Recent Advances of Biogas Production and Future Perspective

1. Introduction

The demand of energy has been increased over the years as the sequence by increasing of the world population [1]. Fossil fuels are exhausting and the emission products of these fuels have been causing some damages to the environment. The scientists in the world are focusing on developing alternative methods of energy production [2]. Bioenergy is an energy obtained from any fuel that is originated from biomass, which includes recently living organisms and their metabolic by-products [3]. Biomass is defined as all animal and plant material on the Earth’s surface. Hence, collecting biomass, such as manure, crops, or trees, and employing it to produce electric power, heat, or motion is bioenergy [4, 5]. If not managed optimally, the large amounts of biomass, livestock’s manure, agro-industrial waste, and slurries produced today as well as the wet organic waste streams represent a constant pollution risk with a potential negative impact on the environment [6]. Biofuels are defined as fuels made from biomass resources, or their processing and conversion derivatives [3, 5]. Biofuels are eco-friendly and renewable resources of energy and hence have been receiving attention as an alternative energy source [2]. The organic part of nearly any form of biomass, involving industrial effluents, sewage sludge, and animal waste, can be decomposed via AD into carbon dioxide and methane mixture called as biogas and is considered an alternative green energy resource. Methane (CH₄) is the most important component of biogas because it has the highest energy density among the biogas components. Therefore, the high CH₄ content of biogas is desired [7, 8]. Biogas was first identified 600 years ago as originating from decomposing organic matter. More recently, in 1884, Louis Pasteur investigated it sourced from animal waste, suggesting it as an appropriate fuel for the lighting of street lamps. Biogas primarily consists of methane (CH₄), in a range of 50–75%, and carbon dioxide (CO₂), at 25–50%, with minor amounts of other compounds, such as hydrogen (0–1%), nitrogen (0–10%), which could originate from air saturated in the influent, vapor water (H₂O) at concentrations of 5–10%, or higher at thermophilic temperatures, derived from medium evaporation, hydrogen sulfide (0–3%), which is produced from reduction of sulfate contained in some waste-streams, ammonia (NH₃) originating from hydrolysis of proteinaceous materials or urine and oxygen (0–2%), which is entering the process from the influent substrate or leakages, hydrocarbons at concentrations of 0–200 mg/m⁻³, trace carbon monoxide (CO), and siloxanes at concentrations of 0–41 mg m⁻³, originating for example from effluents from cosmetic medical industries. The relative content of CH₄ and CO₂ in biogas is mainly dependent on the nature of the substrate and pH of the reactor [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. Typical components and impurities influence the quantity and quality of the biogas. CO₂ and N₂ lowers the calorific value, CO₂ also causes corrosion and damages to alkali fuel cells. H₂S spoils catalysts, causes excessive corrosion and deterioration of lubrication oil, generates harmful environmental emissions, and corrodes the engines of biogas purification machinery. N₂ and NH₃ increase the anti-knock properties of engines and NH₃ also damages fuel cells. Water vapor causes corrosion of equipment and piping system leading to damage instruments and plants. Siloxanes acts like an abrasive and damages engines. Dust blocks nozzle and fuel cell [18, 19, 20, 21, 22, 23].

Biogas is flammable, smokeless, hygienic, colorless, odorless, and has bad eggs odor whether not desulfurized. It has an energy content of 37.3 MJ/m³, explosion limits 6–12% biogas in air, ignition temperature 650–750°C, specific gravity 0.847–1.004, and calorific value 4740–7500 kcal/Nm³ [18, 24, 25, 26, 27, 28]. Biogas is an environmentally friendly, a renewable, clean, cheap, high quality, and versatile fuel which is generated in digesters filled with the feedstock. It is considered an alternative green energy resource. It can be utilized for different energy services like heat, combined heat and power, or a car fuel [7, 8, 29].

Biogas technology is used to convert the organic waste into energy. The use of energy and manure can lead to social economic benefits, green environment, and also contributes towards sustainable development [30, 31, 32]. Biogas technology is also a source of nutrient-rich organic fertilizer and the effluent slurry produced as a result of biogas technology is also helpful for algae growth, fish production, and seed germination [24]. Biogas technique is applied to small-scale and large-scale uses involving electric power production. It is a mixture of gases of which the composition relies on substrates and AD process conditions like retention time, temperature, and pH. Biogas is one of the main products of the AD of organic substances.

Anaerobic digestion (AD) is considered as a biological process that degrades organic substances by the actions of microbial communities in the absence of oxygen. In fact, AD can be divided into four stages, as seen in Figure 1, which are hydrolysis, acidogenesis and this stage is considered as acid-producing, acetogenesis and this phase is also called acetic acid-producing, and methanogenesis and this final step is known as methane-producing [8]. It is quite important to purify raw biogas and upgrade it to a high-quality fuel standard, in order to increase the calorific value and reduce undesired components, such as H₂S and CO₂, which are damaging the utilization systems. This process is well known as biogas cleaning and upgrading [33, 34]. Biogas could be simply upgraded into biomethane or renewable natural gas (RNG), which is similar to natural gas that produced from nonrenewable fuel sources. It contains about 90% or greater of methane. RNG could be replaced for natural gas and could be used as fuel for cars that can run on natural gas and to provide gas to natural gas grid. Upgrading of biogas to biomethane is considered as one of the technologies that has got a lot of attention in the bioenergy industry [8, 35]. Biogas could play a key role in the developing market for renewable energy and the utilization of biogas in the world is expected to be doubled in the next years, ranging from 14.5 GW in 2012 to 29.5 GW in 2022 [20, 36, 37].

