Summary
Anaerobic digestion plays an essential role in decomposing organic waste and converting it into valuable products, ensuring sustainability through waste management, recycling nutrients, and reducing greenhouse gas emissions. To achieve efficient degradation of organic matter, various types of microorganisms are utilized, each with a specific function in the anaerobic digestion process. The process consists of four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. In addition, some process parameters, such as temperature, pH, C/N ratio, retention time, organic loading rate and substrate mixing in the reactor, affect the anaerobic digestion performance. Application of the anaerobic digestion process includes wastewater treatment, biogas production, industrial organic waste and agricultural waste management. Despite its widespread applications, it still faces challenges such as the quality of end products, optimization of process parameters and inhibiting elements for the anaerobic digestion process. To address these challenges, future research should focus on the process optimization, designing and evaluating new digesters, process monitoring and controlling, inhibition management and sustainability assessment. This article aims to discuss the fundamentals of the anaerobic digestion process.
Introduction
Anaerobic digestion is a chemical process in which microorganisms break down organic matter in an enclosed chamber known as a digester. Agricultural wastes, municipal waste, industrial organic waste, livestock manure, and food scraps are all examples of organic matter or feedstock for anaerobic digestion. This digestion provides an ideal anaerobic environment for the microorganisms, which breaks down the organic matter in the absence of oxygen to produce biogas and digestate [1]
The anaerobic digestion process consists of four key biological and chemical stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In the first stage, hydrolytic bacteria break down complex organic matter into simpler soluble compounds. The soluble organic compounds are then converted into volatile fatty acids by acidogenic bacteria in the second stage. In the third stage, acetogenic bacteria convert the volatile fatty acids into acetic acid, carbon dioxide and hydrogen. Finally, methanogenic archaea convert the products from the previous stages into methane gas in the fourth stage [2].
Biogas produced in this process typically consists of 50-75% methane, 25-45% carbon dioxide, and small amounts of hydrogen sulfide and water vapor. The composition of biogas depends on the feedstock used in anaerobic digestion. Carbon dioxide, hydrogen sulfide and other gases are separated from methane, which can be used as fuel for heating or electricity generation as it is the main component of natural gas [3].
The remaining digestate is the solid-liquid residue rich in nutrients like nitrogen, phosphorus and potassium that can be used as organic fertilizer. When applied on agricultural land, digestate improves soil properties and crop yields [4].
Role Of Microorganisms
Microorganisms play an important role in the process of anaerobic digestion of organic wastes. These microorganisms are divided into different groups based on their role in different stages of anaerobic digestion, such as fermentative, syntrophic, acetogenic, and methanogenic microorganisms. They grow and work in communities, and depending upon the process condition of the digester, they follow different pathways to decompose complex organic matter [5, 6].
Fermentative bacteria are used in the initial stages of anaerobic digestion, including hydrolysis and acidogenesis. During hydrolysis, bacteria like Clostridium and Bacteroides secrete enzymes like cellulases, lipases, and proteases to break down complex organic polymers such as cellulose, lipids, and proteins into simpler and soluble compounds like sugars, amino acids, and long-chain fatty acids. Subsequently, these compounds are further degraded into volatile fatty acids (VFAs) like acetate, propionate, and butyrate by acidogenic bacteria in the acidogenesis stage [7].
Syntrophic microorganisms like Syntrophomonas and Syntrophobacter consume intermediate products such as hydrogen, formate, and VFAs through interspecies hydrogen transfer and collaborate with methanogenic archaea to avoid the accumulation of these intermediates, which can inhibit methane production. These types of microorganisms help to stabilize the anaerobic digestion process.
Acetogenic bacteria like Acetobacterium utilize the Wood-Ljungdahl pathway to catalyze the reaction of acetic acid from compounds like hydrogen, carbon dioxide and carbon monoxide which serves as a key substrate for methane production. On the other hand, methanogenic archaea like Methanosarcina and Methanobacterium facilitate methane production by using acetic acid, hydrogen, and carbon dioxide as substrates [8].
