Sustainable Utilization of Biowaste Resources for Biogas Production to Meet Rural Bioenergy Requirements

Abstract

Since the impending warning of fossil fuel inadequacy, researchers’ focus has shifted to alternative fuel generation. This resulted in the use of a wide variety of renewable biomass sources for making biofuels. Biofuels made from biomass are seen as the most promising long-term strategy for addressing issues associated with conventional energy sources, atypical climate change, and greenhouse gas emissions. Hydrocarbons may be efficiently extracted from biomass, which contains a lot of sugars. Biofuels including bioethanol, biodiesel, biohydrogen, and biogas can be produced from biomass for widespread usage in transportation, industry, and households. In recent years, there have been numerous reports of breakthroughs in the manufacturing of biofuels and biogas. This paper examines the big picture of biogas generation, with an emphasis on the many forms of biomass utilization in both commercial and residential settings in rural areas.

1. Introduction

As the world grapples with the challenges of climate change and energy security, sustainable solutions for meeting energy demands are of paramount importance. Many researchers have reported that fossil fuel sources throughout the world will last up to 25 years, and thus, there is a need for alternative sources such as renewable energy [1]. In rural areas, where access to conventional energy sources is limited, alternative energy options such as biogas production offer tremendous potential. The sustainable utilization of biowaste resources for biogas production is a promising solution for meeting rural bioenergy requirements. Biowaste, such as agricultural residues, food waste, and animal manure, represents a significant source of organic material that can be converted into biogas through anaerobic digestion [2]. The fundamental steps in the conversion of biomass to biogas using a biodigester or anaerobic digester are shown in Figure 1. This process involves the decomposition of organic matter by microorganisms in the absence of oxygen, resulting in the production of biogas, primarily composed of methane (CH₄) and carbon dioxide (CO₂).

Rural areas often face challenges in accessing reliable energy sources, leading to heavy dependence on traditional fuels such as firewood and fossil fuels. These energy sources are not only environmentally unsustainable but also contribute to deforestation, air pollution, and greenhouse gas emissions [3]. Biogas production from biowaste offers a viable alternative by utilizing locally available organic resources, reducing waste disposal issues, and providing a decentralized energy solution for rural communities. While the utilization of biowaste resources for biogas production offers numerous benefits, there are challenges to overcome. These include the need for proper waste collection and segregation systems, technological know-how, financing options, and policy support. It is crucial to raise awareness among rural communities about the benefits of biogas and provide training and capacity-building programs to enable the adoption of biogas technologies [4].

To promote sustainable utilization of biowaste resources for biogas production, collaboration between various stakeholders is essential. Governments, non-governmental organizations, researchers, and private entities should work together to develop supportive policies, provide financial incentives, and facilitate the dissemination of knowledge and best practices. Additionally, research and development efforts should focus on improving biogas production efficiency, developing cost-effective technologies, and exploring innovative uses for biogas, such as decentralized power generation, cooking fuel, and transportation fuel [5]. Biogas, a renewable energy source, can be derived from the sustainable utilization of biowaste resources. Though many articles describe the production technologies for biogas, this paper has the novelty of describing the production of biogas and the energy requirement of biogas in rural sectors. This paper explores the significance of utilizing biowaste resources for biogas production to address rural energy needs while promoting environmental and socioeconomic benefits.

2. Bioconversion Process

The bioconversion process for biogas refers to the production of biogas using the decomposition of organic matter by microorganisms in an anaerobic (oxygen-free) environment. It involves the conversion of biomass, such as agricultural waste, animal manure, sewage sludge, and food waste, into biogas, which primarily consists of methane (CH₄) and carbon dioxide (CO₂), along with small amounts of other gases like hydrogen sulfide (H₂S) and traces of nitrogen, oxygen, and water vapor [6]. The bioconversion process typically takes place in a sealed container called a biogas digester or anaerobic digester. The digester provides an oxygen-free environment that encourages the growth of anaerobic microorganisms, primarily methanogenic bacteria. These bacteria break down the complex organic molecules present in the biomass into simpler compounds through a series of biochemical reactions [7].

The transformation and production of biomass into useful biofuel products need a deep knowledge of chemistry, engineering, and control systems. The types of bio-refineries based on varieties of raw materials, technologies, products, and processes are classified as first, second, and third-generation refineries. Some of the common technologies used for the conversion of biomass into biofuel and biogas are biological, physical, chemical, and thermal, depending on the type of product [8].

The quest for enhanced sustainability within the realm of bioenergy unfolds with the establishment of eco-efficient supply chains, heralding a transformative era in biomass utilization. In this context, a fascinating prospect emerges with the utilization of grass, an abundant by-product stemming from landscape management practices, which presents a substantial reserve of biomass with immense potential for integration into the anaerobic digestion (AD) supply chain. This strategic utilization of grass represents a critical step towards attaining more sustainable scenarios within the bioenergy sector, leveraging untapped biomass resources and maximizing their potential using eco-efficient pathways [9].

The biological conversion process can be classified into anaerobic digestion, saccharification, and dark/photo fermentation, which tend to produce biomethane, ethanol, and biohydrogen, respectively. Furthermore, physical conversion can be classified into mechanical extraction, briquetting, and distillation. Bioconversion requires pretreatment and hydrolysis, which tend to release monomeric cellulose and hemicellulose for microbial fermentation. Thermochemical conversion can be classified into pyrolysis, liquefaction, and gasification to produce bio-oil, bio-crude oil, and syngas, respectively [10].

The global adoption of anaerobic digestion technology is rapidly expanding due to its advantageous economic and environmental characteristics. Consequently, numerous studies and research endeavors have been undertaken in recent years to investigate the biogas potential of solid organic substrates. This process entails the hydrolysis of complex high-molecular-weight carbohydrates, fats, and/or proteins into soluble polymers using the enzymatic activity of hydrolytic fermentative bacteria. Subsequently, these polymers are transformed into organic acids, alcohols, H₂, and CO₂. Volatile fatty acids (VFAs) and alcohols are further converted into acetic acid using acetogenic bacteria that produce hydrogen. Ultimately, methanogenic bacteria convert acetic acid and H₂ gas into CO₂ and CH₄ [11].

