Critical insights into psychrophilic anaerobic digestion: Novel strategies for improving biogas production

An integrated review on the role of different biocatalysts, process parameters, bioreactor technologies and data-driven predictive models for upgrading biogas

As energy consumption and waste generation from human activities continue to rise, the technology of anaerobic digestion (AD), which converts waste into bioenergy, has gained popularity. Biogas produced from AD commonly contains 60 % CH4, 40 % CO2 and a minor fraction of impurities. Currently, several anaerobic reactors have been designed to upgrade the biogas with biomethane content above 90 %. This review summarizes the current trends in the biological upgradation of biogas from a bio-circular economy perspective to achieve sustainable energy goals. Examples of applications reporting the latest advancements in treating industrial effluents using high-rate anaerobic reactors have been mentioned. The integrated anaerobic-aerobic hybrid reactor offers a solution to the limitations of traditional methods in treating diverse effluents. A special focus on biological upgradation techniques such as in-situ, ex-situ, and hybrid mechanisms have been briefed. The key advantage of hybrid upgradation is its ability to address the pH rise during in-situ process. Additionally, the applications of artificial neural networks and optimization to upgrade biogas production have been discussed. The review concludes with future research directives with emphasis on the economic viability of the approaches.

Advanced performance and operational strategies of partitioned anaerobic membrane bioreactor combined with ferromagnetic biochar for pesticide wastewater treatment at low-temperature and insufficient hydraulic load

Accompanied by high energy consumption and the difficulty in maintaining process stability, the treatment of pesticide wastewater in cold climates remain a significant challenge due to its low biodegradability and complex composition. Therefore, this study developed a hybrid anaerobic membrane bioreactor (HAnMBR) that incorporates ferromagnetic biochar and a partitioned upflow anaerobic sludge blanket structure. Its performance was evaluated and compared to that of a conventional anaerobic membrane bioreactor (CAnMBR) under varying organic loading rate (OLR) and hydraulic loading rate (HLR) at 10 °C. Results indicated that the maximum average removal rates of COD, pyridine, and trichlorfon in HAnMBR were 17.3 %, 11.2 %, and 6.9 % higher than those in CAnMBR, respectively. The volatile fatty acid (VFA) concentration decreased by 14.8-55.4 %, and the electron transport system activity (ETSA) increased by 1.3-2.6 times. HAnMBR significantly reduced the sludge yield by 21.5-51.0 % and decreased the SVI by 13.3-39.4 %. The operational strategies were subsequently determined, including: an influent COD concentration of 8.0 g/L and a HLR of 0.9 m3/(m2·d); an OLR of 5 kg COD/(m3·d) and a HLR of 0.7 m3/(m2·d); and a HLR of 1.0 m3/(m2·d) and an OLR of 7 kg COD/(m3·d). This study offered a new process for effectively mitigating the impact of load shocks from high-strength wastewater at low temperatures, potentially expanding the application of AnMBRs.

Bioprospecting Bacteria From Psychrophilic Anaerobic Digestate for Potential Plant Growth-Promoting Attributes

The psychrophilic anaerobic digestion (PAD) system is a diverse and underexplored microbial ecosystem that typically harbors cold-adapted microorganisms with possible agronomic potential. The plant growth-promoting bacteria in the residue of PAD have the potential to enhance crop production, particularly during cold winter months. In this context, the characteristics of cultivable, cold-tolerant bacteria isolated from digestate obtained during PAD were investigated. Of the 20 isolates, 12 (60%) were able to solubilize phosphate from insoluble compounds at 15°C. Furthermore, nine (45%) and six (30%) isolates exhibited nitrogen fixation activity and produced indole acetic acid (IAA), respectively, while only two (10%) isolates were capable of producing siderophores. Hydrolytic enzyme production varied with cellulase production observed as a common trait since all isolates produced varying levels of cellulase ranging from 3.3 ± 0.5 to 15.3 ± 4 mm activity diameter. Isolates Comamonas sp._A3-1, Acinetobacter iwoffi_B5-1, and Pseudomonas sp._B5-5 displayed maximum cellulolytic activity with activity diameters of 13 ± 2, 13 ± 1.2, and 15.3 ± 4 mm, respectively. However, only two (10%) of the bacterial isolates produced protease with Pseudomonas sp._B5-5 demonstrating maximum proteolytic activity as depicted by an activity diameter of 11.3 ± 2.5 mm. Nucleotide sequence analysis of seven isolates, possessing multiple plant-beneficial traits, revealed their affiliation to three genera: Acinetobacter (57%), Comamonas (28.7%), and Pseudomonas (14%). Biolog Phenotype MicroArray plates revealed varied catabolic capability among bacterial strains, with isolate B5-5 demonstrating the highest metabolic diversity. The findings of this study revealed that cold-tolerant isolates from low-temperature AD possess promising plant growth-promoting characteristics, which indicates the potential of psychrophilic digestate for application in agriculture.

