Biogas Production and Process Control Improvements

Abstract

Biogas is a sustainable energy produced from biodegradable organic matter through anaerobic digestion. Biogas mainly contains methane (CH4) and carbon dioxide (CO2) and various contaminants, such as water vapor (H2O), ammonia (NH3), hydrogen sulfide (H2S), methyl siloxanes (MSs), nitrogen (N2), oxygen (O2), halogenated volatile organic compounds (VOCs), carbon monoxide (CO) and hydrocarbons whose presence largely depends on the source of biogas. The value of biogas is enhanced further by the production of organic manure as substitutes for chemical fertilizers in crop production. The vast range of applications makes biogas an attractive sustainable energy choice. Bioenergy generation capacity, mainly from biogas globally, has shown promising worth over non-renewable energy options; hence, its production has expanded rapidly during the previous two decades, from 42.38 Exajoules/year in 2000 to about 57.74 Exajoules/year in 2023, an increase of about 36.24%. This study presents the biogas production and potential improvement measures that include factors influencing the process and quality of biogas produces such as C/N (carbon nitrogen ratio, the pH, dilution of feedstock, composition and nutritive value of feedstock, residence period, mixing and stirring, temperature, presence of toxicants, loading time and redox conditions). The main barriers to biogas production and use include lack of technical know-how, lack of subsidies and incentives for biogas production and use. Access to appropriate and affordable biogas technologies, the development of a comprehensive policy on the construction and operation of bio-digesters as well as utilization of biogas and sales of biogas energy products and services are important for an increasing adoption of biogas.

1. Introduction

Anaerobic digestion is an attractive biomass-to-energy and waste management technology due to the significant presence of feedstock in form of the organic wastes, food wastes and energy crops, and technology maturity [1]. Biogas is an important renewable energy resource whose production and use can aid in the mitigation of greenhouse gas (GHG) emissions by substituting fossil fuels and promoting the utilization of biodegradable plant and animal waste feedstocks [2, 3]. With the expected depletion of coal, gas, and oil within the next decade and a half, renewable energy sources like biogas are becoming increasingly important [4]. Another motivation for the use of biogas as an energy resource is the international treaties or agreements that commit countries to reduce their carbon footprint like agenda 21 and Kyoto Protocol [5, 6]. These mechanisms advocated for the transition to renewable and low-carbon sources of energy to reduce global greenhouse gas emissions, particularly from the energy sector which is dominated by fossil fuel sources of energy [7, 8].

Biogas is a viable option to the worldwide energy dilemma and has enormous promise as a sustainable and renewable energy source for commercial, industrial, and residential use [9, 10]. Because of rising environmental concerns and measures in response to concerns over greenhouse gas emissions and climate change, there has been growth in interest in using biomass resources as a renewable feedstock for electricity generation, fuel production, chemical processing, and hydrogen production [11, 12], resulting in increasing energy generation from organic waste production as it reduces the threat to climate change [13]. Electricity generation, thermal applications like cooking, heating, and lighting, and biofuel production are the primary uses for biogas. Biogas is used to produce over 7000 megawatts (MW) of electricity annually [14, 15].

Many developing countries still face the challenge of providing modern energy services to their populations e.g., India alone has about 450 million people with no access to modern energy for electricity [16, 17], yet they have huge biomass resources [18]. There is substantial potential for biomass to play a leading role in global energy generation is evidenced by the 9.7% annual growth rate of biofuels since 1990 [19, 20]. In this context, the role of biogas energy within this huge biomass supply chain cannot be overemphasized. Beyond providing food for all of humanity, agriculture is the primary source of income for more than two-thirds of the world’s population. Additionally, bout 82% of the global population is directly or indirectly involved in smallholder agriculture and other industries, making this economic activity the mainstay of many countries [16, 21]. This shows that biogas is important as a renewable and sustainable energy resource for the energy transition and that agriculture and other carbon transformative processes should boost its production and consumption [22, 23].

Biogas has a significant role to play in the ongoing energy transition as a renewable and sustainable energy resource because of its high energy conversion potential and the widespread availability for power generation, industrial applications as a renewable process feedstock and for thermal energy applications [24, 25, 26, 27, 28]. Besides biogas production, the anaerobic digestion (AD) process can act as a waste treatment and disposal method while the digestate produced is a valuable organic fertilizer that can replace chemical fertilizers in sustainable agriculture [29, 30]. Anaerobic digestion is an important process in global carbon cycle which produces biogas through biodegradation of organic matter that releases 590–800 million tons of methane into the atmosphere annually which can be mitigated through controlled anaerobic digestion that produces useful biogas [25, 31, 32]. Numerous different organic feedstocks are anaerobically digested to produce biogas, which can then be upgraded to produce biomethane which is an important substitute of natural gas. The sustainable technique of turning biomass waste into biogas can lead to advantages including less carbon emissions, improved organic waste management, and increased resource use efficiency [25, 32].

Biogas has the potential to contribute to sustainable fuel for multiple applications as a fuel characterized by a high methane content, which is generated through the process of anaerobic digestion of organic substrate under carefully regulated biochemical process conditions [33, 34]. Indigestible carbohydrates, proteins, and lipids are not present in the biomass substrate utilized for biogas production, which can complicate or slow down the digestion [35, 36]. Biogas can also be used as a feedstock for production of biomethane, carbon dioxide, hydrogen, and various biofuels for a wide range of applications including generation of heat and power and feedstock for various industrial processes [20, 37].

In the period preceding the 1800s, man predominantly relied on biomass as its primary source of energy [38, 39]. With the beginning of the industrial revolution, the energy landscape underwent a significant change that was typified by a predominate reliance on fossil fuels, mainly coal, and later petroleum derivatives like natural gas and diesel. A major increase in oil prices was present between the early 1970s to the late 1980s. This situation made it necessary to investigate and promote ecologically friendly, sustainable, and renewable energy sources. During this time, biogas became one of the most popular choices [40, 41, 42]. According to [43], in order to mitigate environmental degradation and reduce greenhouse gas emissions, it is imperative to engage in environmental conservation practices and adopt proper waste management strategies. Rural communities, comprising a significant proportion of the populace in numerous developing countries and primarily involved in subsistence agriculture, possess the capacity to generate biogas like in India, where approximately 70% of the populace resides in rural regions [17, 44], whereas on a global average basis, the enormous energy potential of biogas is largely unrealized. A sizeable section of the population works in small-scale agriculture in rural areas of many developing countries. These people have the opportunity to use biogas technology to meet a significant amount of their energy needs [45, 46].

There is considerable potential of biogas as a sustainable energy source, both currently and in the foreseeable future, is significant. Despite the significant potential of biogas to make a substantial contribution to the achievement of sustainable development goals and the global transition toward sustainable energy due to its minimal greenhouse gas emissions, widespread availability, and access to raw materials, its incorporation into the global energy and electricity mix has been constrained [20, 44]. Therefore, it is crucial to initiate a change in the current circumstances [7, 47, 48, 49]. The overall objective of this study is to investigate the technological progress and possible uses of biogas in promoting a sustainable worldwide transition and attaining the targets specified in the sustainable development goals. The primary objective of this study is to evaluate the feasibility of incorporating biogas into the existing power grid and other energy applications as a means of reducing emissions. Furthermore, this study presents a comprehensive strategy for the transformation of biogas into electrical energy within the context of a large-scale energy transition. This measure will contribute to mitigating the dual challenge presented by the increasing levels of greenhouse gas emissions and the consequent climate change, in accordance with the emissions and climate goals outlined in the Paris Agreement. This research investigates and evaluates different alternatives, approaches, and feasible technologies to produce electricity using biogas, with a specific emphasis on its implementation in agricultural and industrial contexts. This chapter provides discussion on the potential, prerequisites, and obstacles linked to the production of biogas and the processes to improvement. Furthermore, this study offers an examination of the biogas potentials that can be employed for the purpose of generating heat and electricity [31, 41, 42, 46, 50].

1.1 Problem statement

The heightening need for food and energy resulting from the growth of the global population, particularly in developing countries, has intensified the strain on energy production and consumption. This strain is currently associated with the release of greenhouse gases and the subsequent impact on climate change [16]. Although biogas has a great deal of promise to supply clean energy to both rural and urban populations, the prevalence of inoperable and abandoned biogas facilities has raised concerns about the sustainability of the technology. This poses questions concerning the dependable creation, distribution, and use of biogas fuel [17]. It is technically possible to convert this waste into valuable energy sources, thereby generating additional revenue [34, 51, 52].

