Special Issue "Anaerobic Digestion Processes"

A special issue of Processes (ISSN 2227-9717). This special issue belongs to the section "Green Processes".

Deadline for manuscript submissions: 15 July 2021.

Special Issue Editors

Dr. Pietro Bartocci
E-Mail Website
Guest Editor
CRB Italian Biomass Research Centre, University of Perugia, 06100 Perugia, Italy
Interests: energy and environment; biofuels; LCA; kinetics
Special Issues and Collections in MDPI journals
Prof. Qing Yang
E-Mail Website
Guest Editor
John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
Interests: LCA; environmental impact; energy
Prof. Francesco Fantozzi
E-Mail Website
Guest Editor
Department of Industrial Engineering, University of Perugia, Via G. Duranti 67, 06125 Perugia, Italy
Interests: thermal machines; power plants; bioenergy; energy from waste

Special Issue Information

Dear Colleagues,

Given the growing demand for green energy in developed and developing countries, biogas production from biomass and waste is a technology that can be used to produce renewable methane. This can be obtained as biogas or biomethane (through a successive upgrading step). Different substrates can be used in the anaerobic digestion process, such as energy crops, waste, and residual biomass, as well as food waste. Feedstock logistics, plant operation optimization, and biogas utilization in engines and turbines are key topics that must be carefully analyzed for the successful planning of biogas and biomethane projects. Process optimization should be based on the comprehension of biogas yields from different substrates, the analysis of pre-treatment processes of the raw materials, the optimization of pH in the reactor, the optimization of retention time, the optimization of volatile matter degradation efficiency, and process integration and heat integration to reduce waste heat production (e.g., from CHP units). Together with these conventional aspects, other innovative technologies have to be developed to purify biogas through fermentation, separation of CO2, hydrogenation of CO2, use of biochar in the digester, or for biogas purification.

This Special Issue on "Anaerobic Digestion Processes" aims to curate novel advances in biogas production and use, focusing both on modeling and experimental campaigns. Topics include, but are not limited to, the following:

  • Modeling of anaerobic digestion and biogas production;
  • Organic substrate characterization and pre-treatment;
  • Biogas purification and biomethane production and use;
  • Biogas combustion in engines and turbines.

Dr. Pietro Bartocci
Prof. Qing Yang
Prof. Francesco Fantozzi
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Processes is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • biogas
  • anaerobic digestion
  • biochar
  • biomethane
  • waste food
  • process integration
  • reactors
  • modeling
  • gas cleaning
  • emissions

Published Papers (19 papers)

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Research

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Open AccessArticle
Dry Mesophilic Anaerobic Digestion of Separately Collected Organic Fraction of Municipal Solid Waste: Two-Year Experience in an Industrial-Scale Plant
Processes 2021, 9(2), 213; https://doi.org/10.3390/pr9020213 - 24 Jan 2021
Viewed by 355
Abstract
In this paper, performance analysis over two years’ operation of an industrial anaerobic digestion (AD) plant of a separately collected organic fraction of municipal solid waste is presented. The continuous plug-flow AD plant is still regularly operating and it has been fully operational [...] Read more.
In this paper, performance analysis over two years’ operation of an industrial anaerobic digestion (AD) plant of a separately collected organic fraction of municipal solid waste is presented. The continuous plug-flow AD plant is still regularly operating and it has been fully operational since September 2018. Since then, it has been supplied with 40,000 t/y of pretreated separately collected organic fraction of municipal solid waste from municipalities of the Calabria region in Southern Italy. The AD process is carried out in a mesophilic regime at 40 ± 0.5 °C, using a constant hydraulic retention time (HRT) of 22 days and a substrate with average total solids and average total volatile solids of 30.0% and 22.2%, respectively. In the last two years, the plant produced an average of 191 m3 and 860 m3 of biogas per tonne (t) of organic input material and of total volatile solids, respectively, with an average methane specific production of 508 m3/t (total volatile solids). The average CH4 percentage in the biogas was of 59.09%. The obtained results came out from the combination of high organic content of separately collected organic fraction of municipal solid waste, optimized pretreatment system and operating conditions adopted. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperArticle
Stabilization of Anaerobic Co-Digestion Process via Constant the Digestate Solids Content
Processes 2021, 9(2), 197; https://doi.org/10.3390/pr9020197 - 21 Jan 2021
Viewed by 440
Abstract
The process instability of anaerobic digestion (AD) is a common issue and may result in underperformance or short-term process failure. Extensive research has shown that total solids (TS) content in AD has a significant impact on system stability and performance. However, no study [...] Read more.