The bioconversion process for biogas refers to the production of biogas using the decomposition of organic matter by microorganisms in an anaerobic (oxygen-free) environment. It involves the conversion of biomass, such as agricultural waste, animal manure, sewage sludge, and food waste, into biogas, which primarily consists of methane (CH₄) and carbon dioxide (CO₂), along with small amounts of other gases like hydrogen sulfide (H₂S) and traces of nitrogen, oxygen, and water vapor [6]. The bioconversion process typically takes place in a sealed container called a biogas digester or anaerobic digester. The digester provides an oxygen-free environment that encourages the growth of anaerobic microorganisms, primarily methanogenic bacteria. These bacteria break down the complex organic molecules present in the biomass into simpler compounds through a series of biochemical reactions [7]. The transformation and production of biomass into useful biofuel products need a deep knowledge of chemistry, engineering, and control system. The types of bio-refineries based upon varieties of raw materials, technologies, products, and processes are classified as first, second, and third-generation refineries. Some of the common technologies used for the conversion of biomass into biofuel and biogas are biological, physical, chemical, and thermal, depending on the type of product [8]. Production of methane with respect to various biomass types is shown in Figure 2.

Pretreatment

Biomass, a crucial reservoir of energy, remains an enduring subject of scientific inquiry as researchers persistently delve into its untapped potential. The allure of harnessing energy from biomass stems from its distinguished status as the third most significant primary energy source on the global scale, with only coal and oil surpassing its prominence. The broad spectrum of biomass encompasses two fundamental categories: vegetal biomass, encompassing remnants from forestry and select agricultural cultivations, and animal biomass, encompassing lipids derived from the leather industry. This multifaceted composition of biomass beckons exploration, propelling scientists toward unlocking its profound energy-yielding capabilities [12].

The pretreatment of agricultural by-products prior to anaerobic digestion plays a pivotal role in enhancing various aspects of the waste management process [13,14]. By subjecting the by-products to pretreatment, the biodegradability of the waste is increased, making it more amenable to digestion by microorganisms. Moreover, pretreatment aids in reducing the presence of inhibitors present in the feedstock, thus promoting a more favorable environment for digestion. Additionally, pretreatment improves the accessibility and utilizable organic content of the substrate, making it more readily available to microorganisms involved in the digestion process. This effectively minimizes the impact of known barriers to anaerobic digestion. An in-depth investigation was undertaken to evaluate the potential of hydrothermal pretreatment (HTP) applied to poultry slaughterhouse waste (PSW) sludge. The primary objective was to enhance solubilization, improve physical properties, and augment biogas production using subsequent anaerobic digestion. The study aimed to identify the optimal HTP temperature for achieving these desired outcomes. To assess the efficacy of HTP, various parameters were examined, including capillary suction time (CST), time to filter (TTF), and particle size, which provided insights into solubilization and physical property enhancements. Furthermore, the anaerobic digestion process was evaluated using biochemical methane potential (BMP) tests followed by statistical analysis utilizing the modified Gompertz model. The results revealed that HTP demonstrated a notable capacity to enhance the solubilization of PSW sludge, with the degree of improvement correlating positively with the HTP temperature. This study sheds light on the potential of HTP as an effective pretreatment method to optimize the solubilization, physical properties, and subsequent biogas production from PSW sludge using anaerobic digestion [15]. Furthermore, pretreatment helps to reduce the residence time required during the hydrolysis stage, which is often the rate-limiting step, thereby increasing the overall conversion efficiency. Various approaches can be used for pretreatment, including chemical, thermal (physical), thermochemical, and biological methods. Among these, the thermochemical pretreatment method, which combines thermal and chemical approaches synergistically, has been recognized as an especially advantageous technique in achieving effective pretreatment of agricultural by-products.

In the realm of scientific exploration, fermentation-based methodologies have gained substantial prominence, both as standalone techniques and in conjunction with other treatment modalities, in the noble pursuit of bio-refining food waste. These approaches serve two paramount objectives: firstly, to optimize the reclamation of invaluable nutrients and energy using the art of recycling, and secondly, to curtail the burdensome trifecta of treatment cost, time constraints, and environmental encumbrances. Nonetheless, a perplexing quandary remains unresolved, as a suitable synergy between technology integration and the treatment of mixed food waste, bolstering the concurrent prospects of energy recuperation and nutrient extraction, has yet to be effectively established [16].

A comprehensive investigation was conducted to evaluate the influence of ultrasound treatment on the rheological and dewatering properties of waste-activated sludge (WAS). Furthermore, the study aimed to assess the economic implications of implementing ultrasound treatment within a hypothetical wastewater treatment plant. Using a meticulous analysis, the study provided valuable insights into the overall effects of ultrasound treatment on the physical characteristics and handling efficiency of WAS. Moreover, it shed light on the potential economic benefits and drawbacks associated with the incorporation of ultrasound treatment into wastewater treatment plants. By considering both the technical and economic aspects, this study contributed to a holistic understanding of the impacts of ultrasound treatment on sludge management and wastewater treatment processes [17].

