A Review of Operational Conditions of the Agroforestry Residues Biomethanization for Bioenergy Production Through Solid-State Anaerobic Digestion (SS-AD)

Abstract

Agroforestry residues are a promising source of organic matter and energy. These organic wastes are often poorly managed by incineration or open-air composting, resulting in the emission of greenhouse gases. Solid-state anaerobic digestion has recently attracted considerable attention to converting organic waste with a high total solids content, such as agroforestry residues, into renewable energy. However, the complex structure of these residues is still a defiance to this technology. Their degradation requires a long period, resulting in low heat and mass transfer. In addition, the process is often inhibited by the accumulation of toxic compounds. An efficient management process has remained under development. Comprehending the challenges faced when treating agroforestry waste is necessary to create practical applications. This review provides essential information for more effective management of complex agricultural and forestry residues using the SS-AD process. It covers the different parameters and experiments that have successfully managed these residues for renewable energy production. Various solutions have been identified to overcome the drawbacks encountered. These include co-digestion, which brings together different residues for better sustainability, and the strategies used to improve energy production from these residues at different levels, involving efficient pretreatments and appropriate operational reactor designs.

1. Introduction

Climate change caused by increasing greenhouse gas (GHG) emissions into the atmosphere is one of the greatest environmental challenges of our time. Most GHGs are produced through both natural processes and anthropogenic activities. Industries across various sectors generate huge amounts of solid waste, such as agricultural and forestry, food, and sewage sludge wastes [1]. A large proportion of these residues is incinerated for heat production or handled by direct fertilization, outdoor composting, or landfill, resulting in GHG emissions [2,3]. Other technologies have been used to produce bioenergy from biomass, such as gasification, which produces syngas with a higher energy content, and pyrolysis, which converts biomass into bio-oil and biochar [4]. However, these technologies require processing at higher temperatures, which entails substantial costs. Anaerobic digestion (AD) can be a viable alternative to processing these residues. An AD is a set of biological processes in which bacteria break down biodegradable materials without oxygen, converting organic wastes into biogas, thus making it a natural recycling process suitable for various raw materials [5]. The biogas produced could power industrial activities as fuel, providing clean energy and reducing greenhouse gas emissions [6]. The remaining sludge from the AD process, known as digestate, is a nutrient-rich by-product that could be managed sustainably as a biofertilizer [7,8]. Recovering the digestate produced by AD is part of the circular economy, distinguishing anaerobic digestion from other energy-producing technologies. To classify AD processes, critical operating parameters, and reactor design are considered, such as the process continuity (batch/continuous), the operating temperature, the reactor design (plug flow, complete-mix, and covered lagoons, fixed film, up-flow anaerobic sludge blanket, etc.), and the solids content (wet/dry).

Liquid-state anaerobic digestion (LS-AD) is the most widely used digestion with a feedstock moisture content (MC) of over 85% [9]. Its high water content homogenizes the digester materials and enhances interaction between microorganisms. However, this technology requires large reactors and a substantial amount of substrate to generate a significant volume of gas. It must dispose of a large amount of liquid digestate, resulting in high investment and operating costs [10,11]. For the treatment of residues with a total solids content (TS) higher than 15%, solid-state anaerobic digestion (SS-AD, also known as dry AD) is considered a promising technology [12]. It can handle higher organic loading rates (OLR), requiring less water and a smaller reactor volume [13]. Usually operated at mesophilic or psychrophilic conditions, it has lower energy requirements for heating or mixing and higher volumetric methane productivity, permitting the construction of a low-cost biogas plant directly deployable on the farm [14]. However, SS-AD suffers from drawbacks such as the relatively low reaction rate due to the longer retention time and the lack of mixing, compromising the mass transfer [15]. Furthermore, susceptibility to inhibitor accumulation can limit the process [16].

This review examines the agricultural, livestock, crop, and forestry sectors, which generate substantial amounts of solid organic residues and seek to adopt zero-waste strategies. However, sustainable management remains challenging due to limited data on optimal processing conditions and the complexity of anaerobic digestion systems. This paper attempts to comprehensively explore the possibility of linking these different sectors by valorizing their various residues through an intensive process that solves the problem of difficult management faced and creates a clean, profitable energy source. It explores the principles and the different parameters that influence the anaerobic digestion of solid residues, mainly SS-AD. By analyzing experimental data and successful case studies, this review highlights the benefits of co-digestion, strategies for maximizing biogas yield, and advancements in reactor design. It also presents data-driven solutions to optimize SS-AD performance, improve process stability, and enhance scalability for large-scale applications. Despite recent progress, further research should focus on reactor design improvements to enhance mass transfer and prevent inhibition. Advancing pretreatment methods to increase the biodegradability of lignocellulosic biomass is also essential. Additionally, cost-effective biogas upgrading techniques will be crucial for maximizing energy recovery and supporting the industrial-scale adoption of SS-AD technologies.

2. Principle of Solid-State Anaerobic Digestion (SS-AD)

2.1. Solid-State Anaerobic Digestion Steps

Anaerobic digestion, either LS-AD or SS-AD, is generally carried out in four distinct steps, summarized in Figure 1. During the first step (hydrolysis), strict or facultative anaerobic bacteria, such as the Firmicutes and Bacteroidetes phyla, simplify more complex compounds by making them available for an acid fermentation phase [17]. Hydrolysis is commonly the rate-limiting step for recalcitrant organic substrates such as dairy manure and lignocellulosic biomass operated in SS-AD, particularly due to these materials’ complex nature [18,19]. In this context, potential candidates for lignin degradation were identified, in addition to Firmicutes, belonging to the Pseudomonadata and Chloroflexota phyla. [20]. During the second step (acidogenesis), acidogenic bacteria, generally belonging to the Chloroflexi, Firmicutes, and Proteobacteria phylum, degrade simple organic matter into volatile fatty acids (VFAs) [17]. During the third step (acetogenesis), acetogens, generally belonging to the Firmicutes and Proteobacteria phyla, ensure reductive acetogenesis and syntrophic oxidation of organic acids. They serve as a critical link between the hydrolyzing and the fermenting microbial community and the methanogenic archaea [21]. Methanogenesis is the final stage and involves strictly anaerobic methanogens, which consist exclusively of hydrogenotrophic, acetoclastic, and methylotrophic archaea [22]. In SS-AD, mixotrophic and hydrogenotrophic pathways are the predominant methanogenic pathways, as opposed to the acetoclastic pathway in LS-AD [23]. An optimized and stable process requires different operational approaches for SS-AD than LS-AD. According to recent technical advances, SS-AD occurs in two steps, separating dry fermentation from wet methanogenesis step [24]. A nutrient-rich, pH-neutral effluent is drained into the stacked solids during fermentation. The liquid phase is prevented from accumulating, resulting in a dry process. The rest of the process occurs in the wet phase of methanogenesis. Biogas is formed from the liquid leached from the stack, characterized by a low pH and high VFA. Fermentation and methanogenesis can performed in the same reactor, called single-phase systems, or separately in two-phase systems via segregated reactors (Figure 2).