The process instability of anaerobic digestion (AD) is a common issue and may result in underperformance or short-term process failure. Extensive research has shown that total solids (TS) content in AD has a significant impact on system stability and performance. However, no study has examined the feasibility of stabilizing the AD process by maintaining constant TS content in the digestate. In this study, an innovative control approach based on constant TS content in the digestate during AD was developed using a mass balance equation. Two levels of TS content (desired values of 4% wet basis (w.b.) and 6% w.b.) were compared with conventional control. The process stability was examined by monitoring digestate components and pH. Substrate-specific methane yield (m3 CH4/kg VS) was used to assess the effectiveness of the controlled conditions. The results showed that the digestate TS content during AD can be controlled and that the digestion process can be stabilized by controlled conditions. In addition, constant TS in the digestate (within 1% w.b. of the desired level) gave increased levels of biogas production (10.2%), methane (13.5%), and substrate-specific methane yield (43.3%) at 4% TS, and respective increases of 16.6%, 21.2%, and 20.8% at 6% TS when compared with standard operation. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperArticle
Biomethanation Potential (BMP) Study of Mesophilic Anaerobic Co-Digestion of Abundant Bio-Wastes in Southern Regions of Tunisia
Processes 2021, 9(1), 48; https://doi.org/10.3390/pr9010048 - 29 Dec 2020
Cited by 1 | Viewed by 504
Abstract
Tunisia is a country that suffers from energy demand problems and environmental matters. Thus, Tunisian authorities desire to encourage the development of renewable energy sources, especially from biological processes, like anaerobic digestion. Therefore, this study is focused on the evaluation of biogas and [...] Read more.
Tunisia is a country that suffers from energy demand problems and environmental matters. Thus, Tunisian authorities desire to encourage the development of renewable energy sources, especially from biological processes, like anaerobic digestion. Therefore, this study is focused on the evaluation of biogas and bio-methane yield from the co-digestion of three available and abundant bio-wastes in the southern regions of Tunisia. The three different raw materials are an organic fraction of municipal solid waste, chicken manure, and olive mill wastewater. In this context, experimental work to evaluate the potential of biogas and bio-methane production was carried out at mesophilic temperature 35 °C and batch mode. The present work highlights the possibility of generating biogas from these organic wastes and reducing the amounts of the wastes to dispose of in landfills. The experimental study of the co-digestion process under specific conditions of carbon to nitrogen ratio (C/N), T, pH, and inoculums to substrate ratio ISR provided a high yield of net methane and net biogas, in comparison with other research works. Results showed a higher specific net methane production per kg of volatile solids, which is equal to 0.338 Nm3 methane/kg VS and 0.430 Nm3 methane/kg VS for two studied cases. The obtained volatile solids reduction was found to be 91% of the initial content, for a hydraulic retention time (HRT) of 40 days. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessArticle
Effect of Heavy Metals in the Performance of Anaerobic Digestion of Olive Mill Waste
Processes 2020, 8(9), 1146; https://doi.org/10.3390/pr8091146 - 14 Sep 2020
Cited by 3 | Viewed by 655
Abstract
This study presents an investigation on the effect of heavy metals on the production of biogas during the process of anaerobic digestion (AD) of olive mill waste (OMW). The poisonous effect and the inhibitory influence of Fe, Ni, Pb, Zn, Cu, and Cr [...] Read more.