Sustainable electricity generation is crucial. Hence, sustainable farm-level biogas electricity generation requires appropriate infrastructure design and selection due to susceptibility to biogas system failures [53, 54]. Another thing to consider in such system design is the natural decomposition of agricultural waste that results in the emission of substantial amounts of methane, a potent greenhouse gas, into the atmosphere. Take for instance, in the year 2015, the exclusive contribution of livestock manure to methane emissions in the United States of America amounted to approximately 10%. However, a mere 3% of the total livestock waste underwent recycling via anaerobic digestion [34]. This presents the main issue in utilizing as it lies in its unsteady production value and variations in quality. These factors can potentially disrupt the generation process or hinder the effectiveness of biogas applications, resulting in reduced reliability [55].

This study presents the biogas production, process improvements parameters, and its contribution toward promoting sustainable energy generation for local applications, such as in agriculture, as well as its potential for facilitating the transition of grid electricity toward renewable energy sources. This study focuses on the examination of optimal and sustainable methods for producing biogas. It also proposes sustainable approaches for generating electricity from biogas and producing biofuels such as biohydrogen, methanol, and syngas [42, 56, 57].

1.2 Rationale of biogas production

The main challenges facing biogas production and use as an energy resource include high equipment cost and a lack of governmental incentive programs in many countries, low reliability or lack of guarantee of long term performance of biogas plants due to technological challenges, unpredictable investment environment, limited biogas distribution and storage capacity for some countries like Sweden and low cost of commercial fertilizers as well as low cost of fossil fuels like gas and oil in some countries [58]. A study by Huang et al. [2] on household biogas digester use in rural China showed that there was rapid growth in the early part of the century in the use of household biogas digesters but the operations were never smooth because out of 1743 households interviewed, 42% adopted household biogas digesters, but on average they worked for just 6.66 months each year which is an indicator of underutilization of the digestors.

Both developed and developing countries face challenges with the utilization of significant organic waste and pressure to replace fossil energy resources like natural gas, oil, and coal with renewable energy sources. Anaerobic digestion of organic biomass is now a mature technology while biogas in its raw form or cleaned and upgraded forms have multiple applications as a bioenergy substitute of fossil fuel sources. The production and utilization of biogas have also been growing with capacity growing by more than double between 2009 and 2022 [59]. Methane fermentation occurs naturally in the process of organic matter decay in oxygen-deficient environments like swamps and in landfills leading to the natural emission of methane to the atmosphere as a greenhouse gas. Controlled biogas production can be used to reduce these natural emissions [21, 60].

The human population is steadily growing leading to increased demand for food and energy which simultaneously aggravates the environmental challenges. Substituting fossil fuels with renewable energy alternatives is currently a major global issue of the 21st century and is a key sustainable development objective. Implementation of biogas technologies will significantly transform costly, socially sensitive issues and environmentally damaging fossil fuel dependence, environmental pollution, GHG emissions, and waste conversion into profitable options like electricity generation, heat production and production of biofertilizers. As well as the provision of green vehicular fuel substitute of fossil gas. A national biogas production development program can significantly lead to development of new enterprises, boost income for rural communities and create new jobs to improve people’s socio-economic wellbeing [59].

The implementation of biogas technology enables the harnessing of renewable energy through the utilization of crop and animal waste. The energy source exhibits significant potential for utilization in electricity generation and as a viable thermal energy solution for residential structures. The ultimate outcome of this process can be efficiently harnessed as a valuable fertilizer in rural agricultural environments and households, leading to a reduction in costs related to waste management and disposal [43]. Biogas energy can improve rural people’s lives and economies. Many small-holder farmers in developing nations burn biomass for disposal, even though it can be used as a fertilizer, and with biogas technology, an impoverished organic fertilizer that are more affordable can be made by smallholder farmers [61]. Designing biogas systems so that they can reliably supply most or 100% of their energy demands requires defining a number of factors [45]. The challenges limiting full use of biogas that should be overcome, include lack of national generating technologies, need to clean biogas before use, the challenge economic feasibility that requires incentives, the lack penalties for possible environmental damages from biogas schemes [58, 62].

2. Biogas production

The capacity of biogas plants globally has been on a growth trajectory in many countries because of factors like rising cost of fossil fuels, the easy and affordable supply of biomass feedstock, and growing concerns about emissions and the threat of a global climate emergency. The primary waste sources used in the production of biogas include, but are not limited to, home wastes such food, produce, and fruit as well as animal wastes like dung and bird droppings. Public moist wastes from markets, cafes, restaurants, and food outlets are also frequently used in this procedure, along with biodegradable trash from businesses with high moisture content and degradability. The use of anaerobic digestion to produce biogas acts as a catalyst for improving a country’s energy mix while also significantly advancing the preservation of natural resources and defending the environment [30, 32, 48, 49, 63, 64, 65].

Under optimal conditions, a specific group of bacteria can generate biogas through anaerobic processes having a calorific value ranging from 21 to 24 MJ/m³, thereby establishing it as an environmentally sustainable energy resource [22]. Biodegradation of organic matter generates between 590 and 800 million tons of methane per year, which is then released into the atmosphere without human involvement. Biogas, a byproduct of biomass decomposition, can be recovered and converted into usable energy by use of such devices [6, 22]. The composition of biogas can range from 50 to 70% methane and 30 to 50% carbon dioxide, depending on the substrate employed and the management and regulation of the production process. Nitrogen and hydrogen sulfide are among the extra components that are present. Gas distribution systems can be fitted with enriched biogas from larger plants. The metabolic process that ultimately produces methane and other gases from biomass relies on the action of a suite of bacteria that exists as at least three bacterial communities employed in biogas production systems [22, 66]. The utilization of anaerobic digesters usually occurs in the mesophilic (20–40°C) and thermophilic (above 40°C) temperature ranges [22, 66], depending on the substrate fed into the digester, biogas typically consists of 30–60% methane and 30–50% CO₂. Hydrogen gas is also present in biogas although in much smaller amounts.

Large biogas plants can generate biogas for supply into gas supply networks or mains, with a typical heating value of 21–24 MJ/m³ or 6 kWh/m³. This makes it useful for a wide range of applications, including cooking, heating, lighting, and electricity generating [22, 67, 68]. Over a long period of time, biogas has been used as fuel product anaerobic digestion of waste for sanitation. It is also typical to use biogas plant digested waste as a fertilizer [29, 69].

2.1 Global development of biogas energy resources

About 0.25% of the world’s energy came from biogas in 2011, while biogas accounted for 27% the global biofuel market [35, 70]. Income and standard of living in rural areas can be improved through biogas production and use in two ways by selling electricity derived from to the grid and increasing agricultural productivity by application of organic manure/digestate. Not only may biogas be used for the aforementioned purposes, but it can also be converted into diesel fuel to power appliances in places like houses and classrooms [70, 71]. Biogas can be upgraded to biomethane, and utilized more efficiently as a transportation fuel, a natural gas replacement in commercial and residential settings, and as a feedstock for natural gas distribution networks. Feedstock for greenhouses and a raw ingredient for chemical fuel production, carbon dioxide can be derived from biogas [72]. In 2005, there were around 16 million small household biogas digesters in use worldwide, with China and India accounting for the great majority. A total of 16 million tons of firewood were replaced by biogas in India in 1996, and it met 4% of China’s energy requirements at the time. Around 7 million biogas digesters did the same in the USA [70]. The efficiency of using biogas can be greatly increased through cogeneration, which simultaneously produces heat and power. Excess electricity can be sold to the grid to help stabilize the grid and lessen the consequences of global warming by displacing the generation of energy from fossil fuels [47].

Europe has been a leader in the development of biogas technologies from urban trash. More than 14% of Europe’s municipal solid waste is converted into biogas by the region’s more than 70 active anaerobic digesters [73, 74].

2.2 Challenges of biogas development

The technology for producing biogas from biomass is currently fully developed, but there are several obstacles preventing its adoption. The obstacles include general ignorance about biogas and its application among the very people who can benefit most and lack of a regulatory framework to support the installation of biodigesters and lack of necessary incentives [75, 76]. Statistics show that many biogas digesters are underutilized, with some even laying unused soon after being built [74]. Lack of technical management skills, together with economic and cultural factors, contributes to this. In 2002, during the World Summit on Sustainable Development, it was acknowledged that many countries may benefit from using renewable energy to alleviate poverty [2, 77].

