1. Introduction
The anaerobic digestion (AD) process, an environmental protection technology through organic wastes and wastewater treatment, involves converting biodegradable organic waste into biogas and biofertilizer
[1][2]. It is a biotechnological treatment process that recovers energy (biogas), value-added products, and nutrients (nitrogen, phosphorus, and potassium) from biodegradable organic waste in the absence of oxygen
[3]. Nitrogen (N), phosphorus (P), and potassium (K) are recovered in biosolid form, which may be applied as biofertilizers on agricultural land if the level of the pathogen is very low. The biogas can as well be transformed into electricity and heat
[4]. Despite the noncommercialization of this technology, it remains one of the promising technologies that convert wastes into biogas and odor-free residues that are rich in nutrients that can be used as fertilizers
[5].
2. Classification of the AD Process Technology
The AD process technology can typically be categorized or classified based on the following: (a) the nature of the total solid content, (b) the feeding mode, (c) the operating temperature, (d) the number of operational stages, and (e) the type of digester and reactor configuration (i.e., technical)
[6]. The AD process can be categorized into different types based on various factors. The total solid content determines whether the process is dry or wet, while the feeding mode determines if it is a batch or continuous process. The operating temperature determines if the process is mesophilic or thermophilic. The number of stages involved in the process categorizes it as a single-stage or multistage process. Finally, the type of digester used classifies the AD process as a fixed dome, floating dome, balloon, or garage type.
2.1. Dry and Wet AD Process Technology
The dry and wet AD process technologies are dependent on the amount of total solid content (TSC) in the system. The dry AD (D-AD) process technology which is also referred to as high-solid (HS-AD) or solid-state process technology, is the one that generally operates with a feedstock of 15–40% TSC
[7] or 20–40% TSC
[8]. That is, D-AD process technology works at a higher TSC than wet AD process technology
[8]. This indicates that D-AD allows for treating or handling higher quantities of waste per digester volume
[8]. It has been claimed that D-AD or HS-AD is more advantageous over wet AD process technology based on a number of reasons, such as reduced energy input for heating and stirring, higher volumetric loading capacity, greater ease in handling of the digestate, smaller reactor volume, and abrasion reduction in the reactor from sand and grit
[8][9][10]. Despite the merits of D-AD or HS-AD process technology, it also has some demerits, which include low operational stability, long degradation times, lower biogas and biomethane production when compared to the wet AD process technology, liquid and gas diffusional problems, higher inoculation ratio and the accumulation of toxic and inhibitory components or compounds (volatile fatty acids, ammonia, and heavy metals), which still hinders their wide applications
[7][8][11]. The reasons for these demerits have been attributed to the high TSC
[11] and reduced water content which consequently reduces the availability of substrate to the microorganisms and thus affects their metabolism
[12].
Wet AD (W-AD) process technology, also referred to as liquid AD (L-AD) process technology, is the one in which the feedstock is mixed with a large quantity of water to provide a dilute feed of 10 to 15% dry solids
[13]. That is, the W-AD or L-AD systems typically operate with 0.5–15% TSC
[13][14][15]. Consequently, dilution with water, liquid digestate fraction, or slurry is essential to acquire total solid contents of less than 15%. Some studies, as obtained from the literature, all revealed that an increase in the water content results in an increase in biogas and biomethane yields
[16][17]. This increase in water content produces a better homogenization of the digester’s contents in the AD, increasing the interaction between bacteria and nutrients, reducing the problems of diffusion problems, and diluting any potential inhibitors
[8][17]. The advantages and disadvantages of W-AD have been well documented
[10][13]. However, one of the main disadvantages of the wet process is that a large amount of water is required, and the biogas output is not proportional to the volume of water required; thus, when a large volume of biogas is required, a large reactor volume will be required to accommodate the large volume of water required
[13].
2.2. Batch and Continuous AD Process Technology
The AD is classified on the basis of its feeding mode or operation mode, namely batch and continuous modes. In a batch AD process technology, the digester is fed with fresh raw materials, then tightly closed and sealed, and left for a fixed duration until the overall degradation or digestion is accomplished. The digester is emptied once the digestion is completed, and a new batch of organic feedstock is fed. The effluent or residues are then removed to allow the new process to take place
[18]. Generally, it is essential to have a number of digesters in a batch process so that alternate loading and emptying can be done. A batch digester is technologically simple and straightforward, requires fewer moving parts, is cheap or inexpensive, has a low maintenance cost, and has limited energy losses
[19].
