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Review

Potential of Wheat Straw for Biogas Production by Anaerobic Digestion in South Africa: A Review

by
Reckson Kamusoko
* and
Patrick Mukumba
Faculty of Science and Agriculture, Department of Physics, University of Fort Hare, Ring Road, Alice 5700, South Africa
*
Author to whom correspondence should be addressed.
Energies 2024, 17(18), 4662; https://doi.org/10.3390/en17184662
Submission received: 21 August 2024 / Revised: 9 September 2024 / Accepted: 16 September 2024 / Published: 19 September 2024
(This article belongs to the Topic Chemical and Biochemical Processes for Energy Sources, 2nd Edition)
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Abstract

:
Wheat straw (WS) is a promising substrate for biogas production by anaerobic digestion (AD) due to its high carbohydrate content. An estimated 0.603 million t yr−1 of WS are generated from wheat production systems in South Africa. This is equivalent to an energy potential of 11 PJ. Despite this, WS is still undervalued as a bioenergy resource in South Africa due to its structural complexity and low nitrogen content. WS disposal methods, such as use in livestock bedding, burning and burying into the soil, inter alia, are not sustainable and may contribute to global warming and climate change. The commercialization of the AD of WS needs to be further developed and promoted. Pre-treatment (i.e., physical, chemical, biological and hybrid methods) and anaerobic co-digestion (AcoD) are novel strategies that can support the conversion of WS into biogas and other value-added products. Current and future research should focus on optimizing pre-treatment and AcoD conditions towards industrialization of WS into valuable products. This paper focuses on the potential use of WS for biogas production in South Africa. The aim is to create information that will promote research and development, and encourage policy makers and stakeholders to participate and invest in WS biogas technology. Were WS biogas technology fully adopted, we believe that it would alleviate energy insecurity and environmental degradation, and sustain the livelihoods of citizens in South Africa.

1. Introduction

Lignocellulosic biomass from cereals is believed to be an affordable, abundant and renewable energy resource that can replace around 55% of fossil fuels [1,2]. South Africa is one of the major cereal-producers in Africa and it generates vast amounts of wheat straw (WS) as a by-product of wheat production. The gross biomass production for WS in South Africa is estimated at 0.603 million t yr−1. This corresponds to an energy potential of 11 PJ. If the biomass supply chain is properly managed, there is potential to increase the annual production of WS by 140% to 1.45 million t [3]. Currently, WS is utilized as a feed or livestock bedding or soil amendment, with limited conversion into bioenergy. Burning is not a sustainable option for WS management because it emits greenhouse gas emissions (GHGs) that can pessimistically affect the environment [3,4].
WS holds much promise as a feedstock for biogas production by anaerobic digestion (AD) [5,6]. AD is an eco-friendly technology that can support the efficient utilization and recycling of organic matter whilst transforming it into biogas [7,8]. The energy gain to input ratio in AD is approximately 28.8 MJ MJ−1. This should suffice to outperform most technologies in terms of biomass conversion efficiency [9]. Biogas can be upgraded to methane (CH4) and used for heat and power generation and as a vehicular fuel [5]. CH4 is a high-energy carrier with a low heat value (LHV) (35.832 kJ/m3), which is 0.83-fold that of petrol [2]. The AD of WS is promising because it contains large amounts of carbohydrates (30–40% cellulose and 20–30% hemicelluloses) and relatively small amounts of lignin (15–20%) [4,10]. It is postulated that the AD of WS can reduce GHGs and annual emissions by 90 and 49%, respectively [2]. However, WS has not been fully exploited for biogas production due to its refractory structure that is resistant to biotic and abiotic degradation [9,11].
The AD pathway involves four interrelated steps, namely hydrolysis, acetogenesis, acidogenesis and methanogenesis. The process is performed by diverse groups of anaerobic bacteria. Hydrolysis is considered to be a slow and rate-limiting step in the AD of WS [12,13]. More so, WS is touted to have a high carbon to nitrogen (C/N) ratio residue which may have an effect on digester nutrient balance [14]. The highest theoretical biomethane potential (BMP) of WS is calculated at 308 mL g VS−1 [14]. Conventional AD of WS rarely exceeds this hypothetical value. The current research mantra is to deploy innovative strategies that can enhance the overall digestibility and CH4 yield of WS. Pre-treatment and anaerobic co-digestion (AcoD) are potent strategies that can improve the valorization of WS into biogas [11,15].
Several pre-treatment methods, including biological, physical, chemical and a combination of them, are widely reported in the literature to enhance methane production [11,16,17]. The commercialization of physical and chemical methods has been mainly restricted by intensive energy use and strong corrosiveness to equipment, respectively. In comparison, biological pre-treatment is considered an eco-friendly technology due to low energy demand and mild reactor conditions [16,17]. AcoD is a synchronous reaction, which involves the digestion of WS together with a co-substrate [15]. The ultimate goal is to improve the biodegradation of WS due to synergistic interactions in the digester medium [15,18]. Organic matter derived from agricultural, municipal and industrial activities can be co-digested with WS to enhance biogas production through nutrient balance, toxicity reduction and improved buffering capacity [19,20].
The basis of this paper is that as part of biomass supply mix, WS should play a pivotal role in renewable energy systems in South Africa. With annual production projected to rise to about 1.45 million t, WS is a vast bioresource for the country that can be fully utilized for biogasification. Currently, there are no digesters that utilize crop residues as a feedstock for AD in South Africa. This paper reviews the potential of using WS as a feedstock for biogas production in South Africa. The focal point is to present useful and aggregated information that will drive research, development and national policy imperatives.

