Advancements in Giant Reed (Arundo donax L.) Biomass Pre-Treatments for Biogas Production: A Review

Abstract

Giant reed is a non-food, tall, rhizomatous, spontaneous perennial grass that is widely diffused in warm-temperate environments under different pedo-climatic conditions. In such environments, it is considered one of the most promising energy crops in terms of economic and environmental sustainability, as it can also be cultivated on marginal lands. Owing to its complex and recalcitrant structure due to the lignin content, the use of giant reed as a feedstock for biogas production is limited. Thus, pre-treatment is necessary to improve the methane yield. The objective of this review was to critically present the possible pre-treatment methods to allow the giant reed to be transformed in biogas. Among the studied pre-treatments (i.e., hydrothermal, chemical, and biological), alkaline pre-treatments demonstrated better effectiveness in improving the methane yield. A further opportunity is represented by hybrid pre-treatments (i.e., chemical and enzymatic) to make giant reed biomass suitable for bio-hydrogen production. So far, the studies have been carried out at a laboratory scale; a future challenge to research is to scale up the pre-treatment process to a pilot scale.

1. Introduction

Energy production from biomass can mitigate global warming and foster energy security by displacing the consumption of fossil fuels. Renewable energy carriers in the form of biogas (i.e., biomethane and bio-hydrogen) can be produced from locally available biomass, such as livestock effluents, food waste, dedicated energy crops, agriculture, and forest residues, using microbial processes, notably anaerobic digestion (AD), dark fermentation (DF), or photo-fermentation (PF).

Biogas can be used directly as a source of heat and electricity or upgraded to obtain higher-purity fuel-grade gas that is suitable for transportation or directly injected into the natural gas grid and used by various industries [1,2,3]. Compared with other renewable energy production systems, such as those based on solar and wind energy, biogas production has the advantage of being largely independent of daily or seasonal fluctuations of the sources; moreover, biomethane and bio-hydrogen can be stored using existing facilities [2,4,5,6]. Following biogas production, the residual digestate can be mechanically separated and used as a fertilizer or soil improver [7] if chemical and biological standards are fulfilled.

The utilization of lignocellulosic biomass, which is composed of cellulose, hemicellulose, and lignin, provides the opportunity to convert a substantial amount of renewable resources into fungible energy commodities, such as methane or hydrogen. In fact, if properly converted, the amount of lignocellulosic biomass produced annually theoretically exceeds the existing global energy requirement [8].

The biogas from lignocellulosic biomass is produced mostly from holocellulose, consisting of cellulose and hemicellulose, which are the most abundant biopolymers on Earth [9,10]. Among lignocellulosic species, giant reed (Arundo donax L.) increasingly attracted attention as an energy crop [11]. Giant reed is a non-food, tall, rhizomatous, spontaneous perennial grass [12] that is widely diffused in warm-temperate environments under different pedo-climatic conditions. In such environments, it is considered one of the most promising energy crops [13] in terms of economic and environmental sustainability, as it can also be cultivated on marginal lands (i.e., wetlands, riparian slopes, polluted areas) that are poorly or totally unsuited to annual row crops, avoiding competition with food production or land use change [14,15,16]. As a matter of fact, giant reed can thrive in many soils, ranging from loose sands and gravel soils to heavy clay, and tolerates both high salinity and extended periods of drought [12]. Moreover, it requires low agronomic input in terms of fertilizers, tillage, pesticides, and irrigation water [11]. In particular, it uses nitrogen very efficiently, which is seasonally translocated from the above to the below plant organs and vice versa. Furthermore, giant reed cultivation can provide different ecosystem services. In fact, it provides substantial soil carbon sequestration and can efficiently remove residual soil nitrate across the soil profile [17,18]. However, some authors highlighted that due to its invasiveness, giant reed introduction in fragile ecosystems should be avoided [19].

The biomass yield of multi-annual giant reed plantations can reach an average of 30.3 Mg ha⁻¹ y⁻¹ on a dry weight basis in the Mediterranean environment based on numerous reports [20,21,22,23]. However, the yield is strongly influenced by the climate, water, and nutrient availability, yielding an average of 18.05 Mg ha⁻¹ y⁻¹ in a less favorable continental climate (Hungary) without fertilization or irrigation [20] and reaching up to 64.3 Mg ha⁻¹ y⁻¹ when properly supplied with nutrients and under highly favorable conditions (northern Italy) [24]. The highest ever giant reed yield (i.e., 125 Mg DM ha⁻¹) was achieved in Australia in subsurface flow, gravel-based constructed wetlands supplied by aquaculture wastewater [25]. Giant reed has high investment but low maintenance costs [26], with the price of propagules derived from the rhizomes being one of the most expensive direct costs besides the harvest and transportation [19]. However, stands can be easily established by shoot-cutting propagation when properly managed [14]. Micro-propagated plants also offer the potential for reducing the cost of crop establishment.

Despite the high biomass productivity of giant reed, its use as feedstock for biogas production is limited by its complex and recalcitrant structure, which is typical of lignocellulosic biomass [6]. This implies a low methane yield; therefore, pre-treatment is necessary to improve the conversion of such lignocellulosic biomass into biogas [3].

The main objective of this review was to critically discuss the possible pre-treatment methods to make the giant reed biomass prone to be transformed into biogas. First, the fiber composition of the giant reed biomass is reviewed; then, the anaerobic processes to produce methane (i.e., AD) or bio-hydrogen (i.e., DF and PF) are described. Then, the different pre-treatment methods proposed in the scientific literature to improve biogas production from giant reed are reviewed. Some of the most promising methods not yet tested on giant reed are reported as well.

2. Compositional Features of Giant Reed Biomass

According to studies conducted in Italy, the cellulose content of giant reed ranged between 32.1 and 46.5%; the hemicellulose content fluctuated between 20.5 and 29%; whereas the lignin content exhibited larger variability, varying between 9.6 and 25.9% (Table 1). Studies conducted in the USA reported cellulose content for giant reed ranged between 27.7 and 41.5% and the hemicellulose content varied between 13.9 and 20.5%, whereas the lignin variability fluctuated between 16 and 23.9%. The values of the compositional traits of giant reed reported for other countries (China, Portugal, and Thailand) fall within the ranges listed for the two aforementioned countries (Table 1).