In a continuous AD process technology, fresh raw materials are fed regularly and constantly into the digester to replace or maintain the same amount of digested waste as products are continuously withdrawn
[7]. Typically, a pumping system is fixed for the transportation of the feed into the digester. Any interruption during pumping will affect biogas production. The advantages of using a continuous feeding mode in anaerobic digestion include (i) a constant biogas production rate by maintaining a steady feedstock input, (ii) a smaller land area requirement, (iii) lower operating costs, (iv) uninterrupted digestion, (v) a continuous cycle of input and removal of bio-waste, and (vi) the achievement of steady-state conditions
[6]. However, there are also some disadvantages associated with this method, including (i) higher initial investment costs, (ii) technical difficulties associated with the pump used for loading, and (iii) a requirement for high internal fluidity to ensure a smooth feedstock intake and removal process.
2.3. Mesophilic and Thermophilic AD Process Technology
Based on the operating temperature, as earlier mentioned, AD process technology can be classified as mesophilic AD process technology and thermophilic AD process technology. Mesophilic AD process technology is the technology where the AD process is performed at a temperature range of 20–40 °C in which the mesophilic organisms that are involved in the degradation or digestion process grow and perform optimally. According to Gebreeyessus and Jenicek
[20], the advantages of this technology are: (i) the microorganisms can tolerate greater environmental changes, (ii) it is more stable and easier to maintain, and (iii) it involves the use of smaller or minimal energy, while the disadvantages include longer retention time and lower biogas production. Several studies have been performed with the use of this technology for the biological treatment of sewage sludge
[21], slaughterhouse waste
[22], sugarbeet pulp
[23], cattle manure
[24], corn silage
[25], and fruit and vegetable waste
[26].
Thermophilic AD process technology is the technology in which the AD process is carried out at a temperature range of 50 °C–65 °C, where the thermophilic organisms involved in the digestion or degradation process grow and perform optimally. A review of the advantages and disadvantages of the use of this technology has been made by Gebreeyessus and Jenicek
[20]. According to the review, the advantages of this technology include lower retention time, higher organic loading rate, increased biogas/biomethane generation, and higher pathogen destruction, while the disadvantages are responsive to toxins, the presence of less distinct microorganisms with an attendant less efficient mechanism, difficult maintenance of the system, and higher energy required for heating. Many studies have been conducted with the use of this technology for the biotreatment of organic wastes to produce biogas/biomethane, and such wastes include sewage sludge
[27], sugar beet pulp
[23], cattle manure
[24], solid waste residues from palm oil mill (empty fruit bunches, oil palm fronds, and oil palm trunks)
[28], and food waste
[29].
2.4. Single and Multistage AD Process Technology
AD process technology is further classified based on the number of operational stages, namely, single-stage and multistage AD process technologies. A single-stage AD (SS-AD) process technology is that technology that involves the use of a single biodigester or reactor where all the four steps (i.e., hydrolysis, acidogenesis, acetogenesis, and methanogenesis) involved in microbial digestion simultaneously take place
[30]. This implies that in SS-AD, acidogenic and methanogenic microbial species have to cohabit despite their marked differences with regard to nutritional needs, environmental factors such as temperature and pH, growth factors, and kinetics
[30][31]. It is pertinent to note that most AD applications operate in single-stage systems
[32]. This is because the SS-AD process has proven to possess many advantages, such as simplicity in design, low cost or low capital investment, low maintenance cost, recirculation adaptability, less technical failure, lower volatile solid losses, and smaller reactors required
[13][29][32]. Notwithstanding the advantage, the SS-AD process still has some disadvantages or limitations, such that it cannot alone handle organic waste with TSC under 20%, the possibility of dilution with water is low, restricted bioreactor heights, high fluctuations in biogas/biomethane production, and the loss of biogas during the emptying of the bioreactors
[30]. So far, the SS-AD process technology has been utilized to biologically treat several arrays of feedstock, including food waste
[13][29][33], manure
[34], sewage sludge
[35], vegetable waste
[36], and municipal solid waste (MSW)
[37], and process performance was optimized in the greatest number of cases by recirculating the process digestate back into the reactor
[38].