An in-depth analysis of microbial response to exposure to high concentrations of microplastics in anaerobic wastewater fermentation

The impact of microplastics (MPs) in anaerobic wastewater treatment on microbial metabolism is significant. Anaerobic granular sludge (AS) and biofilm (BF) are two common ways, and their responses to microplastics will have a direct impact on their application potential. This study investigated the microbial reactions of AS and BF to three types of MPs: polyethylene (PE), polyvinyl chloride (PVC), and a mixture of both (MIX). Results exhibited that MPs reduced methane output by 44.65 %, 55.89 %, and 53.18 %, elevated short-chain fatty acid (SCFA) levels by 95.93 %, 124.49 %, and 110.78 %, and lowered chemical oxygen demand (COD) removal by 28.77 %, 36.78 %, and 33.99 % for PE-MP, PVC-MP, and MIX-MP, respectively, with PVC-MP showing the greatest inhibition. Meanwhile, microplastics also facilitated the relative production of reactive oxygen species (ROS, 40.29 %-96.99 %), lactate dehydrogenase (LDH, 20.01 %-75.02 %), and adenosine triphosphate (ATP, 26.64 %-43.80 %), while reducing cytochrome c (cyt c, 23.60 %-49.02 %) and extracellular polymeric substances (EPS, 17.44 %-26.58 %). AS and BF displayed distinct enzymatic activities under MPs exposure. Correspondingly, 16S-rRNA sequencing indicated that AS was mainly involved in acetate generation by Firmicutes, while BF performed polysaccharide degradation by Bacteroidota. Metatranscriptomic analysis showed AS to be rich in acetogens (Bacillus, Syntrophobacter) and methanogens (Methanothrix, Methanobacterium), while BF contained more fermentation bacteria (Mesotoga, Lentimicrobium) and electroactive microorganisms (Clostridium, Desulfuromonas) under MIX-MP. Moreover, BF exhibited higher glycolysis gene expression, whereas AS was more active in methane metabolism, primarily through the acetoclastic methanogenic pathway's direct acetate conversion. This study provides new insights into understanding the microbial response produced by microplastics during anaerobic wastewater digestion.

Second-generation biorefineries: single platform for the conversion of lignocellulosic wastes to environmentally important biofuels

The continuously increasing demands for various fossil fuels to achieve the day-to-day needs of the human population are growing and causing adverse effects on the environment and leading to the depletion of their natural resources. To overcome such drastic problems and minimize the production of greenhouse gases, lignocellulose biomass, which is an abundant and bio-renewable source present on earth with excellent properties and composition, has been used for decades to develop biofuels that can easily take over the place of conventional fuels. Lignocellulose biomass comprises polymeric sugars, i.e., cellulose and hemicellulose, and aromatic polymer, lignin, which are responsible for producing various bio-based products. However, utilizing lignocellulosic wastes for such purposes is needed but their recalcitrant structure makes it difficult to achieve their full usage. For this, several pretreatment approaches are developed to loosen the complexity between sugars and lignin. In some way, few of the conventional pretreatment methods are expensive, non-eco-friendly, and produce undesired by-products, causing a lower yield and reusability of enzymes used in the reaction. Utilizing novel pretreatment strategies that are cost-effective, help in increasing the yield of products, and are environment-friendly is required. Thus, incorporating nanoparticles and nanomaterials in the development of pretreatment and other strategies for the production of bio-based products is currently thriving. This review is designed in such a way that the readers can easily get brief knowledge about the production of important biofuels developed within second-generation biorefineries using lignocellulosic biomass. It also summarizes the importance of nanotechnology in different steps of biofuel development.

