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Current Advances in Anaerobic Digestion Technology

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A special issue of Bioengineering (ISSN 2306-5354).

Deadline for manuscript submissions: closed (30 June 2020) | Viewed by 109676

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Special Issue Editors

Dr. Marcell Nikolausz

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Guest Editor

Department of Environmental Microbiology, Helmholtz Zentrum für Umweltforschung, Leipzig, Germany
Interests: anaerobic digestion; biogas; methanogenesis; mixed cultures; enrichment cultures; environmental biotechnology; waste biorefinery; biomimicry; gut microbiology
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Dr. Jörg Kretzschmar

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Guest Editor

Biochemical Conversion Department, DBFZ Deutsches Biomasseforschungszentrum, Leipzig, Germany
Interests: (bio)electrochemistry; environmental biotechnology; anaerobic digestion; biogas; biofuels; process monitoring; simulation
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Biomass degradation under anoxic conditions includes a variety of natural microbial processes, which have a long tradition of biotechnological utilization. Anaerobic digestion (AD), for instance, has been used for decades to treat various waste streams and to produce methane-rich biogas as an important energy carrier from various wastes and crops. AD is a popular, mature technology, and our knowledge about the influencing process parameters as well as about the diverse microbial communities involved in the process has increased dramatically over the last few decades.

However, the AD sector also faces current challenges, such as feedstock availability and price increases, competitiveness of the products, as well as a lack of process monitoring and automation technologies. Germany is one of the European leaders in biogas technology, regarding the number of large-scale plants and their installed capacity. The produced biogas is mainly used for electricity and heat production. Although more than 8000 German biogas plants still enjoy the generous national subsidy system, this support will be phased out soon, and they have to cope with increasing substrate prices, competition with other renewables, and the requirements of the energy market (e.g., demand oriented power production).

On the other hand, AD offers so much more than biogas alone, and the metabolic flexibility of AD can be the answer to the current challenges. A great variety of substrates can be utilized for AD, including gaseous ones, such as syngas, for example from biomass gasification or hydrogen obtained by the electrolysis of water using electricity from other renewables. AD can be combined with new (bio)technologies, such as (bio)electrochemical technologies, to enhance the efficiency of the process. The product spectrum can also be diversified by shifting AD to anaerobic fermentation, which will lead to the production of carboxylates with a higher economic value. Chemical building blocks can also be obtained by converting methane to methanol, or higher value single-cell proteins can be produced by methanotrophic bacteria. Academic knowledge about the microbiome, the engine driving the AD process, has been accumulating, but the use of this knowledge for the innovation of AD technologies is scarce.

This Special Issue of Bioengineering will publish research articles and review papers dealing with the current advances in the fascinating process of anaerobic digestion and its related processes.

Dr. Marcell Nikolausz
Dr. Jörg Kretzschmar
Guest Editors

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Published Papers (13 papers)

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Editorial

Jump to: Research, Review

3 pages, 178 KiB

Open AccessEditorial

Anaerobic Digestion in the 21st Century

by Marcell Nikolausz and Jörg Kretzschmar

Bioengineering 2020, 7(4), 157; https://doi.org/10.3390/bioengineering7040157 - 7 Dec 2020

Cited by 8 | Viewed by 3175

Abstract

Despite being a mature biotechnological process, anaerobic digestion is still attracting considerable research attention, mainly due to its versatility both in substrate and product spectra, as well as being a perfect test system for the microbial ecology of anaerobes [...] Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

Research

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28 pages, 6481 KiB

Open AccessArticle

Degradation of Veterinary Antibiotics in Swine Manure via Anaerobic Digestion

by Ali Hosseini Taleghani, Teng-Teeh Lim, Chung-Ho Lin, Aaron C. Ericsson and Phuc H. Vo

Bioengineering 2020, 7(4), 123; https://doi.org/10.3390/bioengineering7040123 - 9 Oct 2020

