Biogas Upgrading Using a Single-Membrane System: A Review

In recent years, the use of biogas as a natural gas substitute has gained great attention. Typically, in addition to methane (CH4), biogas contains carbon dioxide (CO2), as well as small amounts of impurities, e.g., hydrogen sulfide (H2S), nitrogen (N2), oxygen (O2) and volatile organic compounds (VOCs). One of the latest trends in biogas purification is the application of membrane processes. However, literature reports are ambiguous regarding the specific requirement for biogas pretreatment prior to its upgrading using membranes. Therefore, the main aim of the present study was to comprehensively examine and discuss the most recent achievements in the use of single-membrane separation units for biogas upgrading. Performing a literature review allowed to indicate that, in recent years, considerable progress has been made on the use of polymeric membranes for this purpose. For instance, it has been documented that the application of thin-film composite (TFC) membranes with a swollen polyamide (PA) layer ensures the successful upgrading of raw biogas and eliminates the need for its pretreatment. The importance of the performed literature review is the inference drawn that biogas enrichment performed in a single step allows to obtain upgraded biogas that could be employed for household uses. Nevertheless, this solution may not be sufficient for obtaining high-purity gas at high recovery efficiency. Hence, in order to obtain biogas that could be used for applications designed for natural gas, a membrane cascade may be required. Moreover, it has been documented that a significant number of experimental studies have been focused on the upgrading of synthetic biogas; meanwhile, the data on the raw biogas are very limited. In addition, it has been noted that, although ceramic membranes demonstrate several advantages, experimental studies on their applications in single-membrane systems have been neglected. Summarizing the literature data, it can be concluded that, in order to thoroughly evaluate the presented issue, the long-term experimental studies on the upgrading of raw biogas with the use of polymeric and ceramic membranes in pilot-scale systems are required. The presented literature review has practical implications as it would be beneficial in supporting the development of membrane processes used for biogas upgrading.

Mini-review: Nanoparticles for enhanced biogas upgrading

This mini-review is intended to explore the innovative applications of nanoparticles (NPs) in biogas upgrading, emphasizing their capacity to enhance biogas quality. Numerous studies underscore how NPs, when applied during anaerobic digestion, can boost not only the quantity but also the quality of the produced biogas, leading to reduce significantly the concentration of hydrogen sulphide or even to remove it completely. Moreover, NPs are proving to be excellent alternatives as adsorbent materials, achieving up to 400 mgH2S g-1 NPs. In addition, new studies are exploring the application of NPs to increase the efficiency of biological treatments thanks to their unique features. This review also emphasizes the potential benefits and addresses the challenges that need to be overcome for these technologies to reach their full potential, ultimately contributing to the development of a sustainable and environmentally friendly energy landscape.

Optimization of the Ex Situ Biomethanation of Hydrogen and Carbon Dioxide in a Novel Meandering Plug Flow Reactor: Start-Up Phase and Flexible Operation

With the increasing use of renewable energy resources for the power grid, the need for long-term storage technologies, such as power-to-gas systems, is growing. Biomethanation provides the opportunity to store energy in the form of the natural gas-equivalent biomethane. This study investigates a novel plug flow reactor that employs a helical static mixer for the biological methanation of hydrogen and carbon dioxide. In tests, the reactor achieved an average methane production rate of 2.5 LCH4LR∗d (methane production [LCH4] per liter of reactor volume [LR] per day [d]) with a maximum methane content of 94%. It demonstrated good flexibilization properties, as repeated 12 h downtimes did not negatively impact the process. The genera Methanothermobacter and Methanobacterium were predominant during the initial phase, along with volatile organic acid-producing, hydrogenotrophic, and proteolytic bacteria. The average ratio of volatile organic acid to total inorganic carbon increased to 0.52 ± 0.04, while the pH remained stable at an average of pH 8.1 ± 0.25 from day 32 to 98, spanning stable and flexible operation modes. This study contributes to the development of efficient flexible biological methanation systems for sustainable energy storage and management.

The Use of Fungi of the Trichoderma Genus in Anaerobic Digestion: A Review

Plant waste biomass is the most abundant renewable energy resource on Earth. The main problem with utilising this biomass in anaerobic digestion is the long and costly stage of degrading its complex structure into simple compounds. One of the promising solutions to this problem is the application of fungi of the Trichoderma genus, which show a high capacity to produce hydrolytic enzymes capable of degrading lignocellulosic biomass before anaerobic digestion. This article discusses the structure of plant waste biomass and the problems resulting from its structure in the digestion process. It presents the methods of pre-treatment of lignocellulose with a particular focus on biological solutions. Based on the latest research findings, key parameters related to the application of Trichoderma sp. as a pre-treatment method are discussed. In addition, the possibility of using the digestate from agricultural biogas plants as a carrier for the multiplication of the Trichoderma sp. fungi, which are widely used in many industries, is discussed.

Microbial and functional characterization of granulated sludge from full-scale UASB thermophilic reactor applied to sugarcane vinasse treatment

Considering the scarcity of data in the literature regarding phylogenetic and metabolic composition of different inocula, especially those from thermophilic conditions, this research aimed at characterizing the microbial community and preferable metabolic pathways of an UASB reactor sludge applied to the thermophilic treatment (55°C) of sugarcane vinasse, by means of shotgun metagenomics. After its metabolic potential was depicted, it was possible to observe several genes encoding enzymes that are of great importance to anaerobic digestion processes with different wastes as substrate, especially regarding the biodegradation of carbohydrates and ligninolytic compounds, glycerolypids, volatile fatty acids and alcohols metabolism and biogas (H2 and CH4) production. The genera identified in higher relative abundances for Bacteria domain were Sulfirimonas (37.52 ± 1.8%), possibly related to the sludge endogenic activity due to its strong relation with a peptidoglycan lyase enzymes family, followed by Fluviicola (5.01 ± 1.0%), Defluviitoga (4.36 ± 0.2%), Coprothermobacter (4.32 ± 0.5%), Fervidobacterium (2.93 ± 0.3%), Marinospirillum (2.75 ± 0.2%), Pseudomonas (2.14 ± 0.2%) and Flavobacterium (1.78 ± 0.1%), mostly related with carbohydrates fermentations and/or H2 production. For Archaea domain, Methanosarcina (0.61 ± 0.1%), Methanothermobacter (0.38 ± 0.0%), Methanoculleus (0.30 ± 0.1%), Thermococcus (0.03 ± 0.0%), Methanolobus (0.02 ± 1.8%), Methanobacterium (0.013 ± 0.0%), Aciduliprofundum and Pyrococcus (0.01 ± 0.0%) were the most dominant ones, being Methanosarcina the most related with methanogenesis. It was concluded that the robust inoculum description performed in this study may subside future biotechnological researches by using similar inocula (UASB sludges), focusing on the obtainment of value-added by-products by means of anaerobic digestion, such as volatile fatty acids, alcohols and biogas (H2 and CH4), by using several types of waste as substrate.