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2. Renewable energy resources

The term energy can be generally defined as the amount of force or power when applied can move one object from one position to another, or it defines the capacity of a system to do the work. The most important characteristic of energy is the possibility to convert one of its forms to another. Generally, the energy technologies are the man-made devices, equipment, and systems used to capture, convert, store, and transport energy from the energy resources [38]. Energy is an important demand in our daily life as a way of enhancing human development leading to productivity and economic growth. Energy is a key driver for agriculture, industries, and service sectors that influence economic development [39]. The term energy sources refer to the output forms of energy from the man-made energy technologies, while the energy resources refer to the naturally available forms of energy [38]. The resources of energy are divided into three groups: nuclear resources, renewable resources, and fossil fuels [40]. Renewable energy is called “renewable” because the sources harnessed to create the energy renew and replenish themselves constantly and within a reasonably short period of time (i.e., months or years, not centuries) [41]. Thus, renewable energy sources renew themselves naturally without being drained in the earth [39, 41]. Renewable energy techniques give a great opportunity for reduction of greenhouse gas emission and decreasing global warming via replacing traditional energy sources [6]. Various types of renewable resources such as hydrothermal, geothermal, solar, wind, ocean (tide and wave), heat from the Earth’s interior, and biogenic (biomass) energies are available and they give the possibility to produce consistent power [38]. These renewable energy sources are also often called alternative sources of energy [42]. Alternative or renewable energy sources include traditional renewable energy technologies (such as wind turbines) as well as innovative new technologies, such as hydrogen internal combustion engines, and hydrogen-based fuel cells [38].

There is a direct relationship between renewable energy and sustainable development via its effect on human development and sustainable economic growth [43]. Renewable energy sources supply a lot of opportunities in reduction of environmental and health impacts, energy access, energy security, climate change mitigation, social, and economic development [6, 38, 44, 45, 46]. Renewable energy resources will play an important role in the world’s future [6, 40]. The technologies developed to exploit renewable energies are known as renewable energy technologies (RET) or clean technologies or green energy. Sustainable economic and industrial growth also requires safe and sustainable energy resources [47, 48, 49]. Maximizing resource (material and energy) recovery and minimizing environmental impacts such as contribution to the global warming are important objectives in the solid waste management (SWM) sector, which is considerably developed over the past century [50, 51].

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3. Sources of biogas

There are a variety of wastes that can be used as sources, or feedstocks, that are fed into the digester to produce biogas. Most of biodegradable organic compounds could be transformed into biogas by anaerobic digestion, and biodegradability is the characteristic indicating to what extent this is possible. Raw materials for producing biogas by anaerobic digestion are biomass feedstocks which include; municipal solid waste (MSW), industrial solid wastes and industrial wastewaters, food waste, livestock manure, sewage sludge, agricultural manures, catch crops, energy crops, and microalgae [12, 52, 53, 54, 55, 56, 57]. The largest resource is represented by animal manure and slurries from cattle and pig production units as well as from poultry, fish, fur, etc. [58, 59, 60, 61]. Manure of animal is considered as a main carbon source for biogas and involves pig manure, cattle, and poultry. The total solids present in animal manures consist of 90% moisture content and volatile solids. It performs as perfect substrate because of its great buffering capacity [62, 63, 64]. In most countries, sewage sludge and agricultural manures have been the principal sources for some time, complemented by slaughterhouse, dairy, and restaurant waste. More recently, biogas plants are increasingly using municipal solid waste (MSW), industrial solid wastes, and industrial wastewaters as feedstocks [12].

Nowadays, sludge from municipal wastewater treatment plants is considered as major source of organic matter for biogas generation in Sweden. Other familiar substrates for biogas generation in co-digestion plants involve source-sorted food waste and manure, slaughterhouse waste, and waste from the feed and food industries [65]. Sewage sludge is usually used as a feedstock to provide energy to power sewage treatment works. For many years, sewage sludge and agricultural manures was the principal inputs, making up over 80% of the total.

However, more lately, the manufacturers have been testing with biogas particular agricultural crops, involving maize and rapeseed. Both the crop itself and the generated fodder (silage) are utilized. Animal waste is increasingly used as a feedstock throughout the world. In the EU, there are now over 750 biogas plants processing animal waste, many of them on a large scale. Organic waste from households and municipal authorities is also an important source of biogas [12].