This study presents an investigation on the effect of heavy metals on the production of biogas during the process of anaerobic digestion (AD) of olive mill waste (OMW). The poisonous effect and the inhibitory influence of Fe, Ni, Pb, Zn, Cu, and Cr on the digestion process are investigated and determined. Biomethanation potential tests are performed for this sake. Adding some of the heavy metals to the AD decreases the efficiency of biogas production and methane concentration and decreases the reduction in the VS, the TCOD, the SCOD, and the organic acid load. A critical increase in the total organic acid and inhibition of methanogenic bacteria was observed due to its toxicity. The toxicity of the heavy metals can be arranged according to increasing order: Cu > Ni > Pb > Cr > Zn > Fe, which leads to rapid poisoning of the active microorganisms. Iron may also exhibit stimulatory effects, but with a low rate and at a certain level. The conclusions of this work are important for the industry and help to understand how to carefully manage the presence of heavy metals in the digestate. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessArticle
New Insights on the Estimation of the Anaerobic Biodegradability of Plant Material: Identifying Valuable Plants for Sustainable Energy Production
Processes 2020, 8(7), 806; https://doi.org/10.3390/pr8070806 - 09 Jul 2020
Cited by 2 | Viewed by 625
Abstract
Based on fifteen European plant species, a statistical model for the estimation of the anaerobic biodegradability of plant material was developed. We show that this new approach represents an accurate and cost-effective method to identify valuable energy plants for sustainable energy production. In [...] Read more.
Based on fifteen European plant species, a statistical model for the estimation of the anaerobic biodegradability of plant material was developed. We show that this new approach represents an accurate and cost-effective method to identify valuable energy plants for sustainable energy production. In particular, anaerobic biodegradability (Bo) of lignocellulosic material was empirically found to be related to the amount of cellulose plus lignin, as analytically assessed by the van Soest method, i.e., the acid detergent fiber (ADF) value. Apart from being theoretically meaningful, the ADF-based empirical model requires the least effort compared to the other four proposed conceptual models proposed, as individual fractions of cellulose, hemicellulose, and lignin do not need to be assessed, which also enhances the predictive accuracy of the model’s estimation. The model’s results showed great predictability power, allowing us to identify interesting crops for sustainable crop rotations. Finally, the model was used to predict Bo of 114 European plant samples that had been previously characterized by means of the van Soest method. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperArticle
Greening the Gas Grid—Evaluation of the Biomethane Injection Potential from Agricultural Residues in Austria
Processes 2020, 8(5), 630; https://doi.org/10.3390/pr8050630 - 24 May 2020
Cited by 2 | Viewed by 1100
Abstract
In order to implement the Paris Climate Agreement, the current Austrian coalition government has included trend-setting targets in its policy statement. “Green gas” plays a key role in this context, as the natural gas grid shall also gradually become renewable. This article analyses [...] Read more.
In order to implement the Paris Climate Agreement, the current Austrian coalition government has included trend-setting targets in its policy statement. “Green gas” plays a key role in this context, as the natural gas grid shall also gradually become renewable. This article analyses the technical biomethane injection potential for agricultural residues based on Integrated Administration and Control System (IACS) data on a municipal level. While a technical biogas potential of 16.2 TWhCH4 from catch crops, farm manure, straw and beet leaves is available, only about half of it can be fed into the gas grid because of technical and economic reasons. Austria’s biomethane injection potential of 7.4 TWhCH4 is mainly produced in arable farming regions. In order to further increase this potential, the investment costs of biogas upgrading plants must be reduced, the use of biogenic waste and energy crops must be further promoted and an investor-friendly legal framework must be created. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessArticle
Optimisation and Modelling of Anaerobic Digestion of Whiskey Distillery/Brewery Wastes after Combined Chemical and Mechanical Pre-Treatment
Processes 2020, 8(4), 492; https://doi.org/10.3390/pr8040492 - 23 Apr 2020
Cited by 5 | Viewed by 1229
Abstract
Whiskey distillery waste streams consisting of pot ale (liquid residue) and spent grain (solid residue) are high strength organic wastes and suitable feedstock for anaerobic digestion (AD) from both economic and environmental stand points. Anaerobic digestion of pot ale and pot ale/spent grain [...] Read more.
