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An Overview of Biogas Production from Anaerobic Digestion and the Possibility of Using Sugarcane Wastewater and Municipal Solid Waste in a South African Context

An Overview of Biogas Production from Anaerobic Digestion and the Possibility of Using Sugarcane... Review An Overview of Biogas Production from Anaerobic Digestion and the Possibility of Using Sugarcane Wastewater and Municipal Solid Waste in a South African Context 1, 1 2 3 Zikhona Tshemese *, Nirmala Deenadayalu , Linda Zikhona Linganiso and Maggie Chetty Department of Chemistry, Steve Biko Campus, Durban University of Technology, Durban 4000, KwaZulu-Natal Province, South Africa Research and Postgraduate Support, Steve Biko Campus, Durban University of Technology, Durban 4000, KwaZulu-Natal Province, South Africa Chemical Engineering, Cape Peninsula University of Technology, Ape Town 7535, Western Cape, South Africa * Correspondence: 22174937@dut4life.ac.za Abstract: Bioenergy production from waste is one of the emerging and viable routes from renewable resources (in addition to wind and solar energy). Many developing countries can benefit from this as they are trying to solve the large amounts of unattended garbage in landfills. This waste comes in either liquid (wastewater and oil) or solid (food and agricultural residues) form. Waste has neg- ative impacts on the environment and, consequently, any form of life that exists therein. One way of solving this waste issue is through its usage as a resource for producing valuable products, such as biofuels, thus, creating a circular economy, which is in line with the United Nations (UN) Sus- tainable Development Goals (SDGs) 5, 7, 8, 9, and 13. Biofuel in the form of biogas can be produced from feedstocks, such as industrial wastewater and municipal effluent, as well as organic solid waste in a process called anaerobic digestion. The feedstock can be used as an individual substrate Citation: Tshemese, Z.; for anaerobic digestion or co-digested with two other substrates. Research advancements have Deenadayalu, N.; Linganiso, L.Z.; shown that the anaerobic digestion of two or more substrates produces higher biogas yields as com- Chetty, M. An Overview of Biogas pared to their single substrates’ counterparts. The objective of this review was to look at the anaer- Production from Anaerobic obic digestion process and to provide information on the potential of biogas production through Digestion and the Possibility of the co-digestion of sugarcane processing wastewater and municipal solid waste. The study deduced Using Sugarcane Wastewater and that sugar wastewater and municipal solid waste can be considered good substrates for biogas pro- Municipal Solid Waste in a South duction in SA due to their enormous availability and the potential to turn their negative impacts African Context. Appl. Syst. Innov. into value addition. Biogas production is a feasible alternative, among others, to boost the country 2023, 6, 13. https://doi.org/ 10.3390/asi6010013 from the current energy issues. Academic Editor: Francesca Valenti Keywords: energy; biogas; anaerobic co-digestion; substrate type; sugarcane processing Received: 14 November 2022 wastewater; municipal solid waste Revised: 8 December 2022 Accepted: 14 December 2022 Published: 16 January 2023 1. Introduction Energy accessibility and waste management are some of the most significant chal- lenges developing countries face, including South Africa [1]. Energy demands exceed the Copyright: © 2023 by the authors. existing energy supply due to the continual increase in population globally. Regularly Licensee MDPI, Basel, Switzerland. used energy resources, such as oil, coal, and natural gas, are diminishing and they emit This article is an open access article greenhouse gases that contribute to climate change [2]. Accordingly, the research focus in distributed under the terms and conditions of the Creative Commons many countries has shifted to finding and implementing efficient and green alternatives, Attribution (CC BY) license such as renewable resources, as solutions to these conventional energy sources, which are (https://creativecommons.org/licenses detrimental and waning [3]. Examples of renewable energy resources are those that can /by/4.0/). Appl. Syst. Innov. 2023, 6, 13. https://doi.org/10.3390/asi6010013 www.mdpi.com/journal/asi Appl. Syst. Innov. 2023, 6, 13 2 of 17 be replenished naturally, such as solar photovoltaic and wind power. However, the en- ergy demands of a constantly growing population using coal-powered stations cannot be supplemented with only solar and wind energy as these are predominantly weather-de- pendent [4]. Waste generation and management have been dealt with in many ways, one of which is using it as a resource for producing valuable products, such as biogas, which contrib- utes to the circular economy [5]. Further, this is in line with the UN’s Sustainable Devel- opment Goals 5, 7, 8, 9, and 13. Biogas can be produced from many substrates, including organic solid waste, wastewater/effluent, etc., as substrates/feedstock in biodigestion. Feedstock is any substrate that can be converted to biogas/methane through anaerobic bacteria. These range from solid wastes to readily degradable wastewater and sludge. This waste must contain a substantial amount of organic matter, which is then converted into biogas [6]. Conventionally, anaerobic digestion is a practical way to treat animal and ag- ricultural waste, macroalgae, and sewage sludge from aerobic wastewater treatment plants [7–9]. However, a change happened after 1970, as soon as environmental conscious- ness grew in connection with the demand for renewable energy reforms and new waste- management strategies [10]. Industrial and municipal waste has also been identified as eligible for anaerobic digestion, as shown in Figure 1. This resource is one of the sustain- able and viable routes to help many developing countries manage massive amounts of the waste left unattended in landfills and discharged into water streams and oceans [11]. This waste negatively impacts the environment and, consequently, life in such an environment, i.e., human and animal lives [12,13]. Agricultural waste Energy crops Anaerobic digestion Industrial waste and Municipal biowaste wastewater Figure 1. Sources of suitable substrates for anaerobic digestion [14]. Clean, renewable energy in the form of biogas can be achieved through anaerobic digestion (AD) of waste matter [15]. AD comprises a series of biochemical reactions that result in the production of biogas, a mixture of methane, carbon dioxide, and negligible traces of other elements. Different waste matter, including the organic part of municipal solid waste, industrial waste, wastewater from manufacturing processing, and agricul- tural waste produced from livestock and crop production, is used in the AD process. No- tably, the biogas formation process produces some by-products, such as slurry (digestate), which provides an added benefit, since the spent waste (slurry) can be used as organic compost by farmers due to its nutrient composition [16]. However, it is crucial to check the safety of this digestate for its use as a fertilizer since there can be potential incidences of heavy metals and pathogenic bacteria [17]. In addition to the environmental benefits of waste management, there are also socio-economic rewards. It is envisaged that countries Appl. Syst. Innov. 2023, 6, 13 3 of 17 would raise their annual turnover for different sectors, generate thousands of jobs, and save billions of dollars a year after fully implementing waste-management solutions [18]. Since research has shown that the digestion of a single substrate produces less biogas than a co-digestion of two or more substrates, municipal solid waste and sugarcane pro- cessing wastewater are deemed to be good co-substrates for biogas production. Further, sugarcane wastewater has low carbon-to-nitrogen ratio, which enables the use of a com- plementary substrate [19]. Both these substrates contain significant amounts of organic content, digested by the anaerobic bacteria [20,21]. This review looks at the anaerobic di- gestion process and seeks to provide information on the potential of biogas production through the co-digestion of sugarcane processing wastewater and municipal solid waste. South Africa is experiencing “load shedding” because energy demand is higher than the currently generated energy; therefore, urgent solutions are required to solve this issue. Biogas production is a viable alternative among others to boost the country from the cur- rent energy issues. 2. Possibility of Generating Renewable Energy from Biogas Using Sugarcane Pro- cessing Wastewater Sugarcane is used for sugar production at the business scale and contributes about 80% of the world’s sugar revenue [22,23]. In South Africa, the provinces of KwaZulu-Natal and Mpumalanga are the major sugarcane producers contributing to the prosperity of the sugar industry’s economy [24]. The same sugarcane that contributes to countries’ econo- mies is characterized by the generation of large quantities of organic wastewater, with excessive chemical oxygen demand that pollutes the environment [25,26]. Therefore, the sustainable advancement of the sugarcane industry requires reducing and treating sugar- cane processing wastewater. One way of treating this sugarcane processing wastewater is by discharging it into wastewater-treatment systems where there would be physical or chemical nutrient removal. Nevertheless, such methods present disadvantages of second- ary pollution, high operation costs, and limitation of nutrient reusability [27,28]. Sugar- cane processing wastewater is an attractive substrate for bacterial cultivation to produce beneficial products, such as biogas, biomass, enzymes, and organic acids, due to its high carbohydrates, minerals, and sugars [26,29]. Many researchers [30–32] have studied the conversion of biogas to electricity. Wang, et al. [33] analyzed the efficiency and sustaina- bility of biogas to electricity production from a large-scale biogas project in China using pig manure. Even though they obtained lower yield results compared to traditional coal and natural gas power plants, electricity generated from biogas still brings more ad- vantages and reduced antithetical environmental effects, as opposed to fossil fuels [33]. 