The global biogas production capacity by volume is over 59 billion m³ biogas or about 35 billion m³ methane equivalent, but the European Union alone accounts for about half of the total capacity. This shows that many developing and some developed countries have unsatisfactory contribution and have a long way to go in biogas production and use compared with developed counterparts [76]. The biggest barrier to the widespread use of biogas technology is the high expenses incurred in digester operation and maintenance. Complex biogas digester designs and a lack of technical knowledge are further obstacles [67, 78]. The key issues with the adoption of technology of biogas are discussed below:

1. Poor maintenance and management

   The efficient operation of a biogas system requires the adoption of suitable management practices for the digester and zero grazing units for animals to achieve maximum efficiency. However, there are a few competing zero grazing management standards that make it difficult to manage the digester effectively. Farmers typically place more importance on the health of their farms than on keeping the digester well maintained. Due to this situation, there is a restricted amount of biogas produced, which may make it more difficult for the farmer to return his costs [67]. Studies show that lack of knowledge on digester maintenance is a leading challenge for biogas production in rural China and other countries, and technical training and availability of technicians would help alleviate the challenge [2].

2. Lack or low technological awareness

   There is a significant knowledge gap among biogas users in terms of technology and feasible applications. It is challenging to run, maintain, and service the biogas digesters and biogas energy systems because of the widespread lack of knowledge and skills. When the needs for biogas are not met to their full potential, production suffers [2, 78].

3. Lack of feedstock

   Studies also show that there is pervasive perception that biogas technology is only ideal for people having sufficient animal and agricultural waste for them to venture into biogas energy production. Therefore, lack of existing supply chain for biogas feedstock limits some small-scale farmers from investing in biogas production. The provision of reliable information on potential feedstock may assist potential investors in decision making [75].

4. High cost of Installation

   The availability of trained workers hinders the widespread implementation of biogas technology. The lack of readily available specific biogas installation supplies necessitates the development of finding alternative materials in the market [79]. A study on challenges biogas adoption in south Africa showed that several respondents are limited by cost of necessary building materials and lack of initial capital to purchase and install biodigesters [75].

5. Systems failure

   Biogas digesters frequently malfunction, and even the ones that do work do not always deliver optimal results. This misunderstanding has given biogas technology a bad name among eco-activists as not being sustainable [80].

6. Lack or poor post-installation support.

   Post-installation support for biogas systems has lagged demand because of a dearth of qualified experts and competent artisans. Service typically stops at the end of the “12-month guarantee period” after installation that is sometimes advertized. The farmers and operators are technologically illiterate, so they have no idea how to keep the systems working efficiently [2].

7. Lack of standards

   Biogas technology operates freely without any laid-out standards to regulate it. This makes it hard to ensure quality control measures are undertaken.

8. Availability of cheaper and convenient alternatives

   Many rural communities are accustomed to using traditional sources of energy like firewood, charcoal kerosene, which is readily available freely in most cases and is considered convenient to use compared to biogas preparation and related investment and operational costs [75].

9. Limited awareness and capacity building

   Awareness among the public is a major challenge in many countries leading to limited adoption of biogas technology in many owing to limited institutional and technical capacity in these countries that would have actively promoted the adoption resulting in its lack of popularity as an energy source [67, 81, 82]. For this technology to gain massive adoption, especially at local or community levels, proper awareness and capability programs are necessary.

10. Lack of economies of scale and sufficient demand

    Households in rural areas having significant feedstock for biogas production are often small with limited energy demand making investment in biogas production unattractive as most of the biogas produced may not be used. Biodigesters running with no biogas consumption leads to emission of highly potent methane which is a greenhouse gas. Connection to grids may create extra demand to sustain higher levels of biogas production [83].

11. There is need for governments and policy developers to provide a comprehensive policy and legislative environment that facilitate biogas digester production, installation and maintenance as well as other financial and non-financial incentives to encourage biogas production and uses [2]. Biodigester acquisition and use is primarily influenced by socio-economic factors like availability of household labor resources, family, or household financial income, and availability and type of feedstock for biogas production. Digestors require labor input of labor for operation and maintenance and provide the energy demand for biogas, hence lack of it has a negative impact. Families with higher income may prefer alternative sources of energy which also negatively affects biogas production and use [2, 84, 85].

2.3 Benefits of biogas production and use

There are several socioeconomic and environmental advantages with biogas production and use [83], these benefits include.

1. Boost for decentralized generation.

   The biogas plants deliver biogas for heating homes and farms, digested manure to boost food production rate; delivery and export excess power to the grid via decentralized generation which improves the economic position farmers [83].

2. Job creation

   Biogas production creates jobs in the supply chain right from equipment manufacturers, dealers and farm works which helps improve the social and economic conditions of local people [83].

3. Better crop yields and sustainable agriculture

   Digestate from biodigesters is proven biofertilizer whose application to fields reduces water needs and improves crop yields while saving cost incurred in buying expensive and polluting chemical fertilizers [83].

4. Biogas forms the basis of energy independence.

   By substituting the fossil fuels with biogas, the oil importing countries and regions acquire some energy self-sufficiency which saves their foreign exchange reserves and a cleaner environment. Biogas production and use is more reassuring for many countries since biodegradable biomass resources are abundant in all countries and regions of the world. By increasing decentralized generation as opposed to centralized energy systems, supply interruptions are easier to manage locally while enhancing the stability of the main grid hence higher reliability of energy supply [13].

5. Reduction in greenhouse gas emissions

   Disposal of agricultural wastes by means of open fires and natural decomposition releases huge quantities GHG and other pollutants to the atmosphere which contributes to global warming and environmental pollution. This is avoided or minimized by controlled biodegradation in digester for biogas production, while producing a clean energy resource to substitute polluting fossil fuels. Since the global warming potential of methane is about 25 times higher than CO₂ it will be scandalous to freely release tons of methane to the atmosphere, when this can be avoided by controlled biodegradation [13, 54]. Controlled biogas production and its use encourages waste accumulation and digestion which avoids methane emissions to the atmosphere from uncontrolled bio digestion of biomass in fields [83].

6. Carbon dioxide sequestration potential by the anaerobic fermentation process

   Biogas use has the possibility to uptake and sequester carbon dioxide to meet the climate target of below 1.5°C temperature rise. Methane from biodigesters has about 30% carbon dioxide. By upgrading biogas to biomethane, this CO₂ is removed and can be used in multiple applications like feedstock for food and biomaterials production [13].

7. Water purification through anaerobic digestion implementation

   Anaerobic digestion can be used to treat effluents with a high biological oxygen demand (BOD) and reduce BOD levels. This is the oxygen demand for microbial metabolism of organic materials (including trash). The BOD levels in wastewater from the dairy industry are 25–40 times greater than those seen in wastewater from the average household. Through bio digestion, between 70 and 90% of the BOD can be removed cost-effectively compared to aerobic systems [13].

8. Health benefits

   Domestic scale biogas production contributes to reduce respiratory and eye diseases that are linked smoke and fumes from firewood and charcoal, in addition to a better living environment of households. Wastewater from animal waste like dung and domestic wastewater sources, can be connected to toilets then to biodigesters for better disposal [13].

9. Reduced cost of waste disposal

   The production of biogas offers solutions, for the treatment and management of digestible animal and plant waste leading to sterilization and production of useful manure and sewage with significant volume and mass reduction which effectively reduces the cost of waste treatment and disposal [30, 64, 86]. According to the French Agency for Environment and Energy Management (ADEME), using manure as raw for anaerobic digestion reduces methane emissions from the storage and use of manure, and biogas reduces fossil greenhouse gas emissions (with an effective reduction in greenhouse gases of more than 751,000 tons of CO₂ equivalent). The global warming potential of carbon dioxide emissions from biogas is smaller than that of methane emissions from fossil fuels. Therefore, biogas production and use reduce greenhouse gas emissions in accordance with the Kyoto protocol and Paris agreements. The negative effect is that reduced use of manure increases use of chemical fertilizers for fertility or nutrient replenishment to the soil [13, 80].

2.4 The biogas cycle

Biogas is generated by anaerobic digestion of biomass with the most exploited feedstock being agricultural wastes like manure, poultry litter, hay, and straw, stillage from ethanol production, activated sludge from wastewater treatment plants, etc. [87]. The biogas cycle is demonstrated in Figure 1.

Figure 1.

The biogas cycle.

Figure 1 shows the biogas cycle which consists of biodegradation of biomass mainly from energy crops and wastes from animal husbandry, biofuel process products, crop harvesting waste, industrial waste and food consumption wastes which are fed to anaerobic digester whose main products are biogas and biofertilizer which is taken back to the soils to support biomass regeneration [88]. The CO₂ released in combustion is absorbed by the vegetation through photosynthesis to form biomass. The organic nitrogen rich residual sludge from the digester is used as fertilizer for the soil [87]. Biogas can be treated and enriched and used as natural gas, but can also be combusted for production of thermal energy or electricity through different generation prime movers [89, 90].