The biodegradability of lignocellulosic biomass is influenced by a multitude of factors, encompassing crystallinity, polymerization grade, surface area, solubility, and lignin content [18]. Extensive research has been conducted to evaluate the efficacy of different physical, chemical, and biological pretreatment methods for enhancing the biodegradation of lignocellulosic biomass and augmenting methane production. The selection of an appropriate pretreatment approach hinges upon the specific characteristics and structure of the biomass, aiming to promote the generation of biodegradable substrates while minimizing overall mass loss during the process. Physical and chemical pretreatment methods encompass a diverse range of techniques, including mechanical treatment (grinding or milling), extrusion, steam explosion, liquid hot water, organosolvents, ionic liquids, and ozonolysis. Mechanical pretreatment seeks to reduce particle size, thereby increasing surface area while decreasing crystallinity and polymerization grade [19]. Thermal pretreatment involves subjecting biomass to elevated temperatures, with liquid hot water and steam explosion processes reaching temperatures of approximately 230 °C and 260 °C, respectively [18]. Liquid hot water pretreatment entails heating water to high temperatures and pressures, while steam explosion is a physicochemical pretreatment method involving the exposure of biomass to high-temperature steam under pressure. The steam explosion process has been proposed as a cost-effective approach for lignocellulosic biomass degradation. However, it may partially degrade the xylan fraction and lead to the formation of inhibitory compounds during the process [20]. Extrusion, another physical pretreatment method, involves the passage of raw biomass through an extruder under conditions of high temperature and pressure, effectively disrupting the biomass structure [21]. The utilization of ozone treatment has demonstrated promising outcomes in the treatment of various biomass types, such as microalgae biomass, resulting in a significant enhancement in methane yield of up to 66% compared to untreated biomass [22]. However, it is important to note that these physical and chemical pretreatment methods often require specialized and costly equipment, and they may also demand substantial energy input. Moreover, these methods can potentially generate inhibitors, including 5-hydroxymethylfurfural (HMF), which may adversely affect subsequent fermentation processes [23]. Biological pretreatment offers an environmentally friendly and viable alternative approach with low energy requirements [24]. This approach involves the utilization of pure microorganisms, consortia, or enzymes to enhance the biodegradability of lignocellulosic biomass, consequently increasing biogas production. Of note, fungal pretreatment stands out as a low-energy method with minimal chemical demands, effectively reducing the generation of undesirable by-products [25]. Various fungi, including white, brown, and soft rot fungi, are used for this purpose, with white rot fungi exhibiting high hydrolytic capacity due to the production of lignin peroxidase, manganese peroxidase, and laccase enzymes. The methods for biomass pretreatment utilizing various technologies are shown in

Table 1. Technological solutions for biomass pretreatment to biogas production.

Methods of Pretreatment	Biomass	Biogas Yield	Parameters	Reference
Thermal	Kitchen waste	2540 mL·L−1	60 °C, 84 h	[26]
	Waste activated sludge	378 mL·g−1 ODS	90 °C, 60 min, 20 days	[27]
	Sludge	383 mL·g−1 VS	70 °C, 30 min	[28]
	Food waste	783 mL·g−1 VS	100 °C, 10 min, 45 days	[29]
	Food waste	383 mL·g−1 VS	100 °C, 30 min	[30]
	Sewage sludge	210 mL·g−1 VS	134 °C, 30 min	[31]
	Sludge	295 mL·g−1 VS	90 °C, 36 h	[32]
Physiochemical	Agave tequilana bagasse	0.26 L CH4 g−1 COD	32 °C, pH 5	[33]
	Corn stover	217.5 mL CH4 g−1 vs. 243 mL CH4 g−1 VS	37 °C for 28 days	[34]
	Wheat straw	305.5 mL CH4 g−1 VS	35 °C, 120 rpm	[35]
Biological	Corn stover silage	265.1 mLCH4 g−1 VS	37 °C, 30 days	[24]
	Corn stover	238.4 mL CH4	30 °C, 6 h	[36]
	Agropyron elongatum	169.2 mL CH4 g−1 V	28 °C, 28 days	[37]

.

In the context of lignocellulosic biomass pretreatment, the utilization of specific fungi such as Pleurotus ostreatus and Trichoderma reesei has shown remarkable potential. Notably, the application of these fungi in the pretreatment of rice straw demonstrated significant improvements in biodegradability. P. ostreatus, in particular, exhibited exceptional efficacy when used at a moisture content of 75%. This resulted in the removal of 33.4% of lignin and a remarkable increase in methane yield, up to 120% higher compared to the control without pretreatment [38]. Another intriguing biological approach involves the implementation of an aerobic upstream process utilizing Trichoderma viride, which led to a three-fold augmentation in methane yield from organic waste mixtures [39]. Furthermore, combined pretreatment strategies have been explored to maximize benefits. For instance, a combination of alkaline and enzymatic pretreatment was evaluated for cassava peels, aiming to enhance bioethanol production followed by biogas production. The combined pretreatment approach exhibited a remarkable improvement in methane yield, achieving a noteworthy 56% increase compared to the control [40]. These studies showcase the potential for biological pretreatment approaches, either using specific fungi or combined strategies, to significantly enhance the biodegradability and subsequent methane production from lignocellulosic biomass.

3. Biogas Production

Biogas production is the process of generating a combustible gas called biogas using the decomposition of organic materials in the absence of oxygen, a process known as anaerobic digestion. This renewable energy source is produced from various organic waste materials such as agricultural residues, food waste, animal manure, sewage sludge, and energy crops. Organic waste materials, such as crop residues, animal manure, and food waste, are collected and transported to the biogas plant. The collected feedstock undergoes pretreatment, which may involve sorting, shredding, and the removal of contaminants such as plastics and metals [41]. This step ensures that the feedstock is suitable for the anaerobic digestion process. The pretreated feedstock is loaded into an anaerobic digester, which is a sealed container where anaerobic bacteria break down the organic matter. This process occurs in the absence of oxygen and produces biogas as a by-product. The digester is typically maintained at a controlled temperature and pH level to optimize microbial activity [42]. During anaerobic digestion, the organic matter is decomposed by bacteria, resulting in the production of biogas. Biogas primarily consists of methane (CH₄) and carbon dioxide (CO₂), along with trace amounts of other gases such as hydrogen sulfide (H₂S), nitrogen (N₂), and water vapor (H₂O). The biogas produced in the anaerobic digester is collected and stored in a gas holder or storage tank. The gas holder allows for storage and balancing of the gas production, as well as providing constant pressure for subsequent use [43].

Anaerobic digestion has traditionally aided low- and middle-income countries, particularly rural economies, to sustainably manage biogenic waste and generate revenues and local employment while producing clean fuels to meet domestic energy demands. The utilization of biogas is similar to natural gas commonly used for cooking, heating, or as a gaseous fuel for vehicles. Mainly, it contains CH₄, CO₂, water vapor, and traces of N₂, NH₃, H₂, and H₂S. The energy content of biogas mainly depends upon the quantity of CH₄ present in it [44]. Hence, a high amount of CH₄ is always desirable. Care must be taken to avoid the water content and CO₂ and to minimize the sulfur content for the engines of vehicles, considering the pollution stage norms. Biogas has an average calorific value of 21–25 MJ/m³, while 1 m³ of biogas is equivalent to 0.5 L of diesel fuel with 6 kWh [45].