3. Municipal Solid Waste A continual increase in the worldwide populace has led to rapid urbanization in many countries. It is estimated that about two-thirds of the world’s population will live in cities by 2025 because more than 150,000 people move to urban areas each day [34]. This rise in urbanization has resulted in cities that generate thousands of tons of municipal solid waste daily and this is projected to increase significantly in the near future [35]. Mu- nicipal solid waste is an amalgamation of waste from households, markets, backyards, street cleaning, institutional establishments, such as hospitals, and industrial and com- mercial wastes. Management of this type of waste in urban areas pertains to its disposal, collection, resource recovery, recycling, and treatment to promote the quality of both the environment and health while supporting the economy’s efficiency and productivity through generating employment and income [36]. The most preferred waste-management method is the one tuned to take the circular economy direction since it leads to sustainable development. The circular economy focuses on the upper ranks of the waste hierarchy, as shown in Figure 2, including prevention, reuse, and recycling, because these promote cleaner production and minimal waste [37]. Particularly, a circular economy has been Appl. Syst. Innov. 2023, 6, 13 4 of 17 adopted globally since it offsets issues of resource depletion and the detrimental environ- mental effects that lead to climate change. Traditionally, the production and consumer approach, which translates to the linear economy, has been used through a typical “take, use, and dispose of” model [38]. Some of the drawbacks of a linear economy are apparent when the consumer generates waste and disposes of it, causing pollution to the environ- ment and depleting resources [39]. On the other hand, a circular economy follows waste generation minimization and pollution reduction, hence, protecting the environment through a “resource-product-waste-resource” model [40]. South Africa has also embarked on an integrated waste-management structure that considers waste prevention, recycling, recovery, and controlled and supervised disposal. This idea will help to efficiently manage and safeguard human health and the environ- ment, with a significant focus on sustainable development economically, socially, and en- vironmentally. It was suggested that this integrated waste management should incorpo- rate hierarchical waste techniques, which focus more on the avoidance of and reductions in waste than on collection, storage, and disposal [41]. On a local scale, a study of the optimization and financial viability of landfill gas to electricity was conducted in Durban. The study demonstrated that the conversion of landfill gas to electricity provides viable projects with options for optimizing and improving the financial feasibility of the devel- opments [42]. The study also suggests that researchers should look at the possibility of sugarcane waste to produce renewable energy. Favourable results from this research may add more value to sugarcane, as a plant boosts the economy and creates jobs. Conse- quently, this review looks at the process of biogas production, with a particular focus on two different substrates, i.e., sugar wastewater and the organic part of municipal solid waste. These substrates are simple biodegradable materials that can be broken down by microorganisms in an AD process, which includes a series of biochemical reactions, as explained in the following section. Figure 2. Waste hierarchy adapted from [43]. Anaerobic digestion and composting are biological treatments used to treat biode- gradable waste. Studies show that waste management leads to lower environmental im- pacts, lower economic costs, and lower energy consumption. It is suggested that energy- rich waste should be prevented because of the low recovery of resources and harmful environmental effects of landfilling [44]. The advantages attached to waste management include reducing solid waste (about 70–80% mass and 80–90% volume), leading to a pre- served landfill space [45], removal of organic contaminants (halogenated hydrocarbons) [46], reduction in greenhouse gases [47], and naturally compatible exploitation of renew- able energy from waste, predominantly when the plant used is designed to generate heat and power [48]. Appl. Syst. Innov. 2023, 6, 13 5 of 17 4. Anaerobic Digestion Process This is a four-step anaerobic biological decomposition of organic substrates, namely, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The brilliance of this process is that all the phases are connected since a by-product of one step becomes the substrate of the next step, all in one system [49]. These biochemical decomposition phases have a series of chemical reactions, as illustrated in Figure 3 and detailed in the subsequent sub- sections. Figure 3. Anaerobic digestion was adapted from Bajpai [50] and Khanal [51]. 4.1. Hydrolysis The hydrolysis phase of anaerobic digestion is where complex biopolymeric com- pounds (lipids, carbohydrates, and proteins) are converted to water-soluble compounds by degradation through Bacteroides, Clostridia, and Bifidobacteria and sometimes Strep- tococci and Enterobacteriaceae [52]. This step is relatively slow and can limit the rate of the overall digestion, mainly when solid material is used as a substrate. As seen in Equa- tion (1), cellulose (C6H10O5) is hydrolyzed to generate glucose (C6H12O6) and hydrogen (H2). This reaction is catalyzed by homogeneous/heterogeneous acids yielding the fer- mentable monosaccharide (C6H12O6). The products (C6H12O6 and H2) are used by the fer- mentative microorganisms in the next phase to form higher-chain organic compounds, such as volatile fatty acids [53,54]. (C H O ) + nH O → n(C H ) + nH (1) 4.2. Acidogenesis This is the second phase, known as the fermentation stage, where the acidogenic bac- teria Streptococcus, Escherichia, Staphylococcus, Pseudomonas, Bacillus, Sarcina, Desulfovibrio, Lactobacillus, and others are active [55]. These bacteria degrade amino acids, lipids, and Appl. Syst. Innov. 2023, 6, 13 6 of 17 glucose into volatile fatty acids, organic acids, carbon dioxide, and hydrogen gas (as illus- trated in Equations (2)–(7) below [53]). Acetic acid (CH3COOH) is the most important or- ganic acid produced in this stage, which serves as the substrate for methanogenic micro- organisms [16]. It is worth noting that volatile fatty acid production is favored when pH is above 5, while ethanol production (CH3CH2OH) is favored by pH lower than 5, with reactions stopping at a pH level that is less than 4 [50]. C H O ↔ 2CH CH OH + 2CO (2) C H O + 2H ↔ 2CH CH COOH + 2H O (3) C H O → 3CH COOH (4) C H O N + 2H O → C H O + NH + CO + 2H + ATP (5) C H O + NH + CO + H + ATP (6) 4CH COCOO + 4H O → 5CH COO + 2HCO + 3H (7) 4.3. Acetogenesis At this stage, the reactions are reversible with a release of hydrogen. Volatile fatty acids, specifically acetic acid and butyric acid, are converted into carbon dioxide gas, hy- drogen, and acetate, as shown in Equations (8)–(10). The active bacteria in this stage are Clostridium, Syntrophomonas wolfeii, and Syntrophomonas wolinii [55]. This conversion of volatile fatty acids is enabled by the presence of water molecules (acting as electron sources) from the previous stages of anaerobic digestion. Equation (9) converts the phase product to acetate and hydrogen, used in the next stage [56]. This stage is equally im- portant since it reflects the biogas production efficiency, given that the reduction of the acetate ion forms about 70% of methane. Acetate is a primary intermediary product of this phase and it accounts for 25% of the products formed together, with approximately 11% of hydrogen [57]. CH CH COO + 3H O ↔ CH COO + H + HCO + 3H (8) C H O + 2H O ↔ 2CH COOH + 2CO + 4H (9) 2CH CH OH + 2H O ↔ CH COO + 2H + H (10) 4.4. Methanogenesis In the final phase of the methanogenesis stage, acetic acid is converted into methane and carbon dioxide using bacteria called methanogens, which are anaerobes with a high vulnerability to limited amounts of oxygen. In addition, carbon dioxide is a product that reacts with hydrogen gas to produce more methane. On the other hand, ethanol under- goes decarboxylation to form methane. Two types of bacteria—the acetophilic methano- genic (with Methanosarcina and Methanosaeta active specie) and the hydrogenophilic meth- anogenic (with Methanospirilum, Methanobacterium formicicum, Methanoplanus, and Meth- anobrevibacterium as the dominant specie)—exist in this stage [55]. The former is responsi- ble for the decarboxylation of acetate to methane and the latter for methane formation through a reaction of carbon dioxide and hydrogen [53]. The final product of the anaerobic digestion process is biogas, which is composed of methane and carbon dioxide. (11) CH COOH → CH + CO (12) CO + 4H → CH + 2H O (13) 2CH CH OH + CO → CH + 2CH COOH Appl. Syst. Innov. 2023, 6, 13 7 of 17 5. Factors Affecting the Anaerobic Digestion Process and Biogas Production Biogas production is influenced by different factors, such as substrate type, temper- ature, pH, organic loading rate, hydraulic retention time, etc. [58]. 