Whiskey distillery waste streams consisting of pot ale (liquid residue) and spent grain (solid residue) are high strength organic wastes and suitable feedstock for anaerobic digestion (AD) from both economic and environmental stand points. Anaerobic digestion of pot ale and pot ale/spent grain mixtures (with mixing ratios of 1:1, 1:3, and 1:5 by wet weight) was performed after implementation of a novel hybrid pre-treatment (combined chemical and mechanical) in order to modify lignocellulosic structure and ultimately enhance digestion yield. Lignin, hemicellulose, and cellulose fractions were determined before and after chemical pre-treatment. Effects of different inoculum rates (10–30–50% on wet basis) and beating times (0–7.5–15 min) on anaerobic digestion of pot ale alone and of pot ale/spent grain mixtures were investigated in lab scale batch mode with a major focus of optimising biogas yield by using response surface methodology (RSM) in Design Expert Software. The highest biogas yields of 629 ± 8.5 mL/g vs. (51.3% CH4) and 360 ± 10 mL/g vs. (55.0 ± 0.4) with anaerobic digestion of pot ale alone and spent grain mix after 1M NaOH and 7.5 min beating pre-treatments with 50% inoculum ratio respectively. The optimum digestion conditions to maximise the biogas quality and quantity were predicted as 10 and 13 min beating times and 32 and 38 °C digestion temperatures for anaerobic digestion of pot ale alone and spent grain mix respectively. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperEditor’s ChoiceArticle
Biodrying of the Light Fraction from Anaerobic Digestion Pretreatment in Order to Increase the Total Recovery Rate
Processes 2020, 8(3), 276; https://doi.org/10.3390/pr8030276 - 28 Feb 2020
Cited by 3 | Viewed by 1050
Abstract
Two pilot-scale tests were carried out to assess if biodrying could be an effective process for the treatment of light fraction produced by an hydromechanical pre-treatment in an anaerobic digestion plant. The trials were performed using two pilot-scale stainless steel cylindrical reactors of [...] Read more.
Two pilot-scale tests were carried out to assess if biodrying could be an effective process for the treatment of light fraction produced by an hydromechanical pre-treatment in an anaerobic digestion plant. The trials were performed using two pilot-scale stainless steel cylindrical reactors of 750 L capacity. Two tests were performed: in Test 1, only the light fraction was used; in Test 2, the light fraction was mixed with a bulking agent composed of garden and pruning waste. In Test 2, the highest temperature (71 °C) in a short time (8 days) was reached. An average water content reduction of 78% in Test 1 and 61% in Test 2 was measured, leading to similar reductions of weight (47–48%) and volume (27–29%). A high biological stability was measured on the final light fraction samples collected from both the tests. Furthermore, the lower heating value obtained after the biodrying treatment complies with the quality specification of the European standard on refuse-derived fuels. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessArticle
Feasibility Assessment of Two Biogas-Linked Rural Campus Systems: A Techno-Economic Case Study
Processes 2020, 8(2), 180; https://doi.org/10.3390/pr8020180 - 05 Feb 2020
Cited by 1 | Viewed by 654
Abstract
The principle of sustainable development is becoming more and more prominent in various schools, and the eco-campus in rural areas often has more room for display. The identification and assessment of cost-effective biomass resources appropriate for recycling represent an opportunity that may significantly [...] Read more.
The principle of sustainable development is becoming more and more prominent in various schools, and the eco-campus in rural areas often has more room for display. The identification and assessment of cost-effective biomass resources appropriate for recycling represent an opportunity that may significantly improve the comprehensive efficiency of an eco-campus system, resulting in remarkable investment savings, pollution reduction, as well as reducing energy consumption and resources waste. The economic feasibility of two biogas-linked rural campus systems (Fanjiazhai Middle School, FJZ and Xidazhai Middle School, XDZ, Yangling, China), as well as their key technologies, is investigated, the two systems respectively represent two biobased agricultural production modes. It is found that the initial investment, operating investment, and total revenue of FJZ system is 1.37 times, 2.39 times, and 1.71 times of XDZ system respectively, thus indicating that FJZ campus is proved to be a “larger” system compared to the other one. The operating costs show that reasonable control of labor and transportation costs should be carried out to optimize the economic feasibility of the system. After considering the system’s economic credits obtained from using biogas slurry flushing system and avoiding waste management, the net present value of XDZ system had increased to its 1.5 times, while the FJZ system had increased to its 135%. From the perspective of revenue to investment ratio, XDZ system has a better profit earning efficiency compared to FJZ system. The sensitivity analysis indicates that biogas price, fruit yield, labor use are main factors that have the greatest impacts on the economic performance of these eco-campuses. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperArticle
Anaerobic Digestate Treatment Selection Model for Biogas Plant Costs and Emissions Reduction
Processes 2020, 8(2), 142; https://doi.org/10.3390/pr8020142 - 22 Jan 2020
Cited by 5 | Viewed by 1604
Abstract
Agriculture is one of the leading sectors on the global level contributing to greenhouse gas (GHG) emissions increase. With the utilization of biogas production technology within the agriculture sector, ecological benefits could be achieved, with immediate economic profit. Therefore, to retain economic profit [...] Read more.