Cited by 10 | Viewed by 5153

Abstract

Antibiotic-resistant microorganisms are drawing a lot of attention due to their severe and irreversible consequences on human health. The animal industry is considered responsible in part because of the enormous volume of antibiotics used annually. In the current research, veterinary antibiotic (VA) degradation, finding the threshold of removal and recognizing the joint effects of chlortetracycline (CTC) and Tylosin combination on the digestion process were studied. Laboratory scale anaerobic digesters were utilized to investigate potential mitigation of VA in swine manure. The digesters had a working volume of 1.38 L (in 1.89-L glass jar), with a hydraulic retention time (HRT) of 21 days and a loading rate of 1.0 g-VS L⁻¹ d⁻¹. Digesters were kept at 39 ± 2 °C in incubators and loaded every two days, produced biogas every 4 days and digester pH were measured weekly. The anaerobic digestion (AD) process was allowed 1.5 to 2 HRT to stabilize before adding the VAs. Tests were conducted to compare the effects of VAs onto manure nutrients, volatile solid removal, VA degradation, and biogas production. Concentrations of VA added to the manure samples were 263 to 298 mg/L of CTC, and 88 to 263 mg/L of Tylosin, respectively. Analysis of VA concentrations before and after the AD process was conducted to determine the VA degradation. Additional tests were also conducted to confirm the degradation of both VAs dissolved in water under room temperature and digester temperature. Some fluctuations of biogas production and operating variables were observed because of the VA addition. All CTC was found degraded even only after 6 days of storage in water solution; thus, there was no baseline to estimate the effects of AD. As for Tylosin, 100% degradation was observed due to the AD (removal was 100%, compared with 24–40% degradation observed in the 12-day water solution storage). Besides, complete Tylosin degradation was also observed in the digestate samples treated with a mixture of the two VAs. Lastly, amplicon sequencing was performed on each group by using the 50 most variable operational taxonomic units (OTUs)s and perfect discriminations were detected between groups. The effect of administration period and dosage of VAs on Phyla Firmicutes Proteobacteria, Synergistetes and Phylum Bacteroides was investigated. These biomarkers’ abundance can be employed to predict the sample’s treatment group. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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12 pages, 1437 KiB

Open AccessArticle

A Small Study of Bacterial Contamination of Anaerobic Digestion Materials and Survival in Different Feed Stocks

by Lauren Russell, Paul Whyte, Annetta Zintl, Stephen Gordon, Bryan Markey, Theo de Waal, Enda Cummins, Stephen Nolan, Vincent O’Flaherty, Florence Abram, Karl Richards, Owen Fenton and Declan Bolton

Bioengineering 2020, 7(3), 116; https://doi.org/10.3390/bioengineering7030116 - 22 Sep 2020

Cited by 7 | Viewed by 3901

Abstract

If pathogens are present in feedstock materials and survive in anaerobic digestion (AD) formulations at 37 °C, they may also survive the AD process to be disseminated in digestate spread on farmland as a fertilizer. The aim of this study was to investigate the prevalence of Salmonella spp., Escherichia coli O157, Listeria monocytogenes, Enterococcus faecalis and Clostridium spp. in AD feed and output materials and survival/growth in four formulations based on food waste, bovine slurry and/or grease-trap waste using International Organization for Standardization (ISO) or equivalent methods. The latter was undertaken in 100 mL Ramboldi tubes, incubated at 37 °C for 10 d with surviving cells enumerated periodically and the T₉₀ values (time to achieve a 1 log reduction) calculated. The prevalence rates for Salmonella spp., Escherichia coli O157, Listeria monocytogenes, Enterococcus faecalis and Clostridium spp. were 3, 0, 5, 11 and 10/13 in food waste, 0, 0, 2, 3 and 2/3 in bovine slurry, 1, 0, 8, 7 and 8/8 in the mixing tank, 5, 1, 17, 18 and 17 /19 in raw digestate and 0, 0, 0, 2 and 2/2 in dried digestate, respectively. Depending on the formulation, T₉₀ values ranged from 1.5 to 2.8 d, 1.6 to 2.8 d, 3.1 to 23.5 d, 2.2 to 6.6 d and 2.4 to 9.1 d for Salmonella Newport, Escherichia coli O157, Listeria monocytogenes, Enterococcus faecalis and Clostridium sporogenes, respectively. It was concluded that AD feed materials may be contaminated with a range of bacterial pathogens and L. monocytogenes may survive for extended periods in the test formulations incubated at 37 °C. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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16 pages, 1926 KiB

Open AccessArticle

Material Characterization and Substrate Suitability Assessment of Chicken Manure for Dry Batch Anaerobic Digestion Processes

by Harald Wedwitschka, Daniela Gallegos Ibanez, Franziska Schäfer, Earl Jenson and Michael Nelles