The biogas production rate of a plant depends upon the design, feedstock, temperature, and holding time [46]. For example, the common feedstocks used in anaerobic digestion are cattle manure and agricultural residues. Biogas plants can be broadly classified into two types: fixed dome plants and floating gas-holder digester plants. Biogas is generally used for cooking, where the gas is supplied through pipes to households from the plant. Biogas has been effectively used as a fuel in industrial high-compression spark-ignition engines. Biogas can also be effectively used as fuel in water heaters by completely removing H₂S during the supply of the gas [47]. Biogas production using selected biowaste resources is shown in

Table 2. Biogas production using selected biowaste resources.

Substrate	DM %	VS % of DM	Biogas Yield per Ton Fresh Matter (m3)	Methane Content (%)	Reference
Pig slurry	3–8	70–80	340–550	65–70	[48]
Cattle slurry	6–12	70–85	90–310	65	[49]
Poultry droppings	10–30	70–80	310–620	60	[50]
Wheat straw	85–90	90–95	200–300	50–60	[51]
Rye straw	85–90	90–95	200–300	59	[52]
Barley straw	85–90	90–95	250–300	59	[53]
Oat straw	20–25	90–95	290–310	59	[54]
Corn straw	30–40	90–95	380–460	59	[55]
Hemp	20–30	90–95	360	59	[56]
Grass	20–30	90–95	280–550	70	[57]
Elephant grass	20–30	90–95	430–560	60	[58]
Sunflower leaves	15–20	90–95	300	59	[59]
Agricultural waste	10–40	75–90	310–430	60–70	[60]
Slaughterhouse residues	35	90–95	550–650	58	[61]
Algae	20–40	90–95	420–500	63	[62]

.

3.1. Fixed Dome Biogas Digester Plants

A fixed dome biogas digester plant consists of a sealed, underground digester tank and a gas holder or dome. The digester tank is typically constructed using reinforced concrete or brickwork, ensuring durability and longevity. Organic waste, such as animal manure, agricultural residues, or kitchen waste, is fed into the digester tank. The anaerobic digestion process takes place within the digester, where microorganisms break down the organic material in the absence of oxygen, producing biogas and nutrient-rich digestate [59]. The fixed-dome design enables the digester to operate under constant pressure, allowing the gas to accumulate in the dome. As the biogas is produced, it displaces the slurry in the dome, causing it to rise. This rise in gas level provides the necessary pressure for the biogas to flow out through a gas outlet pipe. The gas can be collected and utilized for various applications, including cooking, heating, electricity generation, or fuel for vehicles [63].

Generally, the structure of a biogas plant is composed of brick and cement with five components such as a mixture tank, inlet tank, digester, outlet tank, and overflow tank. The mixing tank is located above ground level, whereas the inlet tank is kept underground in an inclined chamber, and the inlet tank is opened just below the large digester tank. The ceiling of the tank is kept as a dome-shaped structure, on which a long pipe is connected using an outlet valve for supplying the biogas. The digester opens from the bottom side of the outlet chamber, and the outlet chamber opens from the top side into a small overflow tank. A typical schematic for a fixed dome-type biodigester plant is shown in Figure 3.

At first, the biomass available in different forms is mixed thoroughly with an equal amount of water in a mixer tank to make a slurry to feed the digester through the inlet chamber. Induction of the slurry into the digester tank is stopped after partial filling, and the biogas plant is then intentionally left for two months without any use. Thereafter, the remaining anaerobic bacteria in the slurry decompose the biomass (fermentation) in water for several weeks to generate a high quantity of biogas, which is collected in the dome above the digester tank. As the collection of biogas increases, the exerted gas pressure increases, which forces the used slurry into the outlet chamber. This slurry overflows from the outlet chamber after it is filled, and then the slurry is manually removed and used to make manure. A gas valve, as shown in Figure 3, controls the supply of biogas using a pipeline connected to the dome. To maintain a continuous supply of biogas through the piping system, it is recommended to feed the slurry continuously to the digester [64].

3.2. Floating Dome Biogas Digester Plant

A floating dome biogas digester plant is a specific type of biogas system that utilizes a floating dome to store and collect biogas produced from the anaerobic digestion of organic waste. This technology has gained popularity, especially in rural areas, due to its simplicity, affordability, and ease of operation. The following are some key aspects and benefits of a floating dome biogas digester plan. The floating dome biogas digester consists of two main components: the digester tank and the floating dome. The digester tank is a sealed, underground, or semi-underground structure made of concrete or reinforced plastic, which acts as a container for the organic waste material and facilitates anaerobic digestion [56]. The floating dome, typically made of high-density polyethylene (HDPE), floats on the surface of the digester tank and captures the biogas produced during the digestion process. The organic waste, such as animal manure, crop residues, or kitchen waste, is fed into the digester tank. As the waste decomposes in the absence of oxygen, it produces biogas, primarily composed of methane and carbon dioxide. The biogas rises to the top of the digester, displacing the floating dome and accumulating underneath it. The dome’s buoyancy ensures that it remains airtight, preventing the escape of biogas and maintaining the pressure necessary for gas storage. Floating dome biogas digester plants offer a practical and sustainable energy solution for rural communities. With their low-cost construction, efficient biogas production, and additional benefits such as organic fertilizer production and waste management, these systems contribute to rural development, energy access, and environmental sustainability. Promoting the adoption of floating dome biogas digesters can lead to improved livelihoods, reduced environmental impact, and a cleaner, greener future for rural areas [60].

A common movable or floating dome-type biogas digester plant or gobar gas plant is shown in Figure 4. First, raw materials like cattle manure and water are mixed properly in the input tank, then the mixture is allowed to enter inside the large digester tank where digestion takes place biologically or anaerobically [65]. The biogas generated using the digester moves upwards and is stored in the gas holder tank or dome. The movement of the gasholder is restricted up to a particular level. With the collection of more and more biogas, greater pressure is likely to be exerted in the slurry. Then, the used slurry is forced from the top of the inlet chamber into the outlet chamber. When the spent slurry fills the outlet chamber, the excess quantity is forced into the overflow tank through the outlet pipe, which is used later on as high-quality manure for plants. A bent pipe is fitted at the top of the dome for biogas flow, which is controlled by opening and closing a gas valve, as shown in Figure 4. Once the production of biogas begins, a continuous supply of gas can be ensured with the regular removal of spent slurry and the introduction of fresh slurry into the inlet chamber. A practical use for such biogas is cooking alone since it requires waste material like cattle manure and other household wastes [66].

