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Anaerobic Digestion Process and Biogas Production

Liangliang Wei, Weixin Zhao, Likui Feng, Jianju Li, Xinhui Xia, Hang Yu, and Yu Liu

State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin, China

1.1 Introduction

The increasing amount of organic wastes worldwide has become problematic for most countries due to the continuous deterioration of land and water conditions, which poses serious risks to the safety of our community [1]. Moreover, the improper treatment of these organic wastes might lead to the undesired release of huge greenhouse gases (GHGs) into the atmosphere [2, 3]. It was estimated by the Intergovernmental Panel on Climate Change (IPCC) and US Environmental Protection Agency (US EPA) that the global anthropogenic methane emission from municipal solid wastes (MSWs) reached 1077 million metric ton of CO2 equivalent in 2020 and is expected to increase by 17% in the year 2030. Mitigation practices have forced global action to adopt a technology that can address anthropogenic methane emissions [4]. Numerous available mitigation opportunities currently include the treatment of the organic portion of MSW in a controlled facility and recovering methane as a fuel for on-site or off-site electricity generation [5].

Energy generation from the MSW and the other alternative sources will benefit climate change mitigation and minimize the alarms posed to the environment [6]. There has been a high uptake of renewable energy technologies (RETs) worldwide to deal with the detrimental effects paused by fossil-related energy generation technologies. For a purpose of increasing the energy accessibility while simultaneously restricting the worldwide temperature increased within 2?°C before 2050, adoption of RETs should be highly encouraged and raised significantly. This growing impetus for alternative avenues for renewable energy demands the consideration of different feedstocks, exploring of novel techniques, and improvements of existing technologies.

Bioenergy has been regarded as the most substantial renewable energy source due to its cost-effective advantages and great potential for substituting nonrenewable fuels. Bioenergy derived from biomass materials, such as biological organic matter obtained from plants or animals, is renewable and green. Generally, those biomass energy sources include but are not limited to terrestrial plants, aquatic plants, timber processing residues, MSWs, animal dung, sewage sludge, agricultural crop residues, and forestry residues. Undoubtedly, bioenergy is one of the most versatile renewable energies because it can be made available in solid, liquid, and/or gaseous forms. Different avenues can be explored to harvest energy from biomass materials. Biomethane has a high heating value ranging between 50 and 55?MJ?m-3 and a low heating value ranging between 30 and 35?MJ?m-3 [7].

Anaerobic digestion (AD) is practiced extensively for the treatment of biodegradable waste for biomethane generation [8]. This technology has the capability of managing the typical organic wastes such as food waste, lignocellulosic biomass and residues, energy crops, and the organic fraction of municipal solid waste (OFMSW) [9], and its environmentally sound features attracted worldwide attention for biogas production. AD is a microbe-driven, multiphase, and complex biochemical process, and four typical biochemical phases such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis are involved in its whole process. Organic matter could be efficiently metabolized by bacteria and archaea and finally converted into methane and carbon dioxide [10, 11]. However, AD processes are always limited by three main factors: (i) hydrolysis of substrates is the rate-limiting factor for the bioconversion phase; (ii) inefficient utilization of key intermediates such as propionic and butyric acid; (iii) slow growth of anaerobes of methanogenesis [12], and finally lead to a low biomethane recovery rate during their practical operation [13]. Thus, the advancements in the AD process are largely aimed toward one goal: improving biogas production and recovery.

There is currently considerable potential for biogas technology to be developed as a RET that addresses energy and environmental issues. Biogas is a critical technology that provides renewable energy from processing a variety of digestible biomass types. Substrates such as straw, forestry residues, animal and poultry manure, and other organic wastes can be treated within AD systems. The purified biomethane can be integrated into conventional fossil energy supply systems and guarantee the AD technology in energy transformation and ecological civilization construction. However, the biogas industry faces many challenges, including low gas productivity, short biogas tank life, high deterioration rates of digesters, difficulty in digestion residue utilization, and limited economic benefits [14, 15]. To improve the biogas and highlight its role in energy and environmental problem-solving, it is necessary to develop new approaches for the purpose of extending the industrial chain and further exploring new models that can promote the commercialization.

1.2 Basic Knowledges of AD Processes and Operations

1.2.1 Fundamental Mechanisms and Typical Processes of AD

AD, full microbiological degradation process under anaerobic conditions, represents one of the most promising processes to convert diverse organic substrates (animal manure, food waste, MSW, and lignocellulosic biomass as agricultural waste) into energy carriers (produced biogas mainly 55-75% CH4 and 25-45% CO2) [16].

Figure 1.1 General biochemical process involved in anaerobic digestion.

Source: D'Silva et al. [17]/with permission of Elsevier.

Microbial ecology in anaerobic digesters is quite complex, and different bacterial and archaeal communities are involved in the digestion process. The AD process is composed of four main steps, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Figure 1.1). The hydrolysis process is the primary step (stage I) in AD where organic polymers (i.e. cellulose, lipids, carbohydrates, polysaccharides, proteins, and nucleic acids) are hydrolyzed into monomers, simple sugars, saccharides, peptides, glycerol, amino acids, and other higher fatty acids, which could be summarized in Eq. (1.1):

Hydrolytic bacteria, also known as primary fermenting bacteria, are facultative anaerobes that hydrolyze the substrate with extracellular enzymes. A wide range of enzymes, i.e. cellulases, hemicellulases, proteases, amylases, and lipases, were generated in this stage and played a great role in the substrate degradation [18]. Undoubtedly, the generation of the aforementioned enzymes enhanced the whole hydrolysis. By contrast, the lack of the suitable enzymes would negatively affect the biogas generation, for instance, the hydrolyzation of lignocellulosic substrates becomes the rate-limiting step of the AD process [18]. During acidogenesis (stage II), primary fermentative bacteria convert hydrolysis products into volatile fatty acids (VFAs), including acetate, propionate, butyrate, valerate, and other acids (i.e. lactate, succinate, and alcohols). Acidogenic bacteria are able to metabolize organic compounds at a very low pH around 4. Methanogenic microorganisms cannot directly use all products from the acidogenic step. Except for acetate, H2 and CO2 and some other micromolecular organic acids were abundantly generated during the so-called acetogenic phase (stage III) by secondary fermenting bacteria, also called obligate hydrogen-producing bacteria (OHPB). However, the thermodynamics of these reactions are unfavorable, and these microorganisms can only live in syntrophy with end-product users, i.e. methanogens.

The methanogenic step (stage IV) corresponds to the final conversion of acetate, carbon dioxide (CO2), and hydrogen (H2) into biogas, and the obligate anaerobic archaea of hydrogenotrophic and acetoclastic methanogens abundantly exist in the digesters and could transform the mixture of CO2/H2 and acetate into methane. Specifically, hydrogenotrophic microorganisms convert H2 and CO2, produced by fermentative bacteria, into CH4 and keep the reactor under a low hydrogen partial pressure and thus enhanced the growth of acetogenic bacteria. The relative abundance of hydrogenotrophic and acetotrophic is variable according to environmental factors (i.e. acetate, ammonia, hydrogen, and hydrogen sulfide concentrations), and operational conditions (i.e. hydraulic retention time [HRT], pH, type of substrate, and source of inoculum), as well as solid contents [19]. It has been reported that the hydrogenotrophic methanogens (i.e. Methanoculleus and Methanobacterium) are predominated during the start-up of anaerobic digesters and lead to a subsequent decline of the H2 concentration; Then, a shift of the methanogens into the acetoclastic methanogens (i.e....