Enhanced anaerobic digestion of freezing and thawing pretreated cow manure with increasing solid content: kinetics and microbial community dynamics

High solid anaerobic digestion has proved the mainstream technology for the treatment of organic wastes. However, the high molecular weight and complex lignocellulosic structure of cow manure (CM) make it indigestible and inefficient, leading to limit the hydrolysis step of anaerobic digestion at high solid content. To mitigate this bottleneck, an improved cost-effective freezing and thawing pretreatment technique was proposed in this study. The freezing and thawing pretreatment of raw CM without any dilution was done for 20 days. The maximum cumulative methane yield (487 mL CH4 g- 1VS) was achieved at a total solid (TS) of 5% followed by TS of 10% and 15%, which was 13%, 20% and 21% higher than obtained from untreated CM, respectively. The kinetic results revealed that the biodegradable materials could be utilized at increasing TS with decreasing hydrolysis rate. The pretreatment significantly enhanced the methylotrophic methanogenic pathway during high solid anaerobic digestion, which was contrary to the general concept that the process is usually dominated by acetoclastic and hydrogenotrophic methanogens. This study is very important to understand the effect of solid content but also important to understand the effect of freezing and thawing pretreatment on process parameters and microbial community dynamics in high solid anaerobic digestion.

Enhancing biohydrogen production from xylose at low temperature (20 °C) using natural FeS2 Ore: Thermodynamic analysis and mechanistic insights

This study investigates the efficacy of pyrite in enhancing biohydrogen production from xylose at low temperature (20 °C). Higher hydrogen yield rates (Rm) and reduced lag time (λ) were achieved across initial xylose concentrations ranging from 2-10 g/L. At an optimal xylose concentration of 5 g/L, pyrite reduced λ by 2.5 h and increased Rm from 1.3 to 2.7 mL h-1. These improvements are attributed to pyrite's ability to enhance the secretion of extracellular polymeric substance and flavins, facilitate NADH and NAD+ generation and transition, and favor biohydrogen production. Thermodynamic analyses and Gibbs free energy calculations further elucidated pyrite's role in the full reaction process and rate-limiting steps at low temperature. This study offers valuable insights into improving the efficiency of biohydrogen production at low temperature, with significant implications for energy conservation.

Potential application of bioelectrochemical systems in cold environments

Globally, more than half of the world's regions and populations inhabit psychrophilic and seasonally cold environments. Lower temperatures can inhibit the metabolic activity of microorganisms, thereby restricting the application of traditional biological treatment technologies. Bioelectrochemical systems (BES), which combine electrochemistry and biocatalysis, can enhance the resistance of microorganisms to unfavorable environments through electrical stimulation, thus showing promising applications in low-temperature environments. In this review, we focus on the potential application of BES in such environments, given the relatively limited research in this area due to temperature limitations. We select microbial fuel cells (MFC), microbial electrolytic cells (MEC), and microbial electrosynthesis cells (MES) as the objects of analysis and compare their operational mechanisms and application fields. MFC mainly utilizes the redox potential of microorganisms during substance metabolism to generate electricity, while MEC and MES promote the degradation of refractory substances by augmenting the electrode potential with an applied voltage. Subsequently, we summarize and discuss the application of these three types of BES in low-temperature environments. MFC can be employed for environmental remediation as well as for biosensors to monitor environmental quality, while MEC and MES are primarily intended for hydrogen and methane production. Additionally, we explore the influencing factors for the application of BES in low-temperature environments, including operational parameters, electrodes and membranes, external voltage, oxygen intervention, and reaction devices. Finally, the technical, economic, and environmental feasibility analyses reveal that the application of BES in low-temperature environments has great potential for development.