3.3. Modelling and Optimization of Anaerobic Digestion

Anaerobic digestion of biomass waste presents versatile possibilities, encompassing both mono-digestion for individual materials and co-digestion for mixtures comprising diverse components. The intricacies of biogas production during this process are governed by a multitude of influential factors, spanning the realms of inoculum composition, digester temperature, pressure dynamics, pH regulation, stirring duration, feedstock pretreatment, co-digestion strategies, loading rates, volatile matter proportions, carbon-to-nitrogen ratios, and hydro retention periods, among others. Co-digestion in anaerobic systems augments the overall digestion efficiency and energy generation by bolstering nutrient availability to microbial consortia and organic load, concurrently mitigating the toxic effects of inhibitory chemicals with co-substrate dilution. Notably, mechanistic models rooted in the well-established anaerobic digestion model No.1 (ADM1) framework have gained prominence within the realm of anaerobic co-digestion modeling. Nonetheless, several pivotal aspects pertaining to present-day anaerobic co-digestion paradigms, particularly the intricate interplay between system performance and co-substrate ratios, as well as the optimal biogas yield-determining properties, remain inadequately explored [67]. The field of anaerobic (co-)digestion has witnessed the application of diverse kinetic models, including the first-order kinetic model [68], Contois kinetic model [69], Haldane kinetic model [70], and dual pooled first-order kinetic model [68], each offering unique insights into the underlying processes [67]. Biogas production stands out among other biofuels due to its remarkable adaptability to a wide range of biodegradable substrates. These substrates include agricultural waste, Eichhornia crassipes (commonly known as water hyacinth), and municipal solid waste, which present abundant and untapped sources for biogas generation. Among the various factors that influence the activities, survival, and growth of microorganisms involved in biogas production, temperature assumes a paramount role [71]. The fundamental catalyst behind biogas formation is the presence of methanogenic bacteria and other microbial species within vegetable biowaste. Furthermore, vegetable waste exhibits a high carbon/nitrogen ratio, which facilitates efficient degradation during the digestion process, making it an ideal choice for researchers exploring suitable feedstocks [72]. The modeling and optimization of substrate treatment processes in biogas energy production were investigated by Onu et al. in 2022, delving into the intricacies of this crucial aspect [73].

The exploration of substrate treatment in biogas production involves the utilization of central composite design (CCD) for experimental design and intelligent modeling techniques such as adaptive neuro-fuzzy inference systems (ANFISs), artificial neural networks (ANNs), and response surface methodology (RSM) [73]. To delve into the kinetics of the anaerobic digestion process, five distinct kinetic models were investigated [73]. Notably, the ANFIS exhibited a slight advantage over the ANN in terms of simulating and modeling anaerobic digestion. By optimizing the ANFIS model, an impressive biogas volume of 356.24 mL was achieved, with specific concentration, time, and temperature parameters set at 0.04 N, 60 s, and 80 °C, respectively [73]. In recent times, anaerobic fermentation techniques have found application in various industrial sectors, particularly in elucidating the intricate interactions and individual impacts within the biochemical conversion process. This endeavor has been accomplished with the integration of response surface methodology (RSM) and artificial neural networks (ANNs) [74].

An artificial neural network (ANN) model is an advanced artificial intelligence model capable of unraveling intricate and multivariate industrial processes. By emulating the functioning of the human neural system, an ANN uses algorithms to stimulate, process, predict, and optimize responses in such processes. Its application has been demonstrated in various studies [75,76,77]. On the other hand, an adaptive neuro-fuzzy inference system (ANFIS) model represents a hybrid artificial intelligence model that combines the valuable attributes of neural networks and fuzzy logic. It excels in decoding complex industrial processes with minimal steady-state error [78]. Response surface methodology (RSM) offers several advantages, including the generation of precise empirical models that accurately predict the optimal response of industrial processes. It achieves this by simultaneously determining the ideal conditions of all process parameters, including their collaborative effects. Numerous studies have utilized RSM to enhance process optimization [73,79]. In the context of anaerobic processes, where feedstock degradability and optimal conditions vary based on composition, substrate composition assumes critical importance [80].

A comprehensive investigation was conducted, wherein representative samples of organic waste and sludge were meticulously procured from a campsite, subsequently blended in various proportions, and subjected to meticulous triplicate analysis to determine their potential for biogas generation. Extensive calorific tests were also meticulously performed to shed light on the energy content of the samples. A fascinating tangential advantage observed during the co-digestion process was the substantial reduction of total solid content in the mixture, effectively aligning it with the recommended levels for wet digestion, all achieved without the need for additional freshwater supplementation. To assess the latent capacity for methane production, cutting-edge technology in the form of the automated methane potential test system (AMPTS) and precisely calibrated graduated tubes within the controlled climatic chamber GB21 were used. Furthermore, the calorific values for the organic waste and sludge were diligently determined, accounting for both dry and wet bases. Notably, the highest biogas production achieved using the AMPTS method was recorded at 153 m³ ton⁻¹ for 100% organic waste and 5.6 m³ ton⁻¹ for 100% sludge, respectively [81].

Response surface methodology (RSM) and artificial neural networks (ANNs) were used to optimize crucial process parameters, including pH, thermophilic temperature (T), organic loading rate (OLR), and agitation time, which significantly influence biogas generation during the anaerobic digestion of vegetable waste (bio-waste) [72]. The ANN model is particularly valuable in developing bioprocess designs, relying entirely on data-driven approaches. Its multilayer architecture enables the approximation of nonlinear interactions between input variables and output-dependent variables. In this investigation, response surface methodology and artificial neural networks were used within the thermophilic temperature range to explore the optimal conditions for biogas and methane production using the anaerobic digestion of vegetable waste [72]. Additionally, Abdel daiem et al. [82] explored the application of the nonlinear autoregressive exogenous (NARX) neural network and seagull optimization algorithm (SOA) in their research. Their findings revealed that anaerobic co-digestion significantly improved the carbon/nitrogen (C/N) ratio, increasing it from 6.64 to 17.85, while also enhancing biogas production by 350% at a 2% mixing ratio compared to mono-sludge digestion [82]. The modeling outcomes demonstrated that the proposed NARX neural network exhibited precise predictions for digested sample characteristics and biogas production [82].