Bioengineering 2020, 7(3), 106; https://doi.org/10.3390/bioengineering7030106 - 7 Sep 2020

Cited by 21 | Viewed by 4612

Abstract

Chicken manure is an agricultural residue material with a high biomass potential. The energetical utilization of this feedstock via anaerobic digestion is an interesting waste treatment option. One waste treatment technology most appropriate for the treatment of stackable (non-free-flowing) dry organic waste materials is the dry batch anaerobic digestion process. The aim of this study was to evaluate the substrate suitability of chicken manure from various sources as feedstock for percolation processes. Chicken manure samples from different housing forms were investigated for their chemical and physical material properties, such as feedstock composition, permeability under compaction and material compressibility. The permeability under compaction of chicken manure ranged from impermeable to sufficiently permeable depending on the type of chicken housing, manure age and bedding material used. Porous materials, such as straw and woodchips, were successfully tested as substrate additives with the ability to enhance material mixture properties to yield superior permeability and allow sufficient percolation. In dry anaerobic batch digestion trials at lab scale, the biogas generation of chicken manure with and without any structure material addition was investigated. Digestion trials were carried out without solid inoculum addition and secondary methanization of volatile components. The specific methane yield of dry chicken manure was measured and found to be 120 to 145 mL/g volatile solids (VS) and 70 to 75 mL/g fresh matter (FM), which represents approximately 70% of the methane potential based on fresh mass of common energy crops, such as corn silage. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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15 pages, 3642 KiB

Open AccessArticle

Processing High-Solid and High-Ammonia Rich Manures in a Two-Stage (Liquid-Solid) Low-Temperature Anaerobic Digestion Process: Start-Up and Operating Strategies

by Prativa Mahato, Bernard Goyette, Md. Saifur Rahaman and Rajinikanth Rajagopal

Bioengineering 2020, 7(3), 80; https://doi.org/10.3390/bioengineering7030080 - 25 Jul 2020

Cited by 15 | Viewed by 4289

Abstract

Globally, livestock and poultry production leads to total emissions of 7.1 Gigatonnes of CO₂-equiv per year, representing 14.5% of all anthropogenic greenhouse gas emissions. Anaerobic digestion (AD) is one of the sustainable approaches to generate methane (CH₄) from manure, but the risk of ammonia inhibition in high-solids AD can limit the process. Our objective was to develop a two-stage (liquid–solid) AD biotechnology, treating chicken (CM) + dairy cow (DM) manure mixtures at 20 °C using adapted liquid inoculum that could make livestock farming more sustainable. The effect of organic loading rates (OLR), cycle length, and the mode of operation (particularly liquid inoculum recirculation-percolation mode) was evaluated in a two-stage closed-loop system. After the inoculum adaptation phase, aforementioned two-stage batch-mode AD operation was conducted for the co-digestion of CM + DM (Total Solids (TS): 48–51% and Total Kjeldahl Nitrogen (TKN): 13.5 g/L) at an OLR of 3.7–4.7 gVS/L.d. Two cycles of different cycle lengths (112-d and 78-d for cycles 1 and 2, respectively) were operated with a CM:DM mix ratio of 1:1 (w/w) based on a fresh weight basis. Specific methane yield (SMY) of 0.35 ± 0.11 L CH₄g/VS_fed was obtained with a CH₄ concentration of above 60% for both the cycles and Soluble Chemical Oxygen Demand (CODs) and volatile solid (VS) reductions up to 85% and 60%, respectively. For a comparison purpose, a similar batch-mode operation was conducted for mono-digestion of CM (TS: 65–73% and TKN: 21–23 g/L), which resulted in a SMY of 0.52 ± 0.13 L CH₄g/VS_fed. In terms of efficiency towards methane-rich biogas production and ammonia inhibitions, CM + DM co-digestion showed comparatively better quality methane and generated lower free ammonia than CM mono-digestion. Further study is underway to optimize the operating parameters for the co-digestion process and to overcome inhibitions and high energy demand, especially for cold countries. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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19 pages, 2029 KiB