The multistage AD (MS-AD) process technology currently includes two-stage, three-stage, and four-stage systems. In the MS-AD process, there is the physical separation of the four biochemical reactions or digestion steps. A two-stage AD (TS-AD) process technology is a technology where the AD process is conducted in two biodigesters or bioreactors in which all the biochemical reactions sequentially occur
[39]. That is, in a TS-AD process system, the first step entails introducing the feedstock into the first digester or bioreactor (acidogenic bioreactor), where hydrolysis, acidogenesis, and acetogenesis occur, and the partially digested feedstock is then removed and fed into the second bioreactor (methanogenic bioreactor), where the biogas/biomethane is finally produced
[32]. This implies that in a TS-AD process, acid fermentation, and methanogenesis are separated into two different bioreactors in order to optimize operating conditions for the acidogenic and methanogenic microbial species. The first (acidogenic) stage is typically performed at a low hydraulic retention time range of two to three days and a pH range of five to six, while the second stage (methanogenic) is operated at a hydraulic retention time of 20 to 30 days and a pH range of six to eight
[15]. Comparing the TS-AD process to the SS-AD process, the TS-AD process allows rapid and efficient biogas/biomethane generation in the second stage
[40]. The multistage AD (MS-AD) process concept that involved a three-stage reactor was developed in the early part of the 1990s
[32]. The distribution of the biochemical reactions or digestion steps in a three-stage AD is hydrolysis/acidogenesis, acidogenesis/acetogenesis, and acetogenesis/methanogenesis
[6]. In this MS-AD (i.e., three-stage AD), the first stage involves the semianaerobic hydrolysis of feedstock at a low hydraulic retention time and the removal and transfer of undegraded waste to the secondary bioreactor for acidogenisis. From the secondary bioreactor, the liquid and solids output are removed and fed into a tertiary bioreactor where biogas/biomethane is finally produced
[32]. The key benefits of the TS-AD, T
hS-AD, or MS-AD processes over the SS-AD process are higher biogas/biomethane yield or better energy recovery, increased volatile solid removal performance, enhanced process stability and reliability, better control of pathogens, reduced retention time, reduced reactor size
[30][31][41]. Nevertheless, the disadvantages lie in the fact that the design is complex (i.e., they are complex systems), biogas/biomethane yield is low if solids are not digested and involve large or high cost of investment, operations, and maintenance. Voelklein et al.
[42] reported that the biomethane production performance of TS-AD is 30% higher than that of SS-AD. TS-AD or MS-AD process technologies are suitable for processing a wide range of wastes. The TS-AD process technology has been applied for the processing of wastes in biogas/biomethane. Such wastes are swine manure and market biowaste
[43], cheese whey and cattle manure
[44], fruit and vegetable waste and food waste
[45], vegetable oil residue and pig manure
[46], and food waste and sewage sludge
[47]. Also, the MS-AD (i.e., three stages) has been utilized to process organic wastes into biogas, and this includes wastes such as food waste
[48], food waste, and horse manure
[49].
3. Types of Anaerobic Digesters
The construction of a biodigester depends on some key factors, such as the type of feedstock, hydrological and geological conditions, and weather or climate conditions
[50]. In the design of a biodigester, the two key parameters that must be chosen correctly are the number of stages and the total solid content since these parameters significantly affect its performance and reliability as well as the overall cost
[6]. Primarily, digester design is concerned with the rate, stability, and completion of biochemical reactions
[51]. There are various types of anaerobic digesters. The variations in their designs are due to climate differences, feedstock types, feedstock amount, feedstock fluid dynamics, structural strength, material availability and cost, design complexity, and process duration
[51]. Each designed digester has its advantages and disadvantages. In anaerobic digester technology, there are several types of digester configurations. The three major types of anaerobic digesters are (a) covered anaerobic-lagoons digesters, (b) plug-flow digesters, and (c) complete-mixed digesters
[52]. The other types of digesters include fixed-dome digesters, floating-drum digesters, and balloon-type digesters
[50].
3.1. Covered Anaerobic Lagoons
Anaerobic lagoons are ponds that are covered in which feedstock is fed at one end, and the residue is removed at another end
[52]. It is used primarily for liquid or diluted waste that contains <2% solids. Plastic with an impermeable cover is used to collect the produced biogas
[51]. It is widely used in cold climate regions for swine or dairy operations and uses a flush system to transport the manure. However, its drawbacks include a low rate of reaction due to the low reaction temperature, no mixing due to a closed lagoon causing coagulation of solids at the bottom of the digester, which results in less contact between the bacteria and feed, and a higher energy requirement to screen out coagulated solids
[53].