2. Availability of Wheat Straw in South Africa

South Africa is the main producer of wheat in the Southern African Development Community (SADC) region and it is ranked sixth in Africa [21]. Wheat is the second largest cereal crop, after maize in South Africa [3,22]. A total of 513,000 ha yr−1 of land are under wheat cultivation in the country [21]. This generates relatively huge amounts of WS from the harvesting and processing of wheat grains. Currently, South Africa produces WS at the capacity of 0.603 million t yr−1. Given the biomass supply chain is optimal, WS generation is expected to rise to around 1.45 million t yr−1 [3]. According to Rocha-Meneses et al. [23], WS is generated at the rate of 3 t ha−1 in South Africa. Table 1 shows the relative abundance of WS biomass in all the provinces of South Africa. It is apparent that the largest amount of WS (~64%) is generated in the Northern Cape Province. This can be attributed to the optimal conditions for wheat production in this region.

3. Structural Composition of Wheat Straw and Limitations for Biogas Production

WS is a biocomposite that consist of cells that make up plant cell walls. The main strengthening compounds of cell walls are macromolecules of cellulose, hemicelluloses and lignin [5,24]. These compounds are linked by covalent and non-covalent forces to form a hydrophobic, tough, heterogeneous and complex structure. The structure is highly resistant to anaerobic biodegradation [1,25]. This often leads to long residence times and restricts the utility of WS for AD. Of the three constituents, cellulose and lignin present the most serious constraints that hinder the AD route [26,27]. The high C/N ratio of more than 90 is a further barrier to the AD of WS [2]. Novel strategies are required to break the lignin seal so that holocellulose (cellulose and hemicelluloses) is readily available for enzymatic hydrolysis and promote nutrient balance. Cellulose and hemicelluloses are the major fermentable sugars found in WS. Lignocellulosic composition of different WS materials is shown in Table 2. Composition of WS may vary according to plant variety, age and conditions for growth and development [27].
Figure 1 depicts the typical structure of lignocellulosic WS. Cellulose is the most abundant polysaccharide in WS. A chain of cellulose is made up of D-glucose units that are joined with β-1,4-gycosidic bonds [26]. Cellulose chains are linked together by hydrogen bonds to form fibers. The crystalline and partially amorphous structure of the fibers increases the resistance of cellulose to biological degradation. Cellulose fibers are further glued by a gel matrix of hemicellulloses, lignin and other polymers to form a biocomposite [11,25]. Hemicelluloses are branched heteropolymers of a mixture of simple sugars, including pentoses, hexoses and sugar acids, in conjunction with xylose. They are strongly linked to cellulose fibers by non-covalent bonds to give lignocellulose its matrix appearance. However, the amorphous shape and low molecular weight of hemicellulose makes it more digestible than cellulose [11,24,26].
Lignin is an aromatic structure composed of monomers of coniferyl, sinapyl and coumaryl alcohols. It covers space between cellulose and hemicelluloses, where it strongly binds cellulosic fibers into a compact structure. Lignin is hydrophobic, amorphous and complex in nature; it thus acts as physical impediment against microbial attack. This is a critical feature of WS which prohibits its suitability for biogas production. Lignin is the most recalcitrant barrier to AD of WS [11,24].

4. Wheat Straw Residue to Biogas

4.1. The Anaerobic Digestion Pathway

AD is a biochemical process by which large organic compounds are converted into fermentable monomeric units using consortia of anaerobic bacteria. Prominent bacteria that perform this reaction are members of the Bacteria and Archaea taxa [33]. Solid-state AD is used to treat WS which contains more than 15% total solids. The purpose is to dispose of the waste in a sustainable, economically viable and eco-friendly way [27,34]. Energy-rich biogas is the core product of the AD of WS. It is a blend of 50–60% CH4, 30–40% carbon dioxide (CO2) and other gasses in trace amounts [35]. Biogas is used for heat, power and electricity generation. A rich-digestate solid waste given off as a by-product of AD is a useful organic fertilizer as well as a substrate for mushroom cultivation, vermicompost and aquaculture [33].
AD is an integrated approach that involves four multipart reactions, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Figure 2). Hydrolysis is the first stage of the AD process. This process utilizes hydrolytic bacteria to transform complex polymers like carbohydrates, proteins and lipids into corresponding units of simple sugars, amino acids and fatty acids. Hydrolysis is often slow, and it is regarded as a rate-limiting step in the AD of WS [34,36,37]. Hence, acceleration of this phase will bear a strong impact on the overall reactor performance. Acidogenesis is the second stage of AD, which is generally characterized by acid production. In this fermentation process, the products of hydrolysis are broken down by acidogens into alcohols, aldehydes, volatile fatty acids (VFAs), acetate (CH3COOH), hydrogen (H2), ammonia (NH3) and CO2 [27,37]. Acetogenesis is the third stage, in which organic acids and alcohols from acidogenesis and hydrolysis are further degraded to produce CH3COOH, CO2 and H2. Acetate and hydrogen-forming bacteria are involved in this process [37]. The last stage in AD is methanogenesis. In this phase, autotrophic methanogens convert substrates from the acetogenesis phase to form CH4 [32,33].

4.2. Biomethane Potential of Wheat Straw

BMP is the measure of anaerobic decomposition of an organic substrate [38]. It predicts the amount of a given substrate that can be converted into biogas. The BMP of several agricultural residues have been extensively reported by many authors [39]. However, only limited studies focused on the AD of WS. For example, Ferreira et al. [4] estimated the BMP of WS at 233 mL g VS−1 while an average CH4 production of 154 mL g VS−1 was observed after the incubation of WS for 30 days [40]. In a study conducted by Tsapekos et al. [26], the CH4 yield of 221 mL g VS−1 was reported from mono-digestion of WS under thermophilic conditions. The CH4 yield of WS is restricted due to high lignin content and imbalanced nutrient composition. These hurdles can be surpassed by deploying appropriate pre-treatment strategies and co-digestion [41].
Table 3 summarizes the capacity of various WS residues to produce CH4 in the world. What is worth noting is that the ability of WS to produce CH4 varies from region to region in accordance with plant type, variety and maturation, and agro-climatic conditions [30]. As such, it is imperative to estimate the BMP for WS of South African origin in an attempt to harness the energy potential bestowed in the local residue.