2.2. Major Inputs and Outputs Determining the Process Efficiency of SS-AD

Like all processes, SS-AD has a wide range of parameters that can positively or negatively affect anaerobic digestion, methane yield, and organic matter reduction. Process parameters such as pH, ammonia, and volatile fatty acids are physicochemical indicators of toxicity level, reactor performance, and methane yield in anaerobic digestion.

In the SS-AD reactors, microorganisms require carbon (C) and nitrogen (N) as essential nutrients. C is an energy source, and N is used for protein and nucleic acid synthesis [25]. The C/N ratio is one of the most influential parameters affecting anaerobic digestion performance [26]. An operating C/N ratio between 20 and 30 is recommended for a good AD process [27]. A too-low C/N ratio caused by C deficiency leads to ammonia-N accumulation, inhibiting methanogenesis [28]. In contrast, a too-high C/N ratio caused by nitrogen deficiency leads to VFA accumulation and pH decrease. In this case, the alkalinity of buffering substances will not be sufficient to prevent inhibition by acidification [29]. To overcome the large amount of nitrogen in certain substrates such as manure, adding co-digestion materials with higher C content can improve the low C/N ratio, increasing methane production. On the other hand, co-digestion of lignocellulosic residues with N-rich substrates can result in a balanced C/N ratio for appropriate anaerobic digestion [27,30,31].

AD temperature is also a significant factor that influences the activity and stability of microbiomes. Research on AD is conducted in either the mesophilic (30–40 °C), thermophilic (50–60 °C), or psychrophilic (10–20 °C) temperature ranges [32]. Thermophilic conditions are more efficient for complex matter degradation, resulting in higher methane production and lower greenhouse gas emissions [33]. However, Li et al. reported that in SS-AD mesophilic reactors, microbial richness and regularity were higher than in thermophilic SS-AD reactors, where Firmicutes made up 60% of the bacteria compared to 82% for thermophilic SS-AD [34]. The study also showed that Methanothermobacter was the most abundant Archaea species in thermophilic SS-AD reactors, and Methanoculleus was the most abundant in mesophilic SS-AD reactors. Recent studies have suggested a preferred temperature for manure co-digestion of 45 °C because of the possibility of increasing syntropic energy between hydrolytic and methanogenic microorganisms, leading to higher methane production than thermophilic regimes at 55 °C [35]. This temperature range is recommended as it combines the advantages of classical mesophilic and thermophilic AD [32]. Su et al. also registered the highest biogas yields at 47.3 °C by varying the temperature range (40–60 °C) during SS-AD of agricultural wastes [26]. After 20 days of operation, it was reported that Firmicutes, Bacteroidetes, Chloroflexi, Synergistetes, and Proteobacteria dominated the bacterial community and that Firmicutes had a competitive advantage over Bacteroidetes at higher temperatures. In northern countries, however, these processes are limited by the energy required to heat the bioreactors and maintain the target temperatures. In these extreme circumstances, a recent study revealed the technical feasibility of psychrophilic anaerobic digestion under cold conditions, where the inoculum used was previously acclimatized to treating organic waste at low temperatures [36].

Furthermore, pH value also plays a crucial role in anaerobic digestion. In multiple stages reactors, hydrolysis and acidogenesis occur in the first reactor under acidic conditions, while acetogenesis and methanogenesis occur in the second digester characterized by a neutral to basic pH [37]. In the first reactor, hydrolytic and acidogenic bacteria are relatively less sensitive to pH, tolerating the range of 4.0–8.5. However, a pH below 5.5 can leads to lactic acid accumulation similar to ensilage, and above 5.5 to dark fermentation according to the dominant microorganisms present [38]. The optimal pH for these microorganisms is between 5.5 and 6.5 [9,39]. In the second reactor, methanogens are very sensitive to pH, and a pH around 7 must be preferred for their highest performance [9]. It is worth mentioning that maintaining an optimal pH for all microorganisms is challenging for a single-stage digester. A pH between 6.8 and 8.0 results in a higher amount of biogas production during dry anaerobic digestion, according to Hossain et al. [39]. Adding urea respecting an optimal C/N ratio, as proposed by Khaled et al., could maintain the pH in the optimal range and result in high biogas production in the anaerobic co-digestion of fallen tree leaves and cattle manure [40].

Another important parameter, the VFAs, are essential intermediates in anaerobic digestion produced during the acidogenesis and acetogenesis steps through the organic matter degradation. Hydrolytic bacteria secrete enzymes that break down complex organic polymers into simpler organic monomers [17], and then acidogens ferment these monomers into VFAs. Lukitawesa et al. mentioned that inoculum acclimatization could be a strategy to enhance VFA production in anaerobic digestion [41].

The solid retention time (SRT) and the organic loading rate (ORL) are other parameters that must be controlled. The SRT is the average time solid materials are retained in the digester. Under mesophilic conditions effective SRT values must be greater than 20 days under mesophilic conditions and between 7 and 15 days under thermophilic conditions [42]. The OLR is the amount of organic matter introduced per unit time and per unit volume of the system regarding chemical oxygen demand. SRT could be associated with OLR in continuous anaerobic digesters; a decrease in SRT increases OLR [43]. These parameters are important as they affect the maintenance of digestion stability. Anaerobic reactors are optimized by controlling SRT and OLR to increase methane production. They should be carefully adjusted to ensure good process stability. Recent research mentioned that an appropriate increase in OLR improved the methane yield due to the high bioconversion activity and the enhanced acidogenesis, acetogenesis, and methanogenesis phases [44,45]. Furthermore, high OLR increases substrate availability, resulting in enhanced VFA production. However, excessive OLR leads to a fast rate of hydrolysis and acidogenesis, resulting in excessive acidification. Sugar-rich substrates are a perfect example of biomatter that generates pronounced acidification [31]. This acidification, due to the accumulation of non-acetic VFA, leads to an irreversible decrease in pH [46]. Adding alkalinity sources is the most common way to relieve excessive acidification, such as sodium hydroxide, metal ions, and zero-valent iron [47]. During anaerobic digestion, nutrient availability is a factor that affects microbial performance. In addition to the basic C and N nutrients for microbial growth, they also require nutrients for the enzyme complex, which include trace elements such as nickel (Ni), iron (Fe), and cobalt (Co) [48]. Methane production processes are more stable and efficient when trace elements are added, while process imbalances are caused by their deficiencies [49]. Adding co-substrates rich in trace elements could enhance biomethanization [50]. Xi et al. confirmed the efficiency of using different herbal-extraction process residues, known by their richness in trace elements, as co-substrates for wheat straw in the presence of pig anaerobic sludge used as inoculum for 30 days under mesophilic conditions [51]. The cumulative production of biogas and methane increased, up to 37% compared to the control. Moreover, in the SS-AD, substrate particles containing trace elements could be both a source of nutrients and a support medium for microorganisms that attach and penetrate the surface [42].