Indeed, the discrepancies in the values are mostly attributable to the analytical method for the fiber fractions characterization (i.e., van Soest or NREL). In fact, when the total of the three individual components (i.e., cellulose, hemicellulose, and lignin) is considered, the differences are less pronounced and most of the values are within the range of 70.3–82.7%.

Cellulose is a polymer that is constituted by long chains of cellobiose (two glucose molecules; Figure 1 [45]) bonded by van der Waals and hydrogen bonds to form microfibrils [46]. Cellulose is commonly found in both crystalline and amorphous forms [47]. The higher the crystallinity, the tougher it is to break it down into cellobiose or glucose units. Thus, the biomass complexity is increased, making it more recalcitrant to biodegradation.

Hemicellulose is formed by various polymers of pentoses and hexoses with a predominance (up to 90%) of xylan-based compounds [48,49]. Both cellulose and hemicellulose (holocellulose) can be degraded during the AD process.

Lignin is an aromatic polymer formed by three phenylpropane-based units. Its molecular structure is inherently highly resistant to microbial attack and oxidative stress; its high hydrophobicity also lowers its biodegradability [48]. Lignin is typically bonded to cellulose, isolating the latter from microbial activity [50], resulting in a low biogas yield.

Aiming to increase the biogas yield, lignocellulosic biomass must be pre-treated to increase the holocellulose accessibility to the microbial attack. Pre-treatment is also mandatory to obtain a sugar-rich substrate suitable for DF [29]. Various pre-treatment strategies were proposed to modify the recalcitrant structure of lignocellulosic biomass and enhance its digestibility for bio-methanogenesis [6,51] or bio-hydrogenesis [52].

3. Anaerobic Digestion Process for Methane Production

The anaerobic digestion process converts organic matter into methane-rich biogas. Anaerobic digestion can be divided into four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis [53] (Figure 2).

During the hydrolysis step, the macromolecules are broken down into smaller compounds characterized by higher solubility (i.e., amino acids, fatty acids, monosaccharides, etc.). These molecules are converted into volatile fatty acids (VFAs), alcohols, methylamines, H₂, and CO₂ in the acidogenic step. Subsequently, these compounds are converted into acetic acid, H₂, and CO₂ during the acetogenic step. The products of the former step are converted into CH₄ and CO₂ in the methanogenesis step. During both the acidogenic and acetogenic steps, H₂ is produced as a by-product, whereas CH₄ is produced only in the methanogenesis step (Figure 2). The large variety of by-products generated during the first three steps of AD is due to the activity of several bacterial groups belonging to the facultative or obligate anaerobes [54,55]. In contrast, methanogenesis is normally operated by only two groups of methanogens: acetoclastic and hydrogenotrophic [53]. However, it is known that Methanosarcina sp. is able to directly convert methanol into CH₄ [56].

4. Fermentation Processes for Bio-Hydrogen Production

Photo-fermentation and dark fermentation processes convert organic matter into hydrogen-rich biogas. Bio-hydrogen can be produced from various organic substrates. Bio-hydrogen is a clean energy carrier that can be widely used in industrial processes and transportation, as well as converted into electricity through fuel cells [29,52,57]. Due to the complex structure of the lignocellulosic biomass, a pre-treatment of the feedstock is essential for efficient bio-hydrogen production [52]. Simple sugars generated from the disruption of the biomass are the ideal substrate for H₂-producing bacteria [52]. Biological hydrolysis using cellulase, α-amylase, β-glucosidase, and endo-xylanase is a prerequisite for H₂ fermentation [29,52,58].

The major processes studied for bio-hydrogen production are PF and DF (Figure 3).

4.1. Photo-Fermentation

Photo-fermentation is a light-dependent anoxic process where photosynthetic bacteria convert the organic matter into H₂ and CO₂ [57,59,60]. Purple nonsulfur bacteria (Chlorobium, Chromatium, Halobacterium, Rhodobacter, Rhodopseudomonas, and Rhodospirillum) are the most studied due to the higher H₂ rate production in comparison with green and purple sulfur bacteria [59,61,62].

These bacteria can degrade fructose and glucose into organic acids (namely, acetic acid) that are used as electron donors to produce H₂ and CO₂ via the metabolic pathway (1) [59,60]:

CH₃COOH + 2H₂O = 4H₂ + 2CO₂(1)

This is a non-spontaneous reaction that requires an external energy source consisting of light. Photo-fermentation is catalyzed by the nitrogenase enzyme, which, in conditions of nitrogen deficiency, allows for the formation of H₂ from protons by consuming Adenosine triphosphate (ATP) instead of forming NH₃ [57,62,63,64]. Indeed, this kind of PF is energetically costly from a metabolic standpoint since the electrons donated by the acetic acid are transported by the ferredoxin to nitrogenase using ATP, which is added to the ATP consumed for the H₂ production, as previously described [59,62,63,64].

This results in a slow bacterial growth rate, which, in turn, causes a low volumetric H₂ production [60,65]. Moreover, the light conversion efficiency is low, and the light intensity affects H₂ production [60,62,63]. The highest efficiency conversion (up to 10%) was obtained with low light intensity and low H₂ production rates [60]. In addition, large areas and water (or very diluted substrates) are required for such a bio-hydrogen-producing process [57,66].

4.2. Dark Fermentation

Dark fermentation is a light-independent bio-hydrogen production process. Carbohydrates (namely, simple sugars) are the most important substrate for DF [58,62,63,67,68]. This process is performed by Enterobacter and Clostridium genera, which convert the organic matter into H₂, CO₂, organic acids, and alcohols, with pyruvate as the key intermediate by-product (Figure 3) [57,63,67,69]. These genera have two diverse hydrogenases (i.e., [NiFe] hyd3 and [FeFe]; Figure 4). [FeFe] hydrogenase, which is typical of Clostridia, is the most H₂-productive (Figure 4b) [62,70].