Agriculture is one of the leading sectors on the global level contributing to greenhouse gas (GHG) emissions increase. With the utilization of biogas production technology within the agriculture sector, ecological benefits could be achieved, with immediate economic profit. Therefore, to retain economic profit and environmental sustainability, implementation of bioeconomy principles is of key importance. This paper examines four options of digestate treatment, which is identified as one of the highest operational cost for the biogas plant. A simple and robust model in Excel Solver was developed to determine the best solution for minimising GHG emissions and maximise profit for the biogas plant operator, through an upgrade of the plant with digestate treatment technologies. The model was implemented on a case of a Croatian biogas plant and the best solution in terms of GHG reduction and profit increase proved to be fertilizer production (Option 1), through a crystallization process of struvite within the digestate. This option obtains a significant reduction in GHG emissions compared to standard biogas production without additional upgrades (Option 4), by over 90%, and increase of profit for the biogas plant operator, which diversifies the income source and creates multiple positive impacts on the environment. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessArticle
Electrical Conductivity for Monitoring the Expansion of the Support Material in an Anaerobic Biofilm Reactor
Processes 2020, 8(1), 77; https://doi.org/10.3390/pr8010077 - 07 Jan 2020
Cited by 2 | Viewed by 887
Abstract
This article describes the use of the electrical conductivity for measuring bed expansion in a continuous anaerobic biofilm reactor in order to prevent the exit of support material from the reactor with the consequent loss of biomass. The substrate used for the tests [...] Read more.
This article describes the use of the electrical conductivity for measuring bed expansion in a continuous anaerobic biofilm reactor in order to prevent the exit of support material from the reactor with the consequent loss of biomass. The substrate used for the tests is obtained from a two-stage anaerobic digestion (AD) process at the pilot scale that treats the liquid fraction of fruit and vegetable waste (FVW). Tests were performed with the raw substrate before anaerobic treatment (S1), the effluent from the hydrolysis reactor (S2), and the effluent from the methanogenic reactor (S3) to evaluate its effect on the electrical conductivity values and its interaction with colonized support material. The tests were carried out in a 32 L anaerobic inverse fluidized bed reactor (IFBR), which was inoculated with colonized support material and using two industrial electrodes at different column positions. The results with the previously digested samples (S2 and S3) were satisfactory to detect the presence of support material at the points where the electrodes were placed since the electrical conductivity values showed significant changes of up to 0.5 V, while with substrate S1 no significant voltage differences were appreciated. These results demonstrate that electrical conductivity can be used as an economic and simple mean for monitoring the support material expansion in order to avoid over expansion in the IFBR. It was also demonstrated that the conditions of the substrate in the methanogenic stage (pH and presence of volatile fatty acids) do not affect the operation of the electrical conductivity detection system. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperArticle
Numerical Comparison of a Combined Hydrothermal Carbonization and Anaerobic Digestion System with Direct Combustion of Biomass for Power Production
Processes 2020, 8(1), 43; https://doi.org/10.3390/pr8010043 - 01 Jan 2020
Cited by 14 | Viewed by 1234
Abstract
Two of the methods for converting biomass to fuel are hydrothermal carbonization (HTC) and anaerobic digestion (AD). This study is aimed at designing and analyzing two scenarios for bioenergy production from undervalued biomass (sawdust). In one of the scenarios (direct combustion or DC), [...] Read more.