The output can be tailored to achieve good biochemical methane potential (BMP), cumulative methane yield, and biogas composition by fine-tuning these parameters. The biochemical methane potential (BMP) is a significant indicator for evaluating the suitability of feedstocks for biogas production. A technique that measures a substrate’s methane generation potential is also known as the anaerobic bio-gasification potential assay [52]. BMP guides biogas engineering feed, optimizes AD equipment, and monitors AD status. An important part of the economic feasibility evaluation is to estimate energy recovery opportunities from organic wastes [53]. The BMP assay is essential for optimizing material proportions for anaerobic co-digestion. Traditional BMP tests take at least 20 days to complete. Rapid evaluation is required to determine optimal feedstock ratios and adequate conditions for methane production. Theoretical methane potential can be calculated using various methods, including chemical oxygen demand (COD), primary composition, and kinetic models [54]. New technologies such as near-infrared spectroscopy (NIRS) combined with chemometrics using variable selection algorithms have recently been investigated. These technologies can realize qualitative analysis and quantitative detection of material composition based on the information about hydric groups such as –CH, –NH, and –OH and can ensure calibration models that quickly evaluate the BMP [55]. Cumulative methane yield is directly related to the volume of methane produced during digestion. It is a normalized value based on the volatile solids content. Typically, the more material available for digestion, the higher the cumulative yield.

Input selection can also influence biogas composition and specific gas production. Biogas is a renewable energy source that is characterized by its chemical composition, which varies with the organic matter digested and contains mainly CH₄ (50–75%) and carbon dioxide (CO₂) (25–50%), as well as traces of hydrogen sulfide (H₂S), oxygen (O₂), ammonia (NH₃), nitrogen (N₂), hydrogen (H₂), and carbon monoxide (CO) [56,57,58]. By using sludge, the expected biogas production is approximately 800–1100 m³·t⁻¹ of converted volatile solids (VS) [59]. However, using generated biogas may be constrained by impurities such as H₂S, CO₂, NH₃, and siloxanes [60]. In fact, biogas contaminants can cause health issues such as pulmonary paralysis, asthma, and other respiratory disorders. Additionally, these impurities have a negative impact on the environment, contributing to global warming and climate change [61]. An upgrading step is required to remove CO₂, H₂S, and other contaminants to purify the biogas into biomethane [58]. Several upgrading methods are available to obtain high-quality biomethane, such as membrane separation, water scrubbing, pressure adsorption, and solvent scrubbing [62]. Recently, biogas recirculation in AD reactors has been reported as a cost-effective and promising method to enhance methane content from the digestion of a mixture of cattle manure, sewage sludge, and fruit waste [63]. It increased methane, reduced harmful compounds such as H₂S, and improved microbial growth. SS-AD processes feedstocks with higher total solids content, operating more effectively at higher organic loading rates and producing higher volumes of biogas. Biogas upgrading improves SS-AD performance and increases the value of end products. Upgrading biogas to higher-value transportation fuels such as compressed natural gas, liquefied biogas, methanol, and ethanol improves the economics of SS-AD from lignocellulosic biomass [64].

In addition to biogas production, SS-AD also produces nutrient-rich digestate, characterized by a lower moisture content, which is much easier to handle than liquid AD, requires sustainable management, and can be recycled as fertilizer or processed into biochar, that can be used as a soil amendment [26,65]. Moreover, the process resolves the problem of unpleasant odors and eliminates various pathogens [66]. Digestate can be used directly or separated into liquid and solid components based on its properties. Recycling the digestate back into the soil properly promotes a circular economy, increases soil fertility, and reduces the need for synthetic fertilizers and environmental issues brought about by using natural feedstock directly [67].

3. Solid-State Anaerobic Digestion of Forestry and Agricultural Biomass

A bibliographic study was conducted to emphasize the cumulative methane yields of forestry, presented in

Table 1. Cumulative methane production of forestry and agricultural biomass in solid-state anaerobic digestion (SS-AD).

Wood Biomass	Source of Added Inoculum	Time and Temperature of Digestion	Cumulative Methane Production (Nm3·t(VS)−1)	Complementary Information	Ref
Maple wood	Effluent from mesophillic LS-AD	30 days at 37 °C	46.9 *	-	[68]
Pine wood	Effluent from mesophillic LS-AD	30 days at 37 °C	17.0 *	-	[68]
Leaves	Effluent from mesophillic LS-AD	30 days at 37 °C	75.3 *	-	[68]
Corn stover	Effluent from mesophillic LS-AD	30 days at 37 °C	131.8 *	-	[68]
Wheat straw	Effluent from mesophillic LS-AD	30 days at 37 °C	123.9 *	-	[68]
Switchgrass	Effluent from mesophillic LS-AD	30 days at 37 °C	116.9 *	-	[68]
Yard wastes	Effluent from mesophillic LS-AD	30 days at 37 °C	49.3 *	-	[68]
Wheat straw	From mesophilic SS-AD pilot	273 days at 35 °C	108.8	A three-level Box–Behnken plan was applied to the TScontent, particle size, and substrate/inoculum (S/I) ratio	[69]
Untreated rice straw	Effluent of a 7000 m3 mesophilic AD	55 days at 39 °C	115.9 ± 12.8 *	-	[70]
Untreated elmwood	Effluent of a 7000 m3 mesophilic AD	55 days at 39 °C	54.2 ± 3.5 *	-	[70]
Untreated pinewood	Effluent of a 7000 m3 mesophilic AD	55 days at 39 °C	38.7 ± 4.1 *	-	[70]
Organosolv-treated rice straw	Effluent of a 7000 m3 mesophilic AD	55 days at 39 °C	113.4 ± 1.6 *	Pretreatment with an ethanol/sulfuric acid at 180 °C and 1 h	[70]
Organosolv-treated elmwood	Effluent of a 7000 m3 mesophilic AD	55 days at 39 °C	93.7 ± 0.9 *	Pretreatment with an ethanol/sulfuric acid at 180 °C and 1 h	[70]
Organosolv-treated pinewood	Effluent of a 7000 m3 mesophilic AD	55 days at 39 °C	71.4 ± 3.7 *	Pretreatment with an ethanol/sulfuric acid at 180 °C and 1 h	[70]
Untreated wood fiber	Derived from a properly operating AD	30 days at 52 °C	No methane production observed	-	[71]
Fungus-treated wood fiber	Derived from a properly operating AD	30 days at 52 °C	75.6 **	Pretreated with P. flavido-alba fungus for 21 days at 30 °C	[71]
Untreated Albizia chips	From mesophilic LS-AD	58 days at 37 °C	33.9 *	-	[72]
Fungal-treated Albizia chips	From mesophilic LS-AD	58 days at 37 °C	123.9 *	Pretreated with C. subvermispora fungus for 48 days at 28 °C	[72]
Untreated pine wood	Effluent of a 7000 m3 mesophilic AD	45 days at 37 °C	34.7	Ratio feed/inoculum 3/1	[73]
Untreated poplar wood	Effluent of a 7000 m3 mesophilic AD	45 days at 37 °C	22.3	Ratio feed/inoculum 3/1	[73]
Untreated berry wood	Effluent of a 7000 m3 mesophilic AD	45 days at 37 °C	80.4	Ratio feed/inoculum 3/1	[73]
Treated pine wood	Effluent of a 7000 m3 mesophilic AD	45 days at 37 °C	25.0	Pretreatment with COSLIF, ratio feed/inoculum 3/1	[73]
Treated poplar wood	Effluent of a 7000 m3 mesophilic AD	45 days at 37 °C	31.1	Pretreatment with COSLIF, ratio feed/inoculum 3/1	[73]
Treated berry wood	Effluent of a 7000 m3 mesophilic AD	45 days at 37 °C	43.9	Pretreatment with COSLIF, ratio feed/inoculum 3/1	[73]
Untreated poplar waste	From biogas plant	30 days at 35 °C	60.8 *	-	[74]
Treated poplar waste	From biogas plant	30 days at 35 °C	81.1 *	Treated with 3% NaOH by weight	[74]
Sorghum vinegar residues	From industrial-scale mesophilic AD	45 days at 35 °C	157.9 *	-	[75]
Spring-harvested Miscanthus	Effluent from operating mesophilic AD	60 days at 37 °C	175 *	Ratio feed/inoculum 2/1 and TS of 20%	[76]
Spring-harvested Miscanthus	Effluent from operating mesophilic AD	60 days at 37 °C	163 *	Ratio feed/inoculum 4/1 and TS of 20%	[76]
Fall-harvested Miscanthus	Effluent from operating mesophilic AD	60 days at 37 °C	172 *	Ratio feed/inoculum 2/1 and TS of 20%	[76]
Fall-harvested Miscanthus	Effluent from operating mesophilic AD	60 days at 37 °C	153 *	Ratio feed/inoculum 4/1 and TS of 20%	[76]