The anaerobic digesters have been used widely to supply biogas by many households [87, 91]. The new trend in biogas production and utilization is the biorefinery concept which mainly uses renewable biomass as energy source and combines with production of chemicals, like plastics, solvents, and synthetic fuels [79, 89, 91]. A typical example is the Danish Bioethanol Concept which comprises the ethanol production from lignocellulosic biomass while biogas is produced from stillage and cellulose waste. Then residual cellulose waste is also recycled after wet oxidation for additional conversion into biogas [39, 87, 92]

2.5 Biogas production process

Different types of organic matter can be digested anaerobically to produce biogas. The final step in the chain of chemical and biological processes used to break down organic matter for the purposes of producing biogas and managing garbage is called anaerobic digestion [73]. The production of biogas involves the controlled breakdown of organic material into smaller molecules by several anaerobic bacteria [73, 77]. Chemical reactions such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis occur during anaerobic digestion, which produces biogas from biomass [35, 73].

Biogas is produced by the microbial action in the digester soon after biomass is prepared and fed after undergoing preparation and a gradual fermentation process; the biomass is introduced into the reactor, where microbial activity within the digester rapidly generates biogas. Consequently, the process is a consequence of bacterial consumption of organic matter, specifically proteins, carbohydrates, and lipids/fats. This microbial digestion gives rise to the production of gases, predominantly methane and carbon dioxide. The process of biogas production encompasses several distinct stages, namely pretreatment, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Prior to introducing feedstock into the digester for the purpose of anaerobic degradation, the initial stage of biogas production involves feedstock processing or pretreatment. The pretreatment of feedstock is an essential and pivotal stage to mitigate instances of failure, enhance the production and enhance the quality of digestate, among other advantageous outcomes.

The bio digestion process for biogas production involves several key stages, namely the pretreatment stage of raw materials, anerobic digestion, purification of raw biogas, final biogas utilization, and post-treatment of digestate. Impurities like sand are, stones, plastics, etc. removed from the feedstock at the pretreatment stage, and the total solids (TS) concentration and temperature of the feedstock moderated for optimum bio-digestion [93]. The digester constitutes the core or the main element of the biogas production plant whose size and performance characteristics should be selected based on specific process and output conditions. At the terminal stage of the biogas production, the residue and slurry are further treated for intended application e.g. biofertilizer [21, 23, 94]. The process flowchart of a biogas plant process is shown in Figure 2.

Figure 2.

Biogas plant operation process and products.

From Figure 2, it is noted that the process of biogas production starts with feedstock collection and pretreatment before it is fed into the anerobic digester. The main products of the anaerobic digestion process are raw biogas and digestate. Biogas can be purified and stored ready for distribution and consumption. The digestate in slurry form is prepared or conditioned and applied in irrigation or controlled discharge as an organic fertilizer. Digested can also be used as a solid fertilizer in addition to slurry form. The various stages of biogas production are discussed as follows.

2.5.1 Pretreatment stage

Pretreatment is implemented as a strategy to enhance the breakdown of substrates, thereby enhancing the overall efficacy of the process. The employed pretreatment methods can be classified into four primary categories: chemical, mechanical, thermal, and enzymatic processes. The aforementioned techniques are designed to expedite the process of decomposition, although it is crucial to acknowledge that they do not necessarily lead to augmented biogas generation [10]. First, the feedstock is washed, then it is macerated, screened, and finally pressed (however, these stages can change depending on the feedstock). Mechanical parts can survive longer if they are free of contaminants like plastic, and magnetic traps are used to effectively eliminate magnetic pollutants. Glass, eggshells, pottery, bones, and sand are among nonmetallic impurities that must be removed during the preparation step. These pollutants do not decompose during digestion and hence accumulate as deposits at the digestor’s base [72].

Enzymes are required for hydrolyzing lignocellulose, which is necessary for digestion. An economic and technical barrier to producing biogas from lignocellulosic waste is its complicated structure. Strong and compact molecular bonds are formed in lignocellulose due to the presence of cellulose, hemicellulose, and lignin. The effectiveness of pretreatment is critical for maximizing lignocellulose’s potential as a biogas source. Pretreatment, in general, hastens reactions, boosts biogas yields, and creates a plethora of novel substrates [10, 95].

Technological advancements in pretreatment have enhanced biogas yields from lignocellulosic feedstocks, resulting in lower methane emissions into the atmosphere. Pretreatment is crucial because it increases biomass digestion and provides more biogas than without it by facilitating microbial breakdown of lignocellulose and its polymers, especially cellulose and hemicellulos. It has been shown that autoclaving and microwave heating can cause the hydrolysis of several non-biodegradable chemicals found in municipal garbage. Biomass pretreatment achieves this by partially or completely breaking down the feedstock to yield fermentable sugars, reducing the lignin barrier, and smoothing out the crystalline structure of cellulose [10, 30, 88, 89].

Table 1 demonstrates the wide range of pretreatment options available, each with its own set of benefits and drawbacks, and cost benefit implications. While pretreatment enhances the efficacy of the process and the biogas yield, it also introduces difficulties such as higher energy input, increased operating and maintenance expenses, and the introduction of inhibitory chemicals.

	Technology	Merits	Demerits
1	Feedstock milling	Does not produce any process inhibitors. Increased methane (5–25%)	High energy requirements High maintenance cost
2	Feedstock Extrusion	Extrusion increases the surface area to volume ratio	Leeds to increase energy consumption. Increases equipment maintenance cost
3	Steam pretreatment/; steam explosion	Increased cellulose fiber reactivity. Can make use of process energy/biogas	Risk of producing inhibitors (e.g., furfural and HMF) Reduces digestible biomass due to lignin condensation. More process energy input and reduced net energy output. Precipitation phenomena
4	Hot water treatment	Solubilized hemicellulose and lignin products are present in lower concentrations. Reduced risk of producing inhibitors like furfural and HMF) Increased enzyme accessibility	High heat demand and hence energy consumption Not effective at all temperature ranges
5	Microwave	Leads to more biogas production by 4–7%	Increased process energy input Increases labor and maintenance cost
6	Diluted or strong acid pretreatment	Used to solubilize hemicellulose component. Methanogens can adapt to inhibiting compounds.	Acids are expensive. Can form inhibiting compounds. Causes corrosion problems.
7	Alkaline pretreatment	Used to solubilize hemicellulose and some lignin. Increased methane production	Can produce inhibitors. Can cause high alkali concentration in reactor

Table 1.

Pretreatment methods and applications [10].

AD have become increasingly popular in producing renewable and sustainable energy from waste. Although synergies do exist between various technologies, the configurations to couple need further development [96].

2.5.2 Processes in anaerobic digestion

1. Hydrolysis

   Hydrolysis is a chemical reaction in which water is broken down into OH⁻ anions and H⁺ cations. In the presence of an acidic catalyst, hydrolysis breaks down the massive biomass polymers present in the substrate [33]. The proteins, carbohydrates, and lipids, all of which are massive organic polymers, are broken down into their component simple sugars, fatty acids, and amino acids to make up biomass [33, 73, 77]. During the process of hydrolysis, fermenting bacteria, such as Bactericides, Clostridia, and Bifidobacterial, efficiently decompose biopolymers, including carbohydrates, proteins, and lipids, into soluble forms such as sugars, fatty acids, and amino acids [97]. Hydrolysis produces acetate and hydrogen, which are put to good use by methanogens in the final stages of anaerobic digestion. Methane formation by acidogenesis necessitates further hydrolysis product breakdown, as these are still quite large molecules [45, 98].

2. Acidogenesis

   Anaerobic activity of acetogens produces mostly organic acids, alcohols, hydrogen gas, and hydrogen sulfide throughout this process [33, 97]. In an acidic environment generated by the fermentative bacteria, hydrolysis products are degraded by acidogenic microorganisms to produce ammonia, carbon dioxide, hydrogen sulfide, carbonic acid, shorter-volatility fatty acids, alcohol, and other trace products based on the substrate composition and products of hydrolysis [97]. Acidogenesis byproducts are too large to be ideal for methane production. Thus, they engage in acetogenesis [67].

3. Acetogenesis

   The process of acetogenesis produces acetate from acetic acid [99]. Through anaerobic digestion, acetic acid, hydrogen, and carbon dioxide are generated during acidogenesis. The results of acetogenesis and other processes are digested to the point where methanogens can produce methane [67].

