Anaerobic Digestion System Research Articles

A comparative life cycle analysis of Sol-Char and anaerobic digestion sanitation systems.

In this study, we compared the Sol-Char sanitation system with an Anaerobic Digestion (AD) system using Life Cycle Assessment (LCA) to evaluate their environmental impacts. Since both systems offer opportunities for human waste treatment and resource recovery, understanding their performance is crucial. This comparison aims to determine their environmental impacts while considering diverse factors, such as energy production and nutrient recovery. The Sol-Char system demonstrated a superior life-cycle environmental performance, showing two to five times lower impacts in categories such as Climate Change (e.g., 127kg CO₂-eq for the Sol-Char system while that 592kg CO₂-eq for the AD system), Non-Renewable Energy Resources, Ionizing Radiation, Land Use, and Water Use. Both systems exhibited significant potential for resource recovery, with the Sol-Char system producing biochar and disinfected urine, and the AD system generating electricity, heat, and digestate. Updated LCA results, after byproduct application, indicated that both systems potentially have a net positive environmental impact (both with reductions exceeding -500kg CO₂-eq per day). Nutrient recovery simulations using SAmpSONS2 revealed that the AD system performed better when utilizing multiple biomass sources. The nitrogen content in the solids was 20.25kg/day after AD and 3.75kg/day for the Sol-Char system. Our results highlight the Sol-Char system is a viable sanitation solution in rural areas. However, the study also identified key challenges, including the absence of uncertainty analysis and the need for a standardized framework that enables more consistent evaluations and comparisons across diverse sanitation systems and contexts.

Genome-based analysis reveals niche differentiation among Firmicutes in full-scale anaerobic digestion systems.

Fermentative Firmicutes species are key players in anaerobic digestion; however, their niche differentiation based on carbohydrate utilization in full-scale systems remains unclear. In this study, we investigated niche differentiation among four major Firmicutes classes using a genome-centric approach, reconstructing 39 high-quality metagenome-assembled genomes. Limnochordia and Clostridia exhibited the broadest substrate versatility, utilizing 24% and 18% of the predicted substrates, respectively. Although common substrates were shared, each class demonstrated unique substrate preferences driven by distinct functional and metabolic differences. Limnochordia and Clostridia possess unique carbohydrate-active enzyme families, such as GH177 and CBM91, which enable xylan and arabinan degradation. Bacilli were abundant with the GH1 and GH3 families, which are critical for cellulose degradation. Overall, the Firmicutes classes showed low overlap in substrate use and functional profiles, confirming significant niche differentiation. Our results demonstrate that Firmicutes occupy distinct dietary niches supporting insights into bacterial coexistence in anaerobic digestion systems.

Comparing sewage sludge vs. digested sludge for starting-up thermophilic two-stage anaerobic digesters: Operational and economic insights.

Despite advances in anaerobic digestion (AD), full-scale implementation faces significant challenges, particularly during the start-up phase, where inoculum selection is crucial. This study examines the impact of inoculum choice on the operational and economic performance of thermophilic digesters during the start-up phase. Methanogenic reactors R3 and R4 were inoculated with digested sludge (DiS) and diluted sewage sludge (DSS), respectively, and fed with hydrolyzed source-sorted organic fraction of municipal solid waste (SS-OFMSW) and thickened sewage sludge, which were processed in R1 and R2, serving as acidogenic reactors. A two-stage AD configuration was employed to mitigate inhibitory effects associated with the undigested inoculum (DSS). This approach enabled the establishment of methanogenic activity in R4 when the AD system is initiated with DSS. However, R3 outperformed R4, achieving 49 % of the feedstock's theoretical methane potential compared to 15 % in R4. Methane production and volatile solids (VS) processing costs in R4 were 18 and 3 times higher than in R3, respectively. R3's superior performance was attributed to DiS's diverse bacterial community, with over 66 % of genera involved in hydrolysis, volatile fatty acid production, and syntrophic methane production. In contrast, DSS was dominated by Trichococcus and Lactococcus (75.4 %), primarily involved in butyrate oxidation and lactate production. This study provides valuable insights into effective inoculum selection for the start-up of full-scale digesters.