5. Potential Strategies to Enhance Biogas Production from Wheat Straw

5.1. Pre-Treatment of Wheat Straw

The choice of pre-treatment is critical in developing a viable scheme for biogasification of WS. Pre-treatment enhances AD hydrolysis and promotes the availability of sugars and other small molecules to microbiota [17]. There are several pre-treatment methods reported in the literature. Pre-treatment methods can be broadly categorized into physical, chemical, biological and physico-chemical processes (Figure 3) [47,48]. Physical and chemical methods are fast and effective; however, they have limited value at the industrial scale due to the high cost of resources, energy and operation and the formation of toxic compounds. Biological pre-treatments are still at their infant stages of development [27,49].

5.1.1. Physical Pre-Treatment

Physical pre-treatment is designed to trim the size of particles of a substrate by mechanical comminution or boost the surface area through mechanical refining. The final goal is to enhance the efficiency of hydrolysis and the yield of biogas [50]. All methods that do not utilize water, chemicals and microorganisms are regarded as physical pre-treatments. These include mechanical, ultrasound and thermal pre-treatment methods [50,51]. Table 4 shows studies that have been performed to ascertain the effect of physical pre-treatment methods on biogas production from WS. Physical methods can increase the AD reaction kinetics of WS, but require more energy and capital [2].
Mechanical pre-treatment includes a gamut of methods, such as grinding, milling, chipping or extrusion. Chipping is believed to be the most suitable mechanical method for treatment of WS. Mechanical pre-treatment reduces the crystallinity of cellulose and its extent of polymerization [50]. Studies performed on mechanical pre-treatment of WS have shown a strong impact on biogasification. For example, Dell’Omo and Spena [52] reported 49.1% more cumulative biogas yield from milled WS than a control. Similarly, multistage knife milling of WS improved CH4 production by 49.3% after 28 d of mesophilic digestion [53]. Size reduction in the range of 0.5–2 mm can enhance heat and mass transfer to achieve adequate levels of biodegradation. Nevertheless, the method is not sustainable because of high energy input [45,52,54].
Thermal pre-treatment can be sub-divided into conventional methods and microwave irradiation. Conventional thermal pre-treatment deconstructs cellulosic material by a combined use of heat and water at a temperature range of 50–250 °C [9]. Heat and water destroy the crystalline structure of cellulose and other lignocellulose matter, and converts hemicelluloses into VFAs and simple sugars [2]. This results in enhanced biomethane production. For example, Bolado-Rodriguez et al. [55] observed 20% higher CH4 yield by exposing WS to thermal heat at 121 °C for 60 min than the control. Similarly, Abdul-Wahab et al. [56] generated 20% more CH4 at 250 °C and 1 min upon pre-treating WS at 150–220 °C for 1–15 min.
The microwave method utilizes thermal energy from stimulation of vibration of molecules by non-ionizing radiation [50]. Thermal heat disrupts the crystal structure of cellulose by cutting the β-1,4-glucan bonds, consequently enhancing the surface area of WS for AD [51]. The microwave method could be a cheap alternative to conventional heating in the foreseeable future. With microwave pre-treatment, high temperatures are attained within a short period, thus saving energy [33]. However, the process is often associated with high processing times, which may lead to sugar degradation [50]. Furthermore, high capital requirements for installation limit its applicability at the industrial scale [54]. Information on microwave processing of WS to enhance biogas seems to be patchy. As an exemplar, CH4 yield of WS was raised by 28% via solubilization with microwave irradiation at 150 °C [57].
Ultrasound pre-treatment uses acoustic energy in the form of high frequency waves to induce cell lysis. Microbubbles are generated due to the cavitation of cells in liquid solutions by high-frequency sonic waves. The disintegration of microbubbles ruptures plant cell walls and exposes cellular contents, resulting in improved hydrolysis [51,58]. Ultrasound pre-treatment has the advantages of short treatment time and low temperature needs, although it integrates the use of chemicals [50]. Few studies were found in the literature on the ultrasound pre-treatment of WS for biogas production. For instance, a 63% increase in CH4 content was recorded from the ultrasound pre-treatment of WS at 20 kHz for 36 h [59].
Table 4. Physical pre-treatment of wheat straw for biogas production.
Table 4. Physical pre-treatment of wheat straw for biogas production.
Physical AgentPre-Treatment ConditionsFindingsReference
MechanicalKnife milling, 0.3–1.2 mm particle sizeMethane yield increased by 49.3%[53]
Roll milling 21% increase in methane yield[14]
Cutting (3–5 cm), milling (<1 mm)5–13% more methane for 3–5 cm particles with faster kinetics[43]
Chopping (2 cm), extruder-grinding (0.2 cm)Size reduction improved methane yield by 26%[60]
Conventional thermal150–220 °C, 1–15 min20% increase in methane yield[43]
200 °C, 5 min27% more methane production[4]
121 °C, 60 min20% increase in methane yield[55]
150–220 °C, 1–15 minMethane yield enhanced by 20%[56]
MicrowavePower of 400–1600 W, 150 °C28% increase in methane yield[57]
200–300 °C, 15 minNo increase in methane yield[61]
Ultrasound4% KOH, 20 kHz, ambient temperature, 36 h63% higher methane yield[59]
Hydrodynamic cavitation, 2300–2700 rpm, 2–6 min145% increased methane yield [62]
4% (w/w) H2O2, 36 °C, 10 min, 25 kHz 64% enhanced methane yield[63]