4. Methanogenesis

   Methane and carbon dioxide are created in the final stage, called methanogenesis, thanks to the activity of acetoclastic methanogens (AM) and carbon dioxide (CO₂) reducing methanogens (CM), respectively. Finally, anaerobic bacteria called methanogens like Methanosarcina barkeri, Methanosaeta concilii, and Methanococcus mazei complete the methanogenesis process by producing methane from the byproducts of acetogenesis and other intermediate products of acidogenesis and hydrolysis [33, 97, 100]. The first three stages of methanogenesis (as shown in Eqs. (1) and (2)) involve the utilization of CO₂ and acetic acid (CH₃ COOH):

$$C{O}_2+4{\text{H}}_2\to C{H}_4+{\text{2H}}_2\text{O}$$E1

$$C{H}_3\text{COOH}\to C{H}_4+C{O}_2$$E2

While (CO₂) might theoretically be turned into water and methane, at this time the acetic acid pathway is the primary mechanism for methane generation. Therefore, (CO₂) and (CH₄) are produced as the primary products of anaerobic digestion via the acetic acid pathway [67]. H₂ and CO₂ gas combinations are transformed into biogas, which is composed of 60–70% CH₄ and 40–60% CO₂ [16]. The four steps of biogas production, together with associated activities and microorganisms, are depicted in Figure 3.

Figure 3.

Summary of the biogas formation process.

Figure 3 shows the main stages in biogas production being hydrolysis, acidogenesis, acetogenesis, and methanogenesis as the four primary stages in anaerobic digestion. Maximal biogas production requires careful management of the four main stages.

2.5.3 Factors influencing digester’s efficiency and performance

There are many factors that influence anerobic digestion and biogas production, e.g., biogas potential of feedstock, nature of the substrates, inoculums, digestion pH, temperature, digester loading rate, hydraulic retention time (HRT), carbon/nitrogen (C:N) ratio, presence of volatile fatty acids (VFA), presence inhibitory substances, etc. [101].

1. Feedstock

   Any biodegradable waste that has fats, proteins, carbohydrates, cellulose, and hemicelluloses is suitable as feedstock in anaerobic digestion. However, the various feedstocks have different theoretical chemical oxygen demand (COD) and the potential biogas. They have different or varied biochemical structure and rate of its biodegradability leading to differences in productivity. The factors in feedstock that influence biogas productivity are BOD, volatile solids, carbon/nitrogen (C:N) ratio, COD, volatile solids (VS) content, and presence of inhibitory substances [78, 101].

2. Temperature

   The bio-digestive process can be accelerated within tolerable limits by heating process. The same holds true for the processes involved in making biogas. The microorganisms engaged in anaerobic digestion perform both thermophilic and mesophilic digestion. Temperatures between 45 and 80°C are ideal for thermophiles, while those between 25 and 40°C are ideal for mesophiles, yet the Psychrophilic bacteria operate below 25°C [67]. A decrease in methane concentration and a rise in CO₂ might impair the heating value of the gas produced if global warming is not controlled. The best number to use for reliable and productive biogas production depends on the organism being used [94, 101].

3. Digester instrumentation

   Anaerobic digesters require continual monitoring of production levels and parameters to ensure optimal process management and, consequently, output is achieved by detecting and responding to changes in biomass quantity, anomalies, and the health of microorganisms [100]. The amount of biomass still to be decomposed and the amount of time spent doing so should be indicated by the gases produced. Knowing when to add new feedstock and the digester’s efficiency can be gauged with this information. Maintaining optimal microbial digestion requires monitoring gas generation in a system where biomass is added regularly [67]. By monitoring the gas produced, issues in the digester, like a shift in pH or temperature, can be identified and remedied [4].

4. The pH

   The pH is an important parameter in the operation of the digester, and the changes in pH value is is different at different states of the digestion. Biogas yield is optimized at pH range of between 6.5 and 7.2 [101, 102]. At a pH of about 5, acid-forming bacteria are at their most productive, while methane-forming bacteria thrive at a pH value of 6.2 or higher [33]. The bacteria population used in the bio methane process does best between 6.8 and 7.2, but may survive in a higher pH range [103]. When the pH of the digester falls outside of that range, the methane-producing bacteria deteriorates, providing an unideal environment for the microorganisms responsible for methane synthesis to survive. With a continuous decrease in pH below 6.2, the biogas productivity is reduced due to the accumulation of VFA, as they are toxic to the methane-producing bacteria. For the digester to generate the required biogas, the microorganism activeness is required as well as regular attention to the pH regulation [99, 104].

5. C/N ratio

   The C/N ratio determines the suitability of biomass for anaerobic digestion. Where the C/N ratio is high indicates low nitrogen content needed for microbial growth which leads to low uptake of nitrogen by methanogens needed for protein production. The result is wastage of carbon and low biogas production [101, 105]. The overall carbon to nitrogen ratio of the substrate. Although very high C/N ratios foster population expansion of methanogens, which has little to no influence on the carbon residue in the substrate and results in poor methane output, the optimal ratio for anaerobic digestion is between 20 and 30. Overabundant ammonia reduces methane generation and poses a threat to methanogenic bacteria when the carbon to nitrogen ratio is too low [99].

6. Toxicants/Inhibitors

   Inhibitors (i.e., which are toxic to the anaerobic digestive process) of methanogenesis, such as antibiotics and other residues, result in less methane synthesis and a rise in the concentration of volatile acids. Avoiding a high nitrogen to carbon ratio is important since it increases the likelihood of creating hazardous circumstances for the bacteria [33].

   The most common inhibitory substances in substrates for the anaerobic digestion process are (CO₂), (NH₃), long chain fatty acids (LCFA), H₂S. Methanogens are more sensitive to toxic material than other groups of bacteria. Short-chain fatty acids being the main inhibitors for methanogens. The concentration of ammonia is important but needs control with less than 200 mg/l being suitable for biogas generation. Nitrogen is a nutrient, hence C:N ratio of 30:1 improves biogas productivity by supplying nutrients to micro-organisms for the microbial activity. The methanogenic phase is also inhibited by the effect of ammonia which is rich in swine waste, poultry waste, swine waste, and high proteinaceous sludge. It is the free NH₃ and not ammonium ion (NH₄⁺) that is responsible for inhibitory effect. Studies show that ammonia with a concentration of 1.77–14 g/l of total ammonia nitrogen can reduce the biogas yield by about 50%. The inhibition by ammonia is influenced by the temperature and the pH. Changes in the temperature and pH can lead to process dynamics disturbance leading to as much as 30% loss in biogas yield. Thermophiles are more sensitive to LCFA inhabitation compared to mesophiles. LCFA are found slaughterhouse waste, agricultural residues, olive oil, food waste, wastewater, etc. which act as inhibitory substances in the process of anaerobic digestion [101].

7. Hydraulic retention time (HRT)

   The hydraulic retention time (HRT) refers to the time taken by the biodegradable material in the bioreactor. Factors influencing the HRT include the temperature inside the digester, technologies applied, and the type of feedstock used. The recommended HRT for mesophilic digester is 10–40 days while the thermophilic digester is about 14 days. A very short retention time may have the bacteria washed out of the digester before getting multiplied which leaves the digester in a state of standstill, while longer retention time increases the volume of the reactor to handle the same feedstock. To reduce the retention time and reactor volume, an optimum loading rate should be maintained for optimizing methane production, with 2–3 weeks being regarded as generally optimum for lignocellulosic material to degrade and produce biogas [101].

8. Co-digestion

   The performance of a digester can significantly be enhanced by co-digestion which helps supply the necessary missing nutrients to micro-organisms for higher efficiency. Common materials for co-digestion includes leftover foods, cow manure, vegetable wastes, and fruit. Co-digestion with properly selected feedstock materials can significantly increase biogas yield [101].

9. Redox conditions

   The digestor’s Redox conditions dictate the degree and velocity of contaminant biodegradation. Aerobic biodegradation rates are typically substantially higher than anaerobic biodegradation rates for most pollutants. While oil that has settled into anaerobic sediments may be quite tenacious, petroleum-based hydrocarbons that penetrate the aerobic zone of waters in lakes and rivers are susceptible to microbial destruction. Highly reduced hydrocarbons, like the alkanes, have low molecular weights and require oxygen to degrade. Methane and other alkanes cannot undergo anaerobic degradation. Hexadecane (C₁₆H₃₄), a high-molecular-weight alkane, may be present in some conditions. These chemicals rarely degrade and are often found in petroleum-contaminated areas. However, heavily chlorinated molecules like perchloroethene (PCE) resist oxygen decomposition. Anaerobic environments can bio transform these substances [106].