The urban solid waste production, or municipal waste, increases with population growth, high economic activity, and goods production. Biogas has the potential to be produced from widely available, abundant raw materials, including agricultural residues (e.g., animal manure), landfill and food waste, and aquatic biomass and lignocellulosic raw materials [66, 67]. Wood, agricultural residues, and dung of animal are the sources of energy for biogas technique [24, 68]. The use of wastewater from inorganic sources, such as chromium, has also been studied as an alternative for energy production, a more environmentally sustainable approach that avoids landfill disposal of these wastes [69, 70]. Industrial waste and wastewater have potential uses in biogas production due to their characteristics, such as high organic load [66, 71]. Algae are considered as a potential biomass feedstock for decreasing our dependence on nonrenewable energy sources for electric power, transportation, and heat production [53].

Livestock manure, i.e., dung of cow is an efficient feedstock for biogas production getting high cumulative biogas yield with steady performance, with a continuous process. Therefore, cow dung is more favorable in the biogas process [72, 73]. In general, there are different kinds of biomass resources to produce biogas, including animal manures, municipal solid wastes, food wastes, industrial wastes, agricultural residues, poultry wastes, forestry wastes, microalgae, and some dedicated energy crops [54, 55, 56, 57].

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4. Biogas production processes

Generation of biogas gives a multiuse carrier of renewable energy, as methane can be utilized for substituting of nonrenewable source of fuels in both heat and electricity production and as a car fuel. AD of wastes, energy crops, and residues is of growing interest in order to decrease the greenhouse gas emissions and to promote a sustainable development of energy supply [74]. Anaerobic digestion is a technology with proven efficiency, being widely used in the stabilization of industrial wastewater, urban solid waste, animal manure, and sewage sludge [66]. There are many benefits associated with anaerobic digestion technology, which include mass reduction, odor removal, pathogen reduction, less energy use, and more significantly, the energy recovery in the form of methane [75, 76]. The aim of anaerobic digestion process is the production of a methane-rich biogas through biological decomposition of organic matter, in an oxygen-free environment. An aerobic digestion is considered as a low-cost an eco-friendly waste management process, thus it reduces the emission of greenhouse gases. In the meantime, it stabilizes and reduces the wastes. One of the major advantages of an aerobic digestion is its adaptability to deal with a wide range of organic substrates. The produced biogas can be used for power and heat production, or can be upgraded and used as vehicle fuel in the transport sector. In addition, the by-product of AD, the “digestate residue,” can be further utilized as a fertilizer on the agricultural land [50]. There are different process types which can be applied for biogas generation, which are classified in dry and wet fermentation systems [21, 77, 78]. Wet digester systems are constantly applied using vertical stirred-tank digester with various stirrer kinds dependent on the source of the feedstock.

Biomass is utilized as substrates for biogas generation as long as it consists of hemicelluloses, cellulose, carbohydrates, proteins, and fats as major constituents. Only powerful lignified organic materials, e.g., wood, are not suitable due to the slowly anaerobic decomposition. The composition of biogas and the methane yield depends on the feedstock type, the digestion system, and the retention time [78]. Maximal gas yields and theoretical methane contents of substrates for biogas production are carbohydrates 790–800 biogas (Nm³/t TS), 50% CH₄ and 50% CO₂, carbohydrates only in the form polymers from hexoses, not inulins and single hexoses, raw protein 700 biogas (Nm³/t TS), (70–71)% CH₄ and (29–30)% CO₂, and finally raw fat 1200–1250 biogas (Nm³/t TS), (67–68)% CH₄ and (32–33)% CO₂ [79].

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5. Operational methods and reactor designs

Digesters established in worldwide differ in their costs, construction materials, and design complexity [121, 122]. In order to design any anaerobic digester, we need to solve three principal requirements such as: to produce a high volume of high-quality biogas; able to continuously handle a high organic loading rate; and to have a short hydraulic retention time in order to have smaller reactor volume. There are various types of digesters, which are mostly used in the industry involving multistage systems, batch, continuous one-stage system, or continuous two-stage. Further configurations, such as the plug-flow systems, anaerobic sequencing batch reactor (ASBR), tubular reactor, baffled digesters (ABR), upflow anaerobic sludge blanket (UASB) reactor, and anaerobic filters, are also present [50, 104, 123]. Normally, the selection of suitable digester kind is relying on the properties of the main feedstock used, specifically total solid. Feedstocks with high amount of total solid and slurry are generally treated in CSTRs; whereas, soluble organic wastes are mainly digested in upflow anaerobic sludge blanket (UASB) reactors, anaerobic filters, and fluidized bed reactors [124]. Co-digestion is principally implemented in wet single step processes (e.g., CSTR). Continuous systems are fed continuously, while the digestate residue is discharged at the same rate, allowing a steady state to occur, leading to a constant gas production rate. In spite of that, this kind of operation is only possible for substrates, which can be pumped for continuous feeding. Otherwise, a semi-continuous process is applied with a discrete amount of feed several times a day [50]. The main advantages of continuous systems are simplicity in design, operation, and have low capital costs, but they have disadvantages including rapid acidification and larger volatile fatty acids (VFA) production [117].