Two of the methods for converting biomass to fuel are hydrothermal carbonization (HTC) and anaerobic digestion (AD). This study is aimed at designing and analyzing two scenarios for bioenergy production from undervalued biomass (sawdust). In one of the scenarios (direct combustion or DC), raw biomass is burned in a combustor to provide the heat that is required by the Rankine cycle to generate electricity. In the other scenario (HTC-AD), the raw biomass first undergoes HTC treatment. While the solid product (hydrochar) is used to produce power by a Rankine cycle, the liquid by-product undergoes an AD process. This results in fuel gas production and it can be used in a Brayton cycle to generate more power. Energy and mass balance analysis of both scenarios were developed for each unit process by using Engineering Equation Solver (EES). The required data were obtained experimentally or from the literature. The performances of the proposed systems were evaluated, and a sensitivity analysis was presented to help in finding the best operational conditions. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperEditor’s ChoiceArticle
An Incubation System to Enhance Biogas and Methane Production: A Case Study of an Existing Biogas Plant in Umbria, Italy
Processes 2019, 7(12), 925; https://doi.org/10.3390/pr7120925 - 04 Dec 2019
Cited by 2 | Viewed by 1160
Abstract
The pre-incubation of digestate and recycling of microbes inside a continuously stirred tank reactor (CSTR) are effective ways to optimize the anaerobic digestion process and improve the performance of biogas and methane production, also in existing biogas plants. In this study, a digestate [...] Read more.
The pre-incubation of digestate and recycling of microbes inside a continuously stirred tank reactor (CSTR) are effective ways to optimize the anaerobic digestion process and improve the performance of biogas and methane production, also in existing biogas plants. In this study, a digestate incubation system using a nutrient mix to boost the activity of microbes was coupled to a CSTR to boost biogas and methane production. This system has been tested both on a lab scale and on an industrial scale. On a pilot scale, the system achieved an increase of +16.47 v% in biogas production with respect to the conventional anaerobic digestion process, and an increase of +2 v% in methane content (from 65.94 v% to 67.84 v%). On an industrial scale, the use of this incubation reactor with a capacity of 1 m3 has led to an increase in methane yield of 12 v%. This system allows to maintain the syntrophic relationship between acid-producing bacteria and methanogens and contemporary push the development of methanogens. Moreover, it is an economic system to be integrated into an existing biogas plant given the small volume and the simplicity of the incubation reactor. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperArticle
A Comparative Study of Biogas Reactor Fluid Rheology—Implications for Mixing Profile and Power Demand
Processes 2019, 7(10), 700; https://doi.org/10.3390/pr7100700 - 03 Oct 2019
Cited by 2 | Viewed by 793
Abstract
Anaerobic digestion (AD) is an established process for integrating waste management with renewable energy and nutrient recovery. Much of the research in this field focuses on the utilisation of new substrates, yet their effects on operational aspects such as fluid behaviour and power [...] Read more.
Anaerobic digestion (AD) is an established process for integrating waste management with renewable energy and nutrient recovery. Much of the research in this field focuses on the utilisation of new substrates, yet their effects on operational aspects such as fluid behaviour and power requirement for mixing are commonly overlooked, despite their importance for process optimisation. This study analysed rheological characteristics of samples from 21 laboratory-scale continuous stirred-tank biogas reactors (CSTBRs) digesting a range of substrates, in order to evaluate substrate effect on mixing efficiency and power demand through computational fluid dynamics (CFD). The results show that substrate and process parameters, such as solids content and organic loading, all have a significant effect on CSTBR fluid rheology. The correlation levels between rheological and process parameters were different across substrates, while no specific fluid behaviour patterns could be associated with substrate choice. Substrate should thus be considered an equally important rheology effector as process parameters. Additional substrate-related parameters should be identified to explain the differences in correlations between rheological and process parameters across substrate groups. The CFD modelling revealed that the rheology differences among the AD processes have significant implications for mixing efficiency and power demand of the CSTBRs, highlighting the importance of considering the substrate-induced effects on CSTBR rheology before including a new substrate. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperArticle
Biogas Generation through Anaerobic Digestion of Compost Leachate in Semi-Continuous Completely Stirred Tank Reactors
Processes 2019, 7(9), 635; https://doi.org/10.3390/pr7090635 - 18 Sep 2019
Cited by 7 | Viewed by 1073
Abstract
The composting process of organic fraction of municipal solid waste, besides to the residual compost, generates a wastewater that is characterized by a high organic load. The application of anaerobic processes represents an advantageous solution for the treatment and valorization of this type [...] Read more.