Among these groups, acetogenic bacteria like Clostridium aceticum and methanogenic archaea like Methanosarcina barkeri significantly influence anaerobic digestion as they are the primary acetic acid and methane-forming microorganisms [9]. Their balance determines the efficiency of anaerobic digestion.
Stages Of The Anaerobic Digestion Process
Anaerobic digestion is divided into four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. For degradation, each stage employed a different species of microbe. The figure shows the reactant and product for each stage of anaerobic digestion.
Figure 1: Reaction in four stages of the anaerobic digestion process [10]
Hydrolysis
Feedstock (Organic matter) for anaerobic digestion mainly consists of large complex polymers such as lipids, carbohydrates, and proteins. These polymers must be depolymerized before being degraded by microbes in subsequent stages. In the presence of water and hydrolytic enzymes, these water-insoluble complex polymers are degraded into smaller water-soluble molecules such as sugar, long-chain fatty acids, and amino acids during the hydrolysis stage [11].
Hydrolysis is the initial stage of anaerobic digestion, and subsequent decomposition of organic waste would be difficult without it. It is a slower step of anaerobic digestion that can contribute to the rate of the anaerobic process. The general form of hydrolysis reaction can be written as [12]:
Biomass + H2O → monomers + H2
Acidogenesis
Acidogenesis is the second stage of anaerobic digestion in which hydrolyzed products such as sugar, long-chain fatty acids, and amino acids are degraded into short-chain fatty acids, including VFAs, other organic acids, and alcohols. Moreover, some quantity of hydrogen, ammonia, carbon dioxide, or other byproducts may also be released during this reaction, either directing or inhibiting the process toward optimal degradation.
The main reaction involved in this stage is given as [12]:
C6H12O6 + 2H2 → 2CH3CH2COOH + 2H2O
C6H12O6 → 2CH3CH2OH + 2CO2
The main products of acidogenesis are VFAs (mainly propionate and butyrate) that serve as a substrate for subsequent stages of anaerobic digestion. The enzymes utilized for this conversion include Bacteroidetes, Chloroflexi, Firmicutes, and Proteobacteria.
Acetogenesis
In this stage, acetogenic bacteria degrade VFAs, organic acids, and alcohol to produce acetic acid, carbon dioxide, and hydrogen gas. The main acetogenic bacteria include syntrophobacter wolinii, syntrophomonos wolfie, clostridium spp., streptococcus anaerobes, and Lactobacillus.
The concentration of hydrogen is a critical parameter that influences the yield and efficiency of acetic acid production. For the reaction to occur, the concentration of hydrogen content should be less than the thermodynamically feasible concentration. Therefore, proper monitoring and controlling of hydrogen levels is essential.
The following is the reaction involved in this stage [12]:
CH3CH2COO– + 3H2O → CH3COO– + H+ + HCO3– + 3H2
C6H12O6 + 2H2O → 2CH3COOH + 2CO2 + 4H2
CH3CH2OH + 2H2O → CH3COO– + 2H2 + H+
2HCO3– + 4H2 + H+ → CH3COO– + 4H2O
Methanogenesis
Methanogenesis is the final stage of anaerobic digestion, in which the products obtained during the acetogenesis reaction are converted into biogas using single-celled bacteria known as methanogen. The resulting biogas consists of methane, carbon dioxide, and a small quantity of other gases or water vapors. Simultaneously, some quantity of digestate is obtained as a byproduct of anaerobic digestion in the form of a liquid and solid mixture containing the material not used by microorganisms and dead bacteria.
The methanogenic bacteria involved in the production of methane are; methanobacterium, methanococcus, methanobacillus, and methanosarcina. And the common reactions of methanogenesis are [12]:
2CH3CH2OH + CO2 → 2CH3COOH + CH4
CH3COOH → CH4 + CO2
CH3OH → CH4 + H2O
CO2 + 4H2 → CH4 + 2H2O
Factors Affecting the Anaerobic Digestion Process
Environmental Factors
Temperature
Anaerobic digestion is a biological process, and temperature is an important factor in all biological processes as it affects the activity of microorganisms and their reaction kinetics. There are three temperature ranges in which anaerobic digestion can occur: psychrophilic (15-25 oC), mesophilic (35-40 oC), and thermophilic (50-60 oC) [13].