Open AccessArticle

Economic Perspectives of Biogas Production via Anaerobic Digestion

by Arpit H. Bhatt and Ling Tao

Bioengineering 2020, 7(3), 74; https://doi.org/10.3390/bioengineering7030074 - 14 Jul 2020

Cited by 102 | Viewed by 17154

Abstract

As the demand for utilizing environment-friendly and sustainable energy sources is increasing, the adoption of waste-to-energy technologies has started gaining attention. Producing biogas via anaerobic digestion (AD) is promising and well-established; however, this process in many circumstances is unable to be cost competitive with natural gas. In this research, we provide a technical assessment of current process challenges and compare the cost of biogas production via the AD process from the literature, Aspen Plus process modeling, and CapdetWorks software. We also provide insights on critical factors affecting the AD process and recommendations on optimizing the process. We utilize four types of wet wastes, including wastewater sludge, food waste, swine manure, and fat, oil, and grease, to provide a quantitative assessment of theoretical energy yields of biogas production and its economic potential at different plant scales. Our results show that the cost of biogas production from process and economic models are in line with the literature with a potential to go even lower for small-scale plants with technological advancements. This research illuminates potential cost savings for biogas production using different wastes and guide investors to make informed decisions, while achieving waste management goals. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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9 pages, 1518 KiB

Open AccessArticle

Performances of Conventional and Hybrid Fixed Bed Anaerobic Reactors for the Treatment of Aquaculture Sludge

by Alessandro Chiumenti, Giulio Fait, Sonia Limina and Francesco da Borso

Bioengineering 2020, 7(3), 63; https://doi.org/10.3390/bioengineering7030063 - 27 Jun 2020

Cited by 11 | Viewed by 3933

Abstract

Aquaculture fish production is experiencing an increasing trend worldwide and determines environmental concerns mainly related to the emission of pollutants. The present work is focused on the improvement of the sustainability of this sector by assessing the anaerobic digestion (AD) of slurry. Wastewater from experimental plants for the production of trout (Udine, Italy) was subject to screening by a drum filter, and then to thickening in a settling tank. The thickened sludge, representing the input of AD, was characterized by total and volatile solids contents of 3969.1–9705.3 and 2916.4–7154.9 mg/L, respectively. The AD was performed in a containerized unit with two digesters (D1 and D2), biogas meters and monitoring of the temperature, pH and redox potential. Both reactors are mixed by a recirculation of the digestate, and reactor D2 is equipped with a fixed bed. The tests were performed at 38 °C with diversified loading rates and hydraulic retention times (HRT). HRT varied from 28.9 to 20.3 days for D1 and from 18.3 to 9.3 days for D2. Methane yields resulted as highest for the hybrid digester with the longest HRT (779.8 NL of CH₄/kg VS, 18.3 days). The conventional digester presented its best performance, 648.8 NL of CH₄/kgVS, with an HRT of 20.3 days. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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13 pages, 2920 KiB

Open AccessArticle

Influence of Enzyme Additives on the Rheological Properties of Digester Slurry and on Biomethane Yield

by Liane Müller, Nils Engler, Kay Rostalsky, Ulf Müller, Christian Krebs and Sandra Hinz

Bioengineering 2020, 7(2), 51; https://doi.org/10.3390/bioengineering7020051 - 4 Jun 2020

Cited by 2 | Viewed by 4120

Abstract

The use of enzyme additives in anaerobic digestion facilities has increased in recent years. According to the manufacturers, these additives should increase or accelerate the biogas yield and reduce the viscosity of the digester slurry. Such effects were confirmed under laboratory conditions. However, it has not yet been possible to quantify these effects in practice, partly because valid measurements on large-scale plants are expensive and challenging. In this research, a new enzyme product was tested under full-scale conditions. Two digesters were operated at identic process parameters—one digester was treated with an enzyme additive and a second digester was used as reference. A pipe viscometer was designed, constructed and calibrated and the rheological properties of the digester slurry were measured. Non-Newtonian flow behavior was modelled by using the Ostwald–de Baer law. Additionally, the specific biomethane yield of the feedstock was monitored to assess the influence of the enzyme additive on the substrate degradation efficiency. The viscosity measurements revealed a clear effect of the added enzyme product. The consistency factor K was significantly reduced after the enzyme application. There was no observable effect of enzyme application on the substrate degradation efficiency or specific biomethane yield. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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13 pages, 1323 KiB