5.1.2. Chemical Pre-Treatment

Chemical pre-treatment is based on substances, including acids, alkalis and ionic liquids to degrade the crystallinity of recalcitrant biomass. It can be classified into acid, alkaline, oxidative and organosolv pre-treatments [11]. The use of chemicals is the most well-known pre-treatment method. Nevertheless, the method has been extensively reported in cellulosic bioethanol production compared to biogas production [34]. Table 5 shows studies that have been conducted on chemical pre-treatment of WS for biogas production. Chemical pre-treatment is intended to improve biogas yield through disintegration of holocellulose.
Alkali pre-treatment can be deployed to solubilize lignin and holocellulose, thus rendering cellulosic materials to biological degradation. The method exploits bases, such as sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2), potassium hydroxide (KOH) and ammonium hydroxide (NH3.H2O) to liquefy and cleave lignin–carbohydrate bonds [54]. The purpose is to destroy the rigidity and structural complexity of WS, and increase the surface area for microbial attack [32,51]. NaOH pre-treatment is the most effective and widely studied alkali method for enhanced AD [34]. For example, a 112% enhanced CH4 yield was reported from NaOH-pre-treated WS [64]. However, NaOH must be treated with caution as it generates sodium ions that can inhibit the AD process [11].
Acid pre-treatment is yet another chemical method considered to be effective against hemicelluloses and lignin [50]. This technique leads to ease of access to cellulose by microbial agents [34]. Nitric acid (HNO3), sulfuric acid (H2SO4), phosphoric acid (H3PO4) and hydrochloric acid (HCl) are typical examples of inorganic acids that can be used for biomass pre-treatment [32,54]. Dilute acid is believed to be more effective than concentrated acid for pre-treatment of lignocellulose. It is possible to solubilize up to 100% hemicelluloses into its monomeric units using dilute acid. Dilute acid can destroy lignin to a high degree, even though it is considered to be less efficient in dissolving the lignin [32]. Concentrated acid is very effective against cellulose, but it is highly toxic, corrosive and requires specialized equipment. H2SO4 is the most extensively studied acidic pre-treatment method [34]. There is limited information on acidic pre-treatment of WS for biogas production. Even so, a 16% rise in CH4 production was obtained from WS pre-treated with H2SO4 [65].
Oxidative pre-treatment is the degradation of lignin and hemicelluloses by oxidants, such as H2O2 and ozone gas [11,17]. This leads to nucleophilic substitution, the destruction of aromatic nuclei, the removal of side chains and the dislocation of alkyl aryl ether bonds. Hydroxyl radicals (-OH) and superoxides (O2−) released from H2O2 promote the delignification of organic matter and release more fermentable sugars [9]. H2O2 is very effective in alkaline solutions (pH 11.5) and does not generate toxic compounds [9,27]. Oxidative pre-treatment of WS with N-methylmorpholine N-oxide (NMMO) increased CH4 yield by 11% [66]. An increase of 27% in CH4 yield was reported from photo-oxidative pre-treatment of WS using titanium oxide (TiO2) [67].
During organosolv pre-treatment, organic solvents are used to destroy internal linkages of lignin and hemicelluloses to ensure pure cellulose in WS is available for AD [11]. Frequently utilized organic solvents include methanol, ethanol, tetrahydrofuranol, acetone and ethylene glycol. The organosolv reaction is catalyzed by acids like H2SO4 and HCl or bases such as NaOH, NH3 and calcium carbonate (CaCO3) [17]. Mancini et al. [66] observed up to 15% enhanced CH4 yield from the pre-treatment of WS with 50% ethanol. Likewise, an improved biogas production (47%) was found from the pre-treatment of WS using NMMO [68]. However, other organic solvents are exempted for WS pre-treatment because they are expensive, flammable, volatile, non-biodegradable and have low lignin removal efficiency [2].
Table 5. Chemical pre-treatment of wheat straw for biogas production.
Table 5. Chemical pre-treatment of wheat straw for biogas production.
Chemical AgentPre-Treatment ConditionsFindingsReference
Acid1% H2SO4, 121 °C, 10–120 minIncreased methane yield by 16% [65]
0.5–5% H2SO4, 90–100 °C, 2 hBiogas yield increased by 32% using 0.5% H2SO4 while 5% H2SO4 did not improve biogas yield[69]
2% HCl, 90 °C, 2 h59% more biogas yield[70]
Alkaline1.6% NaOH, 30 °C, 24 h15% enhanced methane yield [66]
NH3 (2, 4, 6%), 35 °C, 7 d52% increased methane yield[71]
4% NaOH, 37 °C, 5 dBiogas yield increased by 87.5%[64]
7 g KOH, 25 °C, 24 h128% methane yield increment[47]
75 mM NaOH, 16 hMethane yield increased by 23%[72]
0.05 M NaOH, 25 °C, 22 h22% increase in cumulative methane[44]
0.08 M Ca(OH)2, 20 °C, 48 hMethane yield increased by 315%[73]
OxidativeTiO2-assisted photo-oxidationImproved methane yield by 27%[67]
NMMO, 120 °C, 3 h11% methane yield improvement[66]
OrganosolvNMMO, 90 °C, 7 h47% increase in methane production [68]
50% ethanol, 180 °C, 1 h15% improved methane yield[66]
NMMO, 120 °C, 3 h11% enhanced methane yield[66]