Dark fermentation can be carried out in both thermophilic and mesophilic conditions. Although the H₂ yield is higher in thermophilic conditions than in mesophilic conditions, the volumetric productivity is reversed [71].

Typically, DF performed using Clostridia follows two main metabolic pathways (2) and (3), described below, which can occur simultaneously [29,67,72]:

C₆H₁₂O₆ + 2H₂O = 2CH₃COOH + 2CO₂ + 4H₂(2)

C₆H₁₂O₆ = CH₃CH₂CH₂COOH + 2CO₂ + 2H₂(3)

The by-products of DF are acetic and butyric acid. Indeed, DF is a part of the AD process (Figure 5), and it can be separated from the methanogenesis step in a two-stage system that increases the energetic output since the by-products of DF can be used to feed the methanogenesis reactor (Figure 5) [29,52].

In DF, the H₂ rate and volumetric production are higher than in PF [59,62,67]. Yet, smaller reactors are required in DF in comparison with PF [73].

5. Pre-Treatments

The pre-treatments have the specific purpose to divide the holocellulose (in particular, cellulose) from the lignin [74]. This is convenient because lignin, due to its chemical properties, protects the cellulose from microbial degradation (Figure 6).

The types of pre-treatments reported in the literature for giant reed biomass are mechanical, hydrothermal, chemical, and biological; in some cases, they are combined. Table 2 shows the effects of the different pre-treatments on methane production in AD.

5.1. Mechanical Pre-Treatments

Mechanical pre-treatments reduce the particle size and therefore increase the surface area of the biomass exposed to microbial attack. Moreover, the overall porosity of the substrate is increased while the crystallinity is reduced [75,76,77]. The size reduction causes a decrease in the viscosity of the substrate, resulting in a reduction in floating layers, which can cause operational problems [75]. Mechanical pre-treatments are chopping, grinding, hammer-milling, knife-milling, and roll-milling [77,78]. Grinding and milling are the most studied [6]. The major advantage of mechanical pre-treatments is that they do not cause the release of inhibitory compounds [76]. Nevertheless, they do not remove and detach the lignin from the cellulose [78] because their effect is merely a reduction in particle size. Although mechanical pre-treatments are well known to improve biogas yield, they have the disadvantage of being energy-intensive [3,6,75,76,78].

The literature on giant reed mechanical treatments is still limited (Table 2). Dell’Omo et al. [33] applied a two-stage milling system to giant reed biomass. Working in mesophilic batch test these authors reported a +137% methane yield compared with untreated biomass. Nevertheless, it must be considered that the baseline value of untreated giant reed (i.e., 90 mL CH₄ g⁻¹ VS from a 20 mm size) is quite low compared with the range of values reported by other authors (i.e., 165–217 mL CH₄ g⁻¹ VS from a 12–13 mm size) from untreated giant reed in mesophilic batch conditions [38,39,41]. Hence, the reported yield gain might be overestimated. Yet, according to Dell’Omo et al. [33], mechanical pretreatment can be regarded as energy-demanding because it consumed 19% of the energy that was subsequently produced.

5.2. Hydrothermal Pre-Treatments

Hydrothermal pre-treatments can be performed at different combinations of temperature, pressure, humidity, and time. These processes not only use water to primarily solubilize hemicellulose and disrupt the lignin-carbohydrate matrix but can also avoid or cut the use of external mineral acid catalysts; therefore, they do not require particular corrosion-resistant equipment and there is a reduction in hazardous pollutants. The most studied pre-treatments on giant reed are liquid hot water and steam explosion.

5.2.1. Liquid Hot Water Pre-Treatment

Liquid hot water (LHW) pre-treatment is based on variations in temperature, pressure, and time [75]. The temperature adopted is between 50–260 °C [75,76], combined with pressure up to 9 bar for 15–120 min [75]. The aim of LHW pre-treatment is to modify the biomass structure, increasing the solubility in water [76]. In fact, water and heat break down the bonds between crystalline cellulose and the other fiber fractions [75,77], while hemicellulose is depolymerized and degraded into volatile fatty acids (VFAs) and monosaccharides [77]. Thus, the microbial conversion of biomass into biogas is enhanced.

Indeed, the imposed conditions of high temperature and pressure determine the auto-ionization of water that generates hydronium ions, which, in turn, deconstruct the lignocellulosic biomass [78]. The major advantage of LHW pre-treatment is that no use of chemical compounds or particular corrosion-resistant equipment is required [76]. On the other hand, the high temperature generates furans derived from hemicellulose decomposition, which can inhibit anaerobic microbial activities [3,6,79]. The longer the pre-treatment time, the higher the concentration of inhibitory compounds. [80]. Moreover, for these pre-treatments, water and energy requirements are still high [3].

Some authors reported methane yield increases of 23% [34] and 31% [42] in both thermophilic and mesophilic conditions. Owing to the high temperature used (i.e., 180–190 °C for 10 min), such pre-treatments substantially increased the methane yield. Nevertheless, the pre-treatment consumed more energy than the one produced [42].

However, with hot water at a lower temperature (121 °C for 20 min), a negligible increase in methane yield (+3%) was reported [28]. Thus, LHW pre-treatment for giant reed biomass is only effective at about 180 °C.

5.2.2. Steam Explosion Pre-Treatments

Steam explosion is a particular hydrothermal pre-treatment. During the process, biomass is exposed to hot steam for a few minutes, followed by a rapid decompression, which results in an explosion of the biomass with the breakage of the structure of the fiber [3,6,76,78,80] due to the auto-hydrolysis of the acetyl group of hemicellulose [78]. The pre-treatment is carried out at 160–270 °C for 5–15 min at up to 50 bar [3,6,76,77,78]. Therefore, the steam explosion is considered energy-intensive [81,82]. Unfortunately, due to high temperature and pressure conditions, microbial inhibitory compounds (i.e., phenols and furfurals) are generated during biomass degradation [77]. Steam explosion pre-treatment was used on giant reed for bio-hydrogen production [83,84,85]. However, this pre-treatment has not yet been tested on giant reed biomass for biomethane production.