* Values are not normalized at standard conditions (atmospheric pressure and 0 °C). ** Values are not normalized at standard conditions (atmospheric pressure and 0 °C) and are expressed as a function of TS instead of VS. Legend: substrate/inoculum ratio (S/I); total solid (TS); volatile solid (VS); cellulose solvent- and organic solvent-based lignocellulose fractionation (COSLIF).

. The best yield for forestry was found for fungal-treated Albizia chips with a methane yield of 123.9 m³·t(VS)⁻¹. The best yields for agricultural biomass and green waste were obtained for sorghum vinegar residues and harvested miscanthus, with maximum values of 158 and 163 m³·t(VS)⁻¹, respectively. It should be noted that agricultural waste gave a much higher methane yield than wood. For example, Brown et al. noted that the methane yield of plant biomass was directly related to its lignin content [68]. Since lignin is much more difficult for microorganisms to assimilate, reducing its content increases methane production [68]. At a temperature of 37 °C and a retention time of 30 days, pine wood containing 28.3% lignin had a methane yield of only 17 m³·t(VS)⁻¹. In contrast, the methane yield from maple wood, which had 22% lignin, was three times greater than that of pine wood. According to the authors, a pretreatment step of the wood biomass led to an increased methane yield as it degrades the lignin sufficiently, breaking down the lignin to limit its detrimental effects on methane production. Regarding the processing time, the solid retention time of anaerobic digesters reviewed was mostly between 30 and 60 days, as a good compromise between speed and total yield obtainable. The SS-AD proposed by Motte et al. was significantly longer than those reviewed, with a total of 273 days [69]. Almost all the inoculum used comes from digestate or mesophilic AD effluent, except for Motte et al., who used samples from a municipal SS-AD treating municipal waste [69].

4. Advantages Associated with Anaerobic Co-Digestion of Agroforestry Residues

Anaerobic co-digestion has several advantages over mono-digestion. The co-digestion offers a way to overcome the drawbacks of mono-digestion, such as the low-yielding or difficult-to-digest materials by simultaneously degrading two or more feedstocks. Such organic matters with high water and fiber contents resulted in low biogas yields and degradation efficiency. In this context, the co-digestion of a high-protein substrate positively resulted in higher ammonium–nitrogen (NH₄⁺–N) levels than mono-digestion [77]. Several organic materials could be used as co-substrates and enhance methane production by using the optimal ratio of substrate/inoculum [78].

4.1. Case of Binary Co-Digestion of Lignocellulosic Biomass and Bovine Manure

The cumulative methane production of binary-based co-digestion of lignocellulosic biomass and bovine manure is shown in

Table 2. Cumulative methane production of binary-based digestion of lignocellulosic biomass and bovine manure.