5.1. Substrate Type Numerous biomass feedstocks can be used for biogas production, depending on their nutritional composition. Accordingly, these compositions influence biogas yield, methane content, degradation kinetics, and biomass biodegradability [59]. The critical nutritional compositions of biomass substrates suitable for biogas production are carbohydrates, pro- tein, and fats. Theoretical estimations of biogas percentage and methane yield from these nutrients have been reported in the literature and are calculated using the Buswell for- mula [60]. Protein-rich substrates have a high potential for methane yield, but their deg- radation gives off ammonium ions, leading to an alkalinity increase in the AD process. The increase in alkalinity improves the digest value as fertilizer while preventing the ac- tivities of methanogens. This inhibition occurs during the equilibrium shift from ammo- nium to ammonia, typically in changing concentrations. In addition, literature remarks suggest that microorganisms can acclimatize to environments with high ammonia con- centrations while efficiently producing biogas [61,62]. Lipid-rich substrates, such as fats, possess great methane yield potential, such as an- imal and plant tissue waste, biodegradable kitchen and canteen waste, grease and oil mix- ture, etc. [63]. These substrates release long-chain fatty acids during their degradation, which are typically toxic to the microbial environment and cause a drop in pH [64–66]. There are several other types of biomass that are used for biogas production apart from the protein and lipid-rich substrates, for example, substrates with a high degree of ligno- cellulose (wheat straw, sorghum, rice straw, etc.) [67]. This type of biomass is hard to de- grade due to these three reasons: (i) recalcitrant nature, (ii) heterogeneous structure, and (iii) low accessibility by enzymes, such as carbohydrate polymers [68–70]. However, pre- treatment mechanisms can help break down the heterogeneous matrix, thus, increasing the porous and surface area of the lignocellulose biomass and enhancing biogas produc- tion. Characterization of the waste substrates is performed to ascertain the composition of each substrate. This is generally physical and chemical composition regarding volatile sol- ids, total solids, C/N ratio as well as elemental analysis for carbon, nitrogen, hydrogen, and sulfur [71,72]. During substrate characterization, the place (source) where the sub- strate was collected is vital, as waste chemical content is affected by many factors, such as weather conditions and the type of soil where the original substances were grown [73,74]. Physical and chemical compositions depend on the type of substrate, for example, carbo- hydrates have carbon and hydrogen while proteins and lipids have nitrogen as part of their composition [75,76]. These substrate compositions can be analyzed using different analytical techniques. 5.2. Anaerobic Digestion pH The operational pH directly affects both the digestive progress and products formed in the AD process. Literature findings show that the ideal comprehensive pH range for AD should be between 4.0 and 8.5, as per the requirement for the fermentative bacteria, although the limiting range of 6.5–7.2 is favorable for the growth of methanogens [77,78]. The microorganism growth rate is significantly affected by the change in pH and, as such, each microbial group has a specific optimum pH. A comparative abundance of microbial species increased from 6 to 14 at pH 4.0 and 7.0, respectively [79]. Bacterial population dominance differs with changing pH, for example, at pH 6.0, Clostridium butyricum is dominant, while the Propionibacterium spp. thrives during anaerobic acidogenesis at pH 8 [80]. Appl. Syst. Innov. 2023, 6, 13 8 of 17 When the pH level is controlled for the optimal growth of microorganisms, reduc- tions in toxicity, generally from increased concentration of free ammonia (FA), are also achieved. Similarly, pH significantly affects volatile fatty acid (VFA) composition [79]. In an anaerobic reactor, instability typically leads to the accumulation of VFAs, leading to a drop in pH and, therefore, acidification. Nonetheless, this accumulation of VFAs does not always exemplify a pH drop, owing to the buffer capacity of some waste forms. There is an excess of alkalinity in manure, denoting that the VFA growth shall surpass a certain point before it can be determined as a significant change in pH [81]. When the pH in the reactor drops, the concentration of VFAs is possibly very high, and the process may pre- viously have been affected [82]. Hydrogen sulfide and phosphate are other compounds contributing to the buffering capacity [83]. To counteract this pH imbalance, a buffer so- lution may be added to the bioreactor [78]. 5.3. Temperature As this is one of the critical parameters influencing AD, temperature influences the activity of enzymes and co-enzymes and the methane yield and digestate (effluent) qual- ity [84,85]. Anaerobic bacteria generally grow at three temperature ranges, namely the psychrophilic (10–30 °C), mesophilic (30–40 °C) as well as thermophilic (50–60 °C) range [86,87]. Generally, AD performance increases with an increasing temperature [88]. There has been an emphasis on the advantages of the thermophilic operation, which has high metabolic rates, higher rates of destroying pathogens, and higher specific growth rates, collectively leading to higher biogas production [85–87,88]. Gallert, et al. [90] demon- strated that ammonia accumulation inhibition affects thermophilic digestion less than mesophilic digestion. Biogas production under thermophilic conditions (55 °C) has been reported to give more than double the amount produced under psychrophilic conditions (15 °C) by Wei, et al. [91]. Furthermore, other studies show that phosphorus assimilation and organic ni- trogen degradation increase with temperature too [85]. Thermodynamically higher tem- peratures benefit endergonic reactions, such as the breakdown of propionate into acetate, CO2, and H2, though that is not favorable to exergonic reactions, such as hydrogenotrophic methanogenesis [84]. Further, the temperature may influence the passive separation of solids with considerable improvement under thermophilic compared to psychrophilic conditions [92]. There are, however, some shortfalls in thermophilic conditions, for in- stance, being sensitive to environmental changes compared to the mesophilic process [89,93]. 5.4. Organic Loading Rate This is another crucial operational parameter in the biogas production process. This parameter is defined as a measure of the substrate’s amount being added to a constant digester system per unit of volume per day. OLR is frequently presented as grams of total solids, chemical oxygen demand, or volatile solids per litre digester volume per day [94]. This parameter can be calculated according to Equation (14) COD × Q OLR = (14) where CODfeed is the substrate strength in terms of COD concentration (mg/L), Q is the flow rate of the substrate (L/day), and Vr (L) is the working volume of the reactor [95]. Literature studies have looked at how this factor affects the biogas production pro- cess, for example, Jiang, et al. [96] explored its effects on the acidogenesis of food waste. This study was focused on the OLR effects at individual AD steps and it illustrated that high OLR favoured the acetate and valerate percentages while propionate and butyrate percentages were low under the same OLR conditions. Similarly, Lim, et al. [97] con- ducted a comparable study for three OLR 5, 9, and 13 g/L d and observed that the highest Appl. Syst. Innov. 2023, 6, 13 9 of 17 OLR led to a very vicious fermentation broth and reactor became unstable with a lesser yield compared to the lower OLR values in the same study. Both studies agree that higher OLR may lead to an accumulation of unused solid food waste in the reactor and, therefore, lead to a reactor failure. 5.5. Hydraulic Retention Time (HRT) This parameter measures the average retention time of a liquid or dissolved compo- nent in a reactor in a biogas study. This parameter is calculated as the tank volume divided by the influent flow rate. HRT is used to approximate the time a substrate is treated in a process. The mixing controls HRT and the biogas yield greatly depends on how the di- gester is mixed. Other factors that affect the HRT are substrate type used and different processes, with effects observed from a few days to a couple of months [94]. There has been contradicting data on the effects of HRT on anaerobic acidogenesis, for example, some researchers found that acidification increased with the HRT [98]. Demirel and Yenigun [99] studied the effects of variations in HRT with no control of pH and their find- ings revealed that a high degree of acidification was obtained at low HRT. However, the effect of HRT in the overall AD process has been observed to be similar, while longer HRT leads to higher methane content [100]. 5.6. Effect of Inoculation on AD Process Parameters The use of inocula positively reinforces sustainability through the recovery of mate- rial and reduced energy consumption [101,102]. Research has been found that the use of inocula is more significant than alkaline pre-treatment of raw material substrates since inocula have sufficient bacterial content and increase active microorganisms [103]. Since inoculum is highly cellulosic, it is unable to be digested by itself; accordingly, it is suitable to be reused in AD with other substrates. Types of inocula used in biogas production in- clude sludge from wastewater treatment plants, digested silage, paper mill wastewater, digested sewage sludge, etc. [104,105]. Depending on the composition of each inoculum, the influence on biogas production will vary. For example, palm oil mill effluent has been used as an inoculum with cow manure biogas production, resulting in higher biogas pro- duction [106]. Activated digestate from an anaerobic digestion plant that treats crop and agriculture waste was used as an inoculum by Fabbri, et al. [107], where the best biogas production was obtained with an inoculum/substrate ratio of 2:1. Some studies have in- vestigated the effects of different inocula types on specific substrates while others have looked at the effects of mixed inoculation and data that show a positive influence of inoc- ulation are available in literature [108,109]. 5.7. Co-Digestion of Two Substrates In a biogas production process, anaerobic microorganisms have different require- ments of organic and micronutrients for their growth and degradation of substrates. These nutritional requirements of microorganisms are usually not satisfied by the digestion of single substrates. As a result, a combination of two or more substrates can be co-digested. The suitability of substrates for biogas production is determined by their primary nutri- tional composition, including carbohydrates, proteins, and lipids [61]. This nutritional composition greatly influences biogas yield and methane content produced. Suppose a substrate has an imbalance in carbon to nitrogen ratio, such as animal manure. It can be co-digested with a carbon-rich substrate to reimburse for the imbalance, thus, obtaining improved process stability and biogas production [49]. Thus, co-digestion of sugar indus- try wastewater and Tunisian green macroalgae has been conducted to enhance biogas and methane production [110]. Further, Matheri et al. optimized biogas production through co-digestion of the organic part of municipal solid waste and chicken manure [111]. Other examples of substrates used in co-digestion are listed in Table 1. Appl. Syst. Innov. 