5.3. Chemical Pre-Treatments

Chemical pre-treatments are a family of diverse pre-treating agents. The most studied chemical agents to pre-treat lignocellulosic biomass are acids, alkali, ozone, hydrogen peroxide, organic solvents (organosolv), hydrotropes, ionic liquids, and deep eutectic solvents; many of these agents are yet to be tested on giant reed for biogas production.

5.3.1. Acid Pre-Treatment

Acid pre-treatment preferentially solubilizes the hemicellulose by breaking down the van der Waals, hydrogen, and covalent bonds [6,77]. At a higher integration scale, the hemicellulose solubilization generates pores facilitating the bacterial attack on biomass [77,78]. In addition, a small fraction of lignin is removed [3]. However, it was hypothesized that lignin could precipitate on the cellulose surface, resulting in a block of the biodegradation process of the substrate [86].

Acid pre-treatment can be performed using various inorganic (i.e., H₂SO₄, H₃PO₄, HCl, HNO₃) or organic (acetic, formic, oxalic, succinic) acids [3,77,78]. At relatively low temperatures (<100 °C), a high acid concentration is required (30–70% v/v), whereas, at higher temperatures (120–210 °C), diluted acids (0.2–2.5% v/v) can be used [3,76,78]. Diluted acids are more monetarily convenient. Nevertheless, in both cases, there are risks of equipment corrosion and hazards for the people involved due to the utilization of more or less concentrated acid and high temperature [6,48,76]. During the acid pre-treatment, pentoses derived from the hemicellulose decomposition are degraded to inhibitor compounds, such as furfural and hydroxymethyl furfurals (HMFs) [77,78,80].

Acid pre-treatment was tested on giant reed, where 2% w/w H₂SO₄ was used at 150–180 °C; no methane yield increase was obtained [34]. Indeed, 49–100% inhibition was observed in thermophilic conditions (Table 2).

5.3.2. Alkali Pre-Treatment

Alkali pre-treatment is highly effective in de-lignifying biomass [6,59,61]. During the alkali pre-treatment process, the biomass is decomposed and solubilized by the OH⁻ ions. Proteins lose their original three-dimensional shape and the lipids are saponified such that the cellular polymeric structure is destroyed and the intracellular material is released, improving the degradability of the biomass [75].

The solvation separates the hemicellulose from the lignin by removing various uronic acids and acetate structures (acetyl group) [77]. Lignin is solubilized via saponification and disruption of the hydrogen, ester, aryl-ether, and C-C bonds, resulting in the modifications of the lignin side chains and deviations in the aromatic rings [3,75,76,77,78,86,87]. Cellulose is only marginally affected by the alkali pre-treatment [28]. From the lignin depolymerization, several compounds are generated: phenols, syringaldehyde, and vanillin, which were reported to inhibit or block the AD process [77,79]. The most common alkali used are CaO, Ca(OH)₂, KOH, NaOH, Na₂CO₃, and urea [3,75,77,80], typically at room temperature and pressure for at least 24 h, even if higher temperatures can be adopted to reduce the pre-treatment duration.

The major disadvantages of the alkali pre-treatment are the relatively long pre-treatment time, which requires a reactor of adequate size [77]. Moreover, during this kind of pre-treatment, alkali is converted into salts, which are embedded into biomass, and therefore, they cannot be recovered [88]. Thus, the choice of the base used for pre-treatment is crucial because Na⁺ presence in the biomass can hamper microbial activity during the AD process; methanogens, in particular, are highly sensitive to Na⁺ [3,6,77]. Moreover, Na is environmentally harmful because its disposal causes soil degradation [76,77]. Although KOH is about threefold more monetarily expensive compared with NaOH, it is less toxic for microbial activity [3,88]. In addition, KOH can improve the CH₄ yield similarly to NaOH at a fourfold lower concentration [89].

Nevertheless, NaOH treatment is the most studied pre-treatment for giant reed biomass (Table 2). In mesophilic batch conditions, a methane yield increase of 21% was reported using 0.15 N NaOH for 24 h at 25 °C [35], whereas increases of 61 and 63% were reported using 20 g L⁻¹ NaOH for 24 h at 24–25 °C in mesophilic batch conditions [38,42]. In contrast, a methane yield increase of 34% was obtained by pre-treating giant reed biomass with 20 g L⁻¹ Ca(OH)₂ for 24 h at 25 °C [38]. Recently, KOH was used at 121 °C for 20 min, obtaining a 21% methane yield increase in mesophilic batch conditions [28].

Typically, chemical pre-treatments cause partial biomass solubilization. The compounds generated during these pre-treatments are concentrated in the liquid phase and they could inhibit the subsequent biotechnological processes (i.e., enzymatic hydrolysis, bio-methanogenesis, and bio-hydrogenesis).

Therefore, it is common practice to apply a detoxifying washing step to the pre-treated biomass [28,29]. Unfortunately, this practice generates a waste stream (liquid fraction) that is potentially harmful to the environment. The recovery of the compounds from such a liquid fraction would be useful from both environmental and economic standpoints [3,41,78,90]. For giant reed, the recycling of the black liquor generated during the alkali pre-treatment was proposed. Jiang et al. [38] recycled the NaOH used in five successive pre-treatment batches. However, the first batch (without recycling) reached the highest CH₄ yield, while the subsequent batches progressively decreased. More recently, the black liquor derived from NaOH pre-treatment was recycled in DF without particular inhibitory phenomena [29]. Antifungal properties of the liquid fraction derived from KOH pre-treatment can be exploited in horticulture [91]. As an alternative to recycling, the washing step can be avoided, contributing to the organic load of the AD reactor. In fact, no differences were detected in CH₄ yield between washed and unwashed biomass after KOH pre-treatment [28]. These authors reported that due to the biomass loss after the washing step, the actual CH₄ production per unit of initial biomass did not differ from that of untreated giant reed. Thus, the presence of black liquor in the reactor contributed to the overall CH₄ production. In this case, the digestate can be used since K is a macro-nutrient for crop nutrition.