5.1.3. Physico-Chemical Pre-Treatment

Physico-chemical pre-treatment amalgamates different methods to depolymerize lignin and hemicelluloses so that more fermentable sugars in WS are released for AD [33]. The most suitable temperature for physico-chemical pre-treatment varies from 50 to 250 °C [11]. Heat is applied to disrupt hydrogen bonds in WS, thereby increasing the surface area for microbes. It is prudent to recycle heat as a strategy to save energy during physico-chemical processing. Extended pre-treatment times should be avoided to prevent the accumulation of inhibitory by-products [51]. Potent physico-chemical pre-treatments include extrusion, steam explosion and hydrothermal processing [33].
During extrusion pre-treatment, thermal and mechanical methods are combined in a single unit to modify the physical and chemical properties of plant biomass [17]. Biomass is subjected to distressful conditions like heating and mixing with rapid fall in pressure [11]. As biomass is discharged from the extruder, cellulose dissociates from complex polymers by breaking the β-O-4 linkage in lignin and the plant cell wall structure is destroyed [17]. Extrusion results in the deconstruction of cellulose, hemicelluloses, lignin and proteins [74]. The most favorable operational conditions for extrusion are temperature and pressure ranges of 160–250 °C and 0.5–5.0 MPa, respectively [17]. Chen et al. [75] evaluated the effect of extrusion at 37 °C on biogas production from WS. In this study, biogas and CH4 production increased by 23% and 27%, respectively. In a related study, an improved daily CH4 production of 28% was reported from twin-screw extruded WS [14]. However, the BMP value of WS was not significantly improved by extrusion. The AD of extruded-WS for 28 and 90 d enhanced CH4 production by 14–28% and 1–16%, respectively [76].
Steam explosion is a promising eco-friendly strategy for the pre-treatment of WS for AD. In this technology, complex plant polymers are exposed to high pressure (5–50 bar) and saturated steam at 160–250 °C for short residence times [77]. Pressure is then rapidly lowered leading to depolymerization of the plant biomass [11]. The conversion of WS into biofuels and other multiple products via steam explosion pre-treatment has generated interest in the 21st century. For example, steam explosion pre-treatment of WS was studied by Kaldis [78] who reported 20% enhanced CH4 productivity. However, steam explosion pre-treatment of WS did not provide positive results with regard to improvement of biogas production [79,80]. It is possible to enhance the efficacy of steam pre-treatment by adding an acid catalyst. As an example, acid catalyzed steam pre-treatment of pressed WS by exposure to 0.5% H3PO4 at 190 °C for 5 min increased CH4 production by 39% from 0.18 to 0.25 m3 kg VS−1 [81]. Still, acid catalyzed steam pre-treatment is problematic to some extent. Construction of a steam pre-treatment unit incurs high capital and energy costs, while the use of acids like H2SO4 releases large amounts of sulfates that may hinder downstream processes [81].
Hydrothermal liquefaction is an excellent method for enhanced energy recovery from WS to biogas. It is realistic to recuperate about 80% of the energy from WS biomass using this technology [64]. Hydrothermal pre-treatment exploits hot water under high pressure at a reaction temperature of around 200 °C to permeate biomass, solubilize cellulose and destroy hemicelluloses and a portion of lignin [11,64]. Although there is no use of chemicals and rust-proof equipment [64], the method releases phenolics and furfurals that may inhibit AD [11]. Chandra et al. [64] improved biogas and CH4 production by 9.2 and 20.0%, respectively, using hydrothermally pre-treated WS at 200 °C (1.55 MPa equivalent) for 10 min. Around 34% more biogas yield was achieved from hydrothermal and thermal-alkali pre-treatment of WS compared to a control [82]. In another investigation, exposure of WS to hydrothermal pre-treatment at an optimum temperature of 120 °C enhanced CH4 yield by 32% [83]. The optimal operating temperature of 180 °C was found to increase CH4 yield by 53% from hydrothermally treated WS [84].

