Prospects of Bioenergy Production From Organic Waste Using Anaerobic Digestion Technology: A Mini Review

Anaerobic digestion (AD) from organic waste has gained worldwide attention because it offers significant environmental and economic benefits. It can reduce the local waste through recycling which will conserve resources, reduce greenhouse gas emissions, and build economic resilience in the face of an uncertain future for energy production and waste disposal. The productive use of local waste through recycling conserves resources by reducing landfill space, the whole of life impacts of landfilling, and post-closure maintenance of landfills. Turning waste into a renewable energy source will assist the decarbonisation of the economy by reducing harmful emissions and pollutants. Therefore, this mini-review aims to summarise key factors and present valuable evidence for an efficient AD process. It also presents the pros and cons of different AD process to convert organic waste along with the reactor technologies. Besides, this paper highlights the challenges and the future perspective of the AD process. However, it is highlighted that for an effective and efficient AD process, appropriate temperature, pH, a strong inoculum to substrate ratio, good mixing and small particle sizes are important factors. The selection of suitable AD process and reactor is important because not all types of processes and reactors are not effective for processing organic waste. This study is of great importance for ongoing work on renewable energy generation from waste and provides important knowledge of innovative waste processing. Finally, it is recommended that the government should increase their support towards the AD technology and consider the unutilized significant potential of gaseous biofuel production.


INTRODUCTION
Waste production is growing rapidly, and the World Energy Council has projected that more than 6 million tonnes of waste will be produced every day by 2025 (WEC, 2016). When wastes are sent to landfill, they produce methane which is 34 times more potent than carbon dioxides that contribute to global warming and climate change (Atelge et al., 2020). Therefore, it is crucial to develop efficient waste to energy technologies (WtE) to enhance energy recovery from the wastes that currently goes to landfill. The term WtE is used to explain the handling methods or processes used to produce and optimize the output of an energy source like heat, electricity, or waste fuel. While the amount of waste diverted from landfills has generally increased in recent years, there is relatively little energy recovered from waste. Development of high energy fuels including renewable diesel, biodiesel (Mofijur et al., 2020a), bioethanol, and biogas is based on the need to reduce our carbon footprint (Mofijur et al., 2016;Ong et al., 2020) and on the security of our fuel supply (Ong et al., 2019;Muhammad et al., 2021). Energy and value-added products recovery from waste play an important role in the urban waste hierarchy, making better use of waste, and meeting government targets. Researchers around the world are developing different approaches to convert organic waste into value-added products (Ma and Liu, 2019). WtE technologies have been reported to have the potential to tackle waste and related problems, i.e., air pollution, health effects, fuel protection, fossil fuel import dependence, and GHG emissions (Rajendran et al., 2013;Leung and Wang, 2016;Yin et al., 2016;Li et al., 2017b).
Different technical routes are available to produce renewable energy from organic waste (Ma and Liu, 2019;Mofijur et al., 2020b;Zamri et al., 2021). Figure 1 summarizes the ways through which waste is transformed into energy (O'Hara and Melssen, 2013). The technical routes that allow the production of renewable energy include direct combustion, AD, transesterification, hydrothermal liquefaction, fermentation, pyrolysis, and thermal gasification (Rajendran et al., 2013;Yin et al., 2016;Li et al., 2017b;Khan and Kabir, 2020). Direct combustion is the most commonly used form of converting waste into heat or electricity. During this process, the extracted fuel from waste is combusted in the presence of excess oxygen (the oxygen that collects from the air) to produce energy. Waste gasification is a process that converts organic sources into syngas consisting of CO, H 2 , CO 2 , N 2 , CH 4 , etc. at high temperatures and with a regulated oxygen condition (Patel et al., 2016). Pyrolysis is the thermochemical degradation process of organic or inorganic waste at a high temperature (430°C) without oxygen (Pham et al., 2015;Uddin et al., 2018). Anaerobic digestion is the mechanism by which organic waste is broken down to generate biogas and biofertilisers. AD requires a series of processes where microorganisms in the absence of oxygen break down biodegradable material. This can be used for commercial, agronomic, or household waste management and/or the production of clean fuels. Landfill gas combustion is commonly recognized as a WtE process in which electrical energy is produced. Landfills bioreactors are made for facilitating prompt waste digestion using liquid and air injection, leachate treatment, and recirculation resulting in higher landfill gas generation (Environment Protection Authority South Australia, 2017). Enhancing resource recovery and discussing the place of energy recovery.
All technologies are appropriate for the specified organic waste sample; for instance, the biogas production through AD provides greater versatility in the feedstock ranges used. Biogas can be transformed into not only electricity but also heat. Also, it can be transformed into biomethane, which almost the same chemical components as natural gas (Zabed et al., 2020). It can provide essential resources to large populations without causing global warming. It supports societies and helps us to combat the environmental problems we are confronted with today. If successfully introduced, a biogas digester will fuel an entire city from its organic waste. Therefore, this study aims to discuss the progress, challenges, and future perspective of bioenergy production from organic waste through anaerobic digestion that will provide insight into the efficient and costeffective waste to energy technology. Overall, this study is of great significance for ongoing work on the sustainable production of energy from organic waste and offers valuable information that is very useful for researchers, industry experts, and policymakers.