The psychrophilic temperature range is mostly used in cold climate regions where the temperature falls below 20 oC, and additional heat and proper maintenance for an anaerobic system is necessary. The other two temperature ranges are commonly employed for anaerobic digestion.
Mesophilic digestion favors overall process stability while producing less biogas and retaining more pathogens than thermophilic digestion. It occurs between 35 and 40 oC, and may be used to degrade a wide range of feedstocks as it requires less temperature variations [14].
Thermophilic digestion takes place at 50-60 oC, a comparatively higher temperature than mesophilic digestion. This higher temperature causes the reduction of pathogens, improves reaction rates, and increases biogas production compared to mesophilic digestion. However, thermophilic digestion requires extra heat for the system and its proper management to maintain optimal conditions. As a result, the overall process cost increases for this type of digestion [14].
The selection of temperature range for specific feedstock degradation depends upon different parameters such as the nature of feedstock, additional heat requirement, biogas production, process stability, and environmental considerations.
pH
Another essential parameter in anaerobic digestion that influences bacterial growth and its functioning is pH. It regulates the formation of ammonia, hydrogen disulfide, and VFAs, all of which contribute to acidic conditions in the digester. These circumstances hinder microbial activity and reduce biogas generation. As a result, optimal pH is required for successful anaerobic digestion [15].
The pH range that works best for anaerobic digestion is 6.8 to 7.2. Its optimal value varies depending on the stage of anaerobic digestion. For example, the optimal pH value for methanogenesis is between 6.5 and 8.2. While the hydrolysis and acidogenesis processes function optimally at pH levels ranging from 5.5 to 6.5 [16].
C/N Ratio
The C/N ratio is an important factor that affects the stability of the digestion process. Microorganisms require carbon as an energy source and nitrogen for their growth or specific reaction. The optimum value of the C/N ratio ranges from 20 to 30, depending upon the nature of the feedstocks being used for degradation [17].
A low value of C/N means higher nitrogen content compared to a carbon source that leads to the production of access ammonia. Higher ammonia concentration causes the accumulation of VFAs and ultimately reduces the biogas yield.
Conversely, at a higher C/N ratio, a higher carbon source results in the fast decomposition of feedstock during the initial stages. However, lower nitrogen contents reduce the growth of microorganisms and affect biogas production. To optimize the ratio of C/N, carbon or nitrogen-rich organic waste mixed with the feed stream supplied to the digester according to the requirement.
Operational Factors
Retention Time
Retention time is defined as the average time the organic substrates are held in the reaction system. Organic wastes need a specific time to ensure maximum digestion, and this value varies depending upon the nature of the substrate. However, longer retention time requires a larger digester capacity, affecting the overall capital cost.
An anaerobic digester has two main retention times; solid retention time (SRT) and hydraulic retention time (HRT). SRT is the time for which solids and microorganisms are held in the digester, whereas the HRT is the time obtained by dividing the digester volume (m3) by the influent flow rate (m3/day).
To achieve effective digestion, the SRT is generally kept higher than HRT. SRT and HRT are equal for a digestion system without recycle stream [18]. The optimal value of HRT and organic loading rate (OLR) are adjusted according to the demand for biogas production and the nature of the feedstock. There is no increase in digestive efficiency above the optimal value of HRT. However, below this value, the accumulation of VFAs occurs [19].
Organic loading rate
Organic loading rate (OLR) is a critical factor representing the quantity of organic substrate fed to the digester per unit of time continuously. Generally, increasing OLR results in a higher yield of biogas as organic content increases.
However, after a certain limit, further increases in OLR cause the accumulation of VFAs that directly affect the methanogenesis reaction. Therefore, careful monitoring and controlling of OLR must be necessary to ensure a stable digestion process [20].