   For a biodigester, the methanogenic bacteria require redox condition between −300 and − 330 mV for optimal performance [107]. Redox condition is important for fermentative H₂ production process in which anaerobic bacteria are crucial and rate limiting. The efficiency with which substrates are metabolized, proteins and other storage resources are synthesized, and metabolic waste products are excreted, are all impacted by the Redox state. Pyruvate undergoes an oxidation-reduction reaction (AB) that produces VFAs and H₂ under acidic circumstances. Methane and CO₂ are produced by mesophilic bacteria during neutral processes, while solventogenesis occurs during basic operations. The repression of mesophilic bacteria indirectly promotes H₂ producers within the system. The anaerobic bacteria operate below pH 6, but MB optimum range is between 6.0 and 7.5. The pH range of 5.5–6.0 is ideal to avoid both methanogenesis and solventogenesis. Good H₂ is realized at a pH of about 6 compared to near-neutral. However, highly acidic pH (<4.5) inhib-its H₂ production as it inactivates AB. Hydrogenase enzyme activity gets inhibited by maintaining low or high pH beyond optimum range [108].

10. Mixing rate

    The biogas production plants need an appropriate mixing system to provide favorable conditions for microbial community, for process stability and efficiency in anaerobic digestion [96, 109]. The mixing intensity or shear rate and the length of time that shear rate is applied to a mixer determines the degree of mixing realized. Application of uniform shear rate is an important strategy to achieve stability of bio digestion process. In some studies, higher mixing intensity of 67 rpm for 5 min h⁻¹ led to 15–18% higher production output compared 10 rpm and 30 rpm without generating instability in terms of VFA accumulation and dead zones. A higher mixing rate reduces dead zones to less than 2%. Therefore, mixing significantly affects the biogas production rates but the impeller design is the principal factor. The best results were achieved by using a large diameter impeller running at medium mixing rate in anaerobic reactor [109, 110].

11. Inoculum

    The inoculum is used to reduce the start-up time of biodigestion by seeding of the which accelerates biogas formation. By use of inoculum, about 80% biogas can be obtained within the first 10 days of digestion period [101].

12. Moisture

    The substrate dilution or moisture content affects the process of anaerobic digestion with best results in terms of methane yield achieved under 60–80% humidity [89, 101].

13. Organic loading rate (OLR)

    The OLR is the rate at which organic matter is added to the digester volume in relation to either time or the substrate’s biological conversion capacity. The organic loading rate is directly promotional to the quantity of volatile solids loaded in the digester and hence it influences the biogas yield, with more methane production for lesser the OLR. If the reactor has higher concentration VFA’s, then it implies that it is overloaded [101]. The loading rate of a digester is the daily volume of volatile solids given to the digester, and methane production are favorable with high loading rates [33].

14. Biological oxygen demand (BOD) and chemical oxygen demand (COD):

    The BOD and the COD affect biogas production too. The biochemical oxygen demand determines the amount of oxygen needed by the microbes to decompose the substrate. If the BOD is high, then a more rapid organic degradation is achieved which leads to more biogas production. The COD measures all organic and inorganic biodegradable matter in feedstock or substrate. The value of COD of an organic waste helps predict the theoretical methane yield from the substrate. From experiments, if 0.5 l/gm of COD is removed, the approximate methane production is about 0.35 l/g [101, 111].

2.6 Energy requirements for biogas process

The production of biogas necessitates the addition of heat to keep the substrate at an appropriate temperature and the constant stirring of the substrate, either by hand or with an electric motor-driven stirrer [99].

Therefore, process heat energy requirement is determined by the relation in Eq. (3)

$$Q_t=M\ast C\ast \left(T_2-T_1\right)$$E3

where

Q_t = Overall heat required to heat the slurry in Kilojoule (KJ);

M = Mass of the slurry in (Kg);

C = The specific heat capacity of slurry and is expressed in KJ/Kg°C.;

T₂ = Desired digester slurry temperature in °C;

T₁ = Temperature of the digester charge/slurry in °C,

M = Digester size (V) in m³× density of slurry (ρ) Kg/m³; Density of slurry (ρ) = (Density of water + substrate density dung)/2; Specific heat of slurry = (specific heat of water (4.2KJ/Kg°C) + specific heat of the substrate e.g., cow dung (2.8KJ/Kg°C)/2 = 3.5 Kg/Kg°C.

The digestor effluent contains waste heat that can be recovered and used to produce the necessary heat energy. The slurry’s input temperature may rise by a few degrees Celsius as a result. This results in a potential 50% reduction in the total heat required by thermophilic digesters [31, 112]. External heating is most effective for digesters of a large or medium size and requires approximately twice as energy intensive as central heating. A total of 850–1000 W/m² K⁻¹ is the typical range for space heating, and 300–400 W/m² K⁻¹ is the typical range for space cooling [112].

Parameters such as microbe species, feedstock pretreatment, and biogas purification processes, substrate composition, substrate properties, and ideal reactor conditions must be managed to produce biogas profitably. Further investigation is required and have been actively a high interest area toward the enhancement of biogas production and biogas utilization through the application of engineering, biology/biotechnology, and technical innovations in order to achieve cost-effective biogas production [10, 113].

3. Biogas feed stocks

A wide range of biowaste can be used as substrates for biogas production through anaerobic digestion. Significant volumes of lignocellulosic waste are generated from agricultural, municipal, and other activities. The most commonly used materials for biogas production are animal manure and slurry, waste water and sewage sludge, municipal solid waste, and food processing and consumption generated waste [10]. The most used substrates for biogas production are pig and cattle manure, and poultry litter which generally compete with traditional manure [10, 87, 114]. Sewage sludge generated from wastewater treatment plants is the main feedstock for biogas production in places like Gävleborg region and Sweden while the use manure, food waste and industrial waste have been on the rise, although limited. Horse manure was also widely used as a substrate during co-digestion. Policy measures like ban on the landfilling of organic waste and competing applications like manure use can shift the demand for biogas feedstock selection [114, 115].

The quantity and composition of biogas produced by biomass digestion is a function of depending on the substrate used and process parameters control. Different types of feedstocks can be applied for biogas production ranging from animal wastes, agricultural residues, and energy crops [59]. The various feedstock used have different characteristics and hence biogas potential and hence may require different type and level of pretreatment and process conditions during anaerobic digestion [2, 78]. Feedstock biomass can take the form of a solid, a slurry, or a liquid (either concentrated or diluted). Many different materials can be used as feedstocks, including animal manure, straw, biowaste from homes and businesses, waste from bioethanol and biodiesel production, waste from markets and restaurants, sewage sludge from wastewater treatment plants, and energy crops like maize, silage, grass, sorghum, cereals, and sugar beets. The high lignin concentration of wood makes it unsuitable for anaerobic digestion and biogas production [78, 79, 88, 116].

Several different kinds of biomass garbage can be converted into biogas. Litter from farms, city dumps, and other sources can all contribute to the production of lignocellulosic garbage. Common forms of waste that are utilized as feed stock include animal manure, slurry, sewage sludge, municipal solid waste, food waste, and agricultural waste [10]. The biomass feedstock used to create biogas consists of a wide variety of components, including lipids, proteins, carbohydrates, cellulose, and hemicellulose. Co-substrates are added to alter the organic composition and boost biogas generation. Most co-substrates consist of organic industrial waste, food scraps, and municipal biowaste. Although lipids generate more biogas overall, carbohydrates and proteins convert more quickly [10]. Both biogas potential and yield are affected by the feedstock and co-substrates used in production, in addition to the process parameters [60, 84, 86].

There are many substrates that are suitable for anaerobic digestion, which can be characterized and classified into four main groups, namely energy crops, agricultural residues, food production wastes, and organic wastes [30, 117]. In the European countries, about 70% bio digestion feedstock comes from the agricultural sector like manure and agricultural residues. According to the European Biogas Association, agricultural wastes account for 40–60% in biogas production in Germany, Cyprus, Denmark, France, Italy and Poland [59]. The residual digestate is a feasible material for use as organic fertilizers, with huge optimization potential leading to increased farm production and profitability [117]. The various feedstocks for biogas production are discussed below.