A comprehensive techno-economic assessment was conducted to investigate the production of hydrogen from biogas. Specifically, two widely recognized technologies, namely, steam reforming (SR) and autothermal reforming (ATR), were examined for their feasibility and economic viability. The analysis focused on two types of biogases: one derived from landfill sources and the other generated using anaerobic digestion [83]. To determine the optimal hydrogen purification and recovery system, the study compared a pressure swing adsorption (PSA) system with two and four beds, along with a vacuum PSA (VPSA) system consisting of four beds. Using a rigorous evaluation, it was found that the VPSA system outperformed the PSA systems in terms of purity and recovery rates [83]. Remarkably, the VPSA system achieved a remarkable recovery range of 50% to 60% when operated at a vacuum pressure of 0.1 bar. Moreover, it demonstrated an exceptional hydrogen purity of 99.999%. These findings highlight the superior performance of the VPSA system in terms of both recovery efficiency and hydrogen quality, making it the preferred choice for hydrogen production from biogas in that investigation [83].

Over the course of the last few decades, substantial scientific attention has been dedicated to unraveling the immense possibilities offered by algal biomass as a prominent reservoir for liquid and gaseous biofuels. The consensus among researchers is resolute, acknowledging the promising trajectory of algae as a feasible aquatic energy crop that surpasses the energy potential of both terrestrial biomass and municipal solid waste. Nonetheless, the prevailing reality remains that neither microalgae nor seaweed are presently cultivated exclusively for energy production, primarily due to the exorbitant expenses associated with the laborious processes of harvesting, concentrating, and drying. This financial impediment serves as a pivotal barrier, restraining the widespread adoption of algal biomass as a dedicated and cost-effective source of energy [84].

4. Advantages and Challenges of Biogas Plants

Biogas plants have several advantages and challenges associated with their implementation [85].

4.1. Advantages of Biogas Plants

Renewable Energy Source: Biogas is a renewable energy source produced from the breakdown of organic matter such as agricultural waste, animal manure, and sewage. It provides an alternative to fossil fuels and helps reduce dependence on non-renewable energy sources.

Climate Change Mitigation: Biogas production reduces greenhouse gas emissions, particularly methane, which is a potent contributor to climate change. By capturing methane from organic waste and using it as a fuel, biogas plants help to mitigate global warming.

Waste Management: Biogas plants provide an effective waste management solution by converting organic waste into biogas and digestate. This helps to reduce the volume of waste sent to landfills or incineration, minimizing environmental pollution and the release of harmful substances.

Energy Independence: Biogas production provides a localized energy source, reducing dependence on imported energy and increasing energy security. It offers opportunities for decentralized energy generation, especially in rural areas, where biomass resources are abundant.

Agricultural Benefits: The digestate produced as a by-product of biogas production is a nutrient-rich organic fertilizer. It can be used to improve soil quality, increase crop yields, and reduce the need for synthetic fertilizers, thus promoting sustainable agricultural practices.

4.2. Challenges in Biogas Plants

Feedstock Availability and Quality: Biogas plants require a consistent supply of organic waste as feedstock. Ensuring a steady supply of high-quality feedstock can be a challenge, especially in areas where waste collection infrastructure is inadequate or where competition for biomass resources exists.

Investment and Operational Costs: Establishing and operating a biogas plant involves significant upfront costs, including construction, equipment, and maintenance expenses. The economic viability of biogas projects depends on factors such as feed-in tariffs, government incentives, and energy market conditions.

Technology and Infrastructure: Biogas production requires appropriate technology and infrastructure for the anaerobic digestion process. Designing and implementing efficient biogas systems, including feedstock handling, digesters, gas purification, and storage, can be technically complex and may require skilled personnel.

Regulatory and Policy Framework: The regulatory and policy environment can influence the development of biogas plants. Inconsistent or inadequate policies, permitting procedures, or grid connection requirements may pose challenges to the growth of the biogas sector and hinder its integration into the existing energy infrastructure.

Environmental Impacts: While biogas production offers environmental benefits, there are certain potential impacts to consider. These include odors and emissions from the anaerobic digestion process, the transport of feedstock and digestate, and the potential release of air pollutants if gas purification systems are not properly installed or maintained.

Overall, biogas plants have the potential to provide renewable energy, contribute to waste management, and promote sustainable agriculture. However, addressing the challenges associated with feedstock availability, costs, technology, regulations, and environmental impacts is crucial to maximizing the benefits of biogas production [86].

5. Applications of Biogas

The escalating environmental challenges and rapid depletion of conventional energy sources necessitate the exploration of sustainable alternatives. Biogas, as a clean and renewable energy option, emerges as a promising substitute. Unlike traditional energy sources that contribute to ecological and environmental problems, biogas offers a sustainable solution while mitigating adverse ecological impacts. Its renewable nature ensures a continuous supply, reducing dependence on finite resources that are rapidly depleting. By harnessing the potential of biogas, we can address the urgent need for a cleaner and more sustainable energy paradigm. The global biogas production capacity is shown in Figure 5.

Biogas production using the anaerobic digestion process offers a versatile solution for converting a wide array of solid or liquid waste materials into a valuable energy resource. Under anaerobic conditions, diverse organic substances undergo degradation by microorganisms, culminating in the generation of energy-dense biogas. This biogas holds significant potential for diverse applications, including the production of vehicle fuel, electric power, and heat. Furthermore, the organic material utilized in the anaerobic digestion process retains essential plant nutrients within the resulting digestion residue, known as digestate, thereby serving as a valuable plant fertilizer. Despite its inherent advantages as a renewable energy source, the widespread adoption of biogas encounters formidable challenges due to the persistent competition posed by readily available conventional energy sources, which often appear to be more economically viable in the short term. To establish biogas as a dominant renewable energy option, it is imperative to address the existing barriers and highlight the long-term benefits and sustainability it offers [87].