Mathematically organic loading rate can be defined as:
Where Q is the volumetric flow rate of the substrate (m3/day), S is the concentration of substrate (kg VS/m3), and V is the digester volume (m3).
Mixing and stirring
Mixing is also a crucial factor that plays an essential role in the efficient anaerobic digestion process [21]. It directly affects the microorganism activity, VFAs accumulation and biogas entrapment. Proper mixing allows direct contact of the substrate with microorganisms throughout the digester ensuring adequate heat and mass transfer that results in higher biogas production.
On the other hand, poor mixing cause difficulty in heat and mass transfer resulting in instability of the digestion process and decreasing the biogas yield. Due to inadequate slurry mixing, around 44% of anaerobic digester plants fail.
Mixing can be conducted naturally or artificially using mechanical means such as impellers. Natural mixing occurs through the production of biogas in the digester. Artificial mixing is usually preferred after specific intervals for efficient anaerobic digestion.
Applications
Industrial organic waste management
The organic wastes from various industries, such as pulp and paper, pharmaceutical, textile, sugar and starch, leather, and vegetable oil and fat processing, are growing daily and have become environmental concerns recently.
To address these concerns, industrial organic wastes, whether liquid or solid, are subjected to anaerobic digestion for proper management. The biogas produced from industrial wastes is a renewable energy source that can partially replace fossil fuels used in industrial processes, thereby reducing greenhouse gas emissions. For example, biogas from wastewater treatment and other organic residues accounted for over 50% of energy needs in the potato processing industry.
The anaerobic process provides an efficient waste management solution for the growing volumes of organic industrial wastes, enabling circular economies. However, challenges remain regarding pretreatment needs, digester optimization, and enhancing process stability for different industrial waste streams that must be addressed.
Wastewater treatment
Wastewater typically contains high BOD and COD levels and high organic content with high pollution potential. Therefore, it requires proper treatment before being discharged into an open environment.
Anaerobic digestion has been employed as an effective method to treat wastewater. During this process, the organic matter in wastewater is degraded by hydrolytic, acidogenic, acetogenic and methanogenic microorganisms in a series of steps.
The anaerobic digestion of wastewater simplifies sludge disposal and minimizes the environmental impact by reducing the wastewater sludge volume by converting organic solids to biogas and stabilizing the residual sludge [22].
Some key benefits of anaerobic digestion for wastewater treatment include producing energy in biogas, low sludge production, lower energy requirements compared to aerobic treatment, destruction of pathogens in sludge, and lower greenhouse gas emissions. However, challenges like the slow growth rate of methanogenic archaea need to be addressed to make this technology more effective [5].
Agricultural waste management
Agricultural wastes account for significant greenhouse gas emissions, increasing global warming and harming the environment — direct burning of agricultural feedstock results in hazardous gas emissions as well as ash residue.
To overcome these challenges, the anaerobic digestion process was utilized to manage agricultural wastes as it contains considerable organic matter and moisture. During anaerobic digestion, agricultural wastes such as animal manure, crop residues and food waste decompose into biogas and digestate, providing renewable energy resources [23].
The nutrient-rich digestate may be utilized directly as organic fertilizers, providing benefits such as improved soil properties, higher crop yields, and reduced reliance on chemical fertilizers. Moreover, the anaerobic digestion of agricultural waste also protects the environment by minimizing its odor and reducing pathogens present in the waste [1].
Overall, anaerobic digestion offers an environmentally friendly method for sustainably managing large volumes of agricultural waste. However, challenges remain in optimizing digester conditions for different feedstocks, enhancing process stability, and improving biogas yields. Further research on pre-treatment methods, microbial additives, digester configurations and co-digestion blends can help maximize the benefits of agricultural waste anaerobic digestion [24].
Food & Beverage Industry
In the food & beverage industry, significant volumes of organic residues are generated, and their disposal poses a significant challenge. Food processing operations consistently yield diverse residues, encompassing liquid, solid, and semi-solid forms, which can manifest either as a continuous output or seasonally.
Simultaneously, the processing activities demand a substantial supply of energy in the form of heat and power. The specific energy requisites are contingent upon factors such as the production process intricacies, geographic location, and the employed process technologies.