Wood Biomass	Co-Digester	Lignocellulosic Biomass/Co-digester Ratio	Source of Added Inoculum	Time and Temperature of Digestion	Cumulative Methane Production (Nm3·t(VS)−1)	Complementary Information	Ref
PWa	CM	PW/CoM 2/1	From biogas plant	30 days at 35 °C	93.2 *	PW treated with 3% wt. NaOH	[74]
PWa	CM	PW/CoM 1/1	From biogas plant	30 days at 35 °C	98.2 *	PW treated with 3% wt. NaOH	[74]
PWa	CM	PW/CoM 1/2	From biogas plant	30 days at 35 °C	71.9 *	PW treated with 3% wt. NaOH	[74]
Sorghum vinegar residues	CM	Mixed at a volatile solid (VS) ratio of 1/1	From industrial-scale mesophilic AD	45 days at 35 °C	169.4 *	-	[75]
Pine tree sawdust	DM	Not given, sawdust bedding from dairy farm	Not given	85 days at 37 °C	64	Moisture content of 70%	[79]
Pine tree sawdust	DM	Not given, sawdust bedding from dairy farm	Not given	85 days at 37 °C	73	Moisture content of 76%	[79]
Pine tree sawdust	DM	Not given, sawdust bedding from dairy farm	Not given	85 days at 37 °C	90	Moisture content of 83%	[79]
Sawdust	DM	Not given, sawdust bedding from dairy farm	Not given	45 days at 37 °C	73.1	Unmixed	[80]
Sawdust	DM	Not given, sawdust bedding from dairy farm	Not given	45 days at 37 °C	56.3	Mixed every 3 days	[80]
Sawdust	DM	Not given, sawdust bedding from dairy farm	From batch-type mesophilic anaerobic digester	72 days at 39 °C	142.5	-	[81]
Sawdust	CM	Not given, sawdust bedding from beef farm	Not given	49 days at 37 °C	136.2 *	No inoculum	[82]
Sawdust	CM	Not given, sawdust bedding from beef farm	Not given	49 days at 37 °C	143.6 *	S/I 1/1	[82]
Sawdust	CM	Not given, sawdust bedding from beef farm	Not given	49 days at 37 °C	140.3 *	S/I 1/2	[82]
Sawdust	CM	Not given, sawdust bedding from beef farm	Not given	49 days at 37 °C	159.4 *	S/I 1/4	[82]
Sawdust	CM	Not given, sawdust bedding from beef farm	Not given	49 days at 37 °C	158.4 *	S/I 1/50	[82]
EFBOP	CM	EFBOP/CM 2/1	Dairy farm	22 days at 37 °C	94.7 *	-	[83]
EFBOP	CM	EFBOP/CM 2/1	Dairy farm	22 days at 37 °C	211.0 *	-	[83]
CS	DM	DM/CS/inoculum 34.6/32.2/33.2	Mesophilic digester	60 days at 37 °C	53 *	Corn stover with 0.18–0.42 mm particle size	[84]
CS	DM	DM/CS/inoculum 34.6/32.2/33.2	Mesophilic digester	60 days at 37 °C	106 *	Corn stover with 0.42–0.84 mm particle size	[84]
RG	CM	RG/CM 50/50	Experimental farm	32 days at 37 °C	200	Filling in layers, SS-AD in 60 L reactors	[85]
RG	CM	RG/CM 40/60	Experimental farm	32 days at 37 °C	186	Filling in layers, SS-AD in 60 L reactors	[85]
RG	CM	RG/CM 25/75	Experimental farm	32 days at 37 °C	170	Filling in layers, SS-AD in 60 L reactors	[85]
Untreated PW	CM	PW/CM 10/4.3	Not given	24 days at 37 °C	83.9	-	[86]
Pretreated PW	CM	PW/CM 10/4.3	Not given	24 days at 37 °C	208	Pretreated by a combination of acetic acid-hydrothermal and deep-eutectic solvents	[86]
ASWs	CM	ASWs/CM 20/80	Not given	30 days at 35 °C	222.7	S/I = 0.5 gVS/gVS	[87]
ASWs	CM	ASWs/CM 30/70	Not given	30 days at 35 °C	261.4	S/I = 0.5 gVS/gVS	[87]
ASWs	CM	ASWs/CM 40/60	Not given	30 days at 35 °C	232.5	S/I = 0.5 gVS/gVS	[87]
ASWs	CM	ASWs/CM 50/50	Not given	30 days at 35 °C	286.6	S/I = 0.5 gVS/gVS	[87]
ASWs	CM	ASWs/CM 60/40	Not given	30 days at 35 °C	297.9	S/I = 0.5 gVS/gVS	[87]
ASWs	CM	ASWs/CM 80/20	Not given	30 days at 35 °C	291.6	S/I = 0.5 gVS/gVS	[87]

* Values are not normalized at standard conditions (atmospheric pressure and 0 °C). Legend: agricultural solid wastes (ASWs); cattle manure (CM); corn stover (CS); dairy manure (DM); empty fruit bunches of oil palm (EFBOP); roadside grass (RG); poplar wood (PW); poplar waste (PWa).

. Regarding the added inoculum, fewer data were given compared to the simple digestion of lignocellulosic materials. It is possible that the authors consider that the added bovine manure co-digester provides a suited and sufficient microbial flora to achieve a proper SS-AD, which the cumulative methane yields seem to confirm.

Yao et al. studied the influence of “weak-base” treated poplar waste and cattle manure at different ratios (2/1, 1/1, and 1/2) on methane production [74]. Processed by SS-AD in 2 L batches for 30 days at 35 °C, poplar waste alone showed a methane yield of 81.1 m³·t(VS)⁻¹. The best poplar wood/cattle manure ratio was 1/1 with a methane yield of 98.2 m³·t(VS)⁻¹. Based on previous investigations, Almomani et al. confirmed the effectiveness of binary co-digestion of agricultural solid wastes (ASWs) and CM compared to mono-digestion [87]. The experience was performed at the different proportions of ASWs/CM (20/80, 30/70, 40/60, 50/50, 60/40, and 80/20) in 2.5 L anaerobic digesters during 30 days at 35 °C. The cumulative methane production ranged between 227.1 and 297.9 Nm³·t(VS)⁻¹, and the highest production was registered with the AGWs/CM ratio of 60/40. The same research mentioned an improved production with alkalinity treatment using 1.0 g of NaHCO₃/g(VS)⁻¹.

Zhang et al. mixed sorghum vinegar residues with cattle manure during solid-state anaerobic digestion in 1 L batch reactors at 35 °C for 45 days [75]. The S/I ratio was set at 2/1, and the cattle manure/sorghum vinegar residues ratio was fixed for a volatile solid (VS) ratio of 1/1. The obtained methane yield was 169.4 m³·t(VS)⁻¹, 58% higher than the methane yield of cattle manure alone. Cattle manure slightly benefited from sorghum vinegar residues, providing a more biodegradable substance and diluting the different inhibitors, demonstrating an effective synergy for methane production. Co-digestion with vinegar residues was also shown to enrich hydrolytic, acetogenic, and methanogenic bacteria. This enrichment also enhanced the substrate biodegradation and, consequently, methane production. The co-digestion lag phase was less than 1 day (0.84 days), much shorter than that of pure sorghum vinegar residues (4.6 days), approaching the values of cattle manure (0.1 days).

Ajayi-Banji et al. 2020 studied the solid-state anaerobic co-digestion of dairy manure and corn stover [84]. They focused on the effect of the C/N ratio and particle size on co-digestion performance. They found that the best compromise in methane production of 106 m³·t(VS)⁻¹ was achieved with digestate having a C/N ratio of 24 with stover particle size between 0.42 mm and 0.84 mm. Using smaller particles led to lower yields, as they possess a smaller surface area, which would have an impact on the bioconversion rate of VFAs. Manipulating the C/N ratio at 32 significantly reduced the methane yield, barely reaching 2.2 m³·t(VS)⁻¹ after 30 days. These observations of the C/N ratio are consistent with the fact that an excessively high C/N ratio reduces ammoniacal nitrogen concentration and alkalinity, which results in a detrimental effect on digestion. Regarding microbial community analysis, Walter et al. studied the biomethane potential and methanogenic community in a solid-state anaerobic digestion reactor co-digesting cattle manure and empty fruit bunches of oil palm [83]. Digestion was carried out in batch mode in 400 mL reactors over 22 days under mesophilic (37 °C) and thermophilic (55 °C) conditions. Analyses of methanogenic diversity showed that temperature influences methanogenic diversity. Co-digestion under thermophilic conditions favored a consortium of Methanoculleus and Methanosarcina, in contrast to cattle manure and mesophilic conditions dominated by Methanobrevibacter. The methanogens from the genera Methanoculleus and Methanosarcina were positively correlated with good reactor performance, giving a cumulative methane yield of 211 m³·t(VS)⁻¹ in thermophilic conditions, 123% higher than in mesophilic conditions.