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6. Biogas technologies

There are undesired compounds and other gases contained in biogas are unwanted and are considered as biogas pollutants [11]. The concentrations of these impurities are dependent on the composition of the substrate from which the gas was produced [158]. The removal of these harmful components and other non-combustible gases makes biogas a more viable and economical alternative renewable energy source [96, 159]. The energy content of methane described by the Lower Calorific Value (LCV) is 50.4 MJ/kg CH₄ or 36 MJ/m³ CH₄ (at STP conditions). Therefore, the higher the CO₂ or N₂ content is, the lower the LCV in biogas [11, 160]. Developing the quantity and quality of biogas often needs pretreatment to maximize methane yields and/or post-treatment to take out H₂S, which includes higher costs and considerable energy consumption. Therefore, scientific research has performed to develop a low-cost desulfurization process and improve AD conversion. Appealingly, there are a lot of techniques that have been approved to enhance the anaerobic digestion process, like pretreatment procedures using acidic/alkaline, ultrasonic, thermal methods [161, 162, 163]. Lately, there are various treatments targeting at get rid of the trace contaminants and undesired components from the biogas expanding its range of applications [11]. Biomethane involves two major treatment processes; cleaning and CH₄ enrichment (biogas upgrading). The cleaning of the biogas contains elimination of impurities and acidic gases; whereas, the enrichment process is for extraction of CO₂ from biogas [11, 96]. There are three major reasons for gas cleaning; fulfill the requirements of gas appliances (gas engines, boilers, fuel cells, vehicles, etc.), increase the heating value of the gas, and standardization of the gas [58]. Biogas cleaning treatment process includes removal of undesired materials (such as, NH₃, siloxanes, H₂S, volatile organic compounds (VOCs), and CO) to increase the quality of biogas. However, it is practically only H₂S which is mainly targeted and many current biogas plants have H₂S elimination units normally rely on biological H₂S oxidation by aerobic sulfate oxidizing bacteria [11]. Biogas must be desulfurizated and also dried before usage to stop destroys the use of gas units. The concentration of H₂S between 100 and 3000 ppm in biogas generated by cofermentation of manure with harvesting debris or energy crops, in order to prevent an expensive deterioration of lubrication oil and excessive corrosion [21, 22]. CHPs are used for the utilization of biogas need generally levels of H₂S below 250 ppm. The existence of H₂S not only affects the quality and quantity of the biogas generated which can restrict its application, but also produces dangerous environmental emissions and corrodes the motors of biogas purification machinery [20, 23]. Nowadays, biological desulfurization process mainly used to remove of H₂S [21, 22]. Recent study conducted by Register Mrosso [164] reported that red rock (RR) is an available material for biogas purification which used to remove hydrogen sulfide from biogas [164]. The quality of raw biogas can be further improved via various upgrading techniques to remove the non-combustible components and as a result increasing the methane content to approximate natural gas quality (75–98% methane) [96]. Biogas has been upgraded to natural gas composition via methanation using renewable hydrogen [165]. The higher the methane content, the richer the biogas is in energy [12]. Biogas upgrading aims to increase the low calorific value of the biogas, and convert it to higher fuel standard [35]. In case the upgraded biogas is purified to specifications similar to natural gas, the final gas product is called biomethane [11, 166]. Biomethane is a gaseous fuel with physicochemical properties similar to those of natural gas, which makes it possible to inject it into the gas grid [96]. Currently, the specifications of the natural gas composition are depending on national regulations and in some countries >95% methane content is required [11].

Technological development plays an important role in biogas upgradation and purification processes in large-scale commercialization of biogas. There are various cleaning and upgrading techniques to improve the quality of raw biogas which can be categorized into physiochemical and biological technologies. Some of these techniques are conventional methods, including physical absorption, chemical absorption, membrane infiltration and biological methods, and others are considered as new technologies including cryogenic upgradation, membrane enrichment, multistage-, and high-pressurized AD [62, 96, 167, 168].

Physiochemical technologies for cleaning of biogas and its subsequent CH₄ enrichment can be grouped as follows: absorption process (physical and chemical absorption), Hybrid solution (mixed physical and chemical solvent), and physical separation (adsorption on solid surface; membrane; cryogenic) [96]. Novel technologies, such as cryogenic separation, in-situ upgrading, hydrate separation, and biological methods, represent the recent developments in biogas upgrading technologies. Biogas can be used as fuel for domestic stoves, boilers, internal engines, gas turbines, cars, and fuel cells, or injected into natural gas grids to replace gaseous fuel [35]. These techniques have been reported to yield biomethane typically containing 95–99% CH₄ and 1–3% CO₂. At this quality, the spectrum of applications for biogas widens, it can be used to serve the same applications as natural gas [96]. Gas upgrading and utilization as renewable vehicle fuel or injection into the natural gas grid is of increasing interest because the gas can be used in a more efficient way [21]. Types of upgrading plants are available in Sweden, and shows that around 70% of the biogas purification plants apply water-washing technologies [169].