THE PRINCIPAL OF THE AD PROCESS OF ORGANIC WASTE
AD is one of the most efficient and reliable methods of waste management that can treat waste with high moisture content. AD is a set of biological processes where, in the absence of oxygen, microorganisms break down the biodegradable materials. The biological processes include hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Supaphol et al., 2011;Christy et al., 2014). The first step is hydrolysis where substrates are converted into soluble molecules. Extracellular enzymes called hydrolase are used to conduct hydrolysis reactions. Hydrolases can include esterase, glycosidase, and peptidases (Elefsiniotis and Oldham, 1994). This step transforms carbohydrates into sugars and converts lipids and proteins to long-chain fatty acids and amino acids (Dahiya et al., 2015). In the second step, fermentative bacteria transform soluble molecules formed at the hydrolysis stage into volatile fatty acids, lactate, alcohol, and CO 2 (Silva et al., 2013). The main products in this step are acetic acid, propionic acid, and ethanol (Zhou et al., 2018). In the acetogenesis step, acetogenic bacteria oxidize VFAs and alcohols into acetic acid and H 2 (Zinder, 1990). Bacteria that form the acetate by using butyrates and propionates respectively are known as Syntrophobacter wolinii and Smithella propionica. Pelotomaculum schinkii. Clostridium aceticum is another microorganism that develops H 2 and CO 2 acetate. The last step is called methanogenesis where CH 4 and CO 2 are derived from acetogenesis products by methanogenic bacteria (Eryasar and Kocar, 2004).

FACTOR AFFECTING THE AD PROCESS OF ORGANIC WASTE
Different chemical and physical factors affect the AD mechanism (Kwietniewska and Tys, 2014) including seeding, stirring, temperature, pH, C: N ratio, organic loading rate (OLR), hydraulic retention time (HRT), and volatile fatty acids (VFA). Any changes in these parameters can result in a breakdown in the digester process as this changes the environment of microbes and movement inside the digester. Thus, to maximize biogas production, it is important to control these parameters. These parameters should be varied within an appropriate range so that the biogas plant can run effectively and efficiently. The feedstock used for the production of biogas by digestion must promote an atmosphere suitable for the proper development and optimum metabolic functioning of microorganisms involved in this process.
Biological methanogenesis can occur in a wide range of temperatures from 2 to over 100°C (Megonigal et al., 2004). However, for optimum operation in mesophilic and thermophilic environments, temperatures should be about 35 and 55°C respectively (Scaglia et al., 2014). The mesophilic temperature can vary between 32 and 42°C, while the thermophilic temperature may be between 45 and 57°C to increase the substrate degradation. The most successful working temperature of the slurry was stated to be 20-45°C in small biogas facilities (Ortega et al., 2008). Another important factor that influences the AD process is pH. If there is a very low or high pH of the biogas feedstock, neutralization of the feedstock is important before being fed to the plant. The pH can artificially be strengthened by adding a base to the reactor when there is negligible acidification during the anaerobic phase (Netter and Bischofsberger, 1990;Ali, 2015). It has been reported that the optimal pH range for the AD technology is 6.8-7.4 (Mao et al., 2015), 6.8-7.2 (Ward et al., 2008), and 6.8-7.5 (Khalid et al., 2011). Methanogenesis is stated to be most effective at pH 6.5-8.2 with the optimal value of 7.0 (Cerón-Vivas et al., 2019). The optimal pH for hydrolysis and acidogenesis is between 5.5-6.5 and 6.5-8.5, respectively (Mao et al., 2015). An optimum C: N ratio is crucial for an effective AD process. It has been reported that fast AD occurs when C: N is between 25 and 35:1 (Resch et al., 2011). In these ranges, some bacteria release nitrogen from their cells and die to restore equilibrium (Kondusamy and Kalamdhad, 2014).
Biogas production depends on fermentation material concentrations; excess concentrated or diluted materials contribute to decreased production (Abbassi-Guendouz et al., 2012). The ideal solid concentration in sewage sludge digestion varies between 8 and 10%. However, biogas digesters can be built for the digestion of a greater concentration of (up to 40%) solids (Saady and Massé, 2015). Biogas yields typically increase to some degree as OLR increases. The OLR is influenced by some factors such as solids content, active microbial concentration in the digester, the temperature of the digester, and HRT (Guo et al., 2014). Very low OLRs may contribute to deprivation and adversely affect AD (Zhang et al., 2019). Too high OLRs may generate insufficient products that promote microbial growth while large loads contribute to an accumulation of VFA in a fermenter that prevents microbial growth (Dhanya et al., 2020). The optimum OLR for co-digestion of food waste and manure was stated to be 3 gVS/L/d (Agyeman and Tao, 2014); for grass, manure, straw, fruit, and vegetable waste 7.5 gVS/L/d (Ganesh et al., 2013); for sugar beet by-product and pig manure 11.2 gVS/ L/d (Aboudi et al., 2015); and for used coffee grounds and sludge 23.6 gCOD/L/d (Qiao et al., 2013). HRT influences the association between substrates and microorganisms, thereby makes treatment more effective. the shorter HRT is beneficial because it specifically concerns a reduction in capital costs and enhancement in process efficiency (Shi et al., 2017). It has been reported that a minimum of 10 days HRT is required to prevent bacteria from being washed. With compared to the shorter HRTs, longer HRTs have been reported more advantageous in order to produce biogas and methane. Volatile fatty acids (VFAs) are one of the crucial properties that affect the AD process (Magdalena et al., 2019). Approximately 99.9% of VFAs exist in the dissolved form at pH 8.0, while approximately 90% are dissolved at pH 6.0 (Forgács, 2012). However, methanogenesis can be prevented by a rise in VFAs in AD. Therefore, the VFAs as a measure of the effectiveness of the AD process is generally endorsed. Another important factor that influences the AD performance is stirring. This ensures uniformity of microorganisms and equal distribution of heat; thus improves the interaction between bacteria and food. Besides, Stirring also removes gas bubbles, preventing layer formation and settlement (Tian et al., 2015). Stirring can be performed in both electrically and mechanically, with stirrers mounted either vertically or horizontally within the bioreactor.

