀, is known to bind to methyl-S-CoM reductase, catalyzing methane formation from methyl-S-CoM in acetoclastic and hydrogenotrophic methanogens [103]. This coenzyme is part of the methyl-coenzyme M reductase enzyme, which reduces methyl-coenzyme M to methane [95,104]. Besides enhancing the acetate utilization rate, Ni stimulation has also been found to have a connection with the predominance of the genus Methanosarcina [90]. Nickel supplementation can improve methane production, especially in nickel-deficient substrates.
• Cobalt (Co) is a critical component of vitamin B₁₂, which activates carboxypeptidase and is required for the metabolism of certain methanogens. Corrinoids, such as vitamin B₁₂, containing a cobalt ion, bind to methyl-coenzyme M (CoM) reductase, catalyzing methane formation in acetoclastic methanogens and hydrogenotrophic bacteria [95]. The enzyme carbon monoxide dehydrogenase (CODH) also utilizes cobalt [105]. Therefore, cobalt can increase methane yield and improve the metabolic activities of acetoclastic methanogens. Besides stimulating the methanogenesis process, the addition of Co is believed to boost the acetogenesis at the initial stage of anaerobic digestion. This is because Co is one of the growing factors of acetogens [90].
• Molybdenum (Mo) is a cofactor for enzymes. Mo is closely involved in formylmethanofuran dehydrogenase (hydrogenotrophic methanogens) and formate dehydrogenase (syntrophic oxidizing bacteria and hydrogenotrophic methanogens), which participates in the conversion of formate to carbon dioxide [106]. Although Mo is considered to be chemically analogous with tungsten (W) in enzyme formation, Mo cannot be replaced by other trace elements for any methanogenic species. The Mo enzyme is synthesized only when Mo is present in the growth medium [93]. This metal can inhibit sulfate-reducing bacteria, limiting the formation of sulfides. Molybdenum can also stimulate methane production from corn silage and municipal waste substrates [95,107]. In summary, molybdenum enhances formate decomposition, thereby supporting the entire AD process.
• Selenium and tungsten (Se and W) are parts of several selenoproteins that protect cells from oxidative damage and participate in redox reactions [93]. Selenium, like tungsten, is a component of the enzyme formate dehydrogenase (FDH), which catalyzes formate production by propionate oxidizers. Certain methanogenic bacteria contain W and Mo enzymes for the same purpose [108]. Few studies have been conducted on the effects of Se and W on methanogenesis. One study conducted on a laboratory scale with food industry waste showed evidence of increased methane production under the influence of Se and W, and additionally in combination with Co [109]. A study also shows that supplementing Se reduced both the acetic and propionic acid concentrations in the batch incubation, thus enhancing biomethane production. It indicates that Se is involved in common hydrogenases and provides co-enzymes necessary for propionate oxidation and syntrophic hydrogenotrophic methanogenesis. Thus, a lack of Se can slow down the AD process [110].
• Zinc (Zn) is involved in enzyme function, stabilizing protein structures, and regulating gene expression. Zinc is a part of enzymes such as formate dehydrogenase (FDH), superoxide dismutase (SODM), and hydrogenase [93,94]. Zn has been found in remarkably high concentrations (50–630 ppm) in 10 methanogenic bacteria [111]. This metal is necessary for maintaining microbial activity and diversity in AD.
• Copper (Cu) functions in redox reactions and electron transport. In general, the role of copper in methanogenesis is contradictorily perceived. It has rarely been studied, making it difficult to understand the role of Cu in biogas production fully. However, it has not been found to have a noticeable stimulating effect on biogas production [93]. It is important to note that while Cu is essential in small amounts, its excess can be toxic to microorganisms [97].