The energy dynamics vary in diverse industrial contexts, from brewing to slaughtering and dairy processing. For instance, brewing processes demand saturated steam for critical stages like brewing itself and bottle sterilization. At the same time, power is indispensable for facilitating operations like pumping and cooling [25].
The dairy industry’s thermal requirements encompass the treatment of raw milk and sanitation procedures. Furthermore, power is essential for ancillary operations, including pumping, mixing, and cooling, mirroring the energy needs inherent to other industrial sectors.
In the operational paradigm of these industries, a nuanced interplay between heat and power emerges. Given this intricacy, many manufacturing facilities have integrated natural gas into their energy portfolios, serving as a versatile energy vector catering to thermal and mechanical demands.
Considering these circumstances, anaerobic digestion is a valuable technology for degrading indigenous residues from in-house production processes. The conversion of these residues into biogas represents a promising avenue for fulfilling the energy requisites of these industries while aligning with eco-conscious energy practices [26, 27].
Anaerobic Digestion Challenges
Anaerobic digestion provides a solution for managing a wide variety of organic wastes. However, this technology still has several challenges that affect its widespread application. Some of these challenges are given below [27, 28]:
Quality of end products:
The quality of end products of anaerobic digestion, such as biogas and digestate, remains a challenge for efficient anaerobic digestion.
Biogas produced from anaerobic digestion may contain many hazardous gases and moisture content depending upon the composition of the feedstock used in the digester. These impurities directly affect the quality and quantity of the biogas produced. To address this problem, different treatment methods are employed to upgrade the biogas. However, these treatment methods demand additional costs that reduce the overall efficiency of the anaerobic digestion process.
In addition, the digestate obtained as a byproduct of the anaerobic digestion process contains high to low moisture contents depending upon the process conditions and nature of the feedstock. As digestate is used as organic fertilizer for soil, its moisture content may require additional treatment to minimize storage and transportation expenses. However, post-treatment lowers the quality of digestate and increases the expense of the process.
Optimization of process parameters
The efficiency of the anaerobic digestion process is influenced by adequately controlling the process parameters in the digester during digestion, such as pH, temperature, C/N ratio, retention time, liquid-to-solid ratio, and proper mixing. For example, proper slurry mixing during digestion provides homogenous process conditions throughout the digester.
Similarly, the factors like pH, C/N ratio, temperature, and liquid-to-solid ratio, directly affect the growth of microbial activity. Moreover, the optimum retention time required for the process depends upon the type of feedstock or quantity of biogas produced. Complex substrate often requires a long retention time for degradation, leading to greater reactor volume and process cost.
Optimizing all the above process conditions remains challenging as fluctuation in any digestion stage directly impacts the subsequent stage’s reaction rates, reducing biogas yield, and the process becomes more complex.
Inhibiting elements for the process
Inhibitors are substances that influence the efficiency and stability of the anaerobic digestion process. Major inhibitors include high organic loading rates, ammonia, sulfides, and metal ions. A high organic loading rate causes the accumulation of volatile fatty acids (VFAs) like propionic, butyric and valeric acids, which can slow down the hydrolysis of the substrate and inhibit methanogenesis. VFA buildup lowers digester pH and causes toxicity to methanogens.
Protein-rich substrates contain nitrogen that gets converted to ammonia during digestion. Ammonia helps produce VFAs and supplements methanogen growth at low levels. However, high ammonia concentrations over 3000 mg/L disrupt methanogenesis by inhibiting enzyme activities and causing potassium deficiency in methanogens [29].
Sulfides like hydrogen sulfide are produced during the breakdown of sulfur-containing organic matter. Sulfides inhibit methanogens through enzymatic inhibition and depriving iron needed for their growth [7]. High levels above 200 mg/L cause toxicity.
Heavy metals like nickel, cadmium, chromium, lead and zinc entering digesters through contaminated feedstocks cause toxicity to microbes above certain thresholds. Metals damage cell membranes, inhibit enzymes and deprive essential trace metals [30].