2023, 6, 13 10 of 17 Table 1. Previously reported studies on biogas and methane production through a co-digestion of different types of feedstocks at diverse operating parameters. Biogas/Methan Feedstock 1 Feedstock 2 Temperature (℃) Optimal pH HRT (Days) Reference e Yield (L) Fruit and Sewage sludge 20–30 4.1 105 331 [112] vegetable waste Leather flashing MSW - 6.5 30–35 6.518 [113] (LF) Taihu algea Kitchen waste 35 - 1 0.388.6 [114] Horse dung Cow dung 28–33 - 30 0.360 [115] Dairy manure Food waste 35 - 20–30 0.311 [116] Whole stillage Cattle manure 37 5.9–6.6 640 0.310 [117] Coffee-pulp Cow dung 35 7.0 240 - [118] Food waste Straw 35 7.0–7.5 - 0.580 [119] Municipal Poultry waste 35 7.3 34 0.88 [120] wastewater Fruit vegetable Sugarcane - 3.9–7.0 30 2.600 [121] waste bagasse Sugar mill Water hyacinth 30, 40 6.4–8.8 15 6.771 [122] effluent The above table clearly shows that not only in SA but around the globe too there has been a lack of co-digesting sugarcane process effluent and municipal solid waste for bio- gas production. This shows that there is a gap in the research regarding the use of these two substrates as co-substrates, both locally and all around the world. 6. Microorganism Selection, Culturing, and Inhibition In many instances, microorganisms have proven far more cost effective than hydro- lytic enzymes. Microorganisms can convert the substrates’ high-molecular-weight com- pounds into lower-mass compounds through fermentation. Microorganisms involve the synthesis of enzymes and the multiplication of decomposing microorganisms [123]. In this process, it is necessary to consider the conditions of survival and growth of valuable mi- croorganisms, for example, nutrients, inhibitors, pH, temperature, oxygen concentration, etc. [124]. Changes in the structure of the populations of microorganisms used in the sub- strate decomposition are affected by adjusting these parameters. The changes can be made based on the desire and requirements of the biogas process [123]. However, microorgan- isms usually involve a longer retention time, the possibility of growth of unwanted mi- croorganisms, and stricter operating conditions [125]. Therefore, the value of the genera- tion time for the given conditions must be considered for each species. It is also acknowl- edged that the doubling time for bacteria is a lot shorter than for fungi--; thus, microor- ganisms ought to be used after prior studies [123]. Lastly, as suggested by Sawyerr, Trois, Workneh, and Okudoh [54], it is essential to have continued research on the evaluation of different types of biomass feedstock and waste streams, as substrates are critical for developing processes that lead to kinetic reac- tions and increasing methane yield. This is crucial because AD provides multiple ad- vantages over other waste-management methods, such as the technology can be used on both small and large scales, low operating costs, low energy consumption, and reduced environmental impacts through the excess digestate produced, since it can be used to en- hance soil fertility [54,126]. The digestate can work as a biofertilizer, as it is rich in nitro- gen, phosphorus, and potassium, with traces of some elements and heavy metals. The fertiliser value differs according to the nutrients present in the feedstock [127]. Appl. Syst. Innov. 2023, 6, 13 11 of 17 7. Types of Digesters Used A variety of digesters exist for the anaerobic digestion of organic waste material. These digester types depend on operational factors and the nature of waste to be treated, for instance, its solid content. These are classified as covered lagoon digesters (used for treating liquid manure with less than 2% solids), complete-mix digesters (treating manure with 2–10% solids), upflow and downflow fixed-bed biodigesters, batch biodigester, and continuously stirred tank reactors (low solid digesters), as presented in Table 2 [128]. UASB is the most commonly used digester for municipal and industrial wastewaters and it is suitable for both small- and large-scale biogas production. This biodigester has proven to be energetically efficient while it provides operational stability [129]. UASB can also be used for the co-digestion of sugar process wastewater and municipal solid waste as some studies have confirmed it suitable for digestion of more than one substrate [130]. Table 2. Advantages and disadvantages of various digester types used in AD process when one or more feedstocks are used. Biodigester Type Feedstocks Advantages Shortcomings Ref Enhanced mass transfer, usage or generation of improved temperature Continuous Stirred- solids during the Ulva slurry + whey control, facile reaction [131,132] Tank Reactor (CSTR) reaction, plugging optimization, easy problems automation simple and flexible in configuration and long run times, and operation, low Batch Thickened sludge difficulty in defining [133,134] installation and initial conditions operation cost, higher biomass retention delay in start-up and no need for temperature granule formation, Recycled and synthetic Upflow Anaerobic control as heat is inability to remove wastewater containing [129,135,136] Sludge Blanket (UASB) released during pathogens and coloring methanol methanogenesis agents from the wastewater heavy computational relatively cheap, their requirements for multiple Anaerobic Sequencing stepwise nature allows cycles, difficulty in Synthetic wastewater [137] Batch Reactor (ASBR) observation of dynamic, establishing the correct repeatable behavior biomass concentration in the reactor needs hydraulic easy to build, operate, maintenance from 20 to Covered lagoon Palm Oil Mill Effluent [138] and maintain 90 days and wide areas, easy to leak out 8. Discussion Generally, South Africa faces challenges when it comes to biogas production. Around 200 biodigesters have been installed in the last decade, with about 90% of them being for small-scale use. Nonetheless, lack of local research in this field leads to unresolvable fail- ures of the installed biodigesters. Mukumba, et al. [139] explained how research lacks in SA regarding biogas generation and alluded to how there is an absence of data, even from Appl. Syst. Innov. 2023, 6, 13 12 of 17 the currently installed biodigesters. The main reason for this is the lack of financial assis- tance while data collection from the field biodigesters is hindered by some other measures. From the available literature, it can be deduced that thorough characterization of waste substrates must be performed to ascertain the composition of each substrate. This generally gives information on physical and chemical composition regarding volatile sol- ids, total solids, C/N ratio, and elemental analysis for carbon, nitrogen, hydrogen, and sulfur [71,72]. During substrate characterization, the place (source) where the substrate was collected is vital, as waste chemical content is affected by factors, such as weather conditions and the type of soil where the original substances were grown [73,74]. Further, chemical compositions differ greatly depending on the type of substrate. For example, carbohydrates have carbon and hydrogen, while proteins and lipids have nitrogen as part of their composition [75,76]. 9. Conclusions This review can be summarised through the following statements. There is a lack of literature regarding the usage of sugar wastewater as a substrate for biogas production compared with municipal solid waste. It is, therefore, necessary to explore the potential of this substrate and its co-digestion compatibility, particularly in South Africa, as the country is a big sugar producer, making it a hub generating volumes of sugar wastewater in the production process. Anaerobic digestion of single substrates does not lead to max- imum biogas generation; hence, two or more substrates need to be co-digested for better biogas yield and high methane content. Efficient biogas production can be achieved only if there is an excellent synergistic effect between the co-digested substrates. This means an excellent overall balance of nutrients from each substrate, leading to the correct micro- bial community and aiding an enhanced AD process. Different parameters affect biogas production differently; therefore, special attention must be paid to such parameters for thorough parametric analysis. For example, when exploring the hydraulic retention time on biogas generation, analysis must be conducted periodically in 3–5 day intervals to in- vestigate if the AD process is affected thoroughly. Regarding temperature studies, it can be concluded that the thermophilic range leads to higher biogas yields than the psychro- philic and mesophilic ranges. However, the mesophilic range is deemed the best since the thermophilic microorganisms are sensitive to environmental changes. Biogas production is favored by pH in a range of 6.0–8.5, meaning that there should be continuous monitor- ing of this parameter throughout the process. This study also found that a good balance of OLR may help avoid reactor failures. The type of reactor/digester employed for biogas production depends mainly on the type of substrates treated. Finally, sugar wastewater and the municipal solid waste can be considered as good substrates for biogas production in SA due to their enormous availability and the potential to turn their negative impacts into value addition. Biogas production is a viable alternative, among others, to boost the country from its current energy issues. The study was limited regarding available literature in the South African context, which shows that there is a huge gap in the area of waste valorisation in South Africa, even though the country is struggling with waste management. This opens up space for more research to be conducted in this area using two of the country’s most abundant feed- stocks, sugarcane wastewater and municipal solid waste. The specific parameters to be considered for this waste valorisation are highlighted in this review as a foundation. 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An Overview of Biogas Production from Anaerobic Digestion and the Possibility of Using Sugarcane Wastewater and Municipal Solid Waste in a South African Context