To ensure the effectiveness of trace elements, it is essential to follow monitoring, supplementation, and balancing principles. Regular monitoring of trace element concentrations is crucial for maintaining optimal levels. The required concentration of trace metals during anaerobic digestion varies significantly depending on the type of substrate, process temperature (mesophilic or thermophilic), the mode of AD implementation (mono or co-digestion), and the type of methanogens [97,112]. These factors significantly differentiate the biochemical processes related to the dynamics of metals. Different substrates are characterized by varying metal contents, and the temperature of anaerobic systems can also determine different nutritional requirements. As identified in one study [107], the minimum requirements for Ni, Co, Zn, and Fe in thermophilic glucose fermentation are 0.40, 0.45, 2.0, and 3.5 mg/L, respectively. However, these values differ for mesophilic acetate fermentation or mesophilic AD of organic solid waste [107,112].

Proper supplementation is also crucial, aiming to correct deficiencies and improve performance. The form of supplementation (e.g., soluble salts, chelated forms) and dosing strategies are important factors to consider. While supplementation with trace elements enhances AD, as has been repeatedly emphasized, trace elements themselves do not biodegrade and accumulate in biomass, potentially causing inhibition of AD. Therefore, it is important to monitor and balance trace element levels.

5. Biological Additives

In recent years, increasing attention has been given to finding new and innovative solutions that not only enhance the efficiency of biogas and biomethane production but also increase the ecological value of the entire anaerobic digestion process. A promising solution to this problem is the use of biological additives in the form of microorganism cells and/or enzymes they produce (see Table 5), which do not pose a threat to the natural environment or human health [6,113,114]. Biological additives can be introduced directly into the bioreactor where anaerobic digestion is conducted, or they can be used as a pre-treatment method for the substrate preceding the discussed process [115,116].

5.1. Enzyme Supplementation

The role of commercial enzyme preparations and solutions containing extracellular enzymes isolated from microbial cultures, used as biological additives in anaerobic digestion, is to increase the efficiency of the first stage of this process. Hydrolytic metabolites are responsible for converting hard-to-degrade substrate components into soluble micro-molecules of simpler structure [117], thereby accelerating feedstock hydrolysis [118,119]. Enzymes create gaps in the substrate structure through which successive groups of hydrolytic metabolites migrate, thus increasing the surface area available for enzyme and microorganism activity [120,121,122]. This consequently allows more efficient use of the feedstock, accelerates the entire anaerobic digestion process, and increases the amount of produced biogas and biomethane.

Literature reports indicate that enzymatic supplementation is particularly important for lignocellulosic substrates, such as plant biomass, which is primarily composed of cellulose, hemicellulose, and lignin [115,123]. This highly complex lignocellulosic structure protects the plant cell wall from degradation, consequently slowing down feedstock decomposition and reducing energy generation efficiency [116,124]. Therefore, the hydrolytic enzymes most commonly used as biological additives in anaerobic digestion include lipases and peroxidases, which modify and hydrolyze lignin [125], as well as cellulases, which break β-1,4-glycosidic bonds between glucose molecules in cellulose [19,126,127]. Moreover, amylases responsible for breaking down complex sugars and proteases that hydrolyze bonds in proteins, thereby releasing nitrogen molecules that are converted to ammonia, are often used [118,119].

The activity of enzymatic preparations is directly dependent on the reaction environment. Hence, a key challenge in supplementation is the proper preparation of the bioreactor, particularly maintaining the appropriate pH level and adding the necessary cofactors to achieve the desired enzymatic activity. Moreover, it is important to remember that anaerobic digestion occurs under anaerobic conditions, so metabolites used as biological additives must function effectively in an oxygen-free environment [119]. Another critical aspect is the analysis of the structure and composition of the substrates used [116,128], which will determine the selection of the appropriate enzyme [125,127,129], as well as its concentration, as evidenced by studies conducted by Bhatnagar et al. [130]. Their research indicates that excessively high and low enzyme concentrations inhibit the anaerobic digestion process, producing less biogas and biomethane than the control sample. Due to the highly complex structure of feedstock, Liu et al. [119], Speda et al. [126], and Weide et al. [129] suggest that an effective solution is to use preparations composed of various enzyme groups, which will simultaneously break down multiple substrate components.