CLASSIFICATION OF THE AD PROCESS OF ORGANIC WASTE
The AD process typically is classified based on biological, technical, reliability, and overall performance (Mahmudul et al., 2021). Based on the overall solid content, the AD process is categorized as a dry or wet process; according to the feeding mode, it is categorized as a batch or continuous process; according to the operating temperature, it is categorized as a mesophilic or thermophilic; according to the number of the stages, it is categorized as a single-stage or multi-stage; according to the digester, it is classified as a fixed dome, floating dome, balloon, and garage type AD process. The solid content of a dry AD process is between 20 and 40% whereas for a wet AD system it is >15% (Deepanraj et al., 2014;Kothari et al., 2014).
In a batch type AD process, the waste is fed once in the digester, and inoculum is added when it is closed for a fixed period whereas, in a continuous system, the waste is fed continuously in the digester. However, the system is called mesophilic if the operating temperature is between 20 and 40°C and thermophilic if the operating temperature is 50-65°C. AD may either be performed in one or two-stage/ multi-stage systems. In a single-stage system, the digestion occurs in one reactor while two or more reactor is used in the multi-stage system (Lissens et al., 2001). Two-stage AD is effective for the treatment of a large range of waste. Table 1 shows the pros and cons of different AD technologies of organic waste.
Recently a significant number of researchers are working on recovering the bioenergy from organic using different types of AD technology. Research in dry and wet AD processes for the treatment of a wide range of waste shows that the dry process offers a higher CH 4 yield (0.48 L/gVS) and lower VSR rate (85.6%) than the wet process (Yi et al., 2014). The dry AD system enables both VFA and OLR to be enhanced in contrast to the wet process, decreasing the capability for AD inhibition. However, the batch and continuous feeding effects analysis has shown that the AD process with an on-going feeding system retains a stable biogas yield and has better efficiency than the AD process with a batch feeding system (Park et al., 2018). The research into the influence of temperature on the digestion of organic waste suggests that the mesophilic AD system generates less CH 4 (150 mL/gVSCH 4 ) than the thermophilic system (Li et al., 2017a). The efficiency of biomethane production by twostage AD was reported to be 30% greater than single-stage AD output (Voelklein et al., 2017). Zhang et al. (2017) reported that the multi-stage AD system for digesting organic waste offers up to 54% higher CH4 yield and 83.5% VSR efficiency compared to the single-stage AD system.