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Review An Overview of Biogas Production from Anaerobic Digestion and the Possibility of Using Sugarcane Wastewater and Municipal Solid Waste in a South African Context 1, 1 2 3 Zikhona Tshemese *, Nirmala Deenadayalu , Linda Zikhona Linganiso and Maggie Chetty Department of Chemistry, Steve Biko Campus, Durban University of Technology, Durban 4000, KwaZulu-Natal Province, South Africa Research and Postgraduate Support, Steve Biko Campus, Durban University of Technology, Durban 4000, KwaZulu-Natal Province, South Africa Chemical Engineering, Cape Peninsula University of Technology, Ape Town 7535, Western Cape, South Africa * Correspondence: 22174937@dut4life.ac.za Abstract: Bioenergy production from waste is one of the emerging and viable routes from renewable resources (in addition to wind and solar energy). Many developing countries can benefit from this as they are trying to solve the large amounts of unattended garbage in landfills. This waste comes in either liquid (wastewater and oil) or solid (food and agricultural residues) form. Waste has neg- ative impacts on the environment and, consequently, any form of life that exists therein. One way of solving this waste issue is through its usage as a resource for producing valuable products, such as biofuels, thus, creating a circular economy, which is in line with the United Nations (UN) Sus- tainable Development Goals (SDGs) 5, 7, 8, 9, and 13. Biofuel in the form of biogas can be produced from feedstocks, such as industrial wastewater and municipal effluent, as well as organic solid waste in a process called anaerobic digestion. The feedstock can be used as an individual substrate Citation: Tshemese, Z.; for anaerobic digestion or co-digested with two other substrates. Research advancements have Deenadayalu, N.; Linganiso, L.Z.; shown that the anaerobic digestion of two or more substrates produces higher biogas yields as com- Chetty, M. An Overview of Biogas pared to their single substrates’ counterparts. The objective of this review was to look at the anaer- Production from Anaerobic obic digestion process and to provide information on the potential of biogas production through Digestion and the Possibility of the co-digestion of sugarcane processing wastewater and municipal solid waste. The study deduced Using Sugarcane Wastewater and that sugar wastewater and municipal solid waste can be considered good substrates for biogas pro- Municipal Solid Waste in a South duction in SA due to their enormous availability and the potential to turn their negative impacts African Context. Appl. Syst. Innov. into value addition. Biogas production is a feasible alternative, among others, to boost the country 2023, 6, 13. https://doi.org/ 10.3390/asi6010013 from the current energy issues. Academic Editor: Francesca Valenti Keywords: energy; biogas; anaerobic co-digestion; substrate type; sugarcane processing Received: 14 November 2022 wastewater; municipal solid waste Revised: 8 December 2022 Accepted: 14 December 2022 Published: 16 January 2023 1. Introduction Energy accessibility and waste management are some of the most significant chal- lenges developing countries face, including South Africa [1]. Energy demands exceed the Copyright: © 2023 by the authors. existing energy supply due to the continual increase in population globally. Regularly Licensee MDPI, Basel, Switzerland. used energy resources, such as oil, coal, and natural gas, are diminishing and they emit This article is an open access article greenhouse gases that contribute to climate change [2]. Accordingly, the research focus in distributed under the terms and conditions of the Creative Commons many countries has shifted to finding and implementing efficient and green alternatives, Attribution (CC BY) license such as renewable resources, as solutions to these conventional energy sources, which are (https://creativecommons.org/licenses detrimental and waning [3]. Examples of renewable energy resources are those that can /by/4.0/). Appl. Syst. Innov. 2023, 6, 13. https://doi.org/10.3390/asi6010013 www.mdpi.com/journal/asi Appl. Syst. Innov. 2023, 6, 13 2 of 17 be replenished naturally, such as solar photovoltaic and wind power. However, the en- ergy demands of a constantly growing population using coal-powered stations cannot be supplemented with only solar and wind energy as these are predominantly weather-de- pendent [4]. Waste generation and management have been dealt with in many ways, one of which is using it as a resource for producing valuable products, such as biogas, which contrib- utes to the circular economy [5]. Further, this is in line with the UN’s Sustainable Devel- opment Goals 5, 7, 8, 9, and 13. Biogas can be produced from many substrates, including organic solid waste, wastewater/effluent, etc., as substrates/feedstock in biodigestion. Feedstock is any substrate that can be converted to biogas/methane through anaerobic bacteria. These range from solid wastes to readily degradable wastewater and sludge. This waste must contain a substantial amount of organic matter, which is then converted into biogas [6]. Conventionally, anaerobic digestion is a practical way to treat animal and ag- ricultural waste, macroalgae, and sewage sludge from aerobic wastewater treatment plants [7–9]. However, a change happened after 1970, as soon as environmental conscious- ness grew in connection with the demand for renewable energy reforms and new waste- management strategies [10]. Industrial and municipal waste has also been identified as eligible for anaerobic digestion, as shown in Figure 1. This resource is one of the sustain- able and viable routes to help many developing countries manage massive amounts of the waste left unattended in landfills and discharged into water streams and oceans [11]. This waste negatively impacts the environment and, consequently, life in such an environment, i.e., human and animal lives [12,13]. Agricultural waste Energy crops Anaerobic digestion Industrial waste and Municipal biowaste wastewater Figure 1. Sources of suitable substrates for anaerobic digestion [14]. Clean, renewable energy in the form of biogas can be achieved through anaerobic digestion (AD) of waste matter [15]. AD comprises a series of biochemical reactions that result in the production of biogas, a mixture of methane, carbon dioxide, and negligible traces of other elements. Different waste matter, including the organic part of municipal solid waste, industrial waste, wastewater from manufacturing processing, and agricul- tural waste produced from livestock and crop production, is used in the AD process. No- tably, the biogas formation process produces some by-products, such as slurry (digestate), which provides an added benefit, since the spent waste (slurry) can be used as organic compost by farmers due to its nutrient composition [16]. However, it is crucial to check the safety of this digestate for its use as a fertilizer since there can be potential incidences of heavy metals and pathogenic bacteria [17]. In addition to the environmental benefits of waste management, there are also socio-economic rewards. It is envisaged that countries Appl. Syst. Innov. 2023, 6, 13 3 of 17 would raise their annual turnover for different sectors, generate thousands of jobs, and save billions of dollars a year after fully implementing waste-management solutions [18]. Since research has shown that the digestion of a single substrate produces less biogas than a co-digestion of two or more substrates, municipal solid waste and sugarcane pro- cessing wastewater are deemed to be good co-substrates for biogas production. Further, sugarcane wastewater has low carbon-to-nitrogen ratio, which enables the use of a com- plementary substrate [19]. Both these substrates contain significant amounts of organic content, digested by the anaerobic bacteria [20,21]. This review looks at the anaerobic di- gestion process and seeks to provide information on the potential of biogas production through the co-digestion of sugarcane processing wastewater and municipal solid waste. South Africa is experiencing “load shedding” because energy demand is higher than the currently generated energy; therefore, urgent solutions are required to solve this issue. Biogas production is a viable alternative among others to boost the country from the cur- rent energy issues. 2. Possibility of Generating Renewable Energy from Biogas Using Sugarcane Pro- cessing Wastewater Sugarcane is used for sugar production at the business scale and contributes about 80% of the world’s sugar revenue [22,23]. In South Africa, the provinces of KwaZulu-Natal and Mpumalanga are the major sugarcane producers contributing to the prosperity of the sugar industry’s economy [24]. The same sugarcane that contributes to countries’ econo- mies is characterized by the generation of large quantities of organic wastewater, with excessive chemical oxygen demand that pollutes the environment [25,26]. Therefore, the sustainable advancement of the sugarcane industry requires reducing and treating sugar- cane processing wastewater. One way of treating this sugarcane processing wastewater is by discharging it into wastewater-treatment systems where there would be physical or chemical nutrient removal. Nevertheless, such methods present disadvantages of second- ary pollution, high operation costs, and limitation of nutrient reusability [27,28]. Sugar- cane processing wastewater is an attractive substrate for bacterial cultivation to produce beneficial products, such as biogas, biomass, enzymes, and organic acids, due to its high carbohydrates, minerals, and sugars [26,29]. Many researchers [30–32] have studied the conversion of biogas to electricity. Wang, et al. [33] analyzed the efficiency and sustaina- bility of biogas to electricity production from a large-scale biogas project in China using pig manure. Even though they obtained lower yield results compared to traditional coal and natural gas power plants, electricity generated from biogas still brings more ad- vantages and reduced antithetical environmental effects, as opposed to fossil fuels [33]. 3. Municipal Solid Waste A continual increase in the worldwide populace has led to rapid urbanization in many countries. It is estimated that about two-thirds of the world’s population will live in cities by 2025 because more than 150,000 people move to urban areas each day [34]. This rise in urbanization has resulted in cities that generate thousands of tons of municipal solid waste daily and this is projected to increase significantly in the near future [35]. Mu- nicipal solid waste is an amalgamation of waste from households, markets, backyards, street cleaning, institutional establishments, such as hospitals, and industrial and com- mercial wastes. Management of this type of waste in urban areas pertains to its disposal, collection, resource recovery, recycling, and treatment to promote the quality of both the environment and health while supporting the economy’s efficiency and productivity through generating employment and income [36]. The most preferred waste-management method is the one tuned to take the circular economy direction since it leads to sustainable development. The circular economy focuses on the upper ranks of the waste hierarchy, as shown in Figure 2, including prevention, reuse, and recycling, because these promote cleaner production and minimal waste [37]. Particularly, a circular economy has been Appl. Syst. Innov. 