Biogas, as an environmentally friendly fuel, holds promising potential for future vehicle applications, thereby reducing reliance on conventional fuel imports. However, the existing biogas generation technologies often yield limited amounts of gas over prolonged periods with a high percentage of impurities. Undesirable incombustible gases such as CO₂, H₂S, and water vapor diminish the calorific value of biogas, rendering transportation and compression challenging. To obtain biogas with a methane content exceeding 90%, various processes such as adsorption, absorption, cryogenic techniques, and membrane separation can be used. Notably, purified biogas and base compressed natural gas (CNG) exhibit equivalent carbon emissions. Biogas can be safely blended with CNG for use as an engine fuel, offering satisfactory engine performance and reduced emissions. The conversion kit required for CNG enables vehicles to run smoothly on biogas without significant modifications. While raw biogas possesses a lower calorific value compared to existing fuels, its quality can be enhanced using purification methods and blending with CNG. Within the realm of biogas generation, anaerobic digestion stands out as a less energy-intensive process, renowned for its effectiveness in converting organic materials into biogas [88].

In the realm of green and renewable energy solutions, biogas emerges as a highly promising option for power generation in diverse settings, including rural areas, households, and industrial sectors. Its utilization entails relatively lower capital investment and production costs, making it economically feasible. By harnessing agricultural crop residues and other domestic biomass sources as raw materials, biogas can be effectively generated for various applications, such as driving engines for generators and pump sets. However, it is important to note that the composition of biogas is subject to variations based on the specific raw materials used. One particular challenge arises from the higher concentration of carbon dioxide present in biogas, which can lead to combustion variations that may impact engine durability and performance. Hence, careful attention must be given to optimizing the biogas composition to ensure efficient and reliable utilization in power generation systems [89]. A comparison between the properties of various fuel sources and biogas is shown in

Table 3. A comparison between the properties of common biofuels and biogas.

Characteristic	Biogas	Hydrogen	Methane	Gasoline	Butanol	Ethanol	Methanol
Formula	NA	H2	CH4	H, C4–C12	C4H9OH	CH3CH2OH	CH3OH
Molecular weight (kg/kmol)	Mixture *	2.02	16.04	100–105	74.123	46.07	32.04
Auto-ignition temperature (°C)	650 *	585	540	280	343	365	435
Air fuel ratio	11 *	34.3	17.19	14.6	11.2	9.0	6.5
Higher heating value (kJ/g)	23.1	141.9	55.5	47.5	37.3	29.7	20
Motor octane number	130 *	-	120	81–89	78	102	104

* CH₄—60%, CO₂—30%, CO—0.18%, H₂—0.18%.

.

The integration of a direct-biogas solid oxide fuel cell (SOFC) with a micro-gas turbine (MGT) system presents a promising avenue for the development of a decentralized combined heat and power (CHP) system with significant environmental benefits. To assess the feasibility of utilizing biogas as the primary energy source for a direct-biogas SOFC-MGT hybrid CHP system, a sensitivity analysis was performed under various operating conditions. The objective was to examine the impact of key operating parameters on the performance of the hybrid CHP system while also considering operational constraints. Using this analysis, the researchers aimed to understand the intricate interplay between different factors and their influence on the overall system performance. Factors such as fuel composition, operating temperature, and electrical and thermal loads were carefully evaluated to assess their effects on system efficiency and reliability [90]. By conducting this sensitivity analysis, valuable insights were gained regarding the optimal configuration and operating conditions for the direct-biogas SOFC-MGT hybrid CHP system. The findings from this cited study will contribute to the advancement and effective deployment of environmentally friendly and sustainable energy systems, paving the way for greener decentralized CHP solutions [90].

A novel integrated energy recovery system was devised to facilitate the manipulation of feedstock and the regulation of digester operating parameters, ensuring sustainable biogas production in continuously fed digesters. This advanced system encompasses a range of meticulously designed components that have been seamlessly integrated into a cutting-edge energy recovery system. The system has undergone prototyping and subsequent deployment in a real working environment to conduct pilot studies. Notably, the configuration of the system enables the precise measurement of feedstock moisture content. Based on this measurement, the system calculates the optimal amount of water required to be added to the feedstock, subsequently displaying this information to the operator. This innovative approach ensures that the substrate quality is optimized, enhancing the overall efficiency and effectiveness of the biogas production process [91].

6. Biogas as Energy in Rural Sector and Significance

Biogas is increasingly being recognized as a valuable energy source in the rural sector. In rural areas, where access to traditional energy sources may be limited, biogas provides an efficient and sustainable alternative. It is produced by the breakdown of organic matter, such as animal manure, crop residues, and kitchen waste, through a process called anaerobic digestion. This decentralized energy solution has several advantages for rural communities. Firstly, biogas can be used for cooking, heating, and lighting, addressing the energy needs of households and reducing reliance on firewood or fossil fuels. This improves indoor air quality, reduces deforestation, and decreases greenhouse gas emissions. Secondly, biogas production helps in managing organic waste effectively [92]. By converting waste into biogas, it prevents the release of harmful methane gas into the atmosphere, mitigating climate change. Additionally, the by-product of the biogas production process, known as digestate, is a nutrient-rich organic fertilizer that can enhance soil fertility and agricultural productivity. Biogas plants can also create job opportunities and support local economies in rural areas. Overall, biogas holds great potential to promote sustainable development, improve energy access, and contribute to environmental stewardship in the rural sector [93].