Dominant factors analyses and challenges of anaerobic digestion under cold environments

With the development of fermentation technology and the improvement of efficiency, anaerobic digestion (AD) has been playing an increasingly primary role in waste treatment and resource recovery. Temperature is undoubtedly the most important factor because it shapes microbial habitats, changes the composition of the microbial community structure, and even affects the expression of related functional genes. More than half of the biosphere is in a long-term or seasonal low-temperature environment (<20 °C), which makes psychrophilic AD have broad application prospects. Therefore, this review discusses the influencing factors and enhancement strategies of psychrophilic AD, which may provide a corresponding reference for future research on low-temperature fermentation. First, the occurrence of AD has been discussed. Then, the adaptation of microorganisms to the low-temperature environment was analyzed. Moreover, the challenges of psychrophilic AD have been reviewed. Meanwhile, the strategies for improving psychrophilic AD are presented. Further, from technology to application, the current situation of psychrophilic AD in pilot-scale tests is described. Finally, the economic and environmental feasibility of psychrophilic AD has been highlighted. In summary, psychrophilic AD is technically feasible, while economic analysis shows that the output benefits cannot fully cover the input costs, and the large-scale practical application of psychrophilic AD is still in its infancy. More research should focus on how to improve fermentation efficiency and reduce the investment cost of psychrophilic AD.

Metatranscriptome revealed how carbon brush addition affected the fermentation of food wastewater in the low-temperature environment

Improving the anaerobic digestion (AD) performance in low-temperature environments has become a key factor in the development of waste treatment and resource recovery in cold regions. The utilization of external carriers to form a biofilm is the simplest and most practical way to enhance the psychrophilic AD performance in cold regions. In this study, the effect of carrier addition on the fermentation performance of low-temperature (15 ± 2 °C) food wastewater was investigated by forming biofilms with carbon brushes. The results showed that although the biofilm formation enhanced methane yields (15.24%), it also caused more accumulation of propionic acid (306.99-626.89 mg/L), and the concentration of acetic acid (86.78-254.71 mg/L) was relatively low. The microbial community revealed the highest abundance of the fermentative bacterium Firmicutes and the carbon brush carrier significantly increased its relative abundance (23.74%). Metatranscriptomic sequencing revealed that the abundance level of Clostridium, Bacteroides, Sedimentibacter and Pelotomaculum was the highest, reaching 80% in all groups. In addition, the abundance level of electroactive microorganisms in biofilms was higher, while the fermentation bacteria and methanogens were lower. This showed that biofilm can enrich more electroactive microorganisms, and granular sludge needs to enrich more fermentation bacteria and methanogens to ensure metabolic activity. Further studies have found that carbon metabolism had the highest activity (27.86%-30.39%) and H+-transporting ATPase (atp) was the most dominant functional enzyme (85.50%-86.65%) involved in electron transport in low-temperature fermentation of food wastewater. Interestingly, these expression levels of active granular sludge were higher than the biofilm formed by carbon brushes. Meanwhile, analysis of the methanogenic pathway found that active granular sludge tends to be directly metabolized to realize acetate to acetyl-CoA by acetyl-CoA synthetase (ACSS), while biofilms were not significantly different in the two metabolic pathways of acetate. These results deepen the understanding of treating low-temperature food wastewater.

Atrazine decontamination by a newly screened psychrotroph Paenarthrobacter sp. KN0901 in an aquatic system: Metabolic pathway, kinetics, and hydroponics experiment

Atrazine residues running off the fields and entering water resources are a major threat to food security and the ecosystem. In this study, a psychrotrophic functional strain named KN0901 to remove atrazine residues was screened. KN0901 could degrade 30 mg·L^-1 atrazine in 4 days at 15ºC with 10⁵ CFU·mL^-1 incubation. The phylogenetic results showed KN0901 belonged to Paenarthrobacter sp. PCR results showed that the functional genes consist of trzN, atzB, and atzC, suggesting atrazine was transformed to cyanuric acid by KN0901. KN0901 could degrade atrazine without adding exogenous carbon and nitrogen sources. What's more, KN0901 could tolerate extreme low temperature (5ºC) and high atrazine concentration (100 mg·L^-1). When growth and degradation curves were compared, the results indicated the length of lag time showed significant correlation to atrazine degradation rate. The hydroponic experiments showed that the toxicity of atrazine was significantly reduced with KN0901 treatment. The study provided an effective, economic, and eco-friendly bioremediation measure to address atrazine contamination.