Open AccessArticle

The Influence of Pressure-Swing Conditioning Pre-Treatment of Cattle Manure on Methane Production

by Britt Schumacher, Timo Zerback, Harald Wedwitschka, Sören Weinrich, Josephine Hofmann and Michael Nelles

Bioengineering 2020, 7(1), 6; https://doi.org/10.3390/bioengineering7010006 - 30 Dec 2019

Cited by 2 | Viewed by 5876

Abstract

Cattle manure is an agricultural residue, which could be used as source to produce methane in order to substitute fossil fuels. Nevertheless, in practice the handling of this slowly degradable substrate during anaerobic digestion is challenging. In this study, the influence of the pre-treatment of cattle manure with pressure-swing conditioning (PSC) on the methane production was investigated. Six variants of PSC (combinations of duration 5 min, 30 min, 60 min and temperature 160 °C, 190 °C) were examined with regards to methane yield in batch tests. PSC of cattle manure showed a significant increase up to 109% in the methane yield compared to the untreated sample. Kinetic calculations proved also an enhancement of the degradation speed. One PSC-variant (190 °C/30 min) and untreated cattle manure were chosen for comparative fermentation tests in continuously stirred tank reactors (CSTR) in lab-scale with duplicates. In the continuous test a biogas production of 428 mL/g volatile solids (VS) (54.2% methane) for untreated manure was observed and of 456 mL/g VS (53.7% methane) for PSC-cattle-manure (190 °C/30 min). Significant tests were conducted for methane yields of all fermentation tests. Furthermore, other parameters such as furfural were investigated and discussed. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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19 pages, 1616 KiB

Open AccessArticle

Inoculum Source Determines Acetate and Lactate Production during Anaerobic Digestion of Sewage Sludge and Food Waste

by Jan Moestedt, Maria Westerholm, Simon Isaksson and Anna Schnürer

Bioengineering 2020, 7(1), 3; https://doi.org/10.3390/bioengineering7010003 - 23 Dec 2019

Cited by 36 | Viewed by 8289

Abstract

Acetate production from food waste or sewage sludge was evaluated in four semi-continuous anaerobic digestion processes. To examine the importance of inoculum and substrate for acid production, two different inoculum sources (a wastewater treatment plant (WWTP) and a co-digestion plant treating food and industry waste) and two common substrates (sewage sludge and food waste) were used in process operations. The processes were evaluated with regard to the efficiency of hydrolysis, acidogenesis, acetogenesis, and methanogenesis and the microbial community structure was determined. Feeding sewage sludge led to mixed acid fermentation and low total acid yield, whereas feeding food waste resulted in the production of high acetate and lactate yields. Inoculum from WWTP with sewage sludge substrate resulted in maintained methane production, despite a low hydraulic retention time. For food waste, the process using inoculum from WWTP produced high levels of lactate (30 g/L) and acetate (10 g/L), while the process initiated with inoculum from the co-digestion plant had higher acetate (25 g/L) and lower lactate (15 g/L) levels. The microbial communities developed during acid production consisted of the major genera Lactobacillus (92–100%) with food waste substrate, and Roseburia (44–45%) and Fastidiosipila (16–36%) with sewage sludge substrate. Use of the outgoing material (hydrolysates) in a biogas production system resulted in a non-significant increase in bio-methane production (+5–20%) compared with direct biogas production from food waste and sewage sludge. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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14 pages, 1142 KiB

Open AccessArticle

Comparison of Dry Versus Wet Milling to Improve Bioethanol or Methane Recovery from Solid Anaerobic Digestate

by Florian Monlau, Cecilia Sambusiti and Abdellatif Barakat

Bioengineering 2019, 6(3), 80; https://doi.org/10.3390/bioengineering6030080 - 6 Sep 2019