Enzyme supplementation is a promising method for accelerating substrate hydrolysis because enzymes have a relatively broad tolerance to varying environmental conditions and high resistance to stress factors that may occur in the digester [113]. Compared to microbial cells, these metabolites are smaller, more soluble, and more mobile, which allows enzymes easier access to the substrate structure and more efficient hydrolysis of its components [6,114].

5.2. Bioaugmentation

Bioaugmentation involves introducing additional monocultures of microorganisms or consortia with specific properties into the digester [114,119], thereby modifying the bioreactor’s microflora and restoring the balance that may have been disrupted by stress conditions [113], particularly in the early stages of the process [129]. The main goal of this method is to enhance and select specific groups of microorganisms to stimulate their hydrolytic and methanogenic activity [6], ultimately leading to increased biogas and biomethane production.

The groups of microorganisms most commonly used for bioaugmentation include hydrolytic bacteria and fungi [131,132,133]. These microorganisms modify the structure of lignin and hemicellulose through the production of enzymes and secondary metabolites, reducing cellulose’s crystallinity, thereby increasing substrate porosity and breaking down its components [116,134,135]. Another significant group comprises typical methanogenic bacteria (methanogens) responsible for carrying out anaerobic digestion and producing biogas and biomethane [136,137]. Recent literature indicates that an effective method is the use of microbial consortia consisting of different fungal species [138,139] or bacterial species [140,141], as well as consortia containing both bacteria and fungi [114,142,143]. Vinzelj et al. [144] suggest that an innovative solution involves utilizing symbiotic relationships between anaerobic fungi, which produce hydrogen and formate, and methanobacteria, which convert these compounds into methane [145,146]. Furthermore, Swift et al. [147] report that methanogenic bacteria positively interact with hydrolytic enzymes produced by fungi, thereby promoting their activity.

A study by Li et al. [136] indicates that bioaugmentation using a consortium of methanogens stimulated signaling mechanisms and the activity of genes responsible for cell motility, stabilized the pH level in the bioreactor and reduced the concentration of volatile fatty acids [138]. Tian et al. [148] demonstrated that bioaugmentation reduced instant hydrogen partial pressure, thereby creating favorable conditions for acetate oxidation and fatty acid degradation. Lebiocka et al. [140] showed that bioaugmentation using a consortium of various bacterial species increased the efficiency of volatile solids removal despite a reduced hydraulic retention time. Furthermore, the aforementioned authors found that the addition of new groups of microorganisms accelerated the metabolic rate and stimulated microbial growth and activity, thus leading to an increase in the efficiency of the methane fermentation process.

Just like with enzymatic preparations, the functioning of microorganisms and their ability to exhibit desired properties are directly dependent on the environment in which they reside. Therefore, a key task is to select suitable bacterial and/or fungal species that can withstand stress conditions (such as excessive ammonia concentrations [148,149], inappropriate temperature, and pH), ensuring that they properly grow and develop in the fermentation bioreactor [6]. Another important aspect is the correct concentration of the introduced inoculum [150], as well as the type, composition, and moisture level of the feedstock used [116,134]. Furthermore, Romero-Güiza et al. [6] and Paritosh et al. [113] reported that innovative bioaugmentation approaches suggest that microorganisms should be introduced into the digester in the form of cultures immobilized on suitable carriers (e.g., microcapsules), indicating that the properties of the auxiliary material also play a role in the selection of biological additives.

Table 5. Yield of biogas and biomethane in anaerobic digestion using biological additives.