5.3.3. Ozonolysis

Ozone (O₃) is highly reactive with compounds containing conjugated double bonds, such as lignin [76]. The ozonolysis mechanism is twofold: the direct oxidization by O₃ and the indirect reaction of the hydroxyl radicals [75]. In particular, the aromatic structural units of lignin are oxidized, generating keto-acids and aldehydes [77]. Thus, such pre-treatment is highly selective for lignin. In fact, ozonolysis partially affects hemicellulose, whereas cellulose remains intact [78]. Ozonolysis is considered an eco-friendly technology since no chemicals are required [75,77]. Currently, ozonolysis pre-treatment has not yet been tested on giant reed biomass for biogas production.

5.3.4. Hydrogen Peroxide (H₂O₂) Pre-Treatment

Hydrogen peroxide detaches the lignin reacting with soluble fragments [78]: the lignin is oxidized, breaking down the aromatic nuclei, resulting in electrophilic substitutions, dislocation of side chains, and cleavage of alkyl aryl ether bonds [3,88]. The oxidization of lignin into aromatic compounds could inhibit AD [78]. Moreover, an excess of H₂O₂ concentration could lead to an abundant generation of hydroxyl ions (OH⁻) [6], which may inhibit the AD process following the pre-treatment [46]. This pre-treatment is poorly selective and part of the hemicellulose and cellulose are also lost [6]. Hydrogen peroxide pre-treatment is carried out at a temperature of 100–300 °C [6,76]; hence, it should be considered an energy-consuming process. Hydrogen peroxide can be combined with ferrous iron, giving rise to the Fenton process. Ferrous iron reacts with H₂O₂ at pH 2–3, generating HO∙ radicals that attack the organic matter [75]. No references are available in the literature for hydrogen peroxide pre-treatment on giant reed biomass for biogas production.

5.3.5. Organosolv Pre-Treatment

Organosolv pre-treatment is carried out using some organic solvents (glycerol, ethylene, ethyl glycol, methanol, ethanol, acetone, acetic acid, and peracetic acid), at temperatures of 150–195 °C for 30–90 min [3,6,76,77,78]. Organosolv is effective at lignin and hemicellulose removing via cleavage of α-O-aryl, β-O-aryl bonds, and 4-O-methylglucuronic acid ester bonds [78]. Organic solvents can be combined with acid as a catalyst. However, the acid addition can lead to the generation of furfurals [76,77], which can inhibit the AD process. Until now, organosolv pre-treatment has not yet been tested on giant reed biomass for biogas production.

5.3.6. Hydrotropic Pre-Treatment

Hydrotropes are amphiphilic salts characterized by the presence of both hydrophilic and hydrophobic functional groups [78,92,93]. Thanks to their molecular structure, the presence of aromatic anions, and the nature of the functional groups, they mediate the solubilization of hydrophobic compounds [78], notably lignin. The more utilized pre-treating agents are sodium benzoate, sodium cymene sulphonate, sodium naphthalenesulfonate, sodium phenolsulfonate, sodium salicylate, sodium toluenesulfonate, and sodium xylenesulfonate [78]. These compounds are non-toxic and potentially recyclable; thus, their use is considered environmentally friendly [78]. Currently, no studies have been reported on hydrotropic pre-treatment of giant reed biomass for biogas production.

5.3.7. Ionic Liquid Pre-Treatment

Ionic liquids (ILs) are used in emerging pre-treatment processes [75,77]. Ionic liquids are a combination of large organic cations and small organic anions; thus, they can be bio-based salts [77,78]. Cholinium amino acids, cholinium taurate, N-methyl morpholine-N-oxide monohydrate, 3-methyl-N-butylpyridinium chloride, and imidazole-based compounds (1-allyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium acetate, and 3-methyl-1-octylimidazolium hexafluorophosphate) are mainly used [78,90,94,95]. Typically, ILs have a low melting point (below 100 °C), are not volatile, non-flammable, thermally stable, and have a low vapor pressure [75,78]. Ionic liquids alter the biomass structure, disrupting the hydrogen bonds, degrading the non-covalent interactions between cellulose, hemicellulose, and lignin [96,97], and increasing the active surface area of biomass for microbial activity [75]. However, during the pre-treatment process, ILs become viscous, hampering the process [98]. Although bio-based, ILs are scarcely biodegradable and are undesirable for biotechnological processes; due to their high solubility and low volatility, they are persistent and toxic in the environment [77,78,95,99]. In particular, the imidazole-based compounds cause an inflammatory response in the gut of fish and have a neurotoxic effect on rats [99,100,101]. Therefore, they need to be recovered, even if the process is difficult and energy-intensive [78,95]. Ionic liquid pre-treatment was used on giant reed for bio-hydrogen production [102,103], whereas studies about biomethane production are still lacking.

5.3.8. Deep Eutectic Solvents

Deep eutectic solvents (DESs) are an emerging class of green compounds that are suitable as a pre-treatment method [104]. Deep eutectic solvents are similar to ILs but are non-toxic, inexpensive, biodegradable, and easy to prepare [99,104,105]. Deep eutectic solvents are a mixture of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) compounds [78,99]. This combination abnormally lowers the melting point at the “eutectic” point, even at room temperature [99]. Typical DES mixtures are composed of naturally derived compounds, such as quaternary ammonium salts, amides, amino acids, carboxylic acids, and polyalcohols [78,104]. Deep eutectic solvents have solvation properties that enhance the solubilization of lignin polymers without affecting cellulose [106,107]. However, DESs were only recently used in pre-treating lignocellulosic biomass for biotechnological processes [104,107], but specific studies on giant reed are still missing.