2023, 6, 13 4 of 17 adopted globally since it offsets issues of resource depletion and the detrimental environ- mental effects that lead to climate change. Traditionally, the production and consumer approach, which translates to the linear economy, has been used through a typical “take, use, and dispose of” model [38]. Some of the drawbacks of a linear economy are apparent when the consumer generates waste and disposes of it, causing pollution to the environ- ment and depleting resources [39]. On the other hand, a circular economy follows waste generation minimization and pollution reduction, hence, protecting the environment through a “resource-product-waste-resource” model [40]. South Africa has also embarked on an integrated waste-management structure that considers waste prevention, recycling, recovery, and controlled and supervised disposal. This idea will help to efficiently manage and safeguard human health and the environ- ment, with a significant focus on sustainable development economically, socially, and en- vironmentally. It was suggested that this integrated waste management should incorpo- rate hierarchical waste techniques, which focus more on the avoidance of and reductions in waste than on collection, storage, and disposal [41]. On a local scale, a study of the optimization and financial viability of landfill gas to electricity was conducted in Durban. The study demonstrated that the conversion of landfill gas to electricity provides viable projects with options for optimizing and improving the financial feasibility of the devel- opments [42]. The study also suggests that researchers should look at the possibility of sugarcane waste to produce renewable energy. Favourable results from this research may add more value to sugarcane, as a plant boosts the economy and creates jobs. Conse- quently, this review looks at the process of biogas production, with a particular focus on two different substrates, i.e., sugar wastewater and the organic part of municipal solid waste. These substrates are simple biodegradable materials that can be broken down by microorganisms in an AD process, which includes a series of biochemical reactions, as explained in the following section. Figure 2. Waste hierarchy adapted from [43]. Anaerobic digestion and composting are biological treatments used to treat biode- gradable waste. Studies show that waste management leads to lower environmental im- pacts, lower economic costs, and lower energy consumption. It is suggested that energy- rich waste should be prevented because of the low recovery of resources and harmful environmental effects of landfilling [44]. The advantages attached to waste management include reducing solid waste (about 70–80% mass and 80–90% volume), leading to a pre- served landfill space [45], removal of organic contaminants (halogenated hydrocarbons) [46], reduction in greenhouse gases [47], and naturally compatible exploitation of renew- able energy from waste, predominantly when the plant used is designed to generate heat and power [48]. Appl. Syst. Innov. 2023, 6, 13 5 of 17 4. Anaerobic Digestion Process This is a four-step anaerobic biological decomposition of organic substrates, namely, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The brilliance of this process is that all the phases are connected since a by-product of one step becomes the substrate of the next step, all in one system [49]. These biochemical decomposition phases have a series of chemical reactions, as illustrated in Figure 3 and detailed in the subsequent sub- sections. Figure 3. Anaerobic digestion was adapted from Bajpai [50] and Khanal [51]. 4.1. Hydrolysis The hydrolysis phase of anaerobic digestion is where complex biopolymeric com- pounds (lipids, carbohydrates, and proteins) are converted to water-soluble compounds by degradation through Bacteroides, Clostridia, and Bifidobacteria and sometimes Strep- tococci and Enterobacteriaceae [52]. This step is relatively slow and can limit the rate of the overall digestion, mainly when solid material is used as a substrate. As seen in Equa- tion (1), cellulose (C6H10O5) is hydrolyzed to generate glucose (C6H12O6) and hydrogen (H2). This reaction is catalyzed by homogeneous/heterogeneous acids yielding the fer- mentable monosaccharide (C6H12O6). The products (C6H12O6 and H2) are used by the fer- mentative microorganisms in the next phase to form higher-chain organic compounds, such as volatile fatty acids [53,54]. (C H O ) + nH O → n(C H ) + nH (1) 4.2. Acidogenesis This is the second phase, known as the fermentation stage, where the acidogenic bac- teria Streptococcus, Escherichia, Staphylococcus, Pseudomonas, Bacillus, Sarcina, Desulfovibrio, Lactobacillus, and others are active [55]. These bacteria degrade amino acids, lipids, and Appl. Syst. Innov. 2023, 6, 13 6 of 17 glucose into volatile fatty acids, organic acids, carbon dioxide, and hydrogen gas (as illus- trated in Equations (2)–(7) below [53]). Acetic acid (CH3COOH) is the most important or- ganic acid produced in this stage, which serves as the substrate for methanogenic micro- organisms [16]. It is worth noting that volatile fatty acid production is favored when pH is above 5, while ethanol production (CH3CH2OH) is favored by pH lower than 5, with reactions stopping at a pH level that is less than 4 [50]. C H O ↔ 2CH CH OH + 2CO (2) C H O + 2H ↔ 2CH CH COOH + 2H O (3) C H O → 3CH COOH (4) C H O N + 2H O → C H O + NH + CO + 2H + ATP (5) C H O + NH + CO + H + ATP (6) 4CH COCOO + 4H O → 5CH COO + 2HCO + 3H (7) 4.3. Acetogenesis At this stage, the reactions are reversible with a release of hydrogen. Volatile fatty acids, specifically acetic acid and butyric acid, are converted into carbon dioxide gas, hy- drogen, and acetate, as shown in Equations (8)–(10). The active bacteria in this stage are Clostridium, Syntrophomonas wolfeii, and Syntrophomonas wolinii [55]. This conversion of volatile fatty acids is enabled by the presence of water molecules (acting as electron sources) from the previous stages of anaerobic digestion. Equation (9) converts the phase product to acetate and hydrogen, used in the next stage [56]. This stage is equally im- portant since it reflects the biogas production efficiency, given that the reduction of the acetate ion forms about 70% of methane. Acetate is a primary intermediary product of this phase and it accounts for 25% of the products formed together, with approximately 11% of hydrogen [57]. CH CH COO + 3H O ↔ CH COO + H + HCO + 3H (8) C H O + 2H O ↔ 2CH COOH + 2CO + 4H (9) 2CH CH OH + 2H O ↔ CH COO + 2H + H (10) 4.4. Methanogenesis In the final phase of the methanogenesis stage, acetic acid is converted into methane and carbon dioxide using bacteria called methanogens, which are anaerobes with a high vulnerability to limited amounts of oxygen. In addition, carbon dioxide is a product that reacts with hydrogen gas to produce more methane. On the other hand, ethanol under- goes decarboxylation to form methane. Two types of bacteria—the acetophilic methano- genic (with Methanosarcina and Methanosaeta active specie) and the hydrogenophilic meth- anogenic (with Methanospirilum, Methanobacterium formicicum, Methanoplanus, and Meth- anobrevibacterium as the dominant specie)—exist in this stage [55]. The former is responsi- ble for the decarboxylation of acetate to methane and the latter for methane formation through a reaction of carbon dioxide and hydrogen [53]. The final product of the anaerobic digestion process is biogas, which is composed of methane and carbon dioxide. (11) CH COOH → CH + CO (12) CO + 4H → CH + 2H O (13) 2CH CH OH + CO → CH + 2CH COOH Appl. Syst. Innov. 2023, 6, 13 7 of 17 5. Factors Affecting the Anaerobic Digestion Process and Biogas Production Biogas production is influenced by different factors, such as substrate type, temper- ature, pH, organic loading rate, hydraulic retention time, etc. [58]. 5.1. Substrate Type Numerous biomass feedstocks can be used for biogas production, depending on their nutritional composition. Accordingly, these compositions influence biogas yield, methane content, degradation kinetics, and biomass biodegradability [59]. The critical nutritional compositions of biomass substrates suitable for biogas production are carbohydrates, pro- tein, and fats. Theoretical estimations of biogas percentage and methane yield from these nutrients have been reported in the literature and are calculated using the Buswell for- mula [60]. Protein-rich substrates have a high potential for methane yield, but their deg- radation gives off ammonium ions, leading to an alkalinity increase in the AD process. The increase in alkalinity improves the digest value as fertilizer while preventing the ac- tivities of methanogens. This inhibition occurs during the equilibrium shift from ammo- nium to ammonia, typically in changing concentrations. In addition, literature remarks suggest that microorganisms can acclimatize to environments with high ammonia con- centrations while efficiently producing biogas [61,62]. Lipid-rich substrates, such as fats, possess great methane yield potential, such as an- imal and plant tissue waste, biodegradable kitchen and canteen waste, grease and oil mix- ture, etc. [63]. These substrates release long-chain fatty acids during their degradation, which are typically toxic to the microbial environment and cause a drop in pH [64–66]. There are several other types of biomass that are used for biogas production apart from the protein and lipid-rich substrates, for example, substrates with a high degree of ligno- cellulose (wheat straw, sorghum, rice straw, etc.) [67]. This type of biomass is hard to de- grade due to these three reasons: (i) recalcitrant nature, (ii) heterogeneous structure, and (iii) low accessibility by enzymes, such as carbohydrate polymers [68–70]. However, pre- treatment mechanisms can help break down the heterogeneous matrix, thus, increasing the porous and surface area of the lignocellulose biomass and enhancing biogas produc- tion. Characterization of the waste substrates is performed to ascertain the composition of each substrate. This is generally physical and chemical composition regarding volatile sol- ids, total solids, C/N ratio as well as elemental analysis for carbon, nitrogen, hydrogen, and sulfur [71,72]. During substrate characterization, the place (source) where the sub- strate was collected is vital, as waste chemical content is affected by many factors, such as weather conditions and the type of soil where the original substances were grown [73,74]. Physical and chemical compositions depend on the type of substrate, for example, carbo- hydrates have carbon and hydrogen while proteins and lipids have nitrogen as part of their composition [75,76]. These substrate compositions can be analyzed using different analytical techniques. 