Co-production of biohydrogen and biomethane utilizing halophytic biomass Atriplexcrassifolia by two-stage anaerobic fermentation process

The excessive use of fossil has resulted in the drastic exhaustion of natural energy sources, leading to environmental challenges and energy crises. Owing to rising energy demand there is a dire need to shift towards renewable energies from lignocellulosic biomass. The present study assessed the co-production of biohydrogen (H₂) and biomethane (CH₄) by utilizing a less explored halophyte Atriplexcrassifolia. Various reaction parameters were evaluated for their effect on biohydrogen and biomethane production in batch experiments. One parameter at a time experimental strategy was chosen for production optimization. Hydrogen and methane yields along with their production rates were assessed at different incubation times, temperatures, pH, substrate concentrations, and inoculum sizes in acidogenesis and methanogenesis stages, respectively. In the first stage, maximum cumulative hydrogen production of 66 ± 0.02 mL, with hydrogen yield of 13.2 ± 0.03 mL/g, and hydrogen production rate (HPR) of 1.37 ± 0.05 mL/h was attained when the reaction mixture (5 g Atriplexcrassifolia and 10 mL pretreated sewage sludge) was processed at 37°C and pH 5.5 after 48 h of incubation. While in the second stage, maximum cumulative methane production, i.e., 343 ± 0.12 mL, methane yield (MY) of 8.5 ± 0.07 mL/mL, and methane production rate (MPR) of 0.8 ± 0.05 mL/h was achieved after 18 days of incubation of reaction mixture (40 mL of hydrogenic slurry with 80 mL inoculum) at 45°C and pH 8. Furthermore, a 51% and 24% rise in biohydrogen and biomethane production respectively were recorded when the gases were produced at these optimized reaction conditions. The results ensure halophyte Atriplexcrassifolia as an imperative renewable energy resource and proposed that effective optimization of the process further increased the coproduction of biohydrogen and biomethane.

Spent Mushroom Substrate Hydrolysis and Utilization as Potential Alternative Feedstock for Anaerobic Co-Digestion

Valorization of lignocellulosic biomass, such as Spent Mushroom Substrate (SMS), as an alternative substrate for biogas production could meet the increasing demand for energy. In view of this, the present study aimed at the biotechnological valorization of SMS for biogas production. In the first part of the study, two SMS chemical pretreatment processes were investigated and subsequently combined with thermal treatment of the mentioned waste streams. The acidic chemical hydrolysate derived from the hydrothermal treatment, which yielded in the highest concentration of free sugars (≈36 g/100 g dry SMS, hydrolysis yield ≈75% w/w of holocellulose), was used as a potential feedstock for biomethane production in a laboratory bench-scale improvised digester, and 52 L biogas/kg of volatile solids (VS) containing 65% methane were produced in a 15-day trial of anaerobic digestion. As regards the alkaline hydrolysate, it was like a pulp due to the lignocellulosic matrix disruption, without releasing additional sugars, and the biogas production was delayed for several days. The biogas yield value was 37 L/kg VS, and the methane content was 62%. Based on these results, it can be concluded that SMS can be valorized as an alternative medium employed for anaerobic digestion when pretreated with both chemical and hydrothermal hydrolysis.

Optimization of iron-enhanced anaerobic digestion of agro-wastes for biomethane production and phosphate release

In this study, the optimization of additives (polypyrrole magnetite nanocomposites (Ppy/Fe₃O₄) and antagonists (humic acid and arsenic oxide)) for simultaneous recovery of biomethane and phosphate release from enhanced anaerobic co-digestion of okra waste and pig manure was investigated. The pre-determined dosages of additives from our previous studies were used for the batch anaerobic digestion at different ratios under mesophilic conditions based on the two level-four factors central composite design (CCD) response surface methodology (RSM). After the anaerobic digestion processes, the biomethane yields were recorded and the digestates were characterized to determine the quantity of soluble phosphates. Both the independent variables and the responses were used to model and optimize the biogas yield and phosphate release conditions. The result showed that the maximum biomethane yield and P release were respectively 502.743 mLCH4/gVS and 168.674 mg/L at the optimum conditions of Ppy/Fe₃O₄ (20.0014 mg/L), HA (5.0018 mg/L), As (1.448 mg/L) and co-digestion (25.0001%). The response models predicted biomethane yield and P release to be 528.635 mLCH4/gVS and 164.405 mg/L respectively. All the response models were highly significant with appropriate goodness of fit and had prediction differences of 4.90% and 2.597% respectively for both biomethane yield and P release. Although both the accelerants and antagonists had influences on the anaerobic digestion processes by achieving enhanced biomethane production and P release, the influence of long exposure of anaerobic digestion processes to these additives on both responses is recommended for further investigation.

Effect of Pharmaceutical Sludge Pre-Treatment with Fenton/Fenton-like Reagents on Toxicity and Anaerobic Digestion Efficiency

Sewage sludge is successfully used in anaerobic digestion (AD). Although AD is a well-known, universal and widely recognized technology, there are factors that limit its widespread use, such as the presence of substances that are resistant to biodegradation, inhibit the fermentation process or are toxic to anaerobic microorganisms. Sewage sludge generated by the pharmaceutical sector is one such substance. Pharmaceutical sewage sludge (PSS) is characterized by high concentrations of biocides, including antibiotics and other compounds that have a negative effect on the anaerobic environment. The aim of the present research was to determine the feasibility of applying Advanced Oxidation Processes (AOP) harnessing Fenton's (Fe²⁺/H₂O₂) and Fenton-like (Fe³⁺/H₂O₂) reaction to PSS pre-treatment prior to AD. The method was analyzed in terms of its impact on limiting PSS toxicity and improving methane fermentation. The use of AOP led to a significant reduction of PSS toxicity from 53.3 ± 5.1% to 35.7 ± 3.2%, which had a direct impact on the taxonomic structure of anaerobic bacteria, and thus influenced biogas production efficiency and methane content. Correlations were found between PSS toxicity and the presence of Archaea and biogas yields in the Fe²⁺/H₂O₂ group. CH₄ production ranged from 363.2 ± 11.9 cm³ CH₄/g VS in the control PSS to approximately 450 cm³/g VS. This was 445.7 ± 21.6 cm³ CH₄/g VS (1.5 g Fe²⁺/dm³ and 6.0 g H₂O₂/dm³) and 453.6 ± 22.4 cm³ CH₄/g VS (2.0 g Fe²⁺/dm³ and 8.0 g H₂O₂/dm³). The differences between these variants were not statistically significant. Therefore, due to the economical use of chemical reagents, the optimal tested dose was 1.5 g Fe²⁺/6.0 g H₂O₂. The use of a Fenton-like reagent (Fe³⁺/H₂O₂) resulted in lower AD efficiency (max. 393.7 ± 12.1 cm³ CH₄/g VS), and no strong linear relationships between the analyzed variables were found. It is, therefore, a more difficult method to estimate the final effects. Research has proven that AOP can be used to improve the efficiency of AD of PSS.

A mini-review of biomethane valorization: Managerial and policy implications for a circular resource

The green transition requires renewable energy resources, especially the role of biomass is very crucial as it promotes resource circularity if sustainable substrates are used. This mini-review focuses on green gas derived from biomass called biomethane, which appears to be strategic in the face of soaring energy costs. Hence, combined Strengths, Weaknesses, Opportunities and Threats-Analytic Hierarchy Process analysis is used to compare and evaluate the critical factors. The results provide not only methodological insights through the application of the local-global priority method, but also managerial insights that see biomethane as a winning element for the green transition, fighting climate change and reducing dependence on external energy sources. Subsidies have played a key role in pursuing economic sustainability; however, their use should be reduced over time and measured to the actual contribution related to environmental and social improvement. The results of this work highlight that biomethane development is important to tackle climate change and to be self-sufficient from an energy perspective. This development plan, based on circularity of resources, includes subsidies for small-scale plants, substrates from neighbouring territories, citizen involvement in decision-making processes, valorization of suitable waste from an environmental perspective and stability of political choices.