The composting process of organic fraction of municipal solid waste, besides to the residual compost, generates a wastewater that is characterized by a high organic load. The application of anaerobic processes represents an advantageous solution for the treatment and valorization of this type of wastewater. Nevertheless, few works have been focused on the anaerobic digestion of compost leachate. To overcome this dearth, in the present paper an extensive experimental investigation was carried out to develop and analyse the anaerobic treatment of young leachate in completely stirred tank reactors (CSTR). Initially, it was defined a suitable leachate pretreatment to correct its acidic characteristics that is potentially able to inhibit methanogenic biomass activity. The pretreated leachate was fed to the digester over the start-up phase that was completed in about 40 days. During the operational period, the organic load rate (OLR) changed between 4.25 kgCOD/m3d and 38.5 kgCOD/m3d. The chemical oxygen demand (COD) abatement was higher than 90% for OLR values up to 14.5 kgCOD/m3d and around to 80% for applied loads equal to 24.5 kgCOD/m3d. At this OLR, it was reached the maximum daily biogas production of about 9.3 Lbiogas/(Lreactord). The CH4 fraction was between 70%–78% and the methane production yield in the range 0.34–0.38 LCH4/gCODremoved. The deterioration of biogas production started for OLR values that were over the threshold of 24.5 kgCOD/m3d when a volatile fatty acids (VFA) accumulation occurred and the pH dropped below 6.5. The maximum ratio between VFA and alkalinity (ALK) tolerable in the CSTR was identified to be 0.5 gCH3COOH/gCaCO3. Through an economic analysis, it was proven that the digestion of compost leachate could ensure significant economic profits. Furthermore, the produced digestate had characteristics that were compatible for agricultural applications. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Review

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Open AccessReview
Markers for the Comparison of the Performances of Anoxic Biotrickling Filters in Biogas Desulphurisation: A Critical Review
Processes 2021, 9(3), 567; https://doi.org/10.3390/pr9030567 - 23 Mar 2021
Viewed by 388
Abstract
The agriculture and livestock industry generate waste used in anaerobic digestion to produce biogas containing methane (CH4), useful in the generation of electricity and heat. However, although biogas is mainly composed of CH4 (~65%) and CO2 (~34%), among the [...] Read more.
The agriculture and livestock industry generate waste used in anaerobic digestion to produce biogas containing methane (CH4), useful in the generation of electricity and heat. However, although biogas is mainly composed of CH4 (~65%) and CO2 (~34%), among the 1% of other compounds present is hydrogen sulphide (H2S) which deteriorates engines and power generation fuel cells that use biogas, generating a foul smell and contaminating the environment. As a solution to this, anoxic biofiltration, specifically with biotrickling filters (BTFs), stands out in terms of the elimination of H2S as it is cost-effective, efficient, and more environmentally friendly than chemical solutions. Research on the topic is uneven in terms of presenting performance markers, underestimating many microbiological indicators. Research from the last decade was analyzed (2010–2020), demonstrating that only 56% of the reviewed publications did not report microbiological analysis related to sulphur oxidising bacteria (SOB), the most important microbial group in desulphurisation BTFs. This exposes fundamental deficiencies within this type of research and difficulties in comparing performance between research works. In this review, traditional and microbiological performance markers of anoxic biofiltration to remove H2S are described. Additionally, an analysis to assess the efficiency of anoxic BTFs for biogas desulphurisation is proposed in order to have a complete and uniform assessment for research in this field. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessReview
Life Cycle Assessment and Economic Analysis of Biomass Energy Technology in China: A Brief Review
Processes 2020, 8(9), 1112; https://doi.org/10.3390/pr8091112 - 07 Sep 2020
Cited by 3 | Viewed by 737
Abstract
This study describes the technological processes and characteristics of biomass direct combustion power generation, biomass gasification power generation, biomass mixed combustion power generation, and biomass biogas power generation in terms of their importance and application in China. Under the perspective of environmental and [...] Read more.