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7. Biogas applications

Biogas generation serves three important functions: waste removal, environmental management, and energy production [12]. The first and most direct use of biogas is for heating and domestic purposes [184]. Biogas is an excellent fuel with a numerous application [62]. Biogas that is purified and enriched in methane can be used for household applications, automobile fuel (liquefied), or electricity generation [185, 186]. The biogas is mostly utilized as a combined heat and power (CHP) application in the overall world; and apart from it, it can be used in three sides such as fuels for cars, steam generation, and electric power. Biogas obtained from renewable organic waste is counted as an alternative energy for nonrenewable fuels due to its broad applications in fuel and transportation sector [62, 104, 187].

In general, Waste-to-Energy (WtE) technologies can be defined as any waste treatment processes that create energy from a waste source in any forms of energy carrier, i.e., electricity, heat, or transportation fuels [188]. Depending on a statement by World Energy Council, restricted landfilling capacities, rise in the quantity of produced waste, high costs of energy, and rising concerns of environmental issues are the summarized major factors for the growth in WtE market in the past decades. In 2013, the international WtE market encountered a growth of 5.5% and reached a value of 25.32 billion USD with respect to its previous year [50, 188]. Biogas is a flexible energy transporter, appropriate for various applications. One of the simplest applications of biogas is the immediate utilization for lighting, and cooking, but in a lot of countries biogas is currently utilized for combining heat and power generation (CHP) or it is upgraded and fed into natural gas grids, utilized in fuel cells or as car fuel [189]. Biogas is appropriate for production of electricity in combination with heat recovery. Normally, the gas is combusted in motors with internal combustion connected to turbine. The discharged heat (being about 60% of the used energy) is utilized for heating purposes for household requirements or maintenance of the anaerobic reactor. This method is broadly used for the treatment of activated sludge, debris generated from municipal wastewater treatment plants [184, 190, 191]. Electric power generation by gas turbines can be used by biogas as a fuel, hence substituting the natural gas for small-scale applications [184]. There is a large demand to make biogas transportable. This can be simply done only after taking out impurities such as CO₂, H₂S, and water vapor by compressing and filling the cylinders in it after scrubbing and drying processes [185]. Elimination of carbon dioxide from the flue gas assists to get fuel of higher calorific value as well as to remove the GHG [185, 192]. Biogas is an encouraging renewable source of energy. It can be immediately transformed into electricity, e.g., in a fuel cell, or burnt, discharging heat at high temperature, or burnt in a CHP for the simultaneous generation of heat and power, or fed into the natural gas network for energy rescuing purposes or it can used as fuel for cars, being sold by gas stations. Mostly, the biogas should be transported over long distances and must be purified before further utilization [18]. Biogas systems turn the cost of waste management into a revenue opportunity for farms, dairies, and industries. Converting waste into electricity, heat, or car fuel provides a renewable source of energy that can reduce dependence on foreign oil imports [189]. Biogas is mostly used in factory boilers and in engine generator sets to produce electricity and heat. In those cases, where an internal combustion engine is fuelled with biogas to produce electricity, the electricity can either be used by the facility itself or transferred to a local or national power grid [12]. The most profitable way to use biogas may be to convert it into natural gas. In reality, biogas can be utilized in all applications created for natural gas. The major difference between the two fuels is that, further to methane, natural gas consists of a variety of other hydrocarbons, like propane, butane, and ethane, which provide it a higher calorific value than pure methane. Biogas is normally burned in internal combustion motors to produce electric power. An electrical conversion efficiency of up to 25% can be obtained via small-scale internal combustion motors, with a rated capacity of less than 200 W as well as much higher electrical conversion efficiencies, of 30–35% can be provided through larger internal combustion motors (up to 1.5 MW). When biogas is utilized to generate electric power, there is the extra potential for heating water from the engine’s exhaust and cooling systems. Combining hot water generation with electric power production can provide total conversion efficiency as high as 65–85%. An encouraging near-future application for electric power production is the utilization of gas turbines. Combined-cycle power stations are made up of waste heat recovery boilers, gas turbines, and steam turbines that function together to generate electric power in the larger-scale systems. Advanced gas turbine plants tend to be small, environment friendly, greatly efficient, and visually unobtrusive. Units as small as 200 kW are not uncommon, but only those greater than 800 kW have electrical conversion efficiencies that equal or surpass an internal combustion engine-based system. Gas turbines allow a greater fraction of waste heat to be recovered as steam, a critical commodity for many industries, so overall efficiency levels for gas turbines can be up to 75%. Recently, biogas applications are employed as fuel in fuel cells and as fuel for micro-CHP (combined heat and power). When connected with an organic Rankine cycle (ORC) turbine, a biogas-powered CHP can raise electrical efficiency by 8–10%, making total efficiency rate of 45–48% more than reasonable [12, 184]. Another very attractive application of biogas for electricity production is its use in fuel cells. The specialized cells for these purposes are described briefly by [193]. Identical efficiency rates are obviously being accomplished with biogas fuel cell technique. Sweden-based Acumentrics Corporation, for instance, has registered improved performances with its 5000 W fuel cells, known as solid oxide fuel cell (SOFC) systems, which work on biogas rather than hydrogen, which is difficult to handle, high cost, and difficult to store [12]. The utilization of biogas as a fuel for civil transport and road cars in place of natural gas is already widen in United States and Western Europe [194]. There are a lot of automobiles in Sweden turning on biogas in the urban public transport [184, 195]. Biogas is currently used in many developing countries as an alternative and renewable source of energy for wide spread range of applications. In contemporary times, biogas has been used most extensively in India and China. The Biogas Association in Germany, the world’s largest producer country, included the three functions in its recent summary of what it called the national benefits of biogas production: 650 MW of installed electrical capacity comes from biogas, a reduction of 4 million tons per year of CO₂ emissions, revenues of $500 million for biogas farmers from electricity sales annually, and use by the AD process of biomass material that would otherwise end up in landfills. Economic production of biogas can be economically achieved for both large- and small-scale applications. Hence, it can be designed to fit into rural, urban, as well as regional and nationwide energy needs making it a versatile source of energy [12, 96, 107]. All over the world, Europe has registered the highest growth of biogas utilization with a notable 18% raise registered between 2006 and 2007. Sweden and Germany have registered the highest growth levels with Germany leading to brag over 4000 biogas plants, most of them are established on farms for electric power and heat co-generation [21, 96].