Type of Additive	Type of Substrate	Yield of Biogas/Biomethane	References
cellulases, xylanases, β-glucosidases	ensiled forage ley	increase in methane production by 19%	[126]
lipase from Aspergillus	animal fat	increase in methane production by 80.8%	[125]
	arachis oil	increase in methane production by 26.9%	[125]
	floatable grease	increase in methane production by 37%	[125]
lipase from Candida	animal fat	increase in methane production by 157.7%	[125]
	arachis oil	increase in methane production by 53.8%	[125]
	floatable grease	increase in methane production by 40.7%	[125]
bio-additive Digest P3 (carbohydrases, pectinase, xylanase)	poultry litter	increase in biogas production by 59.7% increase in methane production by 91.4%	[151]
bio-additive APD (Aerobacter, Pseudomonas, Alcaligenes, cellulase, lipase)	igniscum silage	increase in biogas production by 6% decrease in methane production by 7%	[19]
	maize silage	increase in biogas production by 53% increase in methane production by 74%	[19]
bio-additive PPT (Pseudomonas, Flavobacterium, Lactobacillus, cellulase, lipase)	igniscum silage	increase in biogas production by 16% increase in methane production by 26%	[19]
	maize silage	increase in biogas production by 62% increase in methane production by 79%	[19]
bio-additive HAP (Clostridium, Micrococcus, cellulase, lipase)	igniscum silage	increase in biogas production by 12% increase in methane production by 30%	[19]
	maize silage	increase in biogas production by 32% increase in methane production by 46%	[19]
bio-additive JENOR (Pichia, Trichoderma, cellulase, lipase)	igniscum silage	increase in biogas production by 13% increase in methane production by 16%	[19]
	maize silage	increase in biogas production by 17% increase in methane production by 26%	[19]
Orpinomyces sp.	barley, triticale, rye, wheat, cow manure	increase in methane production by 33%	[132]
Ochrobactrum sp.	sewage sludge	increase in biogas production by 22.06%	[152]
Caldicellulosiruptor bescii	birch wood chips	increase in methane production by 44%	[135]
Clostridium thermocellum	wheat straw, cow manure	increase in methane production by 39%	[131]
Clostridium cellulolyticum Clostridium cellulovorans Clostridium aceticum Mesotoga infera Methanosarcina barkeri Methanosaeta concilii	Axonopus compressus	increase in methane production by 20.7%	[141]
Neocallimastix sp. Orpinomyces sp. fermentative bacteria	wheat straw	increase in methane production of 290%	[142]
	mushroom spent straw	increase in methane production by 330%	[142]
Aspergillus sp. Trichoderma viride	maize straw	increase in methane production by 31.7%	[143]
Trichoderma atroviride	water hyacinth	increase in biogas production by 65% increase in methane production by 117%	[133]
Trichoderma reesei	rice straw and soybean straw	increase in biogas production by 318% increase in methane production by 807%	[153]
Orpinomyces sp. Piromyces sp. Anaeromyces sp. Neocallimastix frontalis	algal biomass	increase in methane production by 40.6%	[138]
	cow manure	increase in methane production by 60%	[139]

Table 5. Yield of biogas and biomethane in anaerobic digestion using biological additives.