Careful monitoring and control of organic loading rate, feedstock composition, and digester operation are critical to avoid the buildup of these major inhibitors for stable and efficient anaerobic digestion [28, 31].
Future Perspective Of Anaerobic Digestion
The advancement of anaerobic digestion technology requires developments in areas including digester design, feedstock pretreatment, monitoring and control systems to ensure optimal microbial activity, enhanced biogas production, and process stability. Controlling microbial community dynamics in all stages of anaerobic digestion allows for higher biogas yields and process optimization [32].
Co-digestion is a relatively new technique and promising approach to improve biogas yields. It involves pretreating and blending multiple feedstocks to balance carbon, nitrogen, and other nutrients. Other advanced pretreatment methods like thermal, chemical, mechanical and biological pretreatments can enhance the digestibility of feedstocks. Examples of this feedstock include lignocellulosic biomass and waste-activated sludge. These pretreatment methods supplement the production of biogas [33].
It has been suggested that different types of organic wastes should be explored as co-digestion feedstocks for improved waste management and reduced environmental impacts. Likewise, the quantity and quality of biogas produced needs further research and development of pre and post-treatment technologies. Pretreatment increases the solubilization of organic matter. On the other hand, post-treatment technologies like pressurized membrane separation, amine scrubbing, and cryogenic separation can upgrade biogas by removing carbon dioxide to get high-quality biomethane for use as vehicle fuel [34, 35].
The digestate produced from the anaerobic process contains valuable nutrients like nitrogen, phosphorus, and potassium based on the feedstock. Optimizing feedstock characteristics through pretreatment and controlling moisture content can improve digestate quality and prevent impurities. In this regard, digestate processing technologies like mechanical separation, drying, composting, and ammonia stripping should also be advanced to recover nutrients and produce value-added products [36].
Future growth of anaerobic process requires appropriate policymaking keeping in view the following,
- Incentives for low-carbon gas utilization
- Funding for research
- Regulations mandating organic waste recycling using anaerobic processes
The key aim should be to mitigate greenhouse gas emissions and generate renewable energy, especially from small-scale rural plants. Public acceptance of anaerobic technology can be improved through education and emphasizing waste management, agricultural and renewable energy benefits [37].
Overall, advancing the sustainability of anaerobic processes requires an integrated approach combining technological innovations like advanced digester designs (Cioabla et al., 2012[14]), optimized processes, waste management policies, financial mechanisms, and public engagement. Anaerobic technology has immense potential for renewable energy generation, waste management, and nutrient recovery, but realizing this potential requires systematic efforts across various dimensions [38].
Conclusion
In conclusion, anaerobic digestion effectively manages organic wastes and produces valuable resources while maintaining sustainability. It finds application in agricultural waste management, wastewater treatment, and industrial organic waste management. All these wastes are anaerobically digested to produce renewable energy sources, including biogas and digestate. However, there are some challenges that the anaerobic digestion process faces, such as the quality of biogas and digestate, optimization of process parameters, and accumulation or production of inhibitory elements during the digestion process.
Continuous research and advancement in anaerobic digestion are expected to address these challenges and provide anaerobic digestion as a sustainable waste management system worldwide. For this purpose, new technologies are required for the pretreatment of feedstock, monitoring, and control, and designing and evaluating digester. Policymakers should facilitate and encourage the use of anaerobic digestion, even on a small scale, to reduce the global warming impact and provide a solution for local waste management.
References
[1] L. Appels, J. Baeyens, J. Degrève, and R. Dewil, “Principles and potential of the anaerobic digestion of waste-activated sludge,” Progress in energy and combustion science, vol. 34, no. 6, pp. 755-781, 2008.
[2] B. Demirel and P. Scherer, “Trace element requirements of agricultural biogas digesters during biological conversion of renewable biomass to methane,” Biomass and bioenergy, vol. 35, no. 3, pp. 992-998, 2011.
[3] J. D. Murphy and T. Thamsiriroj, “Fundamental science and engineering of the anaerobic digestion process for biogas production,” in The biogas handbook: Elsevier, 2013, pp. 104-130.