Cited by 10 | Viewed by 6175

Abstract

Biogas plants for waste treatment valorization are presently experiencing rapid development, especially in the agricultural sector, where large amounts of digestate are being generated. In this study, we investigated the effect of vibro-ball milling (VBM) for 5 and 30 min at a frequency of 20 s⁻¹ on the physicochemical composition and enzymatic hydrolysis (30 U g⁻¹ total solids (TS) of cellulase and endo-1,4-xylanase from Trichoderma longibrachiatum) of dry and wet solid separated digestates from an agricultural biogas plant. We found that VBM of dry solid digestate improved the physical parameters as both the particle size and the crystallinity index (from 27% to 75%) were reduced. By contrast, VBM of wet solid digestate had a minimal effect on the physicochemical parameters. The best results in terms of cellulose and hemicelluloses hydrolysis were noted for 30 min of VBM of dry solid digestate, with hydrolysis yields of 64% and 85% for hemicelluloses and cellulose, respectively. At the condition of 30 min of VBM, bioethanol and methane production on the dry solid separated digestate was investigated. Bioethanol fermentation by simultaneous saccharification and fermentation resulted in an ethanol yield of 98 g_eth kg⁻¹ TS (corresponding to 90% of the theoretical value) versus 19 g_eth kg⁻¹ TS for raw solid digestate. Finally, in terms of methane potential, VBM for 30 min lead to an increase of the methane potential of 31% compared to untreated solid digestate. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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Review

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29 pages, 1047 KiB

Open AccessReview

Up-Flow Anaerobic Sludge Blanket (UASB) Technology for Energy Recovery: A Review on State-of-the-Art and Recent Technological Advances

by Matia Mainardis, Marco Buttazzoni and Daniele Goi

Bioengineering 2020, 7(2), 43; https://doi.org/10.3390/bioengineering7020043 - 10 May 2020

Cited by 148 | Viewed by 17262

Abstract

Up-flow anaerobic sludge blanket (UASB) reactor belongs to high-rate systems, able to perform anaerobic reaction at reduced hydraulic retention time, if compared to traditional digesters. In this review, the most recent advances in UASB reactor applications are critically summarized and discussed, with outline on the most critical aspects for further possible future developments. Beside traditional anaerobic treatment of soluble and biodegradable substrates, research is actually focusing on the treatment of refractory and slowly degradable matrices, thanks to an improved understanding of microbial community composition and reactor hydrodynamics, together with utilization of powerful modeling tools. Innovative approaches include the use of UASB reactor for nitrogen removal, as well as for hydrogen and volatile fatty acid production. Co-digestion of complementary substrates available in the same territory is being extensively studied to increase biogas yield and provide smooth continuous operations in a circular economy perspective. Particular importance is being given to decentralized treatment, able to provide electricity and heat to local users with possible integration with other renewable energies. Proper pre-treatment application increases biogas yield, while a successive post-treatment is needed to meet required effluent standards, also from a toxicological perspective. An increased full-scale application of UASB technology is desirable to achieve circular economy and sustainability scopes, with efficient biogas exploitation, fulfilling renewable energy targets and green-house gases emission reduction, in particular in tropical countries, where limited reactor heating is required. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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23 pages, 8183 KiB

Open AccessReview

Technologies for Biogas Upgrading to Biomethane: A Review

by Amir Izzuddin Adnan, Mei Yin Ong, Saifuddin Nomanbhay, Kit Wayne Chew and Pau Loke Show

Bioengineering 2019, 6(4), 92; https://doi.org/10.3390/bioengineering6040092 - 2 Oct 2019

Cited by 253 | Viewed by 22990

Abstract

The environmental impacts and high long-term costs of poor waste disposal have pushed the industry to realize the potential of turning this problem into an economic and sustainable initiative. Anaerobic digestion and the production of biogas can provide an efficient means of meeting several objectives concerning energy, environmental, and waste management policy. Biogas contains methane (60%) and carbon dioxide (40%) as its principal constituent. Excluding methane, other gasses contained in biogas are considered as contaminants. Removal of these impurities, especially carbon dioxide, will increase the biogas quality for further use. Integrating biological processes into the bio-refinery that effectively consume carbon dioxide will become increasingly important. Such process integration could significantly improve the sustainability of the overall bio-refinery process. The biogas upgrading by utilization of carbon dioxide rather than removal of it is a suitable strategy in this direction. The present work is a critical review that summarizes state-of-the-art technologies for biogas upgrading with particular attention to the emerging biological methanation processes. It also discusses the future perspectives for overcoming the challenges associated with upgradation. While biogas offers a good substitution for fossil fuels, it still not a perfect solution for global greenhouse gas emissions and further research still needs to be conducted. Full article

(This article belongs to the Special Issue Current Advances in Anaerobic Digestion Technology)

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