Type of Additive	Type of Substrate	Yield of Biogas/Biomethane	References
cellulases, xylanases, β-glucosidases	ensiled forage ley	increase in methane production by 19%	[126]
lipase from Aspergillus	animal fat	increase in methane production by 80.8%	[125]
	arachis oil	increase in methane production by 26.9%	[125]
	floatable grease	increase in methane production by 37%	[125]
lipase from Candida	animal fat	increase in methane production by 157.7%	[125]
	arachis oil	increase in methane production by 53.8%	[125]
	floatable grease	increase in methane production by 40.7%	[125]
bio-additive Digest P3 (carbohydrases, pectinase, xylanase)	poultry litter	increase in biogas production by 59.7% increase in methane production by 91.4%	[151]
bio-additive APD (Aerobacter, Pseudomonas, Alcaligenes, cellulase, lipase)	igniscum silage	increase in biogas production by 6% decrease in methane production by 7%	[19]
	maize silage	increase in biogas production by 53% increase in methane production by 74%	[19]
bio-additive PPT (Pseudomonas, Flavobacterium, Lactobacillus, cellulase, lipase)	igniscum silage	increase in biogas production by 16% increase in methane production by 26%	[19]
	maize silage	increase in biogas production by 62% increase in methane production by 79%	[19]
bio-additive HAP (Clostridium, Micrococcus, cellulase, lipase)	igniscum silage	increase in biogas production by 12% increase in methane production by 30%	[19]
	maize silage	increase in biogas production by 32% increase in methane production by 46%	[19]
bio-additive JENOR (Pichia, Trichoderma, cellulase, lipase)	igniscum silage	increase in biogas production by 13% increase in methane production by 16%	[19]
	maize silage	increase in biogas production by 17% increase in methane production by 26%	[19]
Orpinomyces sp.	barley, triticale, rye, wheat, cow manure	increase in methane production by 33%	[132]
Ochrobactrum sp.	sewage sludge	increase in biogas production by 22.06%	[152]
Caldicellulosiruptor bescii	birch wood chips	increase in methane production by 44%	[135]
Clostridium thermocellum	wheat straw, cow manure	increase in methane production by 39%	[131]
Clostridium cellulolyticum Clostridium cellulovorans Clostridium aceticum Mesotoga infera Methanosarcina barkeri Methanosaeta concilii	Axonopus compressus	increase in methane production by 20.7%	[141]
Neocallimastix sp. Orpinomyces sp. fermentative bacteria	wheat straw	increase in methane production of 290%	[142]
	mushroom spent straw	increase in methane production by 330%	[142]
Aspergillus sp. Trichoderma viride	maize straw	increase in methane production by 31.7%	[143]
Trichoderma atroviride	water hyacinth	increase in biogas production by 65% increase in methane production by 117%	[133]
Trichoderma reesei	rice straw and soybean straw	increase in biogas production by 318% increase in methane production by 807%	[153]
Orpinomyces sp. Piromyces sp. Anaeromyces sp. Neocallimastix frontalis	algal biomass	increase in methane production by 40.6%	[138]
	cow manure	increase in methane production by 60%	[139]

6. AD Additives Research—Summary

This chapter presents a summary of the most commonly used additives. The discussion will start with granulated activated carbon (GAC). This substance has often been used as a conductive material for DIET stimulation so far, as its preparation and application in the reactor are not very problematic, while its properties are typical of carbon-based materials. It is interesting to note that the conductivity of GAC is approximately 1000 times higher than that of biocarbon [14]. In their study on the AD process of wastewater, Park et al. [10] proved that thanks to GAC supplementation, it was possible to shorten the delay time by 29.8% and increase the methane production rate by 23.4% at a low temperature of 25 °C. So far, additive systems introduced into anaerobic bioreactors have been extremely rarely analyzed. Barua et al. [9] were the first to study GAC doped with magnetite, through which the scientist achieved the highest methane production and COD removal (70%) in fed-batch tests. Powdered activated carbon (PAC), similarly to GAC, facilitates electron transfer between microorganisms by acting as the electrical bridge and enhancing both hydrogenotrophic and aceticlastic pathways. However, in the case of that compound, a more favorable effect is noticed at the hydrolysis stage compared to GAC: effective acceleration of volatile fatty acids (VFAs) consumption and thereby, alleviation of acidification and lag phase. The stimulation of hydrolysis by PAC was also demonstrated by Yan et al. [29]. The results revealed that genes coding for hydrolytic enzymes and xenobiotic metabolism were highly expressed in the presence of PAC.