5.2. Anaerobic Digestion pH The operational pH directly affects both the digestive progress and products formed in the AD process. Literature findings show that the ideal comprehensive pH range for AD should be between 4.0 and 8.5, as per the requirement for the fermentative bacteria, although the limiting range of 6.5–7.2 is favorable for the growth of methanogens [77,78]. The microorganism growth rate is significantly affected by the change in pH and, as such, each microbial group has a specific optimum pH. A comparative abundance of microbial species increased from 6 to 14 at pH 4.0 and 7.0, respectively [79]. Bacterial population dominance differs with changing pH, for example, at pH 6.0, Clostridium butyricum is dominant, while the Propionibacterium spp. thrives during anaerobic acidogenesis at pH 8 [80]. Appl. Syst. Innov. 2023, 6, 13 8 of 17 When the pH level is controlled for the optimal growth of microorganisms, reduc- tions in toxicity, generally from increased concentration of free ammonia (FA), are also achieved. Similarly, pH significantly affects volatile fatty acid (VFA) composition [79]. In an anaerobic reactor, instability typically leads to the accumulation of VFAs, leading to a drop in pH and, therefore, acidification. Nonetheless, this accumulation of VFAs does not always exemplify a pH drop, owing to the buffer capacity of some waste forms. There is an excess of alkalinity in manure, denoting that the VFA growth shall surpass a certain point before it can be determined as a significant change in pH [81]. When the pH in the reactor drops, the concentration of VFAs is possibly very high, and the process may pre- viously have been affected [82]. Hydrogen sulfide and phosphate are other compounds contributing to the buffering capacity [83]. To counteract this pH imbalance, a buffer so- lution may be added to the bioreactor [78]. 5.3. Temperature As this is one of the critical parameters influencing AD, temperature influences the activity of enzymes and co-enzymes and the methane yield and digestate (effluent) qual- ity [84,85]. Anaerobic bacteria generally grow at three temperature ranges, namely the psychrophilic (10–30 °C), mesophilic (30–40 °C) as well as thermophilic (50–60 °C) range [86,87]. Generally, AD performance increases with an increasing temperature [88]. There has been an emphasis on the advantages of the thermophilic operation, which has high metabolic rates, higher rates of destroying pathogens, and higher specific growth rates, collectively leading to higher biogas production [85–87,88]. Gallert, et al. [90] demon- strated that ammonia accumulation inhibition affects thermophilic digestion less than mesophilic digestion. Biogas production under thermophilic conditions (55 °C) has been reported to give more than double the amount produced under psychrophilic conditions (15 °C) by Wei, et al. [91]. Furthermore, other studies show that phosphorus assimilation and organic ni- trogen degradation increase with temperature too [85]. Thermodynamically higher tem- peratures benefit endergonic reactions, such as the breakdown of propionate into acetate, CO2, and H2, though that is not favorable to exergonic reactions, such as hydrogenotrophic methanogenesis [84]. Further, the temperature may influence the passive separation of solids with considerable improvement under thermophilic compared to psychrophilic conditions [92]. There are, however, some shortfalls in thermophilic conditions, for in- stance, being sensitive to environmental changes compared to the mesophilic process [89,93]. 5.4. Organic Loading Rate This is another crucial operational parameter in the biogas production process. This parameter is defined as a measure of the substrate’s amount being added to a constant digester system per unit of volume per day. OLR is frequently presented as grams of total solids, chemical oxygen demand, or volatile solids per litre digester volume per day [94]. This parameter can be calculated according to Equation (14) COD × Q OLR = (14) where CODfeed is the substrate strength in terms of COD concentration (mg/L), Q is the flow rate of the substrate (L/day), and Vr (L) is the working volume of the reactor [95]. Literature studies have looked at how this factor affects the biogas production pro- cess, for example, Jiang, et al. [96] explored its effects on the acidogenesis of food waste. This study was focused on the OLR effects at individual AD steps and it illustrated that high OLR favoured the acetate and valerate percentages while propionate and butyrate percentages were low under the same OLR conditions. Similarly, Lim, et al. [97] con- ducted a comparable study for three OLR 5, 9, and 13 g/L d and observed that the highest Appl. Syst. Innov. 2023, 6, 13 9 of 17 OLR led to a very vicious fermentation broth and reactor became unstable with a lesser yield compared to the lower OLR values in the same study. Both studies agree that higher OLR may lead to an accumulation of unused solid food waste in the reactor and, therefore, lead to a reactor failure. 5.5. Hydraulic Retention Time (HRT) This parameter measures the average retention time of a liquid or dissolved compo- nent in a reactor in a biogas study. This parameter is calculated as the tank volume divided by the influent flow rate. HRT is used to approximate the time a substrate is treated in a process. The mixing controls HRT and the biogas yield greatly depends on how the di- gester is mixed. Other factors that affect the HRT are substrate type used and different processes, with effects observed from a few days to a couple of months [94]. There has been contradicting data on the effects of HRT on anaerobic acidogenesis, for example, some researchers found that acidification increased with the HRT [98]. Demirel and Yenigun [99] studied the effects of variations in HRT with no control of pH and their find- ings revealed that a high degree of acidification was obtained at low HRT. However, the effect of HRT in the overall AD process has been observed to be similar, while longer HRT leads to higher methane content [100]. 5.6. Effect of Inoculation on AD Process Parameters The use of inocula positively reinforces sustainability through the recovery of mate- rial and reduced energy consumption [101,102]. Research has been found that the use of inocula is more significant than alkaline pre-treatment of raw material substrates since inocula have sufficient bacterial content and increase active microorganisms [103]. Since inoculum is highly cellulosic, it is unable to be digested by itself; accordingly, it is suitable to be reused in AD with other substrates. Types of inocula used in biogas production in- clude sludge from wastewater treatment plants, digested silage, paper mill wastewater, digested sewage sludge, etc. [104,105]. Depending on the composition of each inoculum, the influence on biogas production will vary. For example, palm oil mill effluent has been used as an inoculum with cow manure biogas production, resulting in higher biogas pro- duction [106]. Activated digestate from an anaerobic digestion plant that treats crop and agriculture waste was used as an inoculum by Fabbri, et al. [107], where the best biogas production was obtained with an inoculum/substrate ratio of 2:1. Some studies have in- vestigated the effects of different inocula types on specific substrates while others have looked at the effects of mixed inoculation and data that show a positive influence of inoc- ulation are available in literature [108,109]. 5.7. Co-Digestion of Two Substrates In a biogas production process, anaerobic microorganisms have different require- ments of organic and micronutrients for their growth and degradation of substrates. These nutritional requirements of microorganisms are usually not satisfied by the digestion of single substrates. As a result, a combination of two or more substrates can be co-digested. The suitability of substrates for biogas production is determined by their primary nutri- tional composition, including carbohydrates, proteins, and lipids [61]. This nutritional composition greatly influences biogas yield and methane content produced. Suppose a substrate has an imbalance in carbon to nitrogen ratio, such as animal manure. It can be co-digested with a carbon-rich substrate to reimburse for the imbalance, thus, obtaining improved process stability and biogas production [49]. Thus, co-digestion of sugar indus- try wastewater and Tunisian green macroalgae has been conducted to enhance biogas and methane production [110]. Further, Matheri et al. optimized biogas production through co-digestion of the organic part of municipal solid waste and chicken manure [111]. Other examples of substrates used in co-digestion are listed in Table 1. Appl. Syst. Innov. 2023, 6, 13 10 of 17 Table 1. Previously reported studies on biogas and methane production through a co-digestion of different types of feedstocks at diverse operating parameters. Biogas/Methan Feedstock 1 Feedstock 2 Temperature (℃) Optimal pH HRT (Days) Reference e Yield (L) Fruit and Sewage sludge 20–30 4.1 105 331 [112] vegetable waste Leather flashing MSW - 6.5 30–35 6.518 [113] (LF) Taihu algea Kitchen waste 35 - 1 0.388.6 [114] Horse dung Cow dung 28–33 - 30 0.360 [115] Dairy manure Food waste 35 - 20–30 0.311 [116] Whole stillage Cattle manure 37 5.9–6.6 640 0.310 [117] Coffee-pulp Cow dung 35 7.0 240 - [118] Food waste Straw 35 7.0–7.5 - 0.580 [119] Municipal Poultry waste 35 7.3 34 0.88 [120] wastewater Fruit vegetable Sugarcane - 3.9–7.0 30 2.600 [121] waste bagasse Sugar mill Water hyacinth 30, 40 6.4–8.8 15 6.771 [122] effluent The above table clearly shows that not only in SA but around the globe too there has been a lack of co-digesting sugarcane process effluent and municipal solid waste for bio- gas production. This shows that there is a gap in the research regarding the use of these two substrates as co-substrates, both locally and all around the world. 