Anaerobic co-digestion of leachate and glycerol for renewable energy generation

Anaerobic digestion is a versatile biotechnology that produces bioenergy, biogas, from wastewater. Biogas production and wastewater treatment can be optimized by associating substrates with complementary characteristics. In this context, the aim of this study was to evaluate the performance of the anaerobic co-digestion of different contents of landfill leachate and crude glycerol compared to the organic matter removal and specific biogas production, the effects of the factors (time, glycerol content and substrate/inoculum ratio) and their interactions on kinetic parameters of specific biogas production using the modified Gompertz model. A Central Composite Rotational Design (CCRD) was performed for the experimental variables: time (16.6, 20, 25, 30 and 33.4 days), glycerol content (0.43, 0.70, 1.10, 1.50 and 1.77%) and substrate/inoculum ratio (0.23, 0.30, 0.40, 0.50 and 0.57 g COD/g VSS). From the optimization, it was possible to maximize the efficiency of organic matter removal (90.15%) and specific biogas production (403.15 mL/g VSS) in the conditions of 33.2 days, glycerol content of 1.71% and substrate/inoculum ratio of 0.37 g COD/g VSS. Concerning the modified Gompertz model of the optimal condition performed, an average of 20.3 times higher specific biogas production was obtained when compared to the monodigestion of leachate. Therefore, the association of leachate and glycerol was found to be feasible in terms of stability, biodegradability and biogas production.

The Influence of Low-Temperature Food Waste Biochars on Anaerobic Digestion of Food Waste

The proof-of-the-concept of application of low-temperature food waste biochars for the anaerobic digestion (AD) of food waste (the same substrate) was tested. The concept assumes that residual heat from biogas utilization may be reused for biochar production. Four low-temperature biochars produced under two pyrolytic temperatures 300 °C and 400 °C and under atmospheric and 15 bars pressure with 60 min retention time were used. Additionally, the biochar produced during hydrothermal carbonization (HTC) was tested. The work studied the effect of a low biochar dose (0.05 g_BC × g_TSsubstrate⁻¹, or 0.65 g_BC × L⁻¹) on AD batch reactors’ performance. The biochemical methane potential test took 21 days, and the process kinetics using the first-order model were determined. The results showed that biochars obtained under 400 °C with atmospheric pressure and under HTC conditions improve methane yield by 3.6%. It has been revealed that thermochemical pressure influences the electrical conductivity of biochars. The biomethane was produced with a rate (k) of 0.24 d⁻¹, and the most effective biochars increased the biodegradability of food waste (FW) to 81% compared to variants without biochars (75%).

Alternative Materials for the Enrichment of Biogas with Methane

Carbonaceous adsorbents have been pointed out as promising adsorbents for the recovery of methane from its mixture with carbon dioxide, including biogas. This is because of the fact that CO₂ is more strongly adsorbed and also diffuses faster compared to methane in these materials. Therefore, the present study aimed to test alternative carbonaceous materials for the gas separation process with the purpose of enriching biogas in biomethane and to compare them with the commercial one. Among them was coconut shell activated carbon (AC) as the adsorbent derived from bio-waste, rubber tire pyrolysis char (RPC) as a by-product of waste utilization technology, and carbon molecular sieve (CMS) as the commercial material. The breakthrough experiments were conducted using two mixtures, a methane-rich mixture (consisting of 75% CH₄ and 25% CO₂) and a carbon dioxide-rich mixture (containing 25% CH₄ and 75% CO₂). This investigation showed that the AC sample would be a better candidate material for the CH₄/CO₂ separation using a fixed-bed adsorption column than the commercial CMS sample. It is worth mentioning that due to its poorly developed micropore structure, the RPC sample exhibited limited adsorption capacity for both compounds, particularly for CO₂. However, it was observed that for the methane-rich mixture, it was possible to obtain an instantaneous concentration of around 93% CH₄. This indicates that there is still much potential for the use of the RPC, but this raw material needs further treatment. The Yoon–Nelson model was used to predict breakthrough curves for the experimental data. The results show that the data for the AC were best fitted with this model.

Upgrading Biogas from Small Agricultural Sources into Biomethane by Membrane Separation

The agriculture sector in Poland could provide 7.8 billion m³ of biogas per year, but this potential would be from dispersed plants of a low capacity. In the current study, a membrane process was investigated for the upgrading biogas to biomethane that conforms to the requirements for grid gas in Poland. It was assumed that such a process is based on membranes made from modified polysulfone or polyimide, available in the market in Air Products PRISM PA1020 and UBE UMS-A5 modules, respectively. The case study has served an agricultural biogas plant in southern Poland, which provides the stream of 5 m³ (STP) h⁻¹ of biogas with a composition of CH₄ (52 vol.%), CO₂ (46.3 vol.%), N₂ (1.6 vol.%) and O₂ (0.1 vol.%), after a pretreatment. It was theoretically shown that this is possible to obtain the biomethane stream of at least 96 vol.% of CH₄ purity, with the concentration of the other biogas components below their respective thresholds, as required in Poland for gas fuel “E”, with methane recovery of up to 87.5% and 71.6% for polyimide and polysulfone membranes, respectively. The energetic efficiency of the separation process is comparable for both membrane materials, as expressed by power excess index, which reaches up to 51.3 kW_th kW_el⁻¹ (polyimide) and 40.7 kW_th kW_el⁻¹ (polysulfone). In turn, the membrane productivity was significantly higher in the case of the polyimide membrane (up to 38.3 kW_th m⁻²) than those based on the polysulfone one (up to 3.13 kW_th m⁻²).

Diversity of Carbonyl Compounds in Biogas and Natural Gas Revealed Using High-Resolution Mass Spectrometry and Nontarget Analysis

Airborne carbonyl compounds such as formaldehyde, acrolein, and methyl ethyl ketone have long been chemicals-of-concern in the environment due to their reactivity and their potential for negative health effects. Standard methods for determining carbonyls in air, which focus on a set of 15 or fewer compounds, involve derivatization to form nonvolatile hydrazones, which can readily be analyzed via liquid chromatography (LC) with ultraviolet detectors. Here, we apply a new LC-high-resolution mass spectrometry (HRMS) method to natural gas and a variety of upgraded biofuels to better assess their total carbonyl profile using the inherent selectivity of the standard sampling methodology and the selectivity and sensitivity of HRMS. The standard method accounted for only 64% of the total carbonyl content in natural gas and between 26 and 45% of the total carbonyl content in biogas sources, with the balance detected by the new LC/HRMS method. An additional 540 compounds with molecular formulas consistent with carbonyl compounds were detected compared to only 14 target compounds using the standard method. These results demonstrate that the established method dramatically under-reports both the total carbonyl load and the diversity of carbonyl species in natural gas and biogas samples.