[4] E. P. Agency. “Basic Information about Anaerobic Digestion (AD).” EPA. https://www.epa.gov/anaerobic-digestion/basic-information-about-anaerobic-digestion-ad (accessed.
[5] B. Demirel and P. Scherer, “The roles of acetotrophic and hydrogenotrophic methanogens during anaerobic conversion of biomass to methane: a review,” Reviews in Environmental Science and Bio/Technology, vol. 7, pp. 173-190, 2008.
[6] L. T. Angenent, K. Karim, M. H. Al-Dahhan, B. A. Wrenn, and R. Domíguez-Espinosa, “Production of bioenergy and biochemicals from industrial and agricultural wastewater,” TRENDS in Biotechnology, vol. 22, no. 9, pp. 477-485, 2004.
[7] B. Demirel and O. Yenigün, “Two‐phase anaerobic digestion processes: a review,” ed: Wiley Online Library, 2002.
[8] H. Drake, K. Küsel, and C. Matthies, “Acetogenic Prokaryotes in The Prokaryotes, Vol. 2,(eds Dworkin, M. et al.) 354–420,” ed: Springer, New York, 2006.
[9] K. S. Smith and C. Ingram-Smith, “Methanosaeta, the forgotten methanogen?,” Trends in microbiology, vol. 15, no. 4, pp. 150-155, 2007.
[10] B. M. B.-A. Digestion, ” Schematic of four phases of biogas production,” ed: The Pennsylvania State University.
[11] P. Bajpai and P. Bajpai, “Basics of anaerobic digestion process,” Anaerobic technology in pulp and paper industry, pp. 7-12, 2017.
[12] C. B. Clifford. “Anaerobic Digestion.” The Pennsylvania State University. https://www.e-education.psu.edu/egee439/node/727 (accessed.
[13] L. ADMIN, “Effect of Temperature on Methane Production from Field-Scale Anaerobic Digesters Treating Dairy Manure,” 2019. [Online]. Available: https://lpelc.org/effect-of-temperature-on-methane-production-from-field-scale-anaerobic-digesters-treating-dairy-manure/.
[14] TheEcoAmbassador. “Anaerobic Digestion -Mesophilic Vs. Thermophilic.” https://www.theecoambassador.com/Anaerobic-Digestion-Temperature.html (accessed.
[15] A. Cerón-Vivas, K. T. Cáceres, A. Rincón, and Á. Cajigas, “Influence of pH and the C/N ratio on the biogas production of wastewater,” Revista Facultad de Ingeniería Universidad de Antioquia, no. 92, pp. 70-79, 2019.
[16] S. Sarker, J. J. Lamb, D. R. Hjelme, and K. M. Lien, “A Review of the Role of Critical Parameters in the Design and Operation of Biogas Production Plants,” Applied Sciences, vol. 9, no. 9, p. 1915, 2019. [Online]. Available: https://www.mdpi.com/2076-3417/9/9/1915.
[17] M. M. Uddin and M. M. Wright, “Anaerobic digestion fundamentals, challenges, and technological advances,” Physical Sciences Reviews, no. 0, 2022.
[18] C. P. Watcher. “Solids and Hydraulic Retention Times.” Climate Policy Watcher https://www.climate-policy-watcher.org/wastewater-sludge/solids-and-hydraulic-retention-times.html#:~:text=The%20key%20parameters%20in%20providing,is%20held%20in%20the%20digester. (accessed.
[19] M. Lisowyj and M. M. Wright, “A review of biogas and an assessment of its economic impact and future role as a renewable energy source,” Reviews in Chemical Engineering, vol. 36, no. 3, pp. 401-421, 2020.
[20] M. Usman, M. Suleiman, and M. Binni, “Anaerobic digestion of agricultural wastes: A potential remedy for energy shortfalls in Nigeria, vol. 4, no. 1,” ed: Scholarena, 2021.