Biochar has great potential to be used as an AD additive to improve efficiency, which could enhance AD by overcoming toxicant inhibitions, enriching robust microbes, and promoting interspecies electron transfer. Additionally, from a biorefinery perspective, a system integrating AD and thermochemical conversion of biochar could be an effective strategy for waste management and biofuel production [154]. As confirmed by literature reports, the high adsorption capacity and numerous redox-active groups of biochar are beneficial for methane production [155]. Adsorption alleviates the inhibition caused by toxic substances, while the redox-active groups can promote electron transfer between anaerobic microorganisms. It should be emphasized that although the advantages of biochar for improving anaerobic digestion are widely accepted, the practical application of biochar in anaerobic digestion has not been fully verified [156].

Another additive selected for this study was carbon nanotubes (CNTs), which are recognizable as cylindrical carbon nanostructures consisting of a hexagonal mesh. Both single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs) are characterized by high electrical conductivity, high mechanical strength, and thermal conductivity. They also stimulate DIET to form methane, although they are toxic to microorganisms. In a study by Li et al. [45] the response of anaerobic sludge to the dose of SWNTs was examined, and substrate utilization and biogas production were monitored. More rapid substrate utilization and CH₄ generation were observed. In turn, Ambuchi et al. [8] investigated the role of MWNTs in enhancing the AD performance of granular-activated sludge. It was demonstrated that MWNTs improved the kinetics and effectiveness of biogas generation. Conductive polymers can replace the carbon-based conductive materials described above.

Graphite and graphene, classified as carbon-based conductive materials, have also been used in biogas and biomethane production. The use of these additives in appropriate doses has positively affected the fermentation process, particularly by stimulating the acetoclastic pathway [53,54] and the direct transfer of electrons and energy between microorganisms in the bioreactor [57]. The ability to initiate the formation of bacterial aggregates [55], protect microorganisms from unfavorable environmental conditions, and increase the activity of enzymes [60] and bacteria that decompose the feedstock [54] are key properties of graphite and graphene, which make them promising solutions for increasing the amount of biogas and biomethane produced.

Nanostructured materials, a nanotechnology product, also have a significant position as effective additives in the AD process [157]. In that category, additional materials for methane fermentation are metal oxide NPs, such as Al₂O₃, Fe₂O₃, Fe₃O₄, and TiO₂. Metal oxide nanoparticles stimulate the growth of methanogenic bacteria and hydrolytic microorganisms capable of degrading organic matter [158]. Additionally, nanostructured materials promote indirect electron transfer (IET) and direct interspecies electron transfer (DIET) [74,79]. Properly selecting a metal oxide nanoparticle is crucial for the success of anaerobic digestion [70], as some ions exhibit strong toxic properties. Nanoparticles such as ZnO, CuO, Mg, and CeO₂ inhibit the activity of enzymes responsible for the decomposition of feedstock [77] and also increase the amount of reactive oxygen species (ROS), thus leading to apoptosis of microorganisms [78].

Trace elements (TEs) are another type of additive that has a significant impact on kinetics and efficiency. Trace elements are key cofactors in many enzymatic reactions of the metabolic methane production pathway. Fe, Ni, and Co are the most often studied NS, since they are essential cofactors of carbon monoxide dehydrogenase, acetyl-CoA decarbonylase, metreductase, and other enzymes involved in the acetoclastic methanogenesis pathway [6]. Furthermore, those metals have also proved to be essential for the acetotrophic pathway of methanogenesis, which is currently gaining importance. For example, Pobeheim et al. [95] noted in their work that Ni and Co deficiency has a negative impact on the stability of the process (i.e., VFA accumulation).

Supplementation of enzymes and microorganisms, known as biological additives, is also widely studied as an alternative to the physicochemical pretreatment of substrates before AD, while their direct introduction into fermentation chambers has received less attention. The main characteristics of microorganisms that influence their key use in the anaerobic digestion process are their very strong cellulolytic and hemicellulolytic properties, which allow them to accelerate the hydrolysis of the polysaccharides that make up the cell wall of plant waste biomass. Despite the abundant supply of lignocellulosic waste biomass on Earth, it is rarely used in AD [18]. When it comes to the use of enzyme preparation, Fugol et al. [19], in their study with the use of different lignocellulosic substrates, introduced four commercial vaccines containing different bacteria species or a yeast and mold mixture into the reactor along with the inoculum and reported improved biogas production. In turn, Noyola and Tinajero [159], after using the lyophilized bacilli system with a solution of micronutrients, managed to increase methane production dramatically, confirming the validity of using appropriate combinations of additives. The presented summary constitutes the basis for formulating reasons why research in the field of additives for anaerobic digestion should be continued in a multi-faceted manner using advanced techniques.