6. Microorganism Selection, Culturing, and Inhibition In many instances, microorganisms have proven far more cost effective than hydro- lytic enzymes. Microorganisms can convert the substrates’ high-molecular-weight com- pounds into lower-mass compounds through fermentation. Microorganisms involve the synthesis of enzymes and the multiplication of decomposing microorganisms [123]. In this process, it is necessary to consider the conditions of survival and growth of valuable mi- croorganisms, for example, nutrients, inhibitors, pH, temperature, oxygen concentration, etc. [124]. Changes in the structure of the populations of microorganisms used in the sub- strate decomposition are affected by adjusting these parameters. The changes can be made based on the desire and requirements of the biogas process [123]. However, microorgan- isms usually involve a longer retention time, the possibility of growth of unwanted mi- croorganisms, and stricter operating conditions [125]. Therefore, the value of the genera- tion time for the given conditions must be considered for each species. It is also acknowl- edged that the doubling time for bacteria is a lot shorter than for fungi--; thus, microor- ganisms ought to be used after prior studies [123]. Lastly, as suggested by Sawyerr, Trois, Workneh, and Okudoh [54], it is essential to have continued research on the evaluation of different types of biomass feedstock and waste streams, as substrates are critical for developing processes that lead to kinetic reac- tions and increasing methane yield. This is crucial because AD provides multiple ad- vantages over other waste-management methods, such as the technology can be used on both small and large scales, low operating costs, low energy consumption, and reduced environmental impacts through the excess digestate produced, since it can be used to en- hance soil fertility [54,126]. The digestate can work as a biofertilizer, as it is rich in nitro- gen, phosphorus, and potassium, with traces of some elements and heavy metals. The fertiliser value differs according to the nutrients present in the feedstock [127]. Appl. Syst. Innov. 2023, 6, 13 11 of 17 7. Types of Digesters Used A variety of digesters exist for the anaerobic digestion of organic waste material. These digester types depend on operational factors and the nature of waste to be treated, for instance, its solid content. These are classified as covered lagoon digesters (used for treating liquid manure with less than 2% solids), complete-mix digesters (treating manure with 2–10% solids), upflow and downflow fixed-bed biodigesters, batch biodigester, and continuously stirred tank reactors (low solid digesters), as presented in Table 2 [128]. UASB is the most commonly used digester for municipal and industrial wastewaters and it is suitable for both small- and large-scale biogas production. This biodigester has proven to be energetically efficient while it provides operational stability [129]. UASB can also be used for the co-digestion of sugar process wastewater and municipal solid waste as some studies have confirmed it suitable for digestion of more than one substrate [130]. Table 2. Advantages and disadvantages of various digester types used in AD process when one or more feedstocks are used. Biodigester Type Feedstocks Advantages Shortcomings Ref Enhanced mass transfer, usage or generation of improved temperature Continuous Stirred- solids during the Ulva slurry + whey control, facile reaction [131,132] Tank Reactor (CSTR) reaction, plugging optimization, easy problems automation simple and flexible in configuration and long run times, and operation, low Batch Thickened sludge difficulty in defining [133,134] installation and initial conditions operation cost, higher biomass retention delay in start-up and no need for temperature granule formation, Recycled and synthetic Upflow Anaerobic control as heat is inability to remove wastewater containing [129,135,136] Sludge Blanket (UASB) released during pathogens and coloring methanol methanogenesis agents from the wastewater heavy computational relatively cheap, their requirements for multiple Anaerobic Sequencing stepwise nature allows cycles, difficulty in Synthetic wastewater [137] Batch Reactor (ASBR) observation of dynamic, establishing the correct repeatable behavior biomass concentration in the reactor needs hydraulic easy to build, operate, maintenance from 20 to Covered lagoon Palm Oil Mill Effluent [138] and maintain 90 days and wide areas, easy to leak out 8. Discussion Generally, South Africa faces challenges when it comes to biogas production. Around 200 biodigesters have been installed in the last decade, with about 90% of them being for small-scale use. Nonetheless, lack of local research in this field leads to unresolvable fail- ures of the installed biodigesters. Mukumba, et al. [139] explained how research lacks in SA regarding biogas generation and alluded to how there is an absence of data, even from Appl. Syst. Innov. 2023, 6, 13 12 of 17 the currently installed biodigesters. The main reason for this is the lack of financial assis- tance while data collection from the field biodigesters is hindered by some other measures. From the available literature, it can be deduced that thorough characterization of waste substrates must be performed to ascertain the composition of each substrate. This generally gives information on physical and chemical composition regarding volatile sol- ids, total solids, C/N ratio, and elemental analysis for carbon, nitrogen, hydrogen, and sulfur [71,72]. During substrate characterization, the place (source) where the substrate was collected is vital, as waste chemical content is affected by factors, such as weather conditions and the type of soil where the original substances were grown [73,74]. Further, chemical compositions differ greatly depending on the type of substrate. For example, carbohydrates have carbon and hydrogen, while proteins and lipids have nitrogen as part of their composition [75,76]. 9. Conclusions This review can be summarised through the following statements. There is a lack of literature regarding the usage of sugar wastewater as a substrate for biogas production compared with municipal solid waste. It is, therefore, necessary to explore the potential of this substrate and its co-digestion compatibility, particularly in South Africa, as the country is a big sugar producer, making it a hub generating volumes of sugar wastewater in the production process. Anaerobic digestion of single substrates does not lead to max- imum biogas generation; hence, two or more substrates need to be co-digested for better biogas yield and high methane content. Efficient biogas production can be achieved only if there is an excellent synergistic effect between the co-digested substrates. This means an excellent overall balance of nutrients from each substrate, leading to the correct micro- bial community and aiding an enhanced AD process. Different parameters affect biogas production differently; therefore, special attention must be paid to such parameters for thorough parametric analysis. For example, when exploring the hydraulic retention time on biogas generation, analysis must be conducted periodically in 3–5 day intervals to in- vestigate if the AD process is affected thoroughly. Regarding temperature studies, it can be concluded that the thermophilic range leads to higher biogas yields than the psychro- philic and mesophilic ranges. However, the mesophilic range is deemed the best since the thermophilic microorganisms are sensitive to environmental changes. Biogas production is favored by pH in a range of 6.0–8.5, meaning that there should be continuous monitor- ing of this parameter throughout the process. This study also found that a good balance of OLR may help avoid reactor failures. The type of reactor/digester employed for biogas production depends mainly on the type of substrates treated. Finally, sugar wastewater and the municipal solid waste can be considered as good substrates for biogas production in SA due to their enormous availability and the potential to turn their negative impacts into value addition. Biogas production is a viable alternative, among others, to boost the country from its current energy issues. The study was limited regarding available literature in the South African context, which shows that there is a huge gap in the area of waste valorisation in South Africa, even though the country is struggling with waste management. This opens up space for more research to be conducted in this area using two of the country’s most abundant feed- stocks, sugarcane wastewater and municipal solid waste. The specific parameters to be considered for this waste valorisation are highlighted in this review as a foundation. Author Contributions: Conceptualization, L.Z.L., M.C. and N.D.; Methodology, Z.T.,L.Z.L.; Inves- tigation, Z.T.; writing—original draft preparation, Z.T..; writing—review and editing, L.Z.L., M.C., Z.T., N.D.; All authors have read and agreed to the published version of the manuscript. Funding: NRF: South Africa, ND; DUT, Research and Postgraduate Support. Data Availability Statement: Raw data is available from Zikhona Tshemese. Appl. Syst. Innov. 2023, 6, 13 13 of 17 Conflicts of Interest: The authors declare no conflict of interest. References 1. Hafner, M.; Tagliapietra, S.; De Strasser, L. Energy in Africa: Challenges and Opportunities; Springer Nature: Berlin, Germany, 2. You, V.; Kakinaka, M. Modern and traditional renewable energy sources and CO2 emissions in emerging countries. Environ. Sci. Pollut. Res. 2022, 29, 17695–17708. 3. Mbungu, N.T.; Naidoo, R.M.; Bansal, R.C.; Siti, M.W.; Tungadio, D.H. 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Journal

Applied System InnovationMultidisciplinary Digital Publishing Institute

Published: Jan 16, 2023

Keywords: energy; biogas; anaerobic co-digestion; substrate type; sugarcane processing wastewater; municipal solid waste

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