Domestic wastewater treatment by constructed wetland and microalgal treatment system for the production of value-added products

The main aim of this study is to treat domestic wastewater in a hybrid Vertical Flow Constructed Wetland (VFCW-4.2 m²) and Microalgal Treatment System (MTS-1 m²). The objective is not only to treat Domestic wastewater (DW) but also to produce value-added products from microalgal biomass. The domestic wastewater was initially treated by VFCW and the VFCW effluent was further phycoremediated by MTS. Canna indica was used for wetland vegetation and resident microalgal consortium from VFCW effluent was used in MTS. The VFCW and MTS was operated at 1 m³/day (HRT-0.25 m³/m²-day, OLR-400 g/m²-day) and 0.03 m³/day (HRT-0.03 m³/m²-day, OLR-400 g/m²-day), respectively. The integrated system was observed to remove 68.9% COD, 77.4% NH₄-N, 75.8% TKN and 63.6% PO₄-P. The harvested Naive Biomass (NB) was observed to contain 16.7% of lipids (W/W). The Residual Biomass after Lipid Extraction (RBLE) was used as a substrate for ethanol production. The observed yield of ethanol using RBLE as a substrate was 33.4%. NB, RBLE, and Residual Biomass after Lipid and Sugar Extraction (RBLSE) indicated net biomethane yield (mL/g VS) of 211.8, 134.6 and 107.7, respectively. The present study demonstrated an initial attempt of demonstrating a hybrid wastewater treatment system for the production of value-added products in terms of biofuel.

Protein biomethanation: insight into the microbial nexus

Biomethanation of carbohydrates (e.g., lignocellulosic biomass) and lipids (e.g., waste oils) has been well studied. However, investigations on the biomethanation of protein-rich biowastes (PRBs) and associated microbial communities have not been reported. This review summarizes the challenges in the metabolic process of anaerobic digestion of PRBs and the microbial instability associated with it. We discuss the diversity of bacterial and archaeal communities via metagenomics under PRB mono- and codigestion. A stable community structure with enhanced metabolic activity is a core factor in PRB biomethanation. The application of strategies such as codigestion of PRBs with carbon-rich biomass and microbial stimulation/augmentation would make PRB biomethanation more feasible.

Enhancing Energy Recovery in Form of Biogas, from Vegetable and Fruit Wholesale Markets By-Products and Wastes, with Pretreatments

Residues and by-products from vegetables and fruit wholesale markets are suitable for recovery in the form of energy through anaerobic digestion, allowing waste recovery and introducing them into the circular economy. This suitability is due to their composition, structural characteristics, and to the biogas generation process, which is stable and without inhibition. However, it has been observed that the proportion of methane and the level of degradation of the substrate is low. It is decided to study whether the effect of pretreatments on the substrate is beneficial. Freezing, ultrafreezing and lyophilization pretreatments are studied. A characterization of the substrates has been performed, the route of action of pretreatment determined, and the digestion process studied to calculate the generation of biogas, methane, hydrogen and the proportions among these. Also, a complete analysis of the process has been performed by processing the data with mathematical and statistical methods to obtain disintegration constants and levels of degradation. It has been observed that the three pretreatments have positive effects, when increasing the solubility of the substrate, increasing porosity, and improving the accessibility of microorganisms to the substrate. Generation of gases are greatly increased, reaching a methane enrichment of 59.751%. Freezing seems to be the best pretreatment, as it increases the biodegradation level, the speed of the process and the disintegration constant by 306%.

Increasing the Content of Olive Mill Wastewater in Biogas Reactors for a Sustainable Recovery: Methane Productivity and Life Cycle Analyses of the Process

Anaerobic codigestion of olive mill wastewater for renewable energy production constitutes a promising process to overcome management and environmental issues due to their conventional disposal. The present study aims at assessing biogas and biomethane production from olive mill wastewater by performing biochemical methane potential tests. Hence, mixtures containing 0% (blank), 20% and 30% olive mill wastewater, in volume, were experimented on under mesophilic conditions. In addition, life cycle assessment and life cycle costing were performed for sustainability analysis. Particularly, life cycle assessment allowed assessing the potential environmental impact resulting from the tested process, while life cycle costing in conjunction with specific economic indicators allowed performing the economic feasibility analysis. The research highlighted reliable outcomes: higher amounts of biogas (80.22 ± 24.49 NL.kg_SV⁻¹) and methane (47.68 ± 17.55 NL.kg_SV⁻¹) were obtained when implementing a higher amount of olive mill wastewater (30%) (v/v) in the batch reactors. According to life cycle assessment, the biogas ecoprofile was better when using 20% (v/v) olive mill wastewater. Similarly, the economic results demonstrated the profitability of the process, with better performances when using 20% (v/v) olive mill wastewater. These findings confirm the advantages from using farm and food industry by-products for the production of renewable energy as well as organic fertilizers, which could be used in situ to enhance farm sustainability.

Anaerobic co-digestion of spent coconut copra with cow urine for enhanced biogas production

Laboratory-scale bioreactors were used to co-digest spent coconut copra (SCC) and cow urine (CU) as a co-substrate (SCC + CU) in a batch mode under thermophilic condition (45 ± 2°C) in order to enhance biogas production. The effect of CU pretreatment on the performance indicators (biogas and biomethane yields, total solids (TS), and volatile solids (VS) reduction, pH and volatile fatty acids (VFAs) concentrations) were also examined. This was compared with mono-digestion of SCC. The experiment was performed with different mixing ratios in reactors labelled as follows: A = 75 g SCC + 5 ml CU; B = 70 g SCC + 10 ml CU; C = 65 g SCC + 15 ml CU; and D (control) = 80 g SCC at a hydraulic retention time of 42 days. Co-digestion (SCC + CU) significantly improved anaerobic digestion (AD) performance resulting in a threefold and fivefold increase in biogas and biomethane production, respectively, with concomitant TS (44.9-57.7%) and VS (55.4-60.3%) removal efficiencies. But for mono-digestion (control experiment), all CU treated and co-digestion assays showed pH stability ranging between 6.6 and 7.4 and VFAs' concentrations ranging from 15-330 mgL^-1. By acting as a buffer, CU effectively enhanced the AD performance of SCC as demonstrated in this study.

Siloxanes in Biogas: Approaches of Sampling Procedure and GC-MS Method Determination

A new approach of siloxane sampling based on impinger, micro-impinger, adsorption on active carbon, and direct TedlarBag methods followed by gas chromatography-mass spectrometry (GC-MS) was developed for the analysis of three linear (L2–L4) and four cyclic (D3–D5) volatile methyl siloxanes (VMSs). Three kinds of organic liquid-medium characterized by different polarities, namely acetone, methanol, and d-decane as siloxanes trap were arranged in the experiment which is widely discussed below. Thus, the GC-MS equipped with SUPELCOWAX-10 capillary column was employed to perform monitoring of VMS content in the analyzed biogas samples originating from landfill, wastewater treatment plants, and agriculture biogas plants. In all samples that have undergone the analysis, cyclic and linear VMSs were found in quantities exceeding 107.9 and 3.8 mg/m³, respectively. Significant differences between siloxanes concentrations depending on biogas origin were observed. Moreover, the high range of linearity (0.1 to 70.06 mg/m³), low LoD (0.01 mg/m³), low LoQ (0.04 mg/m³), and high recovery (244.1%) indicate that the procedure and can be applied in sensitive analyses of silica biogas contaminants. In addition to the above, the impinger method of sampling performed better than active-carbon Tube and TedlarBag, particularly for quantifying low concentrations of siloxanes. Overall, the evaluation of sampling methods for biogas collection simplified the analytical procedure by reducing the procedural steps, avoiding the use of solvents, as well as demonstrated its applicability for the testing of biogas quality.