Molecular insights informing factors affecting low temperature anaerobic applications: Diversity, collated core microbiomes and complexity stability relationships in LCFA-fed systems

Fats, oil and grease, and their hydrolyzed counterparts-long chain fatty acids (LCFA) make up a large fraction of numerous wastewaters and are challenging to degrade anaerobically, more so, in low temperature anaerobic digestion (LtAD) systems. Herein, we perform a comparative analysis of publicly available Illumina 16S rRNA datasets generated from LCFA-degrading anaerobic microbiomes at low temperatures (10 and 20 °C) to comprehend the factors affecting microbial community dynamics. The various factors considered were the inoculum, substrate and operational characteristics, the reactor operation mode and reactor configuration, and the type of nucleic acid sequenced. We found that LCFA-degrading anaerobic microbiomes were differentiated primarily by inoculum characteristics (inoculum source and morphology) in comparison to the other factors tested. Inoculum characteristics prominently shaped the species richness, species evenness and beta-diversity patterns in the microbiomes even after long term operation of continuous reactors up to 150 days, implying the choice of inoculum needs careful consideration. The generalised additive models represented through beta diversity contour plots revealed that psychrophilic bacteria RBG-13-54-9 from family Anaerolineae, and taxa WCHB1-41 and Williamwhitmania were highly abundant in LCFA-fed microbial niches, suggesting their role in anaerobic treatment of LCFAs at low temperatures of 10-20 °C. Overall, we showed that the following bacterial genera: uncultured Propionibacteriaceae, Longilinea, Christensenellaceae R7 group, Lactivibrio, candidatus Caldatribacterium, Aminicenantales, Syntrophus, Syntrophomonas, Smithella, RBG-13-54-9, WCHB1-41, Trichococcus, Proteiniclasticum, SBR1031, Lutibacter and Lentimicrobium have prominent roles in LtAD of LCFA-rich wastewaters at 10-20 °C. This study provides molecular insights of anaerobic LCFA degradation under low temperatures from collated datasets and will aid in improving LtAD systems for treating LCFA-rich wastewaters.

Insights into methanogenesis of mesophilic-psychrophilic varied anaerobic digestion of municipal sludge with antibiotic stress

Methane production through anaerobic digestion (AD) of municipal sludge is economic and eco-friendly, which is commonly affected by temperature and pollutants residues. However, little is known about methanogenesis in psychrophilic AD (PAD) with temperature variations that simulating seasonal variations and with antibiotic stress. Here, two groups of AD systems with oxytetracycline (OTC) were operated with temperature maintained at 35 °C and 15 °C or variation to explore the influence to methanogenesis. The acetic acid was noticeably accumulated in OTC groups initially (P < 0.001). Methane production was noticeably inhibited initially in PAD with OTC, but had been stimulated later at 35 °C. The dominant acetoclastic methanogens Methanosaeta gradually decreased to 15.48% and was replaced by methylotrophic Methanomethylovorans (73.43%) in PAD with OTC. Correspondingly, the abundances of functional genes related to methylotrophic methanogenesis were also higher in these groups. Besides, genes involving in methane oxidation had over 50 times higher abundances in PAD with OTC groups in the second phase. Further investigation is essential to understand the main dynamics of methanogenesis in PAD and to clear the related molecular mechanism for future methane production regulation in sludge systems.

Internal driving mechanism of microbial community and metabolic pathway for psychrophilic anaerobic digestion by microbial electrolysis cell

The system that microbial electrolysis cell coupled anaerobic digestion (termed MEC-AD) with metal organic framework-modified cathode was operated under different voltage levels (0-1.2 V) at 20 °C. The maximum methane yield increased to 0.23 ± 0.01 LCH₄ g^-1COD at 0.9 V, with 28% improvement compared to 0 V (0.18 ± 0.01 LCH₄ g^-1COD). Moreover, total volatile fatty acid and propionate accumulation decreased by 32% and 15% at 0.9 V, indicating the system has potential to alleviate acidity suppression. Acidogens and electroactive microorganisms was clearly enriched with increasing applied voltage. Specifically, the abundance of Smithella increased, which could degrade propionate to acetate. Methanosaeta was dominant, accounting for ca. 40.1%∼55.1% of the archaea community at 0.3-1.2 V. Furthermore, the system reinforced psychrophilic methanogenesis by activating important enzymes involved in related metabolism pathways. Overall, this study provides perspective on the future practical application for the regulation of psychrophilic AD in electrochemically integrated bioreactors.