Advances and insights into modeling extracellular electron transfer in anaerobic bioprocesses.

Extracellular electron transfer (EET) plays an important role in maintaining redox balance in both natural and engineered anaerobic microbial systems, driving key biochemical processes such as energy generation, bioremediation, and waste degradation. While EET has been characterized in a limited number of microbes and applied in anaerobic digestion and bioelectrochemical systems, further research is needed to explore its mechanism across a broader range of microbial species and anaerobic processes. This review highlights advanced modeling frameworks that provide deeper insights into EET mechanisms and dynamics, aiming to optimize research efforts and minimize time and resource expenditure. Mechanistic models, encompassing thermodynamics and kinetics, are discussed for their utility in calculating conduction rates of electroactive microbes and assessing the energetics of medium chain carboxylic acids production. Genome-scale metabolic models are highlighted for elucidating the roles of cytochromes and conductive pili in the EET pathway. Machine learning is presented as a tool to improve model accuracy and predict EET mechanisms. Furthermore, the integration of quantum mechanics/molecular mechanics methods offers molecular-level insights into electron transfer, while quantum computing addresses limitations of classical computers by simulating complex electron transfer processes in multi-heme cytochromes. Developing advanced modeling techniques will complement experimental techniques, enabling precise predictions and optimization strategies for developing innovative and sustainable anaerobic biotechnologies.

Different combinations of operating temperature and solids retention time during two-phase anaerobic digestion impacts removal of antibiotic resistance genes

Two-phase anaerobic digestion (AD) performance is significantly influenced by operating parameters such as temperature and solids retention time (SRT), while their impact on antibiotic resistance genes (ARGs) during the acidogenic (AP) and methanogenic (MP) phases remains unclear. This study assessed the abundance of eight ARGs in full-scale two-phase AD, then operated lab-scale two-phase AD systems to evaluate temperature combinations (thermophilic-thermophilic, thermophilic-mesophilic, mesophilic-thermophilic, and mesophilic-mesophilic) at a constant SRT (AP = 2/MP = 13d) and to further assess different SRTs (AP = 2/MP = 13d and AP = 4/MP = 11d). qPCR results revealed that full-scale two-phase AD reduced total ARGs abundance by 87.70 ± 0.50 %. In lab-scale tests, the thermophilic-thermophilic configuration achieved nearly complete ARGs removal (97.61 ± 0.21 %), while the combination of AP = 4/MP = 11d had the highest removal efficiency (83.39 ± 1.17 %). Network analysis indicated that Firmicutes, Bacteroidota, and Proteobacteria were the primary ARG hosts, with Firmicutes dominant. These findings highlight the optimal operating parameters in two-phase AD for maximizing ARGs removal.

Maximizing resource valorization: mono- and co-digestion of food and agricultural wastes with insights into microbial community dynamics

Abstract The valorization of food processing waste is critical for sustainable development and circular economy frameworks. Although Turkish delight waste (D), a high-volume byproduct of the confectionery industry in Turkey, holds significant potential for valorization within the circular economy framework, this potential remains largely unexplored. This study evaluated the valorization potential of Turkish delight waste in anaerobic digester systems by co-digesting it with sunflower heads (S) and tea stalks (T), common agricultural wastes in Turkey, in addition to mono-digestion. Economic evaluations were also conducted on the products obtained at different stages of anaerobic digestion (AD). The highest methane yield of 388 mL CH4/g VS was obtained from Turkish delight waste in mono-digestion, and this substrate enhanced methane production when co-digested with the other substrates. However, co-digestion in the DST digester combining all three substrates yielded 234 mL CH4/g VS, indicating a limited synergistic effect. Metagenomic analyses revealed substrate-dependent variations in microbial community dynamics, particularly in digesters containing only Turkish delight waste. Acidogenic fermentation aimed at increasing VFA yield resulted in total acetic acid productions of 2828, 1707, and 1261 mg/L for D, DS, and DST, respectively. Economic assessments demonstrated that the value derived from VFA production was nearly double that obtained from methane production, even in cases where co-digestion resulted in lower overall yields. Thus, Turkish delight waste emerges as a promising candidate for both mono- and co-digestion in AD systems, offering a potentially more economically viable alternative to methane production through the generation of value-added chemicals such as VFAs. Graphical Abstract