3.1 Crop residues for biogas

Crop residues, a byproduct of crop husbandry, can be put to several different applications. Combustion, composite manufacture, bio-digestion, and animal feed are just some of the ways that waste materials like these can be repurposed. In terms of surplus agricultural products, India yearly produces around 500 Mt. (million tons) [16]. Nearly 70 million metric tons of organic waste are generated annually in the United States of America. Examples of agricultural waste include spoiled food and water [34]. Agricultural crop residues are a serious concern of farmers and cause significant contributors to GHG generation. Primary agricultural residues remain in fields as by-products after harvesting cereal grain straws, wheat (Triticum), barley, rice (Oryza), corn stovers, stalks, leaves, etc. On the other hand, secondary agricultural residues are products of agricultural resource processing like bagasse, sunflower husks, nutshells etc. Two sustainability issues associated with the use of agricultural wastes for biogas production are the potential competition with feeding in animal husbandry and the probable depletion of organic matter in the soil and nutrients in farmlands [59].

GHG, including CH₄ and N₂O are produced when crop waste is burned on-site. In terms of GHG from crop residues, maize crop residues produce the most (in gigagrams of CO2 equivalent). GHG emissions can be lowered and items like fertilizer can be more profitable if we can find better applications for these wastes [16]. Figure 4 shows that most carbon dioxide emissions from crop residue combustion are caused by the burning of maize residues as found in [16].

Figure 4.

Leading crop residues in terms of CO₂ gigagram equivalent GHG emissions.

Figure 4 shows the main crop residues used for biogas production led by maize, rice, wheat, and sugarcane wastes. The potential for making biofuels from cellulosic waste is enormous. This includes materials like bagasse, energy crops, agricultural wastes, and even sewage. Cellulose, hemicellulose, and lignin are the three primary organic components of lignocellulose [10]. Cellulose is the primary structural component that gives plant cell walls their mechanical stability. Macroscopic hemicellulose is composed of repeating polymers of pentoses and hexoses. Three aromatic alcohols (coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol) are synthesized within lignin. Composition of lignocellulose differs greatly depending on both the source and the time of year. Cellulose is a linearly connected polymer that contains many β-1,4-glycosidic linkages. Crystalline and amorphous elements coexist throughout the structure [10].

By applying temperatures in the range of 320°C and pressures of 25 MPa, crystalline cellulose can be converted into amorphous cellulose. Cellulose is the most common organic component on Earth, making up more than 25% of plant material. Hemicellulose is an important part of biomass because of the dynamic nature of its structure and the vast variety of polymers it contains, such as pentoses like xylose and arabinose, hexoses like mannose, glucose, and galactose, and sugar/uronic acids like glucuronic, galacturonic, and methylgalacturonic acid. Xylan makes up roughly 90% of the hemicellulose structure, though this value varies with the kind and origin of the feedstock being processed. Hemicellulose requires a wide variety of enzymes in order to be thoroughly degraded into free monomers [10]. Hemicellulose is comprised of multiple sugar units arranged in polymer structures that can be readily hydrolyzed. It possesses shorter side chains and a comparatively lower molecular weight when compared to cellulose. Hemicellulose enhances the overall compactness of the cellulose-hemicellulose-lignin network through its interconnection of lignin and cellulose molecules. The solubility range of hemicellulose molecules is typically observed to be between 150 and 180°C, with variations influenced by environmental conditions such as acidity, neutrality, and alkalinity [10, 118, 119, 120].

Lignin is a naturally occurring heteropolymer found in the cell wall, composed of three phenylpropane-based units, namely p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which are interconnected through linkages. The structure of lignin exhibits resistance to microbial degradation and oxidative stress. Lignin exhibits limited solubility in water, resulting in reduced degradability and hindered biogas production. At approximately 180°C in a neutral setting, the dissolution of lignin and hemicellulose in water occurs. The solubility of lignin in acidic, neutral, or alkaline environments is determined by the presence of the phenylpropane-based unit within its molecular structure. Lignin constitutes approximately 30–60% of the composition of wood, whereas agricultural residues and grasses typically contain lignin in the range of 5–30%. The agricultural produce primarily consists of hemicellulose [120].

The composition and structure of lignin have been found to have a positive impact on the hydrolysis process, resulting in an increase in the generation of biogas. Nevertheless, the presence of higher lignin content in the feedstock has been observed to decrease the efficiency of degradation [10, 120].

3.2 Energy crops

Energy crops encompass a wide range of plant species that can be classified into two main categories: herbaceous and woody. Herbaceous energy crops include grass, maize (Zea mays L.), and raps (Brássica nápus L.), while woody energy crops consist of species such as willow (Salix), poplar (Populus), and oak (Quercus). However, it is important to note that woody crops require a specific delignification pretreatment digestion process to be utilized effectively for energy purposes. Lignin, due to its resistance to degradation in anaerobic digestion, presents a favorable characteristic for gasification, incineration, or composting processes when considering woody substrates [86]. Herbaceous energy crops have many important characteristics making them suitable for anaerobic digestion e.g., high solar energy conversion efficiency leading to high yields, need low agrochemical inputs, and have got low nutrient and water requirement due to their extensive rooting system, to hold onto fertilizers and water, and they also have low moisture content at harvest time. Biogas can be made cost effective and high yielding by using crops with perennial growth habits with low establishment costs and fewer field operations and crops providing high biogas output as a feedstock source [59, 121].

3.3 Animal residues

Animal waste is a valuable energy resource containing biodegradable nutrients and renewable energy. However, by practice, most animal waste is deposed off, collected in lagoons or left to decompose in the open land causing environmental hazard [37]. Generally, the term “animal waste” refers to feces, urine, dung, or other excrement, urea, digestive emission, urea, or similar substances emitted by farm animals like livestock, poultry, or fish [88, 89, 122]. Animal waste like manure emit methane, ammonia nitrous oxide, volatile organic compounds, hydrogen sulfide, and particulate matter, which can cause serious environmental concerns and health problem [4, 61].

The primary sources of animal waste commonly encountered include dairy shed effluent, mainly comprising of wash water, urine, dung, residual milk, and waste feed. Additionally, poultry litter, which encompasses water, manure, spilled feed, feathers, and bedding material, is another significant source [21, 60]. Moreover, renderings from slaughterhouses and other byproducts originating from livestock finishing operations contribute to the overall volume of animal waste. Dairy manure and poultry litter are widely recognized as prominent examples of animal wastes that are extensively be used in large-scale biogas energy production [60]. In the past, significant volumes of bovine excrement were subjected to treatment processes and subsequently utilized as a form of agricultural fertilizer, or alternatively, were disposed of through the practice of spreading onto farmland. Nevertheless, numerous countries have implemented rigorous environmental regulations and legislations to regulate the emission of odors, as well as the pollution of surface and groundwater, soil contamination, and nutrient management. These regulations have prompted the adoption of alternative approaches that discourage the disposal of animal manure. Consequently, there is an increasing emphasis on utilizing animal manure in biomass-based production chains, driven by the incentives provided by these regulations [79, 121].

The challenge with poultry waste is high phosphorus (P) run-off to surface water which creates issues like odor and taste problems in drinking water, because of excess algae growth arising from excess phosphorus. For this reason, local land applications of poultry litter are restricted to protect water quality. Poultry waste on the other hand has high methane potential making it an efficient feedstock for biogas production. Slaughterhouses and fish processing factories also generate significant digestible waste for biogas production [78, 121].

3.4 Industrial and process wastes

Increased demand and consumption of food and industrial produce leads to large scale generation of waste. There is need for proper management of organic waste to minims harm to the environment like as climate change, ecosystem damage, and resource depletion [59]. About one-third of food produced for human consumption globally is discarded as waste, which is approximately 1.3 billion tons of waste per year. Anaerobic digestion of waste prevents the spread of pathogens and environmental degradation arising when waste is carried by to runoff water to basins and drain into the oceans. Anaerobic digestion surpasses thermal treatments technologies for bakery wastes such bread, biscuits, donuts, and pizza dough, as wastes like bakery garbage promotes methane production because it includes easily degradable carbs and vitamins [20, 59].

Implementation of anaerobic technologies started with wastewater treatment hence Industrial wastewater is a major source of aquatic pollution that endangers surrounding environments and ecosystems. Anaerobic digestion of industrial waste is more advantageous than aerobic wastewater treatment since it produces useful biogas, and substantially lowers energy requirement, and generates less sludge that requires disposal Brewery wastewater, cassava (Mánihot esculénta Crantz) starch wastewater, palm oil mill effluent, biodiesel wastewater, and bioethanol wastewater (vinasse) have demonstrated significant biogas potential [59].

3.5 Biogas potential of various feedstocks

Different biomass feedstock has different biogas potential in terms of both quality and quantity. Table 2 illustrates gas yields and methane contents for various substrates after 10–20 days of retention at 30°C [37, 61].