Integrated biorefineries for repurposing of food wastes into value-added products

Food waste (FW) generated through various scenarios from farm to fork causes serious environmental problems when either incinerated or disposed inappropriately. The presence of significant amounts of carbohydrates, proteins, and lipids enable FW to serve as sustainable and renewable feedstock for the biorefineries. Implementation of multiple substrates and product biorefinery as a platform could pursue an immense potential of reducing costs for bio-based process and improving its commercial viability. The review focuses on conversion of surplus FW into range of value-added products including biosurfactants, biopolymers, diols, and bioenergy. The review includes in-depth description of various types of FW, their chemical and nutrient compositions, current valorization techniques and regulations. Further, it describes limitations of FW as feedstock for biorefineries. In the end, review discuss future scope to provide a clear path for sustainable and net-zero carbon biorefineries.

Recent advances in biogas production using Agro-Industrial Waste: A comprehensive review outlook of Techno-Economic analysis

Agrowaste sources can be utilized to produce biogas by anaerobic digestion reaction. Fossil fuels have damaged the environment, while the biogas rectifies the issues related to the environment and climate change problems. Techno-economic analysis of biogas production is followed by nutrient recycling, reducing the greenhouse gas level, biorefinery purpose, and global warming effect. In addition, biogas production is mediated by different metabolic reactions, the usage of different microorganisms, purification process, upgrading process and removal of CO₂ from the gas mixture techniques. This review focuses on pre-treatment, usage of waste, production methods and application besides summarizing recent advancements in biogas production. Economical, technical, environmental properties and factors affecting biogas production as well as the future perspective of bioenergy are highlighted in the review. Among all agro-industrial wastes, sugarcane straw produced 94% of the biogas. In the future, to overcome all the problems related to biogas production and modify the production process.

Research trends and strategies for the improvement of anaerobic digestion of food waste in psychrophilic temperatures conditions

The organic fraction of municipal solid waste is mainly composed of food waste (FW), and traditional disposal practices for this fraction are generally considered to have negative environmental and economic impacts. However, the organic characteristics of this fraction could also be exploited through the anaerobic digestion of FW (FW-AD), which represents unique advantages, including the reduction of the area required for final disposal and environmental pollution and the same time the generation of renewable energy (mainly methane gas), and a by-product for agricultural use (digestate) due to its high nutrient content. Although approximately 88% of the world's population resides in areas with temperatures below 8 °C, psychrophilic conditions (temperatures below 20 °C) have hardly been studied, while mesophilic (66%) and thermophilic (27%) ranges were found to be more common than psychrophilic FW-AD (7%). The latter condition could decrease microbial activity and organic matter removal, which could affect biogas production and even make AD unfeasible. To improve the efficiency of the psychrophilic FW-AD process, there are strategies such as: measurement of physical properties as particle size, rheological characteristics (viscosity, consistency index and substrate behavior index), density and humidity, bioaugmentation and co-digestion with other substrates, use of inocula with psychrophilic methanogenic communities, reactor heating and modification of reactor configurations. However, these variables have hardly been studied in the context of psychrophilic conditions and future research should focus on evaluating the influence of these variables on FW-AD under psychrophilic conditions. Through a bibliometric analysis, this paper has described and analyzed the FW-AD process, with a focus on the psychrophilic conditions (<20 °C) so as to identify advances and future research trends, as well as determine strategies toward improving the anaerobic process under low temperature conditions.

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Temperature has a great influence on anaerobic digestion of food waste (FW-AD).

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Studies on the psychrophilic condition are limited, warranting further research.

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Physical properties of the substrate and inoculum influence psychrophilic FW-AD.

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The use of inocula adapted to low temperatures could increase biogas production.

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Changes in reactor configurations could improve biogas yield at low temperature.

An evaluation of solar thermal heating to support a freeze-thaw anaerobic digestion system for human waste treatment in subarctic environments