Biogas is an excellent source of renewable energy in the rural sector. It is produced through the anaerobic digestion of organic materials such as agricultural waste, livestock manure, and food scraps. Biogas is primarily composed of methane (CH₄) and carbon dioxide (CO₂), with small amounts of other gases. When biogas is captured and stored, it can be used as fuel for cooking, heating, and electricity generation. The methane content makes it a valuable energy source. Biogas production offers a sustainable solution for managing organic waste generated in rural areas. Agricultural residues, livestock manure, and food waste can be diverted from landfills and instead used as feedstock for biogas production. This helps in reducing greenhouse gas emissions, controlling odors, and minimizing the environmental impact of waste disposal. In many rural areas, traditional cooking methods rely on biomass fuels like wood and charcoal, which can have adverse health and environmental effects [94]. Biogas can replace these traditional fuels, providing a cleaner and more efficient cooking solution. It reduces indoor air pollution, minimizes deforestation, and saves time and effort in fuel collection. Biogas can be used for decentralized electricity generation in rural areas. Small-scale biogas plants can produce electricity that powers rural households, schools, and healthcare centers. This helps to bridge the energy access gap and improve the quality of life in rural communities. Biogas production generates a nutrient-rich by-product called digestate [44]. This digestate can be used as an organic fertilizer, replacing chemical fertilizers in agricultural practices. It helps improve soil health, enhance crop yields, and reduce dependence on synthetic fertilizers, thereby promoting sustainable agriculture. Biogas production in rural areas can create economic opportunities by generating income and employment. Farmers and rural communities can benefit from selling excess biogas to the grid, providing maintenance and operational services for biogas plants, and producing and marketing digestate as a valuable organic fertilizer. Overall, biogas offers a sustainable energy solution for rural areas by providing clean cooking fuel, decentralized electricity, waste management, and agricultural benefits. Its utilization can contribute to rural development, environmental sustainability, and energy access for rural populations [95]. Biowaste materials act as sustainable sources for various bioenergy purposes [96,97,98,99,100]. Various technological interventions and studies were conducted on waste biomass utilization for bioenergy production [101,102,103]. Anaerobic digestion systems offer significant advantages to developing nations due to their cost-effectiveness relative to alternative technologies, simplicity, low maintenance requirements, and inherent safety. These systems not only provide a reliable source of fuel but also contribute to improved public health and sanitation. Additionally, they alleviate the labor-intensive task of collecting substantial amounts of firewood, thereby enabling individuals to engage in other productive activities. Consequently, biomass-based energy systems play a pivotal role in facilitating rural development.

The implementation of biogas in rural areas also yields environmental benefits. By replacing wood fires, biogas helps curb deforestation rates and mitigates the associated emissions. Furthermore, biogas serves as a clean and renewable energy source, diminishing reliance on fossil fuels. From a chemical perspective, biogas shares similarities with natural gas; however, it differentiates itself as a renewable fuel, whereas natural gas is derived from fossilized sources. If organic waste is left untreated, the methane it contains would be released into the atmosphere through natural processes, whereas the greenhouse gases in natural gas would remain trapped underground. By utilizing biogas as a fuel source, the amount of methane emitted from open decomposition processes is significantly reduced. The utilization of biogas not only addresses energy requirements but also tackles sanitation challenges. The establishment of a biogas system in rural households is considerably less complex compared to other alternatives. This approach presents a viable solution to both energy and sanitation issues, offering a straightforward and practical method for rural communities.

7. Limitations and Future Work on Biogas

Biogas technology represents a highly promising global venture, primarily attributable to its well-established production technologies and applications, as well as its potential for future advancements. Though biogas technology is regarded as a clean method of bioenergy, it contains many bottlenecks such as low technological advantages, impurities, the effect of temperature on biogas, and lower availability in urban areas. The viability and sustainability of biogas technology include the abundant availability of cost-effective feedstocks, the wide range of applications in various sectors, and utilization as raw materials for the production of sustainable chemicals like hydrogen and biofuels. The flexibility of biogas production, ranging from small-scale to large-scale industrial digesters, combined with the diverse range of feasible feedstocks, enables global biogas production regardless of geographic location. The global production and utilization of biogas are on the rise, positioning it as a leading economically viable alternative for renewable bioenergy production [104].

Biogas stands out as a versatile fuel due to its capacity to generate lower greenhouse gas emissions and its renewable nature, being derived from renewable sources. Moreover, biogas production offers the added benefit of treating and reducing excess organic waste while effectively disinfecting pathogens present in biomass. The wide portfolio of energy applications for biogas includes electricity, heat, and cooling applications. Enhancing biogas yield from biomass can be achieved using appropriate pretreatment of the substrate and careful monitoring of digestion parameters such as the carbon-to-nitrogen ratio, temperature, and substrate dilution. The utilization of biogas as a fuel presents significant opportunities for both direct and indirect use in electricity and heat production, contributing to a sustainable energy transition.

Multiple technologies are available for converting biogas into electricity, with trigeneration and combined heat and power systems exhibiting higher conversion efficiencies, while fuel cells offer superior system reliability. Other identified and proposed technologies are small gas turbines, gasoline engines, Stirling engines, biomethane conversion, biofuel processing, and hydrogen production. Biogas plays a pivotal role in substituting fossil fuels for electricity generation and thermal applications. Its integration into grid electricity generation and supply is particularly relevant in decentralized systems, where investors function as both producers and consumers (referred to as prosumers), thereby contributing to the economic sustainability of the global energy transition [105]. From an environmental perspective, the utilization of biogas helps reduce global greenhouse gas emissions, mitigating the threats associated with global warming and climate change. Furthermore, biogas utilization plays a crucial role in preventing detrimental environmental and health impacts arising from the vast quantities of agricultural waste available worldwide. Realizing the sustainable exploitation of biogas energy resources necessitates investment and promotion in both biogas production using technological advancements and investment, as well as creating demand using measures that encourage its consumption within a competitive energy market, where nonrenewable options are often more cost-effective.

8. Conclusions

Biogas has the potential to revolutionize the energy landscape in the rural sector. Its ability to utilize locally available organic waste, environmental benefits, versatile energy applications, agricultural advantages, and economic empowerment opportunities make it an attractive solution for rural communities. Governments, NGOs, and private sector entities should work together to promote biogas use, provide technical support, and facilitate financing options to ensure its widespread implementation. By embracing biogas as an energy source, rural areas can achieve sustainable development, improve living standards, and contribute to a greener future. While the sustainable utilization of biowaste resources for biogas production is promising, challenges such as feedstock availability, technical complexity, and initial investment costs need to be addressed. Further research and development are required to improve digester performance, optimize feedstock utilization, and develop cost-effective technologies for small-scale biogas plants.