5.1.4. Biological Pre-Treatment

Biological pre-treatment entails the use of microbial metabolism or by-products to pre-digest recalcitrant biomass [9]. The main effect is delignification to provide more cellulose and hemicelluloses for fermentation [2]. As shown in Figure 4, a sole or a consortium of microbes is generally applied to degrade polymeric substances. The biological method is more favorable for the pre-treatment of WS than the other pre-treatment methods. It is an eco-friendly technology that does not pollute the environment, generates little or no toxic by-products and has low energy demands. Contrarily, the method is considered to be slow, often with prolonged incubation times [2,9]. Studies that have been conducted on the biological pre-treatment of WS to improve biogas production are shown on Table 6. The predominantly used biological agents for the pre-treatment of organic matter comprise fungi, bacteria and enzymes.
Fungal pre-treatment utilizes white-rot fungi, brown-rot fungi, soft-rot fungi or other fungi to delignify WS [24,33]. Lignin is a potential source of several materials and biochemicals for biorefineries [85]. White-rot fungi are considered to be the most effective fungal pre-treatment agents [33]. Basidiomycetes (Phanerochaete chrysosporium) are the most widely studied white-rot fungi for the delignification of cellulosic biomass [17]. Factors such as moisture content, substrate particle size, temperature, pH, oxygen concentration, incubation time and nutrient availability must be optimized for the efficient degradation of lignin [11]. Fungal pre-treatment do not always promote CH4 production to a larger extent. For example, pre-treatment of WS using fungus Polyporus brumalis BRFM 985 strain increased CH4 yield by merely 18% from 215 to 254 mL g VS−1 [86]. In addition, a slight rise of 31% in biogas yield was reported upon subjecting WS to Chaetomium globosporum pre-treatment [87]. Prominent barriers to fungal pre-treatment are prolonged residence times and the consumption of fermentable sugars by fungi. Hence, full-scale adoption of fungal pre-treatment is still scarce [87,88].
Bacteria pre-treatment involves the use of enzyme-secreting bacteria to combat the polymerization of biomass. Many anaerobic bacteria have a high capacity to hydrate the structure of WS and improve CH4 production [2]. The destruction of WS using bacteria possesses numerous distinct traits over fungal pre-treatment. Bacteria can induce Cα-oxidation and cleave Cβ-Cβ linkages in lignin [11]. They possess a rapid growth rate with shorter incubation periods and are more cost-effective than fungi. In addition, the genome of bacteria can be more easily modified than the fungal genome [89]. Clostridium, Bacillus and Pseudomonas have been found to degrade plant materials through the secretion of cellulases, xylanases and other hydrolytic enzymes. These bacteria occupy diverse extreme conditions, including decomposing forestry matter, compost, agricultural waste, organic matter and soil and hot springs [89]. Bacillus is one of the most promising genus to decompose WS due to its strong cellulose-degrading capacity. Further, the bacteria can tolerate high temperatures and diverse pH conditions [89]. No studies were found in the literature on the pre-treatment of WS using single strains of bacteria. However, pre-treatment of maize straw using B. subtilis generated 17.35% higher CH4 yield than a control [90].
The construction of a microbial consortium was proposed as a panacea to the limited utility of biological pre-treatment at the pilot scale [32]. The method is believed to be more effective than a single microorganism in enhancing the degradation of cellulosic wastes. A microbial consortium is a group of species with distinct delignification efficiencies and it is functional in diverse ecological conditions. It can deploy discrete delignification mechanisms with improved potential to exploit a substrate compared to indigenous microorganisms [11]. Microbia consortia are isolated from natural conditions, where decomposing cellulosic waste is the main substrate [34]. Unlike fungi, which mostly act on lignin, a microbial consortium has a high affinity for holocellulose [17,34]. The advantage of using microbial consortia is that sterilization may not be required. The pre-treatment of WS using microbial consortium improved CH4 production by 80.34% than the un-pre-treated counterpart [91]. A microbial consortium TC-5 offered a rise in CH4 yield of 36.6% after 35 d of AD of WS [92].
Exogenous hydrolytic or oxidative enzymes can promote the degradation of lignocellulosic substrates. The most widely reported classes of enzymes for the pre-treatment of biomass are cellulases and hemicellulases [11,17]. Enzymes have short reaction periods and can reduce the loss of holocellulose during hydrolysis. Moreover, enzymes have ease of access to a substrate with an increased mass transfer rate [11]. However, enzyme-assisted pre-treatment is limited due to the high cost of commercial enzymes [34]. Operational parameters, such as enzyme activity and specificity, enzyme concentration, inhibitor concentration, digester design, residence time, temperature and pH must be optimized for enhanced enzymatic pre-treatment [27]. Combining different enzymes is an approach that can improve the efficacy of enzymatic pre-treatment. Screening enzymes with high specific activity and cross specificity can lower the quantity of enzymes required as well as the pre-treatment cost [93]. Literature seems to be scant considering the enzyme-assisted pre-treatment of WS for biogas production. Even so, a 14% increase in CH4 production was observed after enzymatic pre-treatment of WS using a complex mixture of hydrolytic enzymes [72].
Table 6. Biological pre-treatment of wheat straw for biogas production.
Table 6. Biological pre-treatment of wheat straw for biogas production.
Biological AgentMicrobes and EnzymesPre-Treatment ConditionsFindingsReference
FungiPenicillium aurantiogriseum, Trichoderma reesei, Gilbertellapersicaria, Rhizomucormiehei100 mL batch reactors, 37 °C, 10 dHighest methane yield increase of 48% from P. aurantiogriseum pre-treated wheat straw[94]
Polyporusbrumalis40 L aerobic reactors, 31 °C, 90% moisture, 13 d18% increase in methane yield[86]
Chaetomium globosporumReagent bottles, 36 °C, 81% moisture, 14 d31% enhanced methane yield[87]
Ganoderma lobatum, Gloeophyllumtrabeum250 mL Erlenmeyer flasks, dark, 25 °C, 10–40 d43.6 and 26.1% increase in glucose yield by G. lobatum and G. trabeum, respectively[95]
Ligninolytic fungi250 mL Erlenmeyer flasks, 28 °C, 150 rpm, 7 dFive-fold higher biogas yield[96]
Microbial consortiumMicrobial consortium TC-51 L anaerobic bottles, 50 °C, 3 d22.2 and 36.6% increase in methane yield under mesophilic and thermophilic conditions, respectively[92]
Microbial consortiumBatch, 37 °C, 20 d80.34% improved methane yield[91]
Cow rumen-derived microbial consortium35 °C, 15 d55.5% lignocellulose degradation[1]
Microbial consortium composed of fungi and bacteria 39.24 and 80.34% increase in biogas and methane yield, respectively[91]
EnzymesCellulase, xylanase, arabinase, pectinase, other carbohydrases, β-glucosidase100 mL glass reactors, 50 °C, 16 h14% enhanced methane yield[72]
Laccase, peroxidase30 °C, 60 rpm, 6 and 24 h11% increased methane yield after 6 h pre-treatment and 15% decreased methane yield after 24 h pre-treatment[97]