The Effect of Hydrogen Peroxide on Biogas and Methane Produced from Batch Mesophilic Anaerobic Digestion of Spent Coffee Grounds

This paper aims to explore both experimental and modeling anaerobic digestion (AD) processes as innovative methods for managing the substantial quantities of spent coffee grounds (SCG) generated in Algeria, transforming them into valuable renewable energy sources (biogas/methane). AD of SCG, while promising, is hindered by its complex lignocellulosic structure, which poses a significant challenge. This study investigates the efficacy of hydrogen peroxide (H2O2) pretreatment in addressing this issue, with a particular focus on enhancing biogas and methane production. The AD of SCG was conducted over a 46-day period, and the impact of H2O2 pretreatment was evaluated using laboratory-scale batch anaerobic reactors. Four different concentrations of H2O2 (0.5, 1, 2, and 4% H2O2 w/w) were studied in mesophilic conditions (37 ± 2) for 24 h at room temperature, providing basic data on biogas and methane production. The results showed a significant increase in soluble oxygen demand (SCOD) and total sugar solubilization in the range of 555.96–713.02% and 748.48–817.75%, respectively. The optimal pretreatment was found to be 4% H2O2 w/w resulting in 16.28% and 16.93% improvements in biogas and methane yield over the untreated SCG. Further, while previous research has established oxidative pretreatment efficacy, this study uniquely combines the empirical analysis of H2O2 pretreatment with a detailed kinetic modeling approach using the modified Gompertz (MG) and logistic function (LF) models to estimate kinetic parameters and determine the accuracy of fit. The MG model showed the most accurate prediction, thus making the present investigation a contribution to understanding the performance of the AD system under oxidative pretreatment and designing and scaling up new systems with predictability. These findings highlight the potential of H2O2-pretreated SCG as a more efficient and readily available resource for sustainable waste management and renewable energy production.

Leveraging Municipal Solid Waste Management with Plasma Pyrolysis and IoT: Strategies for Energy Byproducts and Resource Recovery

The escalating challenges of municipal solid waste (MSW) management, exacerbated by the classification of MSW as hazardous waste due to the presence of heavy metals (HMs) and toxic compounds, necessitate innovative treatment strategies. Plasma pyrolysis has emerged as a promising technology for converting MSW into valuable energy byproducts, such as syngas, bio-oil, and slag, while significantly reducing waste volume. However, maintaining optimal operational parameters during the plasma pyrolysis process remains a complex challenge that can adversely affect both the efficiency and the quality and quantity of outputs. To address this issue, the integration of the Internet of Things (IoT) presents a transformative approach. By leveraging IoT technologies, real-time monitoring and advanced data analytics can be employed to optimize the operational conditions of plasma pyrolysis systems, ensuring consistent performance and maximizing resource recovery. This review explores the synergistic integration of plasma pyrolysis and IoT as a novel strategy for MSW management. The slag from plasma treatment can be efficiently channeled into anaerobic digestion (AD) systems, promoting resource recovery through biogas production and the generation of nutrient-rich digestate. This synergy not only mitigates the environmental impacts associated with traditional MSW disposal methods but also paves the way for sustainable energy recovery and resource management. Ultimately, this review presents a comprehensive framework for exploiting plasma pyrolysis and IoT in addressing the pressing issues of hazardous MSW, thereby fostering a circular economy through innovative waste-to-energy solutions.

Roles and opportunities of quorum sensing in natural and engineered anaerobic digestion systems.