[21] B. Singh, Z. Szamosi, and Z. Siménfalvi, “Impact of mixing intensity and duration on biogas production in an anaerobic digester: a review,” Critical reviews in biotechnology, vol. 40, no. 4, pp. 508-521, 2020.
[22] Metcalf et al., Wastewater engineering: treatment and resource recovery. McGraw Hill Education, 2014.
[23] A. C. Wilkie, “Anaerobic digestion: biology and benefits,” Dairy manure management: treatment, handling, and community relations, pp. 63-72, 2005.
[24] K. Möller and T. Müller, “Effects of anaerobic digestion on digestate nutrient availability and crop growth: A review,” Engineering in life sciences, vol. 12, no. 3, pp. 242-257, 2012.
[25] R. Zhang et al., “Characterization of food waste as feedstock for anaerobic digestion,” Bioresource technology, vol. 98, no. 4, pp. 929-935, 2007.
[26] L. Zhang, K.-C. Loh, and J. Zhang, “Enhanced biogas production from anaerobic digestion of solid organic wastes: Current status and prospects,” Bioresource Technology Reports, vol. 5, pp. 280-296, 2019.
[27] E. Ryckebosch, M. Drouillon, and H. Vervaeren, “Techniques for transformation of biogas to biomethane,” Biomass and bioenergy, vol. 35, no. 5, pp. 1633-1645, 2011.
[28] D. P. Van, T. Fujiwara, B. L. Tho, P. P. S. Toan, and G. H. Minh, “A review of anaerobic digestion systems for biodegradable waste: Configurations, operating parameters, and current trends,” Environmental Engineering Research, vol. 25, no. 1, pp. 1-17, 2020.
[29] O. Yenigün and B. Demirel, “Ammonia inhibition in anaerobic digestion: a review,” Process biochemistry, vol. 48, no. 5-6, pp. 901-911, 2013.
[30] Y. Chen, J. J. Cheng, and K. S. Creamer, “Inhibition of anaerobic digestion process: a review,” Bioresource technology, vol. 99, no. 10, pp. 4044-4064, 2008.
[31] T. G. Ambaye, E. R. Rene, A.-S. Nizami, C. Dupont, M. Vaccari, and E. D. van Hullebusch, “Beneficial role of biochar addition on the anaerobic digestion of food waste: a systematic and critical review of the operational parameters and mechanisms,” Journal of Environmental Management, vol. 290, p. 112537, 2021.
[32] M. Carballa, M. Smits, C. Etchebehere, N. Boon, and W. Verstraete, “Correlations between molecular and operational parameters in continuous lab-scale anaerobic reactors,” Applied microbiology and biotechnology, vol. 89, pp. 303-314, 2011.
[33] M. Carlsson, A. Lagerkvist, and F. Morgan-Sagastume, “The effects of substrate pre-treatment on anaerobic digestion systems: a review,” Waste management, vol. 32, no. 9, pp. 1634-1650, 2012.
[34] J. H. Ebner, R. A. Labatut, J. S. Lodge, A. A. Williamson, and T. A. Trabold, “Anaerobic co-digestion of commercial food waste and dairy manure: Characterizing biochemical parameters and synergistic effects,” Waste management, vol. 52, pp. 286-294, 2016.
[35] A. Petersson and A. Wellinger, “Biogas upgrading technologies–developments and innovations,” IEA bioenergy, vol. 20, pp. 1-19, 2009.
[36] M. Manyuchi, C. Mbohwa, and E. Muzenda, “Biogas and Bio solids production from tea waste through anaerobic digestion,” in Proceedings of the International Conference on Industrial Engineering and Operations Management, 2018, vol. 2018, no. JUL, pp. 2519-2525.
[37] K. L. Kovács et al., “Improvement of biogas production by bioaugmentation,” BioMed research international, vol. 2013, 2013.
[38] C. Warrajareansri and J. Wongthanate, “Comparative optimisation of biohythane production from starch wastewater by one-stage and two-stage anaerobic digestion,” International Journal of Environment and Waste Management, vol. 27, no. 3, pp. 292-309, 2021.
To all knowledge