5.4. Biological Pre-Treatment

Biological pre-treatments are based on bacteria (hydrolysis and ensilage), fungi, and selected commercial enzymes. Obviously, all the biological pre-treatments exploit the activities of the enzymes. The biological pre-treatment is considered environmentally friendly due to the utilization of unharmful biological compounds or organisms, and it does not release inhibitory by-products [3,75,76,77,108].

5.4.1. Ensilage

Ensilage is a traditional method of biomass storage that recently was proposed as a pre-treatment method [6]. Ensilage facilitates the conversion of structural carbohydrates (holocellulose) into lactic and acetic acids and ethanol [6]. Nevertheless, the bacteria involved in the ensilage are not able to degrade the lignin [6]. The formation of lactic and acetic acids, as well as ethanol, can improve both the kinetics and the quality of the biogas thanks to the biochemistry that implies the methanogenesis of these acids [109]. This effect of lactic acid has also been observed in ensiled giant reed [110]. Methane yield increases of 18 and 14% in batch mesophilic conditions were reported by applying ensilage as a pre-treatment of the giant reed biomass [39,40].

5.4.2. Fungal Pre-Treatment

Some wood-decay fungi can be utilized for biomass pre-treatment. They can be divided into white-, brown-, and soft-rot fungi, depending on the type of decay they cause [3,75,76,77]. These fungi secrete several types of enzymes, notably laccase, lignin peroxidase, manganese peroxidase, aryl alcohol oxidase, glyoxal oxidase, copper oxidase, pectinase, and cellulase [76,78,111] (Figure 7). Soft-rot fungi belong to Ascomycota. They produce cellulase and are not efficient in de-lignifying biomass. White- and brown-rot fungi belong to Basidiomycota. While white-rot fungi secrete ligninolytic extracellular oxidative enzymes, brown-rot fungi do not. Indeed, brown-rot fungi are characterized by the ability to degrade holocellulose and less extensive demethylation of lignin. Thus, only white-rot fungi are efficient in lignin degradation [50,112,113]. Fungal pre-treatment is economic, has low energy and water demands, and generates low or no inhibitor amounts for the subsequent biotechnological processes [77,78]. Nevertheless, a large pre-treatment area and a long time are necessary for such pre-treatment [75,77,78].

Data reported for fungal pre-treatment of giant reed with white-rot fungi are conflicting. In mesophilic batch conditions, Liu et al. [39] reported a −7.6% methane yield in comparison with untreated giant reed using Ceriporiopsis subvermispora. Similarly, in mesophilic batch conditions, Irpex lacteus showed a −27% methane yield in comparison to the untreated giant reed [31]. In contrast, in the same batch conditions, Pleurotus ostereatus increased the methane yield by 34% [31]. It is important to highlight that the methane yield reported in the latter study for both the untreated and the fungal pre-treatment was very low (98 mL and 131 mL CH₄ g⁻¹ VS, respectively) in comparison to the values reported in the literature for untreated giant reed in mesophilic batch conditions (144–234 mL CH₄ g⁻¹ VS) [24,28,35,38,39,40,114,115]. The time adopted in these fungal pre-treatments was relatively short (30 days) [31,39]; in addition, it is necessary to test further fungal strains to find the most effective in pre-treating the giant reed biomass.

5.4.3. Enzymatic Pre-Treatment for Anaerobic Digestion

The enzymatic pre-treatment consists of the direct addition of commercially available cellulase, α-amylase, β-glucosidase, and endo-xylanase, as well as marginal laccase, laccase peroxidase, and manganese peroxidase [45,111]. These enzymes break down the polymeric structure of both cellulose and hemicellulose into cellobiose, glucose, arabinose, and xylose (Figure 8).

The major advantage of enzymatic pre-treatment is the selective action on the depolymerization of holocellulose without any sugar loss [3], whereas the large quantities of enzymes, the shelf-life, the costs, and the low reusability are the major issues for the enzymatic pre-treatment [75,78]. Nevertheless, enzymatic pre-treatment is seldom used in the AD process since it only marginally affects methanogenesis [3] without significant differences between active or heat-inactivated enzymes added to wheat straw [116]. Because the enzymatic pre-treatment decomposes the holocellulose into simple sugars (hexoses and pentoses), it is more suitable for bio-hydrogen production. Moreover, the hydrogen production stage in DF can be considered a bacterial pre-treatment for methanogenesis. In fact, during DF, by-products such as VFAs, ethanol, and lactic acids are produced by the same bacteria that are involved in the AD process [72,117,118,119]. The DF by-products have a higher biomethanation potential than holocellulose itself [109,120].

5.4.4. Enzymatic and Hybrid Pre-Treatments for Dark and Photo-Fermentation

Both PF and DF were studied for H₂ production from giant reed biomass (Table 3). Before the H₂ production, enzymatic hydrolysis is mandatory; in some cases, one or two pre-treatments were combined with the enzymatic hydrolysis in a hybrid system (Table 3).

On average, DF showed a higher H₂ yield in comparison to PF (217 vs. 86 mL H₂ g⁻¹ VS; Table 3).

Direct enzymatic pre-treatment could be enough to obtain adequate H₂ yield from giant reed biomass both in DF and PF (Table 3). Nevertheless, a hybrid system should allow for an increase in the sugar yield from the biomass and, in turn, the H₂ yield [52,123]. For example, in PF, hybrid NaOH + enzymatic pre-treatment increased the H₂ yield by 69% in comparison to enzyme pre-treatment alone [41], whereas when applying a hybrid [Bmim]HSO₄ + enzymatic pre-treatment, the H₂ yield was 36% higher than using only direct enzyme addition [102].