Remote locations, small communities, and weather prohibit the operation of piped sanitary sewers in many Alaska Native Villages (ANVs). Research was conducted to understand the technical feasibility of installing anaerobic digesters (ADs) in remote ANVs which would be heated by solar thermal collectors. Biochemical methane potential (BMP) assays were conducted to understand the effect of freezing and thawing on methanogenic activity of synthetic human feces. BMPs were frozen at -20 or -80 °C for 7 days and then incubated at psychrophilic (20 °C) or mesophilic (37 °C) conditions. Psychrophilic BMPs frozen at -20 or -80 °C yielded 453 ± 119 and 662 ± 77 mL CH₄/g VS, respectively. Mesophilic BMPs frozen at -20 or -80 °C yielded 337 ± 59 and 495 ± 63 mL CH₄/g VS, respectively. Freezing caused a lag period, but ultimately many of the assays reached yields similar to or even greater than the baseline, unfrozen assays. Monthly solar radiation and air temperature data were used to identify the number of solar thermal collectors that would be required to supplement heat energy to operate the ADs in several locations. Alaskan subarctic locations receive enough solar thermal energy in summer months to support seasonally operated, psychrophilic ADs.

Thermo-chemo-sonic pretreatment of lignocellulosic waste: Evaluating anaerobic biodegradability and environmental impacts

In the present study, yard waste was pretreated by thermo-chemo-sonic pretreatment prior to anaerobic digestion to improve its anaerobic biodegradability. First, the pretreatment conditions were optimized using Box-Behnken design based response surface methodology for the maximum organic matter solubilisation. Then, the possible mechanism of delignification by thermo-chemo-sonic pretreatment was discussed. Moreover, the anaerobic digestion performance of untreated yard waste (UYW) and pretreated yard waste (PYW) was compared. The optimum pretreatment condition based on the increase in soluble COD and volatile solids (VS) was: 2997 kJ/kgTS ultrasonic energy, 74 °C, and 10.1 pH. The highest methane yield of 374 ± 28 mL/gVS_added for the PYW at the optimum condition was achieved, which was 37.5 % higher than the UYW (272 ± 16 mL/gVS_added). Finally, the environmental impacts associated with anaerobic digestion of both UYW and PYW were compared. The life cycle assessment confirmed a positive environmental impact of pretreatment.

Intensification of Acidogenic Fermentation for the Production of Biohydrogen and Volatile Fatty Acids—A Perspective

Utilising ‘wastes’ as ‘resources’ is key to a circular economy. While there are multiple routes to waste valorisation, anaerobic digestion (AD)—a biochemical means to breakdown organic wastes in the absence of oxygen—is favoured due to its capacity to handle a variety of feedstocks. Traditional AD focuses on the production of biogas and fertiliser as products; however, such low-value products combined with longer residence times and slow kinetics have paved the way to explore alternative product platforms. The intermediate steps in conventional AD—acidogenesis and acetogenesis—have the capability to produce biohydrogen and volatile fatty acids (VFA) which are gaining increased attention due to the higher energy density (than biogas) and higher market value, respectively. This review hence focusses specifically on the production of biohydrogen and VFAs from organic wastes. With the revived interest in these products, a critical analysis of recent literature is needed to establish the current status. Therefore, intensification strategies in this area involving three main streams: substrate pre-treatment, digestion parameters and product recovery are discussed in detail based on literature reported in the last decade. The techno-economic aspects and future pointers are clearly highlighted to drive research forward in relevant areas.

Thermally enhanced bioremediation: A review of the fundamentals and applications in soil and groundwater remediation

Thermally enhanced bioremediation (TEB), a new concept proposed in recent years, explores the combination of thermal treatment and bioremediation to address the challenges of the low efficiency and long duration of bioremediation. This study presented a comprehensive review regarding the fundamentals of TEB and its applications in soil and groundwater remediation. The temperature effects on the bioremediation of contaminants were systematically reviewed. The thermal effects on the physical, chemical and biological characteristics of soil, and the corresponding changes of contaminants bioavailability and microbial metabolic activities were summarized. Specifically, the increase in temperature within a suitable range can proliferate enzymes enrichment, extracellular polysaccharides and biosurfactants production, and further enhancing bioremediation. Furthermore, a systematic evaluation of TEB applications by utilizing traditional in situ heating technologies, as well as renewable energy (e.g., stored aquifer thermal energy and solar energy), was provided. Additionally, TEB has been applied as a biological polishing technology post thermal treatment, which can be a cost-effective method to address the contaminants rebounds in groundwater remediation. However, there are still various challenges to be addressed in TEB, and future research perspectives to further improve the basic understanding and applications of TEB for the remediation of contaminated soil and groundwater are presented.