Microbial Communities in Flexible Biomethanation of Hydrogen Are Functionally Resilient Upon Starvation

Ex situ biomethanation allows the conversion of hydrogen produced from surplus electricity to methane. The flexibility of the process was recently demonstrated, yet it is unknown how intermittent hydrogen feeding impacts the functionality of the microbial communities. We investigated the effect of starvation events on the hydrogen consumption and methane production rates (MPRs) of two different methanogenic communities that were fed with hydrogen and carbon dioxide. Both communities showed functional resilience in terms of hydrogen consumption and MPRs upon starvation periods of up to 14 days. The origin of the inoculum, community structure and dominant methanogens were decisive for high gas conversion rates. Thus, pre-screening a well performing inoculum is essential to ensure the efficiency of biomethanation systems operating under flexible gas feeding regimes. Our results suggest that the type of the predominant hydrogenotrophic methanogen (here: Methanobacterium) is important for an efficient process. We also show that flexible biomethanation of hydrogen and carbon dioxide with complex microbiota is possible while avoiding the accumulation of acetate, which is relevant for practical implementation. In our study, the inoculum from an upflow anaerobic sludge blanket reactor treating wastewater from paper industry performed better compared to the inoculum from a plug flow reactor treating cow manure and corn silage. Therefore, the implementation of the power-to-gas concept in wastewater treatment plants of the paper industry, where biocatalytic biomass is readily available, may be a viable option to reduce the carbon footprint of the paper industry.

Effect of ultrasonic post-treatment on anaerobic digestion of lignocellulosic waste

This paper evaluates the effects of ultrasonication (US) applied, individually or in combination with a mechanical treatment, to the effluent of anaerobic digestion (AD) of lignocellulosic waste, on methane (CH₄) production. US of the substrate downstream of AD is a relatively novel concept aimed at improving the degradation of recalcitrant components in order to enhance the overall energy efficiency of the process. US tests were carried out on real digestate samples at different energies (500-50,000 kJ/kg total solids (TS), corresponding to sonication densities of 0.08-0.45 W/ml). AD tests were performed on mixtures of sonicated (S_us) and untreated (S) substrate at two different S_us: S ratios (25:75 and 75:25 w/w), simulating post-sonicated material recycling to the biological process. The US effect was estimated through the solubilization degree of organic matter, as well as the CH₄ production yield and kinetics, which were all found to be enhanced by the treatment. At S_us: S = 75:25 and E_s ≥ 20,000 kJ/kg TS (0.25 W/ml), CH₄ production improved by 20% and the values of the kinetic parameters increased by 64-82%.

Effect of Antibiotics on the Microbial Efficiency of Anaerobic Digestion of Wastewater: A Review

Recycling waste into new materials and energy is becoming a major challenge in the context of the future circular economy, calling for advanced methods of waste treatment. For instance, microbially-mediated anaerobic digestion is widely used for conversion of sewage sludge into biomethane, fertilizers and other products, yet the efficiency of microbial digestion is limited by the occurrence of antibiotics in sludges, originating from drug consumption for human and animal health. Here we present antibiotic levels in Chinese wastewater, then we review the effects of antibiotics on hydrolysis, acidogenesis and methanogenesis, with focus on macrolides, tetracyclines, β-lactams and antibiotic mixtures. We detail effects of antibiotics on fermentative bacteria and methanogenic archaea. Most results display adverse effects of antibiotics on anaerobic digestion, yet some antibiotics promote hydrolysis, acidogenesis and methanogenesis.

Brewer's Spent Grains-Valuable Beer Industry By-Product

The brewing sector is a significant part of the global food industry. Breweries produce large quantities of wastes, including wastewater and brewer’s spent grains. Currently, upcycling of food industry by-products is one of the principles of the circular economy. The aim of this review is to present possible ways to utilize common solid by-product from the brewing sector. Brewer’s spent grains (BSG) is a good material for sorption and processing into activated carbon. Another way to utilize spent grains is to use them as a fuel in raw form, after hydrothermal carbonization or as a feedstock for anaerobic digestion. The mentioned by-products may also be utilized in animal and human nutrition. Moreover, BSG is a waste rich in various substances that may be extracted for further utilization. It is likely that, in upcoming years, brewer’s spent grains will not be considered as a by-product, but as a desirable raw material for various branches of industry.

Influence of Liquid-to-Gas Ratio on the Syngas Fermentation Efficiency: An Experimental Approach

Syngas fermentation by methanogens is a novel process to purify biogas. Methanogens are able to ferment non-desirable CO₂, H_2, and CO to methane. However, to use methanogens on an industrial scale, more research has to be done. There are studies that discuss the growth of methanogens on syngas in combination with acetate. In this research, growth of methanogens on syngas as sole carbon source is discussed. Effluent of an anaerobic fed-batch was selectively cultivated with syngas in 400 mL Eppendorf© bioreactors. After a period of 7 days, fifteen 120 mL flasks were filled with three different liquid-to-gas ratios (1:1, 1:3, 1:5). Results showed that different liquid-to-gas ratios change the metabolic preference of the anaerobic microbial community. Moreover, complete conversion in a four-to-eight-day period, via the carboxidotrophic pathway, was observed in all three liquid-to-gas ratios.

One- and two-stage anaerobic co-digestion of cucumber waste and sewage sludge

The demand for uniformly sized and shaped produce that are aesthetically pleasing results in significant food waste throughout the world. Cucumber waste is a major agricultural waste product in a number of countries, especially areas with high pickle production. Opportunity exists for wastewater treatment plants containing anaerobic digesters to utilize cucumber agricultural and industrial waste for biogas production. The biomethane potential of cucumber waste as a substrate for co-digestion with sewage sludge was assessed. The impact of long-term co-digestion of cucumber was then evaluated using mesophilic continuously stirred tank reactors (CSTRs), in both single- and two-stage anaerobic co-digestion with sewage sludge. Ground cucumber waste was added to sewage sludge at 8% of the volume (4.5-4.6% of the organic load) and CSTRs were maintained for five hydraulic retention times (HRTs). One-stage co-digestion of cucumber waste produced comparable gas levels as CSTRs without cucumbers (averaging 219 and 221 m³/kgVS/h, respectively) after two HRTs. The two-stage cucumber co-digestion CSTR averaged 64% higher specific gas than the control and single-stage digester, although the volumetric gas produced was lower (averaging 152 m³/kgVS/h) likely due to gas loss in the first stage resulting in a lower organic load rate. After four HRTs, relative methanogen content showed dramatic differences in levels of hydrogenotrophic methanogens for the two-stage digester, while the one-stage digester containing cucumber waste showed minor differences relative to the control. Cucumber waste co-digestion with sewage sludge is effective although numerous conditions could be utilized to optimize gas production.