5.2. Anaerobic Co-Digestion

AcoD is an attractive route for the valorization of WS into biogas [15,98]. It involves the concurrent AD of a mixture of two or more substrates in a bioreactor system. The principal remit of AcoD is to enhance biogas yield by the addition of substrates that have higher CH4 potential than the main substrate [99]. Currently, there is a paradigm shift from mono-digestion to AcoD of organic wastes. AcoD has been shown to enhance mono-digestion through nutrient balance, toxicity reduction and improved buffering capacity [19,98]. Generally, AcoD improves biodegradation and biogas yield due to synergistic interactions in digester medium [15,18].
WS is very rich in cellulose and its nutritional content is appropriate for microbial growth and biogas production [41]. However, the sole use of WS for AD systems is not economically viable due to its high C/N ratio and slow degradation. The C/N ratio is a critical factor for controlling the anaerobic digester reaction. A high C/N ratio causes low biogas production due to the limited supply of nitrogen for cellular synthesis and proper functioning of methanogenic bacteria. It also lowers VFAs and NH3 accumulation in the digester. A low C/N ratio promotes the accumulation of VFAs and NH3 that may be toxic to methanogens and negatively impact the AD system [58,99]. The C/N ratio of WS varies from 15 to 151 [100]. However, an ideal C/N ratio for optimum performance of an AD system falls within the range of 20–30 [41]. The AcoD of WS with low carbon substrates is a potent strategy to maintain the C/N ratio at an optimum level [15,98]. Table 7 shows studies that have been performed to utilize WS in an AcoD system with other biosolids. WS has been co-digested with several organic feedstocks, such as animal manure [20,42], sewage sludge, algal biomass, food waste [101], sunflower meal [46], among others. Such studies have provided useful information to biogas operators in the form of enhanced CH4 production and nutrient revitalization from digestate waste. Animal manure appears to be the most suitable co-substrate for WS biodegradation. Despite its high nitrogen content, animal manure has been revealed to be enriched with rumen flora that assists to complete the AD process expeditiously [18]. Wang et al. [102] enhanced CH4 production by 10% through co-digesting 4.6 kg of WS with 1 t of swine manure. In another study, increased biogas yield by 1.6-fold from the co-digestion of pig dung and WS was accomplished [103].
It is pertinent to choose the correct combination of disparate organic substrates in order to operate a sustainable AcoD system. Identifying the optimal reaction mixtures of substrates by repeating several BMP assays is a cumbersome task. As such, mathematical models have been proposed as formidable tools to predict the accurate mixtures of multiple substrates in bioreactors [100,104]. Most researchers have focused on simplified approaches, such as first-order kinetics and modified Gompertz models to simulate mono-digestion parameters [105]. Suitable models should be selected that take into account the decomposition characteristics of multi-substrate systems, such as reactor kinetics, nutrient balance, particle size, pH and so on. Modified models based on the Anaerobic Digestion Model No. 1 (ADM1) approach could proffer a better simulation option for AcoD than non-ADM1-based models [104,105]. The ADM1 is a structured approach that utilizes differential equations to assess the biological and physico-chemical parameters of a process. The advantage of using ADM1-based models is that it is possible to simulate the major phases of AD using a sophisticated and complex approach [104].
Table 7. Methane potential of wheat straw mixed with other substrates in a co-digestion system.
Table 7. Methane potential of wheat straw mixed with other substrates in a co-digestion system.
Co-SubstrateInoculumExperimental ConditionsMethane Potential
(mL g VS−1)		Reference
Food waste, cattle manureSewage sludge from anaerobic digester610 mL glass bottles, 35 °C, 100 rpm, 45 d416[101]
Rapeseed mealEffluent from mesophilic digester150 mL serum glass vials, 42 °C, 30 d375[106]
Herbal extraction process residuesAnaerobic sludge of pig manure250 mL batch digesters, 30 d178[107]
Swine manure [108]
Cattle manureCattle manure1 L glass bottles, 35 °C, 50 d109[109]
Swine manure [102]
Animal manureAnaerobic sludge of dairy manure1 L ground flasks, 3 g magnetite, 35 °C228[42]
Animal manureCow dung1 L aspirator glass bottles, 25–30 °C, 20 d566[20]
Sunflower mealDigested manure300 mL serum bottles, 35 °C, 60 d 591[46]
Rice strawDigested manure300 mL glass bottles, pH 7–7.5, 35 °C, 60 d339[110]

6. Future Prospects

Overall, biogas production lacks installation capacity and it is still underrated as a sustainable energy technology in South Africa. To date, a total of about 30 biogas projects are established in the country, either at the commercial or industrial scale [111]. This limited uptake can be attributed to huge capital demand, complex policies, and a lack of expertise to design, build and run biogas facilities [112]. Key drivers for the maturation of biogas technology in the country are techno-economic, social, environmental and legislative aspects. The drivers are not fully present in South Africa, yet there are many enablers for the technology. Driven by national policy imperatives and regulatory frameworks, the country should invest in research and development (R&D), human capital development and funding biogas projects to move the technology from a stage of infancy to maturity. Current government policies and strategies must be improved to lower installation costs and promote biogas technology as a renewable energy solution. Policies should be revised in such a way as to remove any barriers that may suppress the private sector, which is a major stakeholder of the economic system. The government must also prioritize incentivizing and subsidizing small-holders farmers and other main stakeholders involved in biogas production.
Feedstock availability and market barriers have been pointed out as other impediments to the successful implementation of biogas projects in South Africa [111]. R&D should focus on the diversification of feedstock streams and boosting the market share for biogas. One way to achieve this is to set up many demo plants in the country in order to promote market awareness and dissemination of biogas knowledge. This should be coupled with repeated awareness campaigns and workshops in remote areas to conscientize people about the benefits of biogas. Another way is to design advanced bioreactors that use cellulosic material as feedstocks and integrate WS in the country’s biogas resource base.
With an estimated total energy potential of 11 PJ, WS has not been fully exploited as a bioenergy resource in South Africa. WS management practices, such as use in livestock bedding, burning and burying into the soil are associated with techno-economic bottlenecks and may contribute to GHGs. Currently, WS biogas technology has not been commercialized in South Africa; thus, its research and relevance needs to be promoted. The recalcitrant nature and high C/N ratio of WS are postulated to be the main barriers to successful performance of WS anaerobic digester systems. Hence, pre-treatment and AcoD have been suggested by many authors to complement bioreactors operating on WS feedstocks [20,101].
A gamut of biological, chemical, physical and hybrid pre-treatment methods have been broadly studied for their aptitude to promote biogas production from WS [7,33,54]. However, these pre-treatment methods do not at all times improve the BMP of WS. There are many obstacles that restrain opportunities for the growth of suitable pre-treatment technologies. For instance, the dissociation of compounds during chemical pre-treatment liberates toxic substances that may cause secondary pollution. Noteworthy is the fact that the majority of physical pre-treatments are capital-intensive due to the high energy demand and use of corrosion-resistant equipment [54]. The focal point of research should be to eliminate such drawbacks and adopt sustainable pre-treatment options. Biological pre-treatment is much-admired for its ability to eradicate nearly all constraints caused by physical and chemical pre-treatments. Yet, the method has been scanty due to extended pre-treatment periods and large area required. Moreover, an efficient biological pre-treatment agent still needs to be established [24,54]. Most researchers are now concentrating on pre-treatment of WS in a hybrid system. This method has been indicated to be promising, but it is still faced with some challenges. There is also a paucity of aggregated data concerning the technical, economic and environmental feasibility of WS pre-treatment. Studies should concentrate on assessing the cost supply chain of pre-treatments if we are to lower the installation costs and energy needs of WS biogas.
The issue of high C/N ratio should be urgently addressed towards the valorization of WS digesters. Several authors have reported on the co-digestion of WS with other biodegradable substrates, including food waste, animal manure, sewage sludge, etc. [46]. Although animal manure seems to be a prospective candidate for AcoD with WS, the correct combination of substrates is not yet fully known. Performing many BMP tests to find the correct mixing ratios is a daunting task. In addition, AcoD has its own disadvantages, such as a high chemical oxygen demand, and increased agitation, energy and pre-treatment requirements [100]. Future studies should immensely focus on developing modeling tools that can simulate combinatorial mixtures and optimize operational parameters in silico. Notable is the fact that a number of mathematical models are solely based on mono-digestion. It is a pre-requisite to upgrade mono-digestion tools to advanced computational approaches that are based on AcoD to rigorously predict big data sets in a high-throughput mode. Put together, much attention should be paid on up-scaling lab studies towards full-scale valorization of WS, provided pre-treatment and AcoD conditions are completely optimized.