AD technologies
Solid content (Deepanraj et al., 2014;Kothari et al., 2014;Moya et al., 2017) Feeding mode (Vandevivere et al., 2002;Park et al., 2018;Orhorhoro and Erameh, 2019;Mahmudul et al., 2021) Temperature (Banks et al., 2008;Khalid et al., 2011;Gebreeyessus and Jenicek, 2016;Kang and Yuan, 2017) Stage (Vandevivere et al., 2002;Hagos et al., 2017;Voelklein et al., 2017;Xu et al., 2018 Digester type (Axaopoulos et al., 2001;Nzila et al., 2012;Vögeli et al., 2014)  VS respectively. The author suggested that the construction cost can be reduced by altering the reactor ratios. Tomei et al. (2016) used two different types of reactors: aerobic and anaerobic reactors. The authors varied the organic loading time for the reactors. The retention time was varied for the aerobic reactor from 9 to 12 days while the retention time for the anaerobic was set at 15 days. The authors reported 66% of methane yield. Boe and Angelidaki (2009) compared a single thermophilic CSTR with a serial CSTR configuration. The authors selected retention time of 15 days for both the system. The authors reported that the serial CSTR configuration was able to yield 11% more biogas compared to the single CSTR and thus established that serial CSTR configuration may improve biogas production from manure.

CHALLENGES OF AD PROCESS OF ORGANIC WASTE
Given several advantages, the processing of bioenergy from waste via AD has many severe challenges. Social acceptance is also influenced by environmental and health issues. Biogas contains unwanted and hazardous substances such as H 2 S, Si, VOCs, CO, and NH 3 (Kristensen et al., 2004;Nielsen et al., 2010). H 2 S and NH 3 are harmful and highly corrosive, causing harm to combined heat and power units and metal parts (Angelidaki et al., 2018). The presence of H 2 S affects the quantity and quality of the generated biogas as well as emits harmful emissions and corrode the biogas purification system (Farghali et al., 2020). Since the biogas generated by AD contains impurities, it typically needs preventative treatment to enhance methanol yields and post-treatment to eliminate H 2 S. These processes are energy-intensive as well as expensive (Farghali et al., 2020). Also, climate effects on biogas production are likewise to other renewable energy sources.

CONCLUSION AND FUTURE PERSPECTIVE
This paper reviewed the progress and challenges of the AD process of organic waste. AD provides an effective solution to treat organic waste, meet local energy demand, reduce waste, and improve energy security and air pollution. AD method yields a second life to materials which are otherwise considered waste. Biogas is a multipurpose renewable green energy source and it can be used as a substitute for fossil fuels for heat and power generation, and as a vehicle's fuel. The stimulating paradigms in the AD technique of organic waste are analyzing and characterizing the organic materials, biodegradability, involving manifold microbial activities, accessibility, fixing the exact limiting factors and steps. Optimum operating conditions in the large scale of AD process biogas plant demand adjustment based on the environmental factors and the availability of raw materials. This is because of the miscellany of feedstock composition. In this circumstance, the local information and data of the feedstocks such as data availability, accessibility and degradability, and design of universal anaerobic reactors are inevitable. There are some unwanted elements and other gases comprised in biogas which are taken into consideration as biogas pollutants. Enhancing the quality and quantity of biogas generally demands pre-treatment to maximize methane production and/or post-treatment to abolish H 2 S. In biogas production plants, pretreatment of organic waste is considered the key process.
Biogas has definite benefits over other clean energy alternatives. It can be generated when needed and easily stored. It can be delivered via existing gas pipelines and used in the same applications as natural gas. In addition to using renewable electricity and heat, biogas will substitute fossil fuels in the transport sector. However, proper process control with essential parameter measurements can optimize the process and increase the biogas output. Moreover, the potential production of biogas systems should be aimed at decreasing the cost of capital and management.
Finally, for allowing the AD technique to encounter its full potential, the policymaker should consider a standardization process which patronizes the redirection of "waste to landfill" to "waste to re-use", and encourage the use of low carbon gas for energy generation. The government should increase its support for biogas production and consider the available unused remarkable potential for biogas production. With sustained efforts, biogas will be a remarkable solution for the depletion of GHG emissions, management of waste disposal, and production of renewable energy.

AUTHOR CONTRIBUTIONS
MU original draft. SKS original draft. MM original draft. FD supervision. MH review and editing, PS supervision, SA review and editing, Y-MC review and funding.