On the other hand, in a hybrid system, a detoxification step is usually performed before the enzymatic hydrolysis and DF to remove possible inhibitors, such as furfurals and phenols, that are released in the liquid phase due to the pre-treatment [52,58,79]. Such a detoxification step generates liquid waste that has to be managed. In DF, a high inhibitory effect was reported for the furfurals released after the steam explosion used for the giant reed pre-treatment [83,84,85]. Indeed, the H₂ yield from the giant reed directly enzymatically hydrolyzed was 18% higher in comparison to steam-exploded giant reed followed by enzymatic hydrolysis (Table 3) [85]. A low concentration of furfurals and 5-HMF (0.2 g L⁻¹) can improve PF, whereas a higher concentration has a detrimental effect [124]. Vasmara and colleagues [29] reported that the adjustment of the pH recycling the black liquor derived from NaOH pre-treatment did not inhibit DF, despite its high polyphenol concentration.

This is hardly a surprise because it is widely known that in suitable conditions, the phenolic compounds are degraded into benzoate or caproate by Clostridia [125,126]. The latter compounds can be transformed in CH₄ during the AD process [79]. An improvement in the fermentation process can be helpful to enhance H₂ yield, both in DF and PF. For instance, in DF the initial pH of 8.7 enhanced the H₂ yield by 36% compared to the unadjusted initial pH (5.3) [29]. In PF, the addition of glycerol (15 g L⁻¹) allowed for an H₂ yield improvement of almost fourfold [121]. The highest H₂ yield in DF (363 mL H₂ g⁻¹ VS) was obtained with a pre-treated inoculum adapted after three sequential batch fermentations on giant reed hydrolyzate [85]. In PF, the highest H₂ yield (126 mL H₂ g⁻¹ VS) was obtained with a light-intensity perturbation from 3000 to 6000 lux [122].

6. Concluding Remarks, Challenges, and Recommendations

This review highlights that alkali pre-treatment is the most effective at improving the anaerobic degradation process of giant reed biomass among the diverse methods studied.

Considering that all studies reported in the literature were carried out in laboratory batch mode, a scale-up of the processes is strongly needed in view of the practical applications. This would also allow for a better evaluation of energy efficiency, as well as economic and environmental issues.

Several kinds of pre-treatment still have to be explored for giant reed to improve biogas production. In particular, biological pre-treatment based on fungi (notably white-rot fungi) can be regarded as one of the most promising and merits further scrutiny. Several advantages and disadvantages can be identified for such a pre-treatment, as follows:

Advantages: (i) it is monetarily inexpensive; (ii) little energy and water are required; (iii) no waste is generated; (iv) little, if any, inhibitors for subsequent processes are generated.

Disadvantages: (i) large areas for pre-treatment are needed and (ii) long pre-treatment times are required (several weeks).

These issues of large areas and long pre-treatment times are not unbeatable. On the one hand, large storage areas are needed anyway in the biogas plants and the timespan for a typical AD process of lignocellulosic biomass is on the order of 60–90 days; hence, there is no urgency to have freshly pre-treated material.

An interesting challenge for the valorization of giant reed biomass is the implementation of a two-stage AD process. In this process, DF (producing H₂) is separated from methanogenesis. Thus, H₂ can be valorized as fuel gas, whereas the by-products (VFAs, ethanol) of DF can be exploited in the subsequent methanogenesis stage, enhancing the CH₄ yield.

Similarly, DF can be coupled with PF to increase the overall H₂ production. In fact, the bacteria involved in the PF process produce H₂ from acetic acid generated in the DF process, whereas the other by-products of DF can be exploited in a bio-refinery process.

Another challenge is coupling the pre-treatment of giant reed biomass and co-digestion with animal waste. This is a promising way to enhance CH₄ production while keeping the reactor volume constant.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Lignocellulosic biomass structure (modified from [45], published under CCBY 4.0 license).

Figure 2. The anaerobic digestion process.

Figure 3. Photo-fermentation and dark fermentation processes (reproduced from [57], published under CCBY 4.0 license).

Figure 4. Metabolic pathways of Enterobacter spp. (a) and Clostridium spp. (b) in dark fermentation. ATP: adenosine triphosphate; NAD(H): nicotinamide adenine dinucleotide; ldhA: lactate dehydrogenase; pfl: pyruvate formate lyase; fdh: formate dehydrogenase; mdh: malate dehydrogenase; adhE: aldehyde–alcohol dehydrogenase; ackA: acetate kinase; fum: fumarate; frd: fumarate reductase; FADH2: flavin adenine dinucleotide; Fd: ferredoxin; ADP: adenosine diphosphate.

Figure 5. Dark fermentation integrated within the anaerobic digestion process.

Figure 6. Pre-treatment effect on the lignocellulose structure.

Figure 7. Biochemical activity of laccase (a), lignin peroxidase (b), and manganese peroxidase (c) (reproduced from [45], published under CCBY 4.0 license).

Figure 8. Enzymatic hydrolysis mechanism on cellulose (a) and hemicellulose (b) (reproduced from [45], published under CCBY 4.0 license).

Table 1

Fiber composition of giant reed biomass.

Harvesting Time	Country	Cellulose (%TS)	Hemicellulose (%TS)	Lignin (%TS)	References
n.a.	China	33.7	30.1	10.1	[27]
January 2017	Italy	43.1	23.6	11.4 a	[28]
January 2017	Italy	39.1	23.1	11.7 a	[29]
January 2017	Italy	41.2	22.5	10.9 a	[30]
February 2021	Italy	36.9	29.0	10.4 a	[31]
March 2006	Italy	33.4	28.9	14.0 a	[32]
July 2016	Italy	46.5	23.3	12.3 a	[33]
September 2016	Italy	40.9	22.9	10.9 a	[24]
September2020	Italy	35.9	29.1	9.6 a	[31]
October 2010	Italy	32.9	23.3	25.9 b	[34]
October 2010	Italy	32.1	20.5	22.9 b	[35]
October 2017	Italy	39.5	25.4	10.2 a	[24]
October 2018	Italy	43.5	23.3	10.4 a	[24]
n.a.	Portugal	33.9	20.3	24	[36]
n.a.	Thailand	39.1	24.4	19.2 b	[37]
August 2016	USA	34.9	17.4	20.1 b	[38]
August 2016	USA	31.0	16.1	16.0 b	[39]
October 2014	USA	27.7	15.6	16.7 b	[40]
October 2014	USA	29.0	15.4	16.4 b	[41]
November 2015	USA	32.5	13.9	23.9 b	[42]
November 2016	USA	28.8	14.4	16.9 b	[39]
December 2014	USA	41.5	20.5	18.4 b	[40]
December 2016	USA	30.3	15.3	17.8 b	[39]

TS: total solids; n.a.: not available; ^a van Soest method [43]; ^b National Renewable Energy Laboratory method (NREL) [44].