Future transport policy designs for biomethane promotion: A system Dynamics model

In this paper, we present a dynamic simulation tool for biomethane policy design in Latvia. A new System Dynamics based computer simulation model covering the whole biogas-to-biomethane value chain including bioresource availability, biogas production and biogas utilization pathways is proposed. The goal of the model is to evaluate the efficiency of a set of policy instruments on increasing the transition from biogas for electricity production to biogas upgrading to be used as a transportation fuel thus contributing to national renewable energy targets in the transport sector by 2030. According to the modeling results, policy design should be focused on two directions, reducing the existing support mechanisms for electricity producers and subsidizing biomethane production, namely. Additional benefits are brought by tax policies aiming to increase the price of competing fossil fuels.

Microbial Resource Management for Ex Situ Biomethanation of Hydrogen at Alkaline pH

Biomethanation is a promising solution to convert H₂ (produced from surplus electricity) and CO₂ to CH₄ by using hydrogenotrophic methanogens. In ex situ biomethanation with mixed cultures, homoacetogens and methanogens compete for H₂/CO₂. We enriched a hydrogenotrophic microbiota on CO₂ and H₂ as sole carbon and energy sources, respectively, to investigate these competing reactions. The microbial community structure and dynamics of bacteria and methanogenic archaea were evaluated through 16S rRNA and mcrA gene amplicon sequencing, respectively. Hydrogenotrophic methanogens and homoacetogens were enriched, as acetate was concomitantly produced alongside CH₄. By controlling the media composition, especially changing the reducing agent, the formation of acetate was lowered and grid quality CH₄ (≥97%) was obtained. Formate was identified as an intermediate that was produced and consumed during the bioprocess. Stirring intensities ≥ 1000 rpm were detrimental, probably due to shear force stress. The predominating methanogens belonged to the genera Methanobacterium and Methanoculleus. The bacterial community was dominated by Lutispora. The methanogenic community was stable, whereas the bacterial community was more dynamic. Our results suggest that hydrogenotrophic communities can be steered towards the selective production of CH₄ from H₂/CO₂ by adapting the media composition, the reducing agent and the stirring intensity.

Biological hydrogen methanation systems - an overview of design and efficiency

The rise in intermittent renewable electricity production presents a global requirement for energy storage. Biological hydrogen methanation (BHM) facilitates wind and solar energy through the storage of otherwise curtailed or constrained electricity in the form of the gaseous energy vector biomethane. Biological methanation in the circular economy involves the reaction of hydrogen – produced during electrolysis – with carbon dioxide in biogas to produce methane (4H₂ + CO₂ = CH₄ + 2H₂), typically increasing the methane output of the biogas system by 70%. In this paper, several BHM systems were researched and a compilation of such systems was synthesized, facilitating comparison of key parameters such as methane evolution rate (MER) and retention time. Increased retention times were suggested to be related to less efficient systems with long travel paths for gases through reactors. A significant lack of information on gas-liquid transfer co-efficient was identified.

Technologies for Biogas Upgrading to Biomethane: A Review

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.

Steam Explosion Conditions Highly Influence the Biogas Yield of Rice Straw

Straws are agricultural residues that can be used to produce biomethane by anaerobic digestion. The methane yield of rice straw is lower than other straws. Steam explosion was investigated as a pretreatment to increase methane production. Pretreatment conditions with varying reaction times (12–30 min) and maximum temperatures (162–240 °C) were applied. The pretreated material was characterized for its composition and thermal and morphological properties. When the steam explosion was performed with a moderate severity parameter of S₀ = 4.1 min, the methane yield was increased by 32% compared to untreated rice straw. This study shows that a harsher pretreatment at S₀ > 4.3 min causes a drastic reduction of methane yield because inert condensation products are formed from hemicelluloses.

Biomethane Production From Lignocellulose: Biomass Recalcitrance and Its Impacts on Anaerobic Digestion

Anaerobic digestion using lignocellulosic material as the substrate is a cost-effective strategy for biomethane production, which provides great potential to convert biomass into renewable energy. However, the recalcitrance of native lignocellulosic biomass makes it resistant to microbial hydrolysis, which reduces the bioconversion efficiency of organic matter into biogas. Therefore, it is necessary to critically investigate the correlation between lignocellulose characteristics and bioconversion efficiency. Accordingly, this review comprehensively summarizes the anaerobic digestion process and rate-limiting step, structural and compositional properties of lignocellulosic biomass, recalcitrance and inhibitors of lignocellulose and their major effects on anaerobic digestion for biomethane production. Moreover, various type of pretreatment strategies applied to lignocellulosic biomass was discussed in detail, which would contribution to cell wall degradation and improvement of biomethane yields. In the view of current knowledge, high energy input and cost requirements are the main limitations of these pretreatment methods. In addition to optimization of fermentation process, further studies should focus much more on key structural influence factors of biomass recalcitrance and anaerobic digestion efficiency, which will contribute to improvement of biomethane production from lignocellulose.

A Citrus Peel Waste Biorefinery for Ethanol and Methane Production

This paper deals with the development of a citrus peel waste (CPW) biorefinery that employs low environmental impact technologies for production of ethanol and methane. Three major yeasts were compared for ethanol production in batch fermentations using CPW pretreated through acid hydrolysis and a combination of acid and enzyme hydrolysis. The most efficient conditions for production of CPW-based hydrolyzates included processing at 116 °C for 10 min. Pichia kudriavzevii KVMP10 achieved the highest ethanol production that reached 30.7 g L⁻¹ in fermentations conducted at elevated temperatures (42 °C). A zero-waste biorefinery was introduced by using solid biorefinery residues in repeated batch anaerobic digestion fermentations achieving methane formation of 342 mL g_VS⁻¹ (volatile solids). Methane production applying untreated and dried CPW reached a similar level (339–356 mL g_VS⁻¹) to the use of the side stream, demonstrating that the developed bioprocess constitutes an advanced alternative to energy intensive methods for biofuel production.

Two-stage thermophilic anaerobic co-digestion of corn stover and cattle manure to enhance biomethane production

Two-stage thermophilic anaerobic co-digestion of cattle manure and corn stover was conducted to increase biomethane production. The first stage pre-digestion of corn stover was studied based on the following treatment variables: corn stover to liquid fraction of digestate (CS:LFD) ratio (1:7, 1:10, 1:13, 1:14), digestion temperature (55 °C, 60 °C) and digestion time (3, 7, 14 days). The reduction in lignin, cellulose and hemicellulose (LCH) was between 3.97% and 11.98%, which increased the biodegradability of corn stover. Corn stover pre-digested with a CS:LFD ratio of 1:10 at 55 °C for a period of 3 and 7 days was subjected to anaerobic co-digestion with cattle manure. The highest biomethane yield was observed on day 21 with a value of 357.41 mL/g volatile solids (VS) for untreated corn stover, 446.84 mL/g VS for corn stover pre-digested for 3 days and 518.58 mL/g VS for corn stover pre-digested for 7 days with LFD. The VS conversion efficiency for co-digestion of cattle manure with untreated corn stover, corn stover pre-digested for 3 days and 7 days was 42.8%, 43.3% and 51.8%, respectively, on day 21, which was higher than that (34.0%) of cattle manure only.