A large part of the binary co-digestion reviewed concerns cattle bedding. As bedding contains both cattle manure and sawdust, this type of waste is directly suitable for biomethanization in co-digestion. Choi et al. analyzed the SS-AD of bedded-pack barn dairy manure [81]. Tests were conducted in 1.4 L batches at 39 °C. After 72 days, the cumulative methane content was 142.5 ± 6 N m³·t(VS)⁻¹, with biogas containing a final methane content of around 62%. The methane production peak was reached around 24 days with a value of 6.1 ± 0.4 N m³·t(VS)⁻¹·day⁻¹ according to a modified Gompertz equation. The authors noted that cellulose and hemicellulose contents decreased during SS-AD, with a higher removal rate for hemicellulose. As reported in the literature, the lignin content increased in agreement with the microbial degradation difficulties of such organic matter. Lee et al. studied the optimum between substrate and inoculum levels of cattle manure and sawdust bedding mixtures [82]. Tests carried out for 49 days at 37 °C showed that adding substrate increased the methane yield. The cumulative methane yield raised from 136.2 ± 0.7 m³·t(VS)⁻¹ for the bedding mixture to 159.4 ± 3.7 m³·t(VS)⁻¹ for the mixture/inoculum at a ratio of 4/1. However, the authors noted that a ratio favoring the inoculum too heavily limited the methane production, as there was less organic matter to convert to methane. An optimal ratio was required to enhance the process. As shown, numerous benefits of co-digestion have been highlighted by significant research, including enhanced methane yield, digestibility associated with higher process stability, synergistic effects of co-substrates, and higher nutrient value of the produced co-digestate.

4.2. Case of Ternary Co-Digestion

Table 3. Cumulative methane production of ternary-based digestion of lignocellulosic biomass and bovine manure.

Residues	Ratios	Source of Added Inoculum	Time and Temperature of Digestion (°C and Days)	Cumulative Methane Production (m3·t(VS)−1)	Complementary Information	Ref
DM/CS/TR	33/13/54	From a mesophilic LS-AD	45 days at 35 °C	175	-	[88]
DM/CS/TR	33/27/40	From a mesophilic LS-AD	45 days at 35 °C	280	-	[88]
DM/CS/TR	33/40/27	From a mesophilic LS-AD	45 days at 35 °C	312	-	[88]
DM/CS/TR	33/54/13	From a mesophilic LS-AD	45 days at 35 °C	249	-	[88]
DM/CS/TR	13/33/54	From a mesophilic LS-AD	45 days at 35 °C	245	-	[88]
DM/CS/TR	27/33/40	From a mesophilic LS-AD	45 days at 35 °C	310	-	[88]
DM/CS/TR	40/33/27	From a mesophilic LS-AD	45 days at 35 °C	355	-	[88]
DM/CS/TR	54/33/13	From a mesophilic LS-AD	45 days at 35 °C	416	-	[88]
DM/CS/TR	24/16/60	Digested sludge from a mesophilic LS-AD	45 days at 35 °C	129	-	[89]
DM/CS/TR	36/24/40	Digested sludge from a mesophilic LS-AD	45 days at 35 °C	170	-	[89]
DM/CS/TR	48/32/20	Digested sludge from a mesophilic LS-AD	45 days at 35 °C	220	-	[89]
CM/RS/SS	-	Digestate from LS-AD plant	65 days at 37 °C	115	TS = 15%	[90]
CM/RS/SS	-	Digestate from LS-AD plant	65 days at 37 °C	105	TS = 20%	[90]
CM/RS/SS	-	Digestate from LS-AD plant	65 days at 37 °C	81	TS = 25%	[90]
CM/RS/SS	-	Digestate from LS-AD plant	65 days at 37 °C	86	TS = 30%	[90]
CM/RS/ChM	-	Digestate from LS-AD plant	65 days at 37 °C	158	TS = 15%	[90]
CM/RS/ChM	-	Digestate from LS-AD plant	65 days at 37 °C	247	TS = 20%	[90]
CM/RS/ChM	-	Digestate from LS-AD plant	65 days at 37 °C	119	TS = 25%	[90]
CM/RS/ChM	-	Digestate from LS-AD plant	65 days at 37 °C	47	TS = 30%	[90]

Legend: cattle manure (CM); chicken manure (ChM); corn stover (CS); dairy manure (DM); rice straw (RS); sewage sludge (SS); tomato residues (TR).

presents the cumulative methane production of ternary co-digestion in SS-AD, including lignocellulosic biomass and bovine manure. As far as we know, only three articles exist on such SS-AD conditions. LS-AD plants were the source of inoculum. Li et al. proposed a ternary anaerobic co-digestion between dairy cattle manure, tomato residues, and corn stover [88]. Different mixtures were prepared to optimize methane production. The batch reactor conditions were 20% total solids for 45 days with an applied temperature of 35 °C. The highest vs. reduction (46.2%) and methane yield (416 m³·t(VS)⁻¹) were obtained with the ternary blends of 33% corn stover, 54% dairy manure, and 13% tomato residues. These results were far superior to those of individual components, showing that combining these components was beneficial for methane production. Results also indicated that using a ternary mixture allows tomato residues to be diluted, reducing tomato’s inhibitory effect on final methane production.

Two years later, the same team continued the anaerobic co-digestion studies of tomato residues/dairy manure/corn stover at three different mixing ratios: 20/48/32, 40/36/24, and 60/24/16 [89]. In addition, the solid-state method was compared to liquid- and semi-solid-state co-digestion. SS-AD were run into 1 L batches at 35 °C and left for 45 days. The methane yield was inversely related to the tomato residue concentration for all reactors. Too high tomato residues reduced the methane yield through an overproduction of VFA and a pH decrease. The best yields were obtained with a cattle manure/corn stover/tomato residues ratio of 48/32/20, with a final methane yield of up to 220.3 m³·t(VS)⁻¹. Although the solid-state methane yield was not the most efficient, the economic analysis carried out in parallel showed that it had the shortest payback period (10.1 years) as well as the highest net present value, indicating that the SS-AD technology pathway is financially attractive and feasible under the analytical conditions tested. Recently, Mothe et al. took a different approach from Li’s team by proposing to test ternary mixtures based on food waste, cattle manure, sewage sludge, and chicken manure [90]. The component’s proportions were fixed to ensure a C/N ratio of 25 for each condition. Of all the tests, the highest methane production was achieved by mixing cattle manure with rice straw and chicken manure with a TS of 20%, producing approximately 247 m³·t(VS)⁻¹. This ternary mixture, irrespective of TS, was also the highest average methane content measured. However, this ternary mixture was also strongly impacted by the TS. Moving from a TS of 20 to 30% reduced the cumulative methane yield by almost 80%, highlighting the significance of this parameter.