Table 2

Effects of different pre-treatments of giant reed biomass on methane yield in batch systems.

Type of Pre-Treatment	Pre-Treatment Agents and Conditions	Digestion Temperature (°C)	Methane Yield(mL CH4 g−1 VS)	Maximum MethaneIncrease a	References
Mechanical	Two stage milling	38	213	137%	[33]
Hydrothermal	Steam cooking (150–180 °C, 10–20 min)	53	337	23%	[34]
Hydrothermal	Liquid hot water (170–230 °C, 5–15 min)	37	284	31%	[42]
Hydrothermal	Autoclaving (121 °C, 20 min)	35	198	3%	[28]
Chemical	NaOH (0.05–0.15 N), 25 °C, 24 h	35	243	21%	[35]
Chemical	NaOH (1–20 g L−1), 25 °C, 24 h	37	354	63%	[42]
Chemical	NaOH (20 g L−1), 25 °C, 24 h	37	353	61%	[38]
Chemical	Ca(OH)2 (1–20% g−1 TS), 25 °C, 24 h	37	285	34%	[38]
Chemical	KOH (13.5% g−1 VS), 121 °C, 20 min	35	232	21%	[28]
Chemical	H2SO4 (2% w/w), 150–180 °C, 10–20 min	53	n.a.	−49%	[34]
Biological	Ensilage, 25 °C, 90 d	37	173	18%	[40]
Biological	Ensilage, 25 °C, 60 d	37	188	14%	[39]
Biological	Ceriporiopsis subvermisporab, 28 °C, 30 d	37	153	−7.6%	[39]
Biological	Irpex lacteusb, 26 °C, 30 d	38	77	−27%	[31]
Biological	Pleurotus ostreatusb, 26 °C, 30 d	38	131	34%	[31]

^a Compared with the untreated control; ^b fungal agent.

Table 3

Hydrogen production from giant reed biomass after hybrid or enzymatic pre-treatment.

Pre-Treatment Method	1st Pre-Treatment	Enzymatic Pre-Treatment	Fermentation System	Temperature(°C)	H2 Yield(mL g−1 VS)	References
Hybrid (SE + Enz.)	Steam explosion, 210 °C, 6 min	Cellulase 15 FPU g−1, cellobiase 30 U g−1, 50 °C, 72 h	DF	35	43	[83]
Hybrid (SE + Enz.)	Steam explosion, 210 °C, 6 min	Cellulase 15 FPU g−1, cellobiase 30 U g−1, 50 °C, 72 h	DF	38	161	[84]
Hybrid (SE + Enz.)	Steam explosion, 210 °C, 6 min	Cellulase 15 FPU g−1, cellobiase 30 U g−1, 50 °C, 72 h	DF	38	307	[85]
Hybrid (alkali + Enz.)	NaOH, 1–20 g L−1 24 °C, 24 h	Cellic CTec2 20 FPU g−1d.a.r.	PF	30	101 a	[41]
Hybrid (alkali + Enz.)	Ca(OH)2, 1–20 g L−1 24 °C, 24 h	Cellic CTec2 20 FPU g−1d.a.r.	PF	30	80 a	[41]
Hybrid (alkali + Enz.)	NaOH, 12 g L−1 121 °C, 20 min	Cellic CTec2 25 FPU g−150 °C, 144 h	DF	35	237	[29]
Hybrid (ILs + Enz.)	[Bmim]BF4, 2–16 g L−1, 80 °C, 3 h	Cellic CTec2 20 FPU g−1d.a.r.	PF	30	97 a	[102]
Hybrid (ILs + Enz.)	[Bmim]HSO4, 2–16 g L−1, 80 °C, 3 h	Cellic CTec2 20 FPU g−1d.a.r.	PF	30	108 a	[102]
Hybrid (ILs + Enz.)	[Amim]Cl, 2–16 g L−1, 80 °C, 3 h	Cellic CTec2 20 FPU g−1d.a.r.	PF	30	100 a	[102]
Hybrid (ILs + Enz.)	[Bmim]acetate, 6 g L−1, 70 °C, 4 h	Cellulase 20 FPU g−1d.a.r.	PF	30	69 a	[103]
Enzymatic	n.a.	Cellic CTec2 20 FPU g−1d.a.r.	PF	30	82 a	[102]
Enzymatic	n.a.	Cellic CTec2 20 FPU g−1d.a.r.	PF	30	59 a	[41]
Enzymatic	n.a.	Cellulase 20 FPU g−1d.a.r.	PF	30	81 a	[121]
Enzymatic	n.a.	Cellulase 20 FPU g−1d.a.r.	PF	30	126 a	[122]
Enzymatic	n.a.	Cellulase 20 FPU g−1d.a.r.	PF	30	46 a	[103]
Enzymatic	n.a.	Cellulase 15 FPU g−1, cellobiase 30 U g−1, 50 °C, 72 h	DF	38	189	[84]
Enzymatic	n.a.	Cellulase 15 FPU g−1, cellobiase 30 U g−1, 50 °C, 72 h	DF	38	363	[85]

^a Calculated; DF: dark fermentation; PF: photo-fermentation; d.a.r.: direct addition of the enzymes in the fermentation reactor; Enz.: enzymatic; SE: steam explosion; ILs: ionic liquids; n.a.: not applicable.

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