Carbon-to-nitrogen and substrate-to-inoculum ratio adjustments can improve co-digestion performance of microalgal biomass obtained from domestic wastewater treatment

This study comparatively evaluated the effect of co-substrates on anaerobic digestion (AD) and biochemical methane potential of wastewater-derived microalgal biomass, with an emphasis on carbon-to-nitrogen (C:N) and substrate-to-inoculum (S:I) ratios. A semi-continuous photobioreactor was inoculated with Chlorella vulgaris and the nutrient recovery potential was investigated. Derived microalgal slurry was subjected to AD in the absence and presence of co-substrates; model kitchen waste (MKW) and waste activated sludge (WAS). The results revealed that up to 99.6% of nitrogen and 91.2% of phosphorus could be removed from municipal wastewater using C. vulgaris. Biomethane yields were improved by co-digestion with both MKW and WAS. The maximum biomethane yield was observed as 523 ± 25.6 ml CH₄g VS_added^-1, by microalgal biomass and MKW co-digestion in 50:50 ratio, at an initial chemical oxygen demand (COD) concentration of 14.0 ± 0.1 g l^-1, C:N ratio of 22.0, and S:I ratio of 2.2. The observed biomethane yield was 80.7% higher than that of the mono-digestion. The highest improvement achieved by 50:50 co-digestion of microalgal biomass and WAS was 15.5%, with biomethane yield of 272 ± 11.3 ml CH₄g VS_added^-1 at an initial COD concentration of 14.0 ± 0.1 g l^-1, C:N ratio of 13.0, and S:I of 2.3.

Inclusion of Saccharina latissima in conventional anaerobic digestion systems

Loading macroalgae into existing anaerobic digestion (AD) plants allows us to overcome challenges such as low digestion efficiencies, trace elements limitation, excessive salinity levels and accumulation of volatile fatty acids (VFAs), observed while digesting algae as a single substrate. In this work, the co-digestion of the brown macroalgae Saccharina latissima with mixed municipal wastewater sludge (WWS) was investigated in mesophilic and thermophilic conditions. The hydraulic retention time (HRT) and the organic loading rate (OLR) were fixed at 19 days and 2.1 g l^-1 d^-1 of volatile solids (VS), respectively. Initially, WWS was digested alone. Subsequently, a percentage of the total OLR (20%, 50% and finally 80%) was replaced by S. latissima biomass. Optimal digestion conditions were observed at medium-low algae loading (≤50% of total OLR) with an average methane yield close to [Formula: see text] and [Formula: see text] in mesophilic and thermophilic conditions, respectively. The conductivity values increased with the algae loading without inhibiting the digestion process. The viscosities of the reactor sludges revealed decreasing values with reduced WWS loading at both temperatures, enhancing mixing properties.

Assessment of Novel Routes of Biomethane Utilization in a Life Cycle Perspective

Biomethane, as a replacement for natural gas, reduces the use of fossil-based sources and supports the intended change from fossil to bio-based industry. The study assessed different biomethane utilization routes for production of methanol, dimethyl ether (DME), and ammonia, as fuel or platform chemicals and combined heat and power (CHP). Energy efficiency and environmental impacts of the different pathways was studied in a life cycle perspective covering the technical system from biomass production to the end product. Among the routes studied, CHP had the highest energy balance and least environmental impact. DME and methanol performed competently in energy balance and environmental impacts in comparison with the ammonia route. DME had the highest total energy output, as fuel, heat, and steam, among the different routes studied. Substituting the bio-based routes for fossil-based alternatives would give a considerable reduction in environmental impacts such as global warming potential and acidification potential for all routes studied, especially CHP, DME, and methanol. Eutrophication potential was mainly a result of biomass and biomethane production, with marginal differences between the different routes.

Screening of biomethane production potential from dominant microalgae

The use of microalgae for biomethane production has been considerably increasing during the recent years. In this study, four dominant species belonging to the genera Scenedesmus, Chlorella, Dunaliella and Nostoc were selected. The influence of different genera with several morphological, structural and physicochemical characteristics on methane production was assessed in biochemical methane potential (BMP) tests. The ultimate methane yield values were 332 ± 24, 211 ± 2, 63 ± 17 and 28 ± 10 mL CH4/g VSadded for Scenedesmus obliquus, Chlorella sorokiniana, Dunaliella salina and Nostoc sp., respectively. The highest methane production was achieved by microalga species that had no complex cell wall or wall basically composed by proteins and simple sugars such as in S. obliquus, whereas lower methane yields were found for D. salina and Nostoc sp., due to the salinity effects and cell wall composition in terms of complex polysaccharide and glycolipid layers, respectively. Kinetic constant values obtained in the BMP tests ranged between 1.00 ± 0.08 and 0.097 ± 0.005 days(-1) for D. salina and S. obliquus, respectively.

A Review on the Valorization of Macroalgal Wastes for Biomethane Production

The increased use of terrestrial crops for biofuel production and the associated environmental, social and ethical issues have led to a search for alternative biomass materials. Terrestrial crops offer excellent biogas recovery, but compete directly with food production, requiring farmland, fresh water and fertilizers. Using marine macroalgae for the production of biogas circumvents these problems. Their potential lies in their chemical composition, their global abundance and knowledge of their growth requirements and occurrence patterns. Such a biomass industry should focus on the use of residual and waste biomass to avoid competition with the biomass requirements of the seaweed food industry, which has occurred in the case of terrestrial biomass. Overabundant seaweeds represent unutilized biomass in shallow water, beach and coastal areas. These eutrophication processes damage marine ecosystems and impair local tourism; this biomass could serve as biogas feedstock material. Residues from biomass processing in the seaweed industry are also of interest. This is a rapidly growing industry with algae now used in the comestible, pharmaceutical and cosmetic sectors. The simultaneous production of combustible biomethane and disposal of undesirable biomass in a synergistic waste management system is a concept with environmental and resource-conserving advantages.

Biogas cleaning and upgrading with natural zeolites from tuffs

CO2 adsorption on synthetic zeolites has become a consolidated approach for biogas upgrading to biomethane. As an alternative to synthetic zeolites, tuff waste from building industry was investigated in this study: indeed, this material is available at a low price and contains a high fraction of natural zeolites. A selective adsorption of CO2 and H2S towards CH4 was confirmed, allowing to obtain a high-purity biomethane (CO2 <2 g m(-3), i.e. 0.1%; H2S <1.5 mg m(-3)), suitable for injection in national grids or as vehicle fuel. The loading capacity was found to be 45 g kg(-1) and 40 mg kg(-1), for CO2 and H2S, respectively. Synthetic gas mixtures and real biogas samples were used, and no significant effects due to biogas impurities (e.g. humidity, dust, moisture, etc.) were observed. Thermal and vacuum regenerations were also optimized and confirmed to be possible, without significant variations in efficiency. Hence, natural zeolites from tuffs may successfully be used in a pressure/vacuum swing adsorption process.

Anaerobic Digestion of Laminaria japonica Waste from Industrial Production Residues in Laboratory- and Pilot-Scale

The cultivation of macroalgae to supply the biofuel, pharmaceutical or food industries generates a considerable amount of organic residue, which represents a potential substrate for biomethanation. Its use optimizes the total resource exploitation by the simultaneous disposal of waste biomaterials. In this study, we explored the biochemical methane potential (BMP) and biomethane recovery of industrial Laminaria japonica waste (LJW) in batch, continuous laboratory and pilot-scale trials. Thermo-acidic pretreatment with industry-grade HCl or industrial flue gas condensate (FGC), as well as a co-digestion approach with maize silage (MS) did not improve the biomethane recovery. BMPs between 172 mL and 214 mL g(-1) volatile solids (VS) were recorded. We proved the feasibility of long-term continuous anaerobic digestion with LJW as sole feedstock showing a steady biomethane production rate of 173 mL g(-1) VS. The quality of fermentation residue was sufficient to serve as biofertilizer, with enriched amounts of potassium, sulfur and iron. We further demonstrated the upscaling feasibility of the process in a pilot-scale system where a CH₄ recovery of 189 L kg(-1) VS was achieved and a biogas composition of 55% CH₄ and 38% CO₂ was recorded.