7. Conclusions

Huge amounts of WS are produced in South Africa, which can be a potential resource for bioenergy. AD technology is increasingly rising in South Africa to supply energy, manage waste, reduce GHGs and abate environmental degradation. Biogas continues to be one of the leading constituents of bioenergy supply mix in South Africa. WS being abundant, renewable and endowed with organic matter could widen the resource base of substrates for biogas production in South Africa. Its energy potential needs to be fully exploited as it is a highly recalcitrant substance. WS biogas technology has never materialized at the industrial level and still needs to be further advanced. It is critical to research the pre-treatment of WS and establish methods that are cost-effective and sustainable. Many pre-treatment methods, such as physical, chemical, biological and hybrid technologies have been suggested to enhance CH4 production from WS. At present, the industrialization of physical and chemical pre-treatments is hindered by high capital costs and energy demands, the requirement of corrosive-resistant equipment and the release of secondary pollutants. Biological pre-treatment is still a nascent technology. More work should delve into developing biological agents that can effectively hydrolyze WS. The AcoD of WS and other biosolids (sewage sludge, animal manure, food waste, etc.) is a viable strategy to lower the C/N ratio, balance nutrients, promote synergism and, finally, improve the CH4 yield of WS. The most critical aspect of AcoD is to know the appropriate combination of mixing ratios, which is absolutely a mammoth task using BMP tests. It has been proposed to explore and apply advanced computational tactics, for example, modified ADM1 models, so as to optimize AcoD reactions.

Author Contributions

Conceptualization, R.K. and P.M.; writing—original draft preparation, R.K.; writing—review and editing, R.K. and P.M.; and supervision, P.M. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the Department of Research and Innovation (DRI) at the University of Fort Hare, Department of Science and Innovation (DSI), Technology Innovation Agency (TIA), National Research Foundation (NRF), Eskom TESP and Research Niche Area: Renewable Energy-Wind, for their financial support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Lignocellulosic structure of wheat straw [24].
Figure 1. Lignocellulosic structure of wheat straw [24].
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Figure 2. Flow diagram of the anaerobic digestion process.
Figure 2. Flow diagram of the anaerobic digestion process.
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Figure 3. Pre-treatment methods for enhanced anaerobic digestion of wheat straw.
Figure 3. Pre-treatment methods for enhanced anaerobic digestion of wheat straw.
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Figure 4. Microbial pre-treatment of polymeric plant biomass [11].
Figure 4. Microbial pre-treatment of polymeric plant biomass [11].
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Table 1. The annual potential of wheat straw in South Africa.
Table 1. The annual potential of wheat straw in South Africa.
ProvinceQuantity (Million t yr−1)
Western Cape0.002
Northern Cape 0.385
Free State0.009
Eastern Cape0.003
KwaZulu Natal0.025
Mpumalanga0.019
Limpopo0.099
Gauteng0.005
North West0.055
Source: [3].
Table 2. Cellulose, hemicelluloses and lignin composition of various wheat straw materials.
Table 2. Cellulose, hemicelluloses and lignin composition of various wheat straw materials.
Cellulose (%)Hemicelluloses (%)Lignin (%)Reference
30–5518–3710–30[1]
35–4520–308–15[25]
33–4020–2515–20[28]
30–4020–3015–20[4]
35–3923–3012–16[29]
35–3820–2816–24[30]
27–4211–2714–21[31]
30–4922–347–22[32]
30–4020–2520–25[20]
35–5015–2510–15[33]
Table 3. Biomethane potential of wheat straw residues.
Table 3. Biomethane potential of wheat straw residues.
CountryInoculumReactor ConditionsBMP (mL g VS−1)Reference
SpainActivated sludge2 L borosilicate glass, 35 °C, 45 d 233[4]
USAInoculum from food waste thermophilic digester1 L anaerobic reactors, 50 °C, 25 d179[42]
SpainMixed sludge from municipal wastewater treatment plant2 L borosilicate glass, 35 °C, 40 d226[43]
DenmarkCo-digested mixture of animal manure and ethanol wastes337 mL glass bottles, thermophilic conditions221[26]
GermanyInoculum from pilot plant treating cow manure and maize silageAutomated Methane Potential Testing System II, 37 °C, 30 d154[40]
PolandDigested sewage sludge from wastewater treatment plant2 L glass bioreactors, 37 °C, 40 d339[44]
DenmarkSludge from wastewater treatment plant digester500 mL bottles, 37 °C, 35 d, stirring at 150 rpm237[14]
DenmarkInoculum from mesophilic anaerobic digester500 mL bottles, 35 °C, 96 d217[45]
PakistanDigested manure300 mL serum bottles, 35 °C, 45 d365[46]
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Kamusoko, R.; Mukumba, P. Potential of Wheat Straw for Biogas Production by Anaerobic Digestion in South Africa: A Review. Energies 2024, 17, 4662. https://doi.org/10.3390/en17184662

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Kamusoko R, Mukumba P. Potential of Wheat Straw for Biogas Production by Anaerobic Digestion in South Africa: A Review. Energies. 2024; 17(18):4662. https://doi.org/10.3390/en17184662

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Kamusoko, Reckson, and Patrick Mukumba. 2024. "Potential of Wheat Straw for Biogas Production by Anaerobic Digestion in South Africa: A Review" Energies 17, no. 18: 4662. https://doi.org/10.3390/en17184662

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