A comparative life cycle assessment of different pyrolysis-pretreatment pathways of wood biomass for levoglucosan production

In order to identify the most environmental-friendly pretreatment for pyrolsis of wood residue to levoglucosan (LG), for the first time a comparative life cycle assessment (LCA) was carried out for hot water treatment (HWT), torrefaction, acid pretreatment (AP) and salt pretreatment (SP) pathways. Since LG production can facilitate both resource recovery (RR) and wood residue handling (WRH), two different functional units (FUs), i.e., 1 kg LG production and 1 kg wood residue handling were considered. AP was found to generate the least global warming potential of 134.60 kg CO₂-eq and human carcinogenic toxicity of 0.59 kg 1,4-dichlorobenzene-eq. for RR perspective. However, for WRH perspective, HWT was found to be the best pretreatment (6.39 kg CO₂-eq; 0.03 kg 1,4-dichlorobenzene-eq.). Sensitivity analysis revealed that a reduction in electricity consumption by 15% could reduce the overall impacts by 14.00-14.82 %. This study also highlights the impact of goal and FU selection on LCA.

Effects of temperature and HRT on biogas production in moving and fixed bed of a novel upflow anaerobic hybrid (UAHB) reactor

The upflow anaerobic hybrid (UAHB) reactor combines the advantages of a upflow anaerobic sludge blanket (UASB-type) reactor and an anaerobic filter in a single compartment. A novel configuration of the UAHB reactor, composed of two three-phase separators (3PHS), was proposed to evaluate the biogas production in the moving and fixed bed in the treatment of synthetic sewage at a temperature range of 14-21 °C and hydraulic retention time (HRT) of 12, 10 and 8 h. The bench-scale reactor was operated in three different phases with organic loading rate (VOLR) of 0.6 (0.3-0.7), 0.7 ± 0.2, and 1.1 ± 0.1 kg COD m^-3 d^-1, respectively, for 225 days. The average removal efficiency of chemical oxygen demand (COD_t) was 78 (42-89)%, and the total biogas yield was 3090 (1704-4782) mL d^-1, with 66% of the lower 3PHS (moving bed) and 34% of the upper 3PHS (fixed bed). However, no significant difference was observed between the biogas yield on the 3PHS (p-value = 0.5048), thus confirming the influence of temperature in the biogas production. The average percentage of methane was 76 (60-82)% for both beds, and the filter media increased the production by 21%. Thus, it can be concluded that the fixed bed suppressed the instability of the moving bed regarding the biogas production and contributed to the final quality of the effluent.

Anaerobic digestion characteristics and key microorganisms associated with low-temperature rapeseed cake and sheep manure fermentation

In this study, gas production from mixed anaerobic fermentation of rapeseed cake and sheep manure at low temperature (15.2-17.8 °C) was investigated in Qinghai rural household biogas digesters to understand the temporal dynamics of key microbial populations involved in fermentations. Different raw material ratios resulted in significantly different effects on biogas yields and microbial community compositions over 40 days. When the dry weight ratio of sheep manure to rapeseed cake was 1:2, the highest level of cumulative gas production was observed (122.92 m³·t^-1). Firmicutes, Proteobacteria, Bacteroidetes, and Actinobacteria were the dominant bacterial phyla among the 29 digester samples (total relative abundances > 79.23%), followed by Synergistetes (4.09-10.7%). Lactobacillus was the most abundant genus in the biogas digesters with high rapeseed cake contents (average relative abundances: 14.68%), while Peptoniphilus exhibited higher abundances (12.69%) in the mixed treatments. In addition, unclassified Synergistaceae abundances (6.64%) were positively associated with biogas production variation among treatments. Bacteroides (5.74%) and Pseudomonas (5.24%) both accounted for larger proportions of communities in the digesters that used more sheep manure. Methanomicrobiales (66.55%) was the most dominant archaeal group among digesters, with Methanogenium (41.82%) and Methanoculleus (16.55%) representing the main gas-producing archaeal genera; they were more abundant in biogas digesters with higher sheep manure contents and higher rapeseed cake contents, respectively. VFAs and pH were the main factors associated with differences in microbial communities among the 29 samples. Specifically, VFA concentrations were positively correlated with Lactobacillus, Methanoculleus and Methanothrix abundances, while pH was positively correlated with Bacteroides, Pseudomonas, and Methanobacterium abundances.