In Sweden, there is currently great interest in the biogas process, since it can stabilize and reduce various types of organic waste while producing renewable and environmentally friendly energy in the form of biogas. There is also increasing interest in both the production of biogas from municipal sewage treatment plants and on-farm biogas production within agriculture [196]. Biogas is also burned in boilers to produce hot water and steam in a variety of settings, including hotels, warehouses, factories, schools, prisons, and other public buildings. The forest-product segment is perhaps the largest user of biomass (combustion) energy in the industrial sector. In addition, in many countries, biogas is viewed as an environmentally attractive alternative to diesel and petrol for operating busses and other local transport vehicles [12]. The food and drinks industries are the largest users of AD for wastewater pretreatment. In 2006, 3400 GW of biogas power was generated in Germany, equivalent to 0.6% of the country’s total energy consumption, reducing carbon dioxide emissions by 2.5 million tons. Countries such as Sweden is considered pioneered in the utilization of upgrading biogas as a light duty car fuel, and the use of biogas in the country has already exceeded natural gas [12, 197]. Identical attempts are also being made in Germany which presently turns on roughly 5000 anaerobic reactors for generation of bioenergy [198]. In the UK, fears over the utilization of biogas as fuel stem from an insufficiency of quality standards and infrastructure, as well as contest with other utilizes of biogas [199, 200]. At the moment, close to 50 biogas plants, mainly small, farm-scale ones, are in operation in Austria, Currently, around 25 biogas plants operate in Denmark, with capacities ranging from 50 to 500 tons of biomass feedstock per day. The resulting biogas is mainly used in heat and power generation applications, while the digested biomass is redistributed to farms as fertilizer. Swedish company Svensk Biogas has developed a passenger train that runs exclusively on biogas. The train has a range of 600 km and can attain speeds of 130 kph. There are also up to 100 municipal busses running on biogas. The Swedish agricultural sector is also increasingly using the residues from the anaerobic digestion of crops and clean organic waste in order to return nutrients to the soil and reduce its dependence on mineral fertilizers. Biogas can also be used to generate electricity alone or with heat (co-generation). Biogas can also be used, like pure methane, as a fuel for motor vehicles [12, 201]. Biogas can be considered as alternative green energy carrier for harnessing electricity, heat, and as a transport fuel [62, 202]. Biogas is a renewable source of energy that can be used as a substitute for natural gas or liquefied petroleum gas. Biogas is a clean, efficient, and renewable green source of energy, which can be used as a substitute for other fuels in order to save energy in rural areas [86, 203, 204]. A series of zero-waste technologies are presented. They are similar to the “five zeros” of the Olympic logo which are zero waste in the product life cycle, zero emissions, zero waste in activities, zero use of toxics, and zero resource waste. This design, firstly invented by Lakhal and H’Mida [205] was titled the Olympic Green Chain model. Lately, Khan and Islam [206] suggested a method for zero-waste (mass) utilization for an ideal urban setting, involving processing and regeneration of gas, liquid, and solid. In this process, kitchen sewage waste and waste are used for diverse purposes, involving generation of biogas, water heating from flue gas, good fertilizer for agricultural production and desalination. The carbon dioxide produced from biogas burning is used for the desalination plant. This process gets zero-waste in mass utilization. The technology development in this line has no negative impact on global warming. It is estimated that biogas usage in the world will be doubled in the coming years, increasing from 14.5 GW in 2012 to 29.5 GW in 2022 [37, 189, 207].