Table 6. Properties, advantages, and disadvantages of the additives.

Type of Additive	Properties	Advantages	Disadvantages
granular activated carbon and powdered activated carbon	high electron conductivity large specific surface area	promote the syntrophic metabolism of alcohols and volatile fatty acids stimulates hydrolysis activity stimulates methanogenic activity shortening lag phase reducte phenolic compounds maintaining the appropriate composition of the microorganism community	an inappropriate dose may inhibit the production of biogas and biomethane acumulation of volatile fatty acid cost
biochar	highly porous carbonaceous material high adsorption capacity numerous redox-active groups	stimulate IET and DIET shortening lag phase reduce ammonia and volatile fatty acid levels alleviate the inhibition caused by toxic substances environmentally friendly maintaining the appropriate composition of the microorganism community	an inappropriate dose may inhibit the production of biogas and biomethane difficulty in obtaining biochar with appropriate properties some types of biochar may contain toxic substances or heavy metals cost
single-walled nanotubes and multiwalled nanotubes	small dimensions large specific surface area high electrical conductivity high mechanical and thermal strength	stimulate DIET acceleration of syntrophic electron transfer improve operational stability reduce ammonia levels	toxic to microorganisms negative impact on the natural environment problem with utilization
graphite and graphene	large adsorption surface high electrical and thermal conductivity properties high mechanical strength	stimulate the growth of hydrolytic bacteria stimulate DIET increase the activity of enzymes responsible for methanogenesis initiates the formation of microorganism aggregates minimize the risk of acidification shortening lag phase protects microorganisms from adverse environmental conditions	an inappropriate dose may inhibit the production of biogas and biomethane negative impact on the natural environment cost
metal oxide nanomaterial	small particle size large specific surface area and density high tensile strength resistance to degradation by microorganisms or chemicals high-temperature superconductivity capacity to penetrate through cell membranes	promote the synthesis of enzymes and cellular components essential for the proper growth and development of microorganisms stimulate IET and DIET shortening lag phase low cost recovery post-fermentation available in various forms	ZnO and CuO increase the amount of reactive oxygen species and heighten the risk of oxidative stress ZnO and CuO inhibit the activity of enzymes responsible for the degradation of feedstock risk of acidification
trace elements	key cofactors in many enzymatic reactions of the metabolic methane production pathway	supporting the metabolic activity of microorganisms preventing the accumulation of inhibitors (e.g., volatile fatty acids and ammonia) preventing toxicity caused by excessive accumulation of certain metals combinations of different trace elements can exhibit synergistic effects	trace elements are non-biodegradable trace elements can accumulate in biomass
enzymes	high hydrolytic properties relatively broad tolerance to varying environmental conditions high resistance to stress factors smaller, more soluble, and more mobile than microbial cells	acceleration of feedstock hydrolysis degradation of difficult-to-decompose feedstock (e.g., lignin, hemicellulose) increase the ecological value of the entire process safe for human and animal health environmentally friendly possibility of using different groups of enzymes at the same time	difficulty in creating and maintaining an appropriate reaction environment cost
microorganisms	high hydrolytic properties high methanogenic properties	maintaining the appropriate composition of the microorganism community acceleration of feedstock hydrolysis degradation of difficult-to-decompose feedstock (e.g., lignin, hemicellulose) increase the ecological value of the entire process safe for human and animal health environmentally friendly possibility of using different groups of microorganisms at the same time	sensitivity of microorganisms to environmental conditions difficulty in creating and maintaining an appropriate reaction environment cost

summarizes the properties, advantages, and disadvantages of the additives discussed in this article.