To summarize the various digestions and co-digestions reported, Figure 3 shows the cumulative methane yield as a function of total digestion time for the mono, binary, and ternary co-digestions presented in Table 1, Table 2 and Table 3. This figure underlines that binary and ternary co-digestion in the presence of cattle manure greatly improved the methane yields, with production significantly higher for ternary co-digestion. In addition, wood generally gives lower yields than other lignocellulosic materials, probably due to its higher lignin content, which inhibits digestion of the material. This literature search also provided a useful list of viable strategies to enhance the biomethanization process, involving either an improvement in reactor design or feedstock pretreatment.

5. Viable Strategies to Enhance Biomethanization

5.1. At the Matter Level: The Pretreatment Step

For some residues, the complex cross-linking structure of lignocellulose and the high crystallinity of cellulose molecules make direct and effective degradation by microorganisms challenging. Pretreatment using aerobic hydrolysis can decompose insoluble macromolecular organic matter into small molecules under the action of extracellular enzymes. Pretreatment can improve the degradation rate of lignocellulose and provide the cellulose for subsequent fermentation. Qu et al. studied the effect of different aerobic hydrolysis processing times on corn stover biotransformation [91]. The optimal condition was obtained with aerobic hydrolysis for 16 h at 43 °C, where the cumulative methane yield reached 268.75 m³·t(VS)⁻¹. Zhang et al. also investigated the anaerobic digestion of organic matter before aerobic hydrolysis [92]. Different components of corn stover, including leaves, piths, and rinds, were pretreated and digested. Different cellulose degradation rates were detected due to differences in tissue structure and composition of the various parts of the plant. In addition, methane yields increased by 8%, 35%, and 30% after a pretreatment of 8 h for the leaves and after 12 h for the pith and rind, respectively, confirming the findings of Qu et al., 2021 [91]. The aerobic hydrolysis reduced the fermentation period by 4–5 days compared to controls.

Regarding fungal delignification, Lopez et al. demonstrated that delignification with ligninolytic P. flavido-alba fungus increased the biogas yield [71]. According to them, non-delignified wood fiber produced no biogas, whereas delignified wood fiber produced a methane yield of 75.6 m³·t(VS)⁻¹. The effect of fungal delignification was attributed to the enhanced availability of hemicellulose. Ge et al. also found increased methane yield by applying a fungal pretreatment of Albizia wood chips [72]. However, applying pretreatment to all types of wood systematically is not an obvious solution. By studying the effect of chemical lignocellulose pretreatment (COSLIF) on three different types of wood/pine wood (softwood), poplar wood (soft hardwood), and berry wood (hardwood), the team found that only pine wood pretreatment increased methane production [73]. For berry wood, the cumulative methane production decreased by 45%, from 80.4 to 43.9 Nm³·t(VS)⁻¹. According to the authors, this decrease could be due to an overload of digestible material in the reactor, reducing the methane yield by too much VFA, increasing the acidification, and decreasing the reactor’s pH.

To remove wood lignin, Mirmohamadsadeghi et al. suggested an “organosolv” pretreatment of the hardwood and softwood biomass for an efficient biomethanization [70]. The organosolv pretreatment consisted of a mixture of ethanol and a sulfuric acid catalyst in a high-pressure batch reactor with temperatures up to 180 °C and times up to 1 h. According to the authors, the organosolv pretreatment effectively reduced the lignin content of both types of wood (up to −27% lignin reduction for softwood and up to −21% lignin reduction for hardwood). For both woods, the effect of pretreatment was positive. For hardwood, only the effect of temperature was significant for the methane yield. Regarding the pH effect, fruit or vegetable waste is difficult to convert into methane as it releases high levels of VFA, which lowers the pH and can inhibit digestion. Using a neutral or a “weak base” lignocellulosic material such as poplar wood can improve SS-AD’s process performance [74]. In addition to studying the influence of wood biomass and cattle manure content, Yao et al. proved that treating a “weak-base” poplar with NaOH increased the methane yield of poplar/cattle manure binary digestion [74]. For all conditions, the pH values after the pretreatment remained within the optimal ranges for biomethanization (7.4–7.5). Moreover, sodium hydroxide (NaOH)-treated poplar wood gave a higher methane yield than untreated poplar wood. With a treated poplar waste/cattle manure ratio of 1/1, a yield of 98.2 m³·t(VS)⁻¹ was even achieved, which is 101% higher than the untreated condition. Another research studied the anaerobic co-digestion of poplar waste and dry cattle manure using a novel methodology that involves a two-step pretreatment process coupled with solid-state anaerobic digestion to transform the co-substrates into biomethane and valuable products [86]. The poplar residues received a two-step treatment of acetic acid-hydrothermal treatment and a three-constituent deep eutectic solvent (3c-DES) treatment, formed by a neutral choline chloride (ChCl)/glycerol duo and an acidic electron donor. This 3c-DES solvent can be considered a new green solvent [86]. The pretreated poplar residues were mixed with dry cattle manure and anaerobic sludge at 37 °C for 24 days. The highest cumulative methane (208 m³·t(VS)⁻¹) was obtained under the optimal conditions of hydrothermal treatment (4% acetic acid, 170 °C, 40 min) coupled with 3c-DES treatment (80 °C, 60 min), resulting in a 148% improvement compared to raw poplar and corresponding to 49% of delignification. However, excessive pretreatment severity was not beneficial for methane production, as it would raise the delignification percentage, leading to a rapid accumulation of VFA, which inhibits the metabolism of methanogenic microbes.

Other parameters may also influence methane production; Ma et al. investigated the methane yield in the anaerobic co-digestion of dairy manure and rapeseed straw [93]. The reactors used were 0.7 L batches for a 60-day digestion at 37 °C. The authors found that the best methane yield was 209.1 m³·t(VS)⁻¹ for the S/I, 2/3 ratio, and the best volumetric methane production was observed for the 2/1 ratio. The authors also noted that a too-high S/I ratio resulted in excessive accumulation of volatile fatty acids and a low pH for efficient methane production. In addition, the moisture content of the biomass can influence the methane yield of SS-AD. Kim et al. studied the methane yield of jack pine sawdust used for dairy cattle bedding in 1.5 L batch reactors at 37 °C for 85 days [79]. They found that moisture content improved the methane yield. After 85 days, samples with 70, 76, and 83% moisture had final methane yields of 64, 73, and 90 Nm³·t(VS)⁻¹, respectively. These results were consistent with the total volatile solids removed during methanization. Moreover, the yield kinetics accelerated with increasing moisture content. The T₉₅ (the time required to produce 95% of the total methane potential) decreased from 67 days for MC = 70% to 43 days for MC = 83%. These results demonstrate that humidity is crucial in material pretreatment and production. Furthermore, André et al. studied the seasonality of grass cut from Breton roadsides and mixed with cattle manure in a 60 L SS-AD [85]. The highest methane yield occurred during the test conducted in spring, with methane production of 202.9 Nm³·t(VS)⁻¹, compared to 167.9 Nm³·t(VS)⁻¹ obtained in autumn. They noted that the seasonality impacted the composition of cattle manure and the roadside grass, and thus the methane yield.