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8. Advantages and limitations of biogas technologies

Biofuels are produced from biomass for a wide range of applications, such as cooking, heating, cooling, and transport. Biofuels can be solid (e.g., fuel-wood), liquid (e.g., bioethanol, biodiesel) or gaseous (e.g., biogas) [208, 209]. Biogas technology utilizes organic wastes for energy production, followed by recycling of the digested substrate as fertilizer [189]. Biogas can be used to generate heat or electricity, or as fuel for manufacturing or transport [210]. Electrical energy and heat generation from biogas is a source of green, environmentally friendly energy. At the same time, there is a reduction in methane emission from the decomposition of unmanaged biomass (especially animal droppings) [211, 212, 213]. Main benefits of biogas technology are to transform waste material into a valuable resource thus reducing waste, and providing valuable green energy [86, 189].

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9. Laws and guidelines concerning biogas plants

Most countries are in the process of instituting legislation to regulate the biogas industry [12]. Mostly, all parts of the plant must be checked out and licensed by the authorities. This involves installations such as tanks for liquid manure, bioreactors, gasholders, ignition oil tanks, stores, and combined heat and power stations (CHP). In biogas plants, the formation of explosive gas mixtures can happen. Thus, a system for plant security has to be present relating to installation and operation of electrical instruments in regions with high risk of explosions [18].

In Africa and the Middle East, the alternative energy market is new, so there are few government regulations and formal incentives. With inexpensive and abundant energy sources—coal in South Africa and oil in the Middle East—interest on the part of the state sector in renewable energy has been minimal. Although there are few laws or regulations pertaining to the biogas sector—reflecting the undeveloped state of the industry, due primarily to abundant and inexpensive sources of traditional fossil fuels, which gives little or no incentive to look for alternative energy sources, South Africa is leading the way in the region.

The South African Minerals and Energy Department, for instance, published its energy efficiency and renewable strategy statement in 2006, which involved statements and targets on biogas. These involved a “target requirement” to generate 4% nearly 10,000 GWh of the country’s electric power from renewable sources in 2013. In some of African countries, involving Egypt, South Africa, and Morocco, there are disagreement of interest over who has the lawful right to utilize municipal and common, specially maize (corn), or tribal, land to cultivate biogas crops [12].

In Europe, legislation is well developed, reflecting the relatively high level of biogas production in many EU markets, such as Germany and the UK, where biogas is the fastest-growing segment of the renewable fuels industry [12]. Europe is the most advanced biogas market in the world and legislation is much more developed than in other regions, with laws and regulations that not only relate to requirements to treat organic waste in certain ways, but also reach an unusual level of detail regarding machine, plant, and process specifications [18]. The European Commission is proposing an increase in the use worldwide of renewable energy sources to 20% of the total demand, which would limit global temperature changes to no more than 2°C above pre-industrial levels. The Commission has stated, rather vaguely, that it seeks, via both voluntary and legal means, to improve the EU’s energy efficiency by 20%, in order to “make the EU the most energy-efficient region in the world” [12].

In Germany, there are a lot of state laws, regulations, norms, and guidelines of branch institutions were released in order to preserve a secure and smooth biogas plants operation. These cover their waste management, installation, operation, and supply. A lot of offices are included in managerial decision for the a plant construction, e.g., the office for noise control-traffic-energy-climatic protection, the planning department and building control office, the water regulatory authority, the natural conservation authority, food control, the authority for nutrition, the authority of agriculture, the office for veterinary matters, the office for technology and plant safety, the public order office, etc. [18]. In Germany, for instance, biogas is the fastest-growing segment in the alternative energy industry. Thus, both local and national governments are beginning to oversee the sector at a much more detailed level than before. Laws requiring more cooperation between biogas producers and public utility companies, to avoid electrocution of workers who may shut down power to an electric cable but the farm-based generator continues to feed energy into it, have come into force in most states since 2006. Most states now have laws requiring electricity utilities to buy excess biogas production, either via established gas distribution networks or directly through national pipelines. EU energy vision includes a cut in carbon dioxide emissions by at least 20% by 2020 [12]. The German law of biowaste biomass specifies the biomasses and the technical processes supported. Approved biomass consists of pure herbaceous products. Not approved are fossil fuels, mixed wastes, mud, sewage sludge, and port sludge.

Debris from biogas plants are undergo to the German law of fertilizers, involving when domestic waste water undergo fermentation or when it is blended with agricultural substrates. The German law of fertilizers control single guidelines like temporary permissions to use fertilizer, the determination of fertilizing requirements, the maximum limits for the utilized amount of fertilizer, methods to use, and much more. The given name of these regulations is the “principles of the good and professional execution of fertilizing.” In Germany, no liquid secondary raw material fertilizer or liquid manure are permitted to be utilized from November 15th to January 15th. This is due to the soil is chilled and the product of fermentation cannot go through the soil. The distribution of debris from biogas plants is also managed. Maximum amount of total nitrogen from industrial fertilizer are allowed on pasture land is not more than 210 kg/ha.a and on ground used agriculturally is not more than 170 kg/ha.a. For phosphates, 120 kg/ha.a as maximum limits for both on pasture as well as on ground used agriculturally, for calcium the limit is regulated to 360 kg/ha.a. Farms with greater than 10 ha of ground utilized agriculturally are compelled to make a written fertilizer balance to keep track of their utilization [18].