3.2. Plug-Flow Digester
The plug-flow digester consists of a long tubular digester or tank with varying sizes (2.4–7.5 m
3), which has a constant volume that produces biogas at variable pressures
[50]. It can be fixed either vertically or horizontally. The digester consists of an inlet and two outlet pipes, which are fixed at opposite ends above ground level. The outlet pipes are connected to the digestate extraction system unit. As the fresh feedstock is introduced into the digester through the inlet, the digestate moves towards the other end of the tank and comes out through the outlet pipes into the digestate extraction unit. It is best suited for feedstocks such as cattle manure with high total solid content in the range of 11% to 14%
[50][53]. Plug-flow digesters may have fewer moving parts
[52] or no moving parts
[50], thus requiring less maintenance. Since the plug-flow digester is a growth-based system, cleaning the reactor is inexpensive
[51]. The main advantages of plug-flow digesters are their ease of use, adaptability to extreme conditions, ease of installation, and low maintenance costs
[50].
3.3. Total-Mixed Digester
In this type of digester, all the organic wastes are combined together into a single tank, and an agitation system is introduced to mix the content while it is being digested
[53]. Various agitators can be used, such as mechanical mixers or recirculation pumps. The most efficient type, in terms of power consumed per gallon mixed, is the mechanical mixer. This system is suitable for handling manures with 3% to 10% solids
[52]. The advantage of the completely-mixed reactor is that it is a proven technology that achieves reasonable conversion of solids to gas
[52]. This process is widely used in industries to convert waste into biogas
[51].
3.4. Fixed-Dome Digester
The fixed-dome digester, also known as the hydraulic digester, is the most prevalent model developed for biogas production
[50]. It is characterized by a simple construction that does not include any movable components. The digester comprises a dome-shaped chamber equipped with inlet and outlet pipes, as well as a gas pipe attached at the top of the dome chamber. The substrates are loaded through the inlet pipe until they reach the bottom of the chamber, and the resulting biogas collects in the upper storage part of the digester. Modified versions of the fixed-dome digester have been created in many countries worldwide
[50]. Generally, these digesters are constructed underground and require minimal space
[54]. Thus, it is expected that this type of digester can be utilized for a long number of years. Fixed-dome digesters take a longer time to warm up. The digester’s size depends on the amount of substrate available daily and the location and number of households that will make use of it
[50]. In general, if well constructed, fixed-dome digesters have advantages, including lower manufacturing or investment cost, low maintenance costs, long life span, less variation in temperature (due to being built underground), and less space requirement
[55]. Nevertheless, some disadvantages could be that a skilled technician will be required for the construction, it might be hard to repair since it is built underground, fluctuation in gas pressure depending on the stored gas volume, and difficulty in constructing it in bedrock.
3.5. Floating-Drum Digester
A floating-drum digester, also known as a Gober gas plant, comprises a cylindrical or dome-shaped chamber, an underground digester (either cylindrical or dome-shaped), and an inverted movable steel drum or gas holder
[50]. The steel drum, positioned on the digester, serves as a gas storage tank that separates gas accumulation from the production process, thereby maintaining a constant gas pressure
[56]. Floating-drum digesters generate biogas with a variable volume at a steady pressure
[50]. The pressure required for gas flow through the pipeline for utilization is achieved by the weight of the steel drum
[57]. The inverted steel drum moves up and down depending on the quantity of accumulated gas stored at the top of the digester, necessitating regular painting to prevent rusting
[50]. The advantage of this type of biodigester is its simplicity, ease of construction, and operation. However, it has several drawbacks, such as high steel material costs, regular maintenance and repair expenses, and a relatively short lifespan
[58].
3.6. Balloon Digester
The balloon biodigester consists of a polyethylene tubular film sealed at both ends along with inlet and outlet polyvinyl chloride (PVC) pipes with rubber straps of recycled tire tubes wound around them
[58]. The digester also consists of an installed PVC pipe at the top of it to allow the generated biogas to be released into a reservoir collection bag. A hydraulic level is created within the digester between the inlet and outlet in such a way that the amount of digestate that leaves the outlet is equal to the quantity of organic matter (a mixture of feedstock and water) that is added. The advantages of this type of digester lie in the fact it is easy to construct with low cost for construction, it has a shallow installation depth that makes it suitable in areas with high groundwater tables, and not being complicated in digester emptying and maintenance
[58]. Its demerits are that the digester is susceptible to mechanical damage, scum cannot be removed from it, and it relatively has a short life span
[58].