5.2. At the Level of Reactor Design

The SS-AD inhibition tendency could be avoided using an inoculum and spreading percolate on top of the mixture [94]. The recirculation of percolate can minimize mass transfer limitations. In this context, the reactor filling can impact the methane yield. André et al. also studied the impact of filling different cattle manure blends with cut grass on Breton roadsides for proportions of 50/50, 60/40, and 75/25 by volume, respectively [85]. For all proportions, the methane yield was dependent on the filling method. A layered filling method provided a higher yield than a homogeneous filling method due to a decrease in the permeability of the homogeneous filling, reducing draining of the liquid phase and system percolation. The best compromise was achieved for a CM/grass ratio 60/40 using the layered filling method. This arrangement provided a compromise between inhibition and production. Coutu al. 2022 characterized the recirculation flow within the leaching bed during anaerobic co-digestion of cattle manure and layered roadside grass [95]. The impact of recirculation flow of leach bed complexity, substrate stratification at different times during the SS-AD (after 0, 15, and 30 days), and the impact of recirculation flow on the evolution of microporosity and macroporosity throughout the SS-AD were analyzed. Stratification and processing time significantly impacted percolation flow and leach bed complexity, accounting for up to 36% of residence time and 110% of leach bed complexity. Furthermore, stratification may impose a different percolation flow for each layer, disrupting preferential channels. The authors encourage a change in recirculation strategy to minimize water and energy costs in the context of reducing operating costs. This study could also be applied to other substrates and experimental conditions. It should be tested under pilot-scale conditions to observe scale-up to see if these improvements increase methane yield through fluid circulation.

Regarding the recirculation and immersion parameters during the SS-AD, Coutu et al. also proposed an optimization of the anaerobic co-digestion of manure and roadside grass to increase the methane yield [96]. The co-digestion was performed in batches of 2.65 L on 15 different devices. Statistical analysis was also used to optimize the experiments. They demonstrated that when the wet grass content was less than 50%, the recirculation frequency and immersion rate positively affected methane production. However, the trends were more complex when the wet grass content was less than 50%. Methane yields increased when the immersion rate was low, and the recirculation frequency was high, and the reverse is also true.

As mentioned earlier in the review, the SS-AD can be differentiated based on its operating regime. A comparison between the discontinuous (D) and semi-continuous (SC) methods was proposed by Wang et al. [97]. The digestate used in experiments consisted of cattle manure, corn straw, and tomato residues. The SC reactors required less inoculum to start the reactions, with a cumulative methane increase of 45%. However, digestates degraded much more slowly in a semi-continuous process with an overall daily methane yield 35% lower than in SS-AD discontinues. Although the best yield was found at 283.3 m³·t(VS)⁻¹ with 40 mm straw particles, the effect of straw particle size in the SC reactor was insignificant. Authors have also proposed an economic analysis on a commercial scale, simulating a 1000 m³ industry by comparing different scenarios (SS-AD discontinuous, semi-continuous, and the two combined). According to them, it could be profitable to start up a reactor with an SS-AD D system and then switch to SS-AD SC. Recently, Hernandez-Shek et al. studied the rheological evolution of biomass as a function of time in bovine manure/straw mixture in continuous SS-AD [98]. They found that the digestion process’s consumption of cellulose and hemicellulose was linked to decreased viscosity and yield strength of the digestate. They found a biogas flow equilibrium at 11.3 NL_methane⋅h⁻¹. A verification continuous cycle validated the robustness of the results obtained by giving similar results. It should also be noted that the cumulative biogas and methane volume produced in continuous mode tends to follow a linear regression overtime after 20 days, which is an optimal case for long-term production. These findings were interesting for a better prediction and design of future SS-ADs. Despite the high viscosity imposed by SS-AD reactors, it would be possible to imagine reactors in continuous mode. Li et al. investigated the rheological behavior of cattle manure/corn straw mixtures in continuous SS-AD reactors [99]. The experiment was run over 100 days, with TS increasing from 20% to 30% during digestion. Three different cattle manure/corn straw mixes (2/1, 1/1, and 1/2) were compared. Results showed that regardless of the mix, shear stress was reduced with the digestion time, improving the flowing property. The TS and the proportion of cattle manure increased shear stress, which, when too high, impacted CH₄ production and created “dead zones” in the reactor. This confirms the importance of understanding the relationship between material viscosity/microstructure and biomethanization potential.

In addition, Jo et al. showed the role of agitation for good methane production in an SS-AD process [80]. Tests were carried out using a cattle manure and sawdust bedding mix in 22 L batches for 45 days at 37 °C. After 45 days, methane production in non-mixed batches reached 73.1 m³·t(VS)⁻¹, compared with 56.3 m³·t(VS)⁻¹ in batches mixed every 3 days. In addition, the removal of total volatile solid (TVS) and biodegradable volatile solid (BVS) was higher in the unmixed than in the mixed batches. However, the authors emphasized the importance of performing these tests on industrial digesters to confirm this observation.

6. Conclusions

In this review, various studies have detailed the importance of using agroforestry residues to produce methane-rich biogas as a rich source of organic matter. As indicated in this review, the SS-AD was the most appropriate technology for this complex matter. However, challenges such as effective mass transfer, recalcitrant biomass, and inhibitor accumulation are encountered in SS-AD reactors. The steps and actors involved in the process were meticulously outlined to understand the SS-AD technology better. Hydrolysis was identified as the rate-limiting step due to the complex nature of these materials. The major inputs and parameters, such as the optimal temperature ranges, pH, C/N ratio, and organic loading, were illustrated to achieve agroforestry residues’ biotransformation into valuable products. Various mono-digestion experiments and the resulting methane yields were mentioned to highlight the efficiency of treating these residues. Thereafter, different cases of substrates co-digested were illustrated to reveal the advantages of this technique over mono-digestion. The highest methane production (416 m³·t(VS)⁻¹) was registered with ternary co-digestion. Furthermore, the various viable strategies, such as pretreatment and percolate recirculation, may reduce mass transfer limitations, improve process stability, and enhance reactor performance. In addition, several parameters require consistent monitoring to ensure process efficiency. These include selecting an appropriate reactor design, such as layered filling, while optimizing mass transfer, structural integrity, and microbial activity. Additionally, applying suitable pretreatment methods, determining optimal co-digestion ratios, and continuously monitoring physicochemical factors as a part of process automation are essential. These elements play a crucial role in maximizing efficiency and energy production. Future research should focus on refining these key aspects to further streamline and enhance anaerobic digestion processes. Various companies are developing commercial SSADs, proving the effectiveness of this approach to waste management. However, pretreatment and co-digestion on a pilot scale still need investigation. Although this review has not adopted a specific methodology, it has gathered the most interesting data, and its approach has been designed to open horizons for different actors in the agricultural and forestry sectors to explore the application of this sustainable technology.