In biogas plants, substances able to contaminate water are treated. In most countries, it is banned by law to contaminate water. Best available measures have to be taken in biogas plants to preserve water from pollution. The distribution of debris in agriculture should be done as stated in specified techniques [18].

In North America, the main US biogas legislation is the Biogas Production Incentives Act of 2007. A widespread unwillingness on the part of electricity companies in both the US and Canada to cooperate with biogas farm-scale producers has discouraged the development of the biogas sector. In response, many states are introducing legislation to oblige electricity providers to work with biogas producers and to buy any excess electricity. Electricity utilities, though, are generally unhappy with the arrangement, as the electricity produced by the biogas plant to run the farm is considered by them to be lost revenue. In Canada, the electric power prices provided to farmers generating biogas are still not enough to make production feasible. The Standard Offer Contract (SOC) program lately launched in Ontario, though, whereas not providing immediate financial inducements, is a first stage in the direction of promoting biogas generation though the utilization of energy crops and anaerobic reactors [12].

In Latin America, laws on quality standards for agricultural and organic waste are now being introduced, although in most cases these have not yet been clearly defined. Governments are only now beginning to make use of tax and investment incentives to encourage production. Latin America Argentina, Brazil and Colombia have the most developed legislative frameworks for the biofuels industry overall, each having set minimum requirement levels for the percentage of renewable fuels in petrol, diesel oil, and fuel oil. In Argentina, for example, the Biofuels Act of May 2006 was the first law to grant tax incentives to alternative fuel producers. The law will initially be in effect for 15 years. It stipulates that biogas can be produced from raw materials in the agricultural, agro-industrial or organic waste sectors provided they meet the government’s quality standards which have yet to be clearly identified [12].

In Asia, Asian governments are beginning to encourage the biogas sector, basically by providing financial and legal incentives to produce energy from organic waste sources in large municipal waste dumps, the existence of which is coming under heavy local opposition in many cities. In India and Mongolia, household and industrial waste laws were legislated in early 2007, requiring the separation and treatment of both kinds of waste at an early processing stage, with a view to using organic and biodegradable waste for energy production. Recently, the Indonesian government issued a statute requiring minimum levels of biogas production from new waste disposal sites that are under construction. The way in which the gas is produced, whether it be by composting, landfill, or anaerobic digestion, can be selected by the producer [12].

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10. Conclusions

Most of industrial and chemical processes produce wastes. Raw materials for producing biogas by anaerobic digestion are biomass feedstocks which include; municipal solid waste (MSW), industrial solid wastes and industrial wastewaters, food waste, livestock manure, sewage sludge, agricultural manures, catch crops, energy crops, and microalgae. Biogas production serves three important functions: waste removal, environmental management, and energy production. Biogas is a versatile renewable green energy source, which can be used for replacement of fossil fuels in power and heat production, and as gaseous vehicle fuel. Biogas technology is considered an alternative green energy resource. The use of energy and manure can lead to social economic benefits, green environment, and also contributes toward sustainable development. Renewable energy technologies provide an excellent opportunity for mitigation of greenhouse gas emission and reducing global warming through substituting conventional energy sources. Anaerobic digestion of organic waste is the most desirable management method, and this research discussed it in detail. There are undesired compounds and other gases contained in biogas which are considered as biogas pollutants. Improving the quality and quantity of biogas usually requires pretreatment to maximize methane yields and/or post-treatment to remove H₂S. The pretreatment of organic waste is the key process step in biogas production plants. Biomethane involves two major treatment processes; cleaning and CH₄ enrichment (biogas upgrading). The cleaning of the biogas consists of removal of acidic gases and impurities, while the enrichment process is for separation of CO₂ from biogas. It is noted that, there are different kinds of digesters, typically digester type is to be selected depending on the characteristics of the major feedstock used, particularly total solid. The study also concluded that, there are various cleaning and upgrading techniques to improve the quality of raw biogas which can be categorized into physiochemical and biological technologies. Some of these techniques are conventional methods, including physical absorption, chemical absorption, membrane infiltration, and biological methods, and others are considered new technologies, including cryogenic upgradation, membrane enrichment, multistage-, and high-pressurized AD. Novel technologies, such as cryogenic separation, in-situ upgrading, hydrate separation, and biological methods, represent the recent developments in biogas upgrading technologies. The biological biogas upgrading technologies can be classified into chemoautotrophic and photosynthetic. Physicochemical methods are in general at high technology readiness levels, while biological methods are still new and not commercial yet. It is reported that, most countries are in the process of instituting legislation to regulate the biogas industry. Europe is the most advanced biogas market in the world and legislation is much more developed than in other regions. Biogas is considered to be the future of renewable and sustainable energy.