This study describes the technological processes and characteristics of biomass direct combustion power generation, biomass gasification power generation, biomass mixed combustion power generation, and biomass biogas power generation in terms of their importance and application in China. Under the perspective of environmental and economic sustainability, the life cycle assessment (LCA) method and dynamic analysis method based on time value are used to simulate and evaluate the environmental loads and economic benefits of different power generation processes. By comparing with coal-fired power generation systems, the environmental and economic benefits of different biomass power generation technologies are illustrated. The results shows that biomass gasification power generation has the best environmental benefits, with a total load of 1.05 × 10−5, followed by biomass biogas power generation (9.21 × 10−5), biomass direct combustion power generation (1.23 × 10−4), and biomass mixed combustion power generation (3.88 × 10−4). Compared with the environmental load of coal-fired power generation, the reduction rate was 97.69%, 79.69%, 72.87%, and 14.56% respectively. According to the analysis of the technical economy evaluation results, when the dynamic pay-back period and IRR (internal rate of return) were used as evaluation indicators, the biomass direct combustion power generation has the best pay-back period (7.71 years) and IRR (19.16%), followed by the biogas power generation, with higher dynamic payback period (12.03 years), and lower IRR (13.49%). For gasification power generation and mixed-combustion power generation, their dynamic payback period is long, and the IRR is low. If net present value (NPV) is selected as the evaluation index, the biogas power generation appears to be the best because its net present value per megawatt is 11.94 million yuan, followed by direct combustion power generation (6.09 million yuan), and the net present value of mixed-combustion power generation and gasification power generation is relatively low. Compared with coal-fired power generation, direct combustion power generation and biogas power generation present significant economic benefits. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
Open AccessReview
Approaches in Design of Laboratory-Scale UASB Reactors
Processes 2020, 8(6), 734; https://doi.org/10.3390/pr8060734 - 24 Jun 2020
Cited by 1 | Viewed by 1102
Abstract
Up-flow Anaerobic Sludge Blanket (UASB) reactors are popular tools in wastewater treatment systems due to the ability to work with high feed rates and wastes with high concentration of organic contaminants. While full-scale industrial applications of UASB reactors are developed and described in [...] Read more.
Up-flow Anaerobic Sludge Blanket (UASB) reactors are popular tools in wastewater treatment systems due to the ability to work with high feed rates and wastes with high concentration of organic contaminants. While full-scale industrial applications of UASB reactors are developed and described in the available literature, laboratory-scale designs utilized for treatability testing are not well described. The majority of published studies do not describe the laboratory UASB construction details or do use reactors that already had developed a trophic network in microbial consortia under laboratory environment and therefore are more stable. The absence of defined guidelines for geometry design, selection of materials, construction, operation rules, and, especially, the start-up conditions, significantly hamper researchers who desire to conduct treatability testing using UASB reactors in laboratory scale. In this article, we compiled and analyzed the information available in the refereed literature concerning UASB reactors used in laboratory environment, where information on geometry and/or operational conditions were provided in detail. We utilized the information available in the literature and the experience gained in our laboratory (Sustainable Waste to Bioproducts Engineering Center) to suggest a unified operation flowchart and for design, construction, operation, and monitoring for a laboratory-scale UASB reactors. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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Open AccessFeature PaperReview
Review of Anaerobic Digestion Modeling and Optimization Using Nature-Inspired Techniques
Processes 2019, 7(12), 953; https://doi.org/10.3390/pr7120953 - 13 Dec 2019
Cited by 11 | Viewed by 1257
Abstract
Although it is a well-researched topic, the complexity, time for process stabilization, and economic factors related to anaerobic digestion call for simulation of the process offline with the help of computer models. Nature-inspired techniques are a recently developed branch of artificial intelligence wherein [...] Read more.
Although it is a well-researched topic, the complexity, time for process stabilization, and economic factors related to anaerobic digestion call for simulation of the process offline with the help of computer models. Nature-inspired techniques are a recently developed branch of artificial intelligence wherein knowledge is transferred from natural systems to engineered systems. For soft computing applications, nature-inspired techniques have several advantages, including scope for parallel computing, dynamic behavior, and self-organization. This paper presents a comprehensive review of such techniques and their application in anaerobic digestion modeling. We compiled and synthetized the literature on the applications of nature-inspired techniques applied to anaerobic digestion. These techniques provide a balance between diversity and speed of arrival at the optimal solution, which has stimulated their use in anaerobic digestion modeling. Full article
(This article belongs to the Special Issue Anaerobic Digestion Processes)
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