7. Feasibility and Perspectives

The incorporation of an increasingly diverse range of waste materials with varying chemical compositions and origins into the anaerobic digestion process often leads to technical challenges, negatively affecting the kinetics and efficiency of the process [1,160]. These challenges primarily include poor biodegradation of the fermented material due to inhibition, low microbial activity, high activation energy, and other factors. As a result, the optimal extraction of chemical (primary) energy from the substrates is hindered [161]. These difficulties are often addressed by using effective yet largely unrecognized additives in the AD process. The literature on such additives is frequently speculative, underscoring the need for continued research and expansion of knowledge.

The previously cited reports suggest that attempts to combine additives have yielded very promising results, confirming that this approach is promising for improving the kinetics and effectiveness of AD. Successfully integrating functional additives into AD requires a comprehensive strategy that takes into account the interactions between additives, optimizes their application through controlled dosing and real-time monitoring, and ensures both economic and regulatory compliance. Understanding synergistic and antagonistic effects is crucial for developing additive systems. Some additives may enhance each other’s effects, leading to improved efficiency. For instance, adding both trace elements and specific enzymes can result in a more effective breakdown and conversion of organic matter. Conversely, other additives may interfere with each other, reducing effectiveness or even inhibiting the process. To avoid such conflicts, careful selection and dosing are essential. Thus, by strategically integrating additives, AD systems can achieve higher efficiency, stability, and biogas production, contributing to more sustainable and effective waste management and energy generation.

To sum up, additives have the potential to significantly enhance anaerobic digestion by improving biogas yield, process stability, and the quality of digestate. However, their use must be carefully managed to balance performance benefits with economic and environmental considerations. Additives must also comply with regulatory standards for environmental safety and public health. It is crucial to ensure that additives do not introduce harmful contaminants into the digestate, which could affect its suitability as a fertilizer. Continued research and technological advancements are expected to expand the range of effective and safe additives available for AD.

8. Conclusions

The supplementation of various additives, including those discussed in this paper—conductive carbon materials, metal oxide nanomaterials, trace metals, and biological additives—in anaerobic digesters is currently a topic of significant interest. The primary objective of research and attempts to implement additive-based technologies is to increase biogas production, while secondary goals include reducing process inhibition and foaming and improving the rheology of the digestate. Most studies have been conducted in laboratory-scale batch reactors, with less emphasis on continuous reactor operations, which partly limits the ability to model and extrapolate results to industrial scales.

The results presented largely confirm the positive impact of the tested additives, particularly conductive carbon and iron-based materials, as well as trace metals and selected metal oxide nanomaterials. Inorganic additives are the subject of numerous research studies. Trace elements, as cofactors in various enzymatic reactions, are often studied individually and in combinations, yielding promising results. Notably, iron has been frequently reported as a promising additive due to its low cost. However, not all types of nanomaterial oxides have produced the expected results—some, such as CuO and ZnO, are toxic to microorganisms and thus unsuitable for use. On the other hand, bioaugmentation has been the focus of more research than enzymatic supplementation with hydrolytic properties. The results clearly indicate that bioaugmentation can be a useful technique for improving AD performance, though its industrial application remains limited, mainly due to economic factors and the relatively small number of studies conducted so far.

Overall, research on additives focuses on understanding their mechanisms of action, optimizing dosages, and application methods. Advanced analytical techniques, such as metagenomics and proteomics, are employed to study microbial communities and their interactions with additives. Developing new additives is driven by environmental and economic considerations, sustainability, and cost-effectiveness. These trends reflect a holistic approach to improving AD processes by enhancing microbial activity, substrate availability, and process stability, ultimately leading to higher biogas yields and more efficient waste processing.