Substrate	Gas Yield; (L/kg Vs^*)	Methane Content; (%)
Pig manure	340–550	65–70
Cow manure	90–310	65
Poultry droppings	310–620	60
Wheat straw	200–300	50–60
Rye straw	200–300	59
Barley straw	250–300	59
Oats straw	290–310	59
Corn straw	380–460	59
Flax	360	59
Hemp	360	59
Grass	280–550	70
Elephant grass	430–560	60
Sunflower leaves	300	59
Agricultural waste	310–430	60–70
Fallen leaves	210–290	58
Algae	420–500	63

Table 2.

Methane content and gas yield for different biomass [37, 101].

As can be seen in Table 2, there is a wide range in both the quantity and quality of biogas produced from various forms of biodegradable biomass feedstocks. According to Table 2, biogas can be produced from a variety of sources, including algae, cow manure, pig manure, agricultural solid wastes, and grass. Among the many potential sources of biogas, algae have the highest output, followed by sugar beets, elephant grass, sewage sludge, clover, elephant grass, and maize straw. There is not much potential for biogas production from materials like reed, sugarcane bagasse, rice seed coat, or rice seed straw. Therefore, a farmer must carefully select his biomass feedstock, or the plants and animals he raises, to achieve maximum biogas output [100].

3.6 Estimation of biogas potential

According to [100, 123], biogas can be used to generate heat, the cost of which can be roughly calculated using Eq. (4) as shown:

$$E_{pi}=E_m^{cd}\ast {\eta }_i$$E4

where:

$E_{pi}$ – Production of the energy (heat only, electricity only or heat and electricity in co-generation), [GWh/year].

${\eta }_i$–efficiency of energy conversion process.

$E_m^{cd}$– Possible production of total biogas energy.

Total biogas energy potential from livestock dung can be determined using the following relation in Eq. (5):

$$E_m=\sum \left(N_{i.n}\ast M_{i.n}\ast oD{M}_{i.n}\ast C{H}_{4i.n}\right)\ast NCV\ast 365\ast {10}^{-6}$$E5

where:

E_m - Expected production from livestock manure available in the region, [GWh/year].

N_i.n--Animals of the same category (cattle, pigs, poultry etc.) in the region, [number of animals].

M_{i.n -} Manure outcome of each category of animals, [kg of manure/animal/day].

oDM_i.n -Organic dry matter content in the manure for each category of animals, [%].

CH_4i.n-Methane outcome, [Nm³ CH4/kg organic dry weight of the animal manure].

NCV – Net Calorific Value [kWh/Nm³].

3.7 Biogas production and fertilizer application

The composition of the feedstock and the stability of the biogas plant’s operations are two factors that influence the biogas yield, biogas quality, status of digestate, and stability of plant operations [124], and the quality of the sludge from anaerobic digestion, that can be used as an organic fertilizer [22], to improve soil quality, boost crop yields, and aid in the advancement of sustainable agriculture [61]. Biodigester slurry can boost agricultural output by 10–20% by enhancing soil fertility and productivity. It is advised that 5 tons of the digestate be applied per hectare on farms that do not use irrigation, and that 10 tons be applied per hectare on farms that do use irrigation [125].

The chemical and nutrient composition of the biodigester substrate and digestate is determined by the efficacy of the process and the feedstock composition. Very little preparation is needed before spraying the farm with the substrate or digestate or utilizing it as bedding for the animals [34]. Digester slurry is a 25% more effective fertilizer than just putting manure on the farm. This suggests that increasing agricultural yield by employing digestate in this way. In comparison to applying manure directly, biogas slurry makes it easier for plants to absorb the nutrients they need [61].

Slurry from a biodigester can be used as a fertilizer because of its high nutritional content and low environmental impact compared to synthetic alternatives. The digestate can be used as a foliar fertilizer on the farm if it is mixed with water and sprayed on the right leaves. Slurry is collected from the digester via a ditch as gas pressure increases after the biogas plant is refueled with fresh feedstock. Overflow and back pressure can be avoided in a biodigester system with regular ditch emptying by the operator [126].

Digestate consists of water and any unreacted feedstock or co-substrates. Digestate management is necessary for monitoring and controlling biogas production. A farm can either use the digestate as fertilizer or store it in a lagoon or storage tank. The wastewater from a biogas plant that has not been filtered for coarse fiber reduces bio digestion rates and gas production. Bedding and compost can be made from the screened-out material. After fibers have been extracted, the liquid organic residue is known as filtrate. Dry matter nitrogen, phosphorous, and potassium levels in manure filtration concentrate range from 3 to 4.5%. The crops can be sprayed with nutrient-rich sewage water.

The filtrate can be refined into a liquid concentrate and a solid product known as filter cake. This is especially important when bringing garbage from off-site locations into the digester’s sphere of impact. Plants’ basic nutrients, nitrogen, phosphorous, and potassium, may be found in abundance in these wastes. Thus, it is important to think about how this would affect the final disposal site’s overall nutrient management strategy. Technology for recovering nutrients to control digestate nutrient levels [127].

As a result, biogas generation can be a more economical alternative to costly chemical fertilizers by boosting agricultural output and yields while saving money for farmers and contributing to long-term energy security. Farmers can earn revenue by selling the digestate or filtrate [30, 60, 65, 66].

4. Conclusion

Anaerobic digestion is a biological process where biodegradable matter is degraded to by the action of microorganisms in the absence of oxygen to produce biogas, which is a clean fuel from renewable feedstocks with potential use in heat and electricity generation. Factors affecting bio digestion include type of raw materials, temperature, pH, mixing, level preparation and pretreatment, residence time, redox effect, hydraulic pressure, etc. The efficiency of anaerobic digestion can be evaluated in terms of bio-methane potential, gross energy yields, process energy demand, net energy yield, and energy efficiency. Energy crops are the most used substrate for biogas production.

Concerns about greenhouse gas emissions and depletion of fossil fuels have increased demand for sustainable renewable sources. Biogas production is one of the innovative and viable for production of energy from biowastes and biomaterials, with an extra benefit of producing biofertilizer to support sustainable agriculture. Huge quantities of biodegradable waste are generated by humanity globally, which can pose a serious threat to human, animal health, and their environment. There are many wastes disposal and treatment methods employed to avoid environmental pollution. Among the many options, anaerobic digestion is a popular non-conventional energy source used to produce renewable energy from biodegradable biomass.

Biogas is a product of the anaerobic digestion process with many applications as in generation of renewable energy. The main component of biogas with energy value is methane, but has impurities like moisture, carbon dioxide, siloxanes, hydrogen sulfide, siloxanes, hydrocarbons, oxygen, ammonia, oxygen, carbon monoxide and nitrogen whose presence is undesirable as they reduce the calorific value of biogas and create operational problems in the energy systems. This necessitates biogas cleaning and application of multi-stage technologies to produce upgraded biogas called biomethane. Biomethane gas is a flexible and easy to store fuel with similar properties and applications to natural gas with no need to modify any equipment settings for natural gas devices and equipment.

The factors influencing the process and quality of biogas produces include the C/N ratio, the pH, dilution of feedstock, composition and nutritive value of feedstock, residence period, mixing and stirring, temperature, presence of toxicants, loading time and redox conditions. The main challenges facing biogas production and use include lack of technical training on biogas production and use, lack of subsidies and incentives. Therefore, besides easy access to appropriate and affordable biogas technologies, there is a need for developing a comprehensive policy on construction and operation of biodigesters as well utilization of biogas and sale of biogas energy products and services. Biogas education and exposure, availability of simple and affordable biodigesters and facilitating legal and policy environment are key tools needed to increase the adoption of biogas technology and its sustainable use.

Raw or cleaned biogas which is free from harmful contaminants has significant energy value and applications. The multifunctional uses of biogas produce include the production of biofuels and renewable energies, reduction in greenhouse gas emissions mainly methane and carbon dioxide, reduction in nuisances resulting from odors and flies associated with uncontrolled biodegrading of biomass, economic or financial savings for farmers, sustainable waste treatment and recycling of organic waste, minimization of water and air pollution, rural poverty alleviation, better of social conditions of rural or remote settlements like gender, balance as women who bear the responsibility of looking for firewood are relived of the struggle and conservation of forests and vegetation cover due reduced use of firewood and charcoal.

Acknowledgments

The authors are grateful to the Durban University of Technology for providing the resources to undertake this study.

Conflict interest

The authors declare that they have no conflict of interest.

Funding

The article publication charges were paid by the Durban University of Technology.

Availability of data

All the data used for the study have been declared.