Use of a marine microbial community as inoculum for biomethane production

Marine substrates are prominent candidates for the production of biofuels, especially for biogas, which is a well-established technology that accepts different types of substrates for its production. However, the use of marine substrates in bioreactors may cause inhibition of methanogenic bacteria due to the addition of seasalts. Here, we explore a simple and economically viable way to circumvent the problem of inoculum inhibition. Based on the current knowledge of the diversity of microorganisms in marine sediments, we tested the direct use of methanogenic bacteria from an anoxic marine environment as inoculum for biomethane production. Both marine and freshwater substrates were added to this inoculum. No pretreatment (that may have enhanced methane production, but would have made the process more expensive) was applied either to the inoculum or to the substrates. For comparison, the same substrates were added to a standard inoculum (cow manure). Both the marine inoculum and cow manure produced methane by anaerobic digestion of the substrates added. The highest methane production (0.299 m(3) kg VS(-1)) was obtained by adding marine microalgae biomass (Chlorella sp. and Synechococcus sp.) to the marine inoculum. No inhibitory effects were observed due to differences in salinity between the inocula and substrates. Our results indicate the potential of using both marine inoculum and substrates for methane production.

Anaerobic digestion of bio-waste: A mini-review focusing on territorial and environmental aspects

Scientific and industrial experiences, together with economical and policies changes of last 30 years, bring anaerobic digestion among the most environmental friendly and economically advantageous technologies for organic waste treatment and management in Europe. In this short review, the role of anaerobic digestion of organic wastes is discussed, considering the opportunity of a territorial friendly approach, without barriers, where different organic wastes are co-treated. This objective can be achieved through two proposed strategies: one is the anaerobic digestion applied as a service for the agricultural and farming sector; the other as a service for citizen (biowaste, diapers and wastewater treatment integration). The union of these two strategies is an environmental- and territorial-friendly process that aims to produce renewable energy and fertiliser material, with a low greenhouse gas emission and nutrients recovery. The advantage of forthcoming application of anaerobic digestion of organic wastes, even for added value bioproducts production and new energy carriers, are finally discussed. Among several advantages of anaerobic digestion, the role of the environmental controller was evaluated, considering the ability of minimising the impacts exploiting the biochemical equilibrium and sensitivity as a quality assurance for digestate.

Evaluation of integrated anaerobic digestion and hydrothermal carbonization for bioenergy production

Lignocellulosic biomass is one of the most abundant yet underutilized renewable energy resources. Both anaerobic digestion (AD) and hydrothermal carbonization (HTC) are promising technologies for bioenergy production from biomass in terms of biogas and HTC biochar, respectively. In this study, the combination of AD and HTC is proposed to increase overall bioenergy production. Wheat straw was anaerobically digested in a novel upflow anaerobic solid state reactor (UASS) in both mesophilic (37 °C) and thermophilic (55 °C) conditions. Wet digested from thermophilic AD was hydrothermally carbonized at 230 °C for 6 hr for HTC biochar production. At thermophilic temperature, the UASS system yields an average of 165 LCH4/kgVS (VS: volatile solids) and 121 L CH4/kgVS at mesophilic AD over the continuous operation of 200 days. Meanwhile, 43.4 g of HTC biochar with 29.6 MJ/kgdry_biochar was obtained from HTC of 1 kg digestate (dry basis) from mesophilic AD. The combination of AD and HTC, in this particular set of experiment yield 13.2 MJ of energy per 1 kg of dry wheat straw, which is at least 20% higher than HTC alone and 60.2% higher than AD only.

Biohydrogen, biomethane and bioelectricity as crucial components of biorefinery of organic wastes: a review

Biohydrogen is a sustainable form of energy as it can be produced from organic waste through fermentation processes involving dark fermentation and photofermentation. Very often biohydrogen is included as a part of biorefinery approaches, which reclaim organic wastes that are abundant sources of renewable and low cost substrate that can be efficiently fermented by microorganisms. The aim of this work was to critically assess selected bioenergy alternatives from organic solid waste, such as biohydrogen and bioelectricity, to evaluate their relative advantages and disadvantages in the context of biorefineries, and finally to indicate the trends for future research and development. Biorefining is the sustainable processing of biomass into a spectrum of marketable products, which means: energy, materials, chemicals, food and feed. Dark fermentation of organic wastes could be the beach-head of complete biorefineries that generate biohydrogen as a first step and could significantly influence the future of solid waste management. Series systems show a better efficiency than one-stage process regarding substrate conversion to hydrogen and bioenergy. The dark fermentation also produces fermented by-products (fatty acids and solvents), so there is an opportunity for further combining with other processes that yield more bioenergy. Photoheterotrophic fermentation is one of them: photosynthetic heterotrophs, such as non-sulfur purple bacteria, can thrive on the simple organic substances produced in dark fermentation and light, to give more H2. Effluents from photoheterotrophic fermentation and digestates can be processed in microbial fuel cells for bioelectricity production and methanogenic digestion for methane generation, thus integrating a diverse block of bioenergies. Several digestates from bioenergies could be used for bioproducts generation, such as cellulolytic enzymes and saccharification processes, leading to ethanol fermentation (another bioenergy), thus completing the inverse cascade. Finally, biohydrogen, biomethane and bioelectricity could contribute to significant improvements for solid organic waste management in agricultural regions, as well as in urban areas.

Towards a metagenomic understanding on enhanced biomethane production from waste activated sludge after pH 10 pretreatment

BACKGROUND

Understanding the effects of pretreatment on anaerobic digestion of sludge waste from wastewater treatment plants is becoming increasingly important, as impetus moves towards the utilization of sludge for renewable energy production. Although the field of sludge pretreatment has progressed significantly over the past decade, critical questions concerning the underlying microbial interactions remain unanswered. In this study, a metagenomic approach was adopted to investigate the microbial composition and gene content contributing to enhanced biogas production from sludge subjected to a novel pretreatment method (maintaining pH at 10 for 8 days) compared to other documented methods (ultrasonic, thermal and thermal-alkaline).

RESULTS

Our results showed that pretreated sludge attained a maximum methane yield approximately 4-fold higher than that of the blank un-pretreated sludge set-up at day 17. Both the microbial and metabolic consortium shifted extensively towards enhanced biodegradation subsequent to pretreatment, providing insight for the enhanced methane yield. The prevalence of Methanosaeta thermophila and Methanothermobacter thermautotrophicus, together with the functional affiliation of enzymes-encoding genes suggested an acetoclastic and hydrogenotrophic methanogenesis pathway. Additionally, an alternative enzymology in Methanosaeta was observed.

CONCLUSIONS

This study is the first to provide a microbiological understanding of improved biogas production subsequent to a novel waste sludge pretreatment method. The knowledge garnered will assist the design of more efficient pretreatment methods for biogas production in the future.