Anaerobic digestion (AD) is a biological process in which anaerobic microorganisms convert organic matter into methane-rich gas, contributing to the cycling of carbon and other nutrients. Quorum sensing (QS), a microbial communication mechanism, plays a critical role in regulating population-level behaviors within AD systems. This review systematically examines the roles and applications of QS in AD, emphasizing its importance in enhancing process efficiency. The review begins by exploring the pathways and characteristics of QS in key functional microorganisms involved in AD. We analyze the response mechanisms of QS to key environmental variables and their effects on the structure and function of microbe communities and extracellular polymeric substances secretion. Potential applications of QS in engineered AD systems are discussed, with a focus on promoting system startup, improving operational efficiency, and enhancing resistance and stability. The use of exogenous signaling molecules and quorum quenching reagents to optimize AD performance is also evaluated. Additionally, the ecological significance of QS in natural environments, such as seafloor sediments and wetlands, is explored, emphasizing its role in regulating AD-related microorganisms within complex microbial communities. Finally, the review identifies current knowledge gaps and outlines future research directions in AD, including QS database development, QS-engineered bacteria excavation, and advanced analytical methods assistants. This comprehensive review aims to bridge existing gaps in QS-related knowledge in AD and provide fresh perspectives for studying microbial communication and collaboration through QS.

Resilience and response of anaerobic digestion systems to short-term hydraulic loading shocks: Focusing on total and active microbial community dynamics.

Anaerobic digestion is known to be sensitive to operational changes, such as hydraulic loading shock, yet the impact on the microbiome, particularly the active RNA-based community, has not been fully understood. This study aimed to investigate the performance of anaerobic reactors and their microbial communities under short-term hydraulic loading shocks. Using synthetic wastewater, the reactor was subjected to 24-h shocks at three-fold and seven-fold the baseline loading rate, followed by DNA and RNA analyses to assess the system's resiliency and microbial responses. The research focused on shifts in major microbial groups and their functions, paying close attention to the active RNA community during loading shock events to better reflect the system's immediate condition. Findings indicated that although the microbial community structure, particularly among the archaea, was altered, the reactor quickly regained its balance. Differences were observed between DNA and RNA profiles and between regular and shock loadings; however, the alpha diversity and functions of the overall community were sustained. This study offers important insights for the design and operation of wastewater treatment plants, with the goal of achieving stable and efficient anaerobic digestion systems.

Combination of anaerobic digestion and sludge biochar for bioenergy conversion: Estimation and evaluation of energy production, CO2 emission, and cost analysis.

Waste-to-energy technologies involve the conversion of several wastes to useful energy forms like biogas and biochar, which include biological and thermochemical processes, as well as the combination of both systems. Assessing the economic and environmental impacts is an important step to integrate sustainability and economic viability at anaerobic digestion systems and its waste management. Energy production, CO2 emissions, cost analysis, and an overall process evaluation were conducted, relying on findings from both laboratory and pilot-scale experiments. The digestate generated during anaerobic digestion proved to be an effective approach for mitigating some CO2 emissions while managing sludge waste within the anaerobic digestion and CHP system. Incorporating biochar production and application in soil into the process led to a 3.5 % reduction in CO2 emissions, which contributed to a more sustainable form of energy production while offering the potential for the generation of carbon credits through a carbon-negative process. The employment of digestate biochar for energy production seems a feasible way to reduce the amount of residue to final disposal in landfill with a minimal reduction of profit per GWh and a slight increase in the CO2 emissions by 2.7 %.

Effects of Calcium-Oxide-Modified Biochar on the Anaerobic Digestion of Vacuum Blackwater.

The increasing global population and urbanization have led to significant challenges in waste management, particularly concerning vacuum blackwater (VBW), which is the wastewater generated from vacuum toilets. Traditional treatment methods, such as landfilling and composting, often fall short in terms of efficiency and sustainability. Anaerobic digestion (AD) has emerged as a promising alternative, offering benefits such as biogas production and digestate generation. However, the performance of AD can be influenced by various factors, including the composition of the feedstock, pH levels, and the presence of inhibitors. This review investigates the effects of calcium oxide (CaO)-modified biochar (BC) as an additive in AD of VBW. Modifying BC with CaO enhances its alkalinity, nutrient retention, and adsorption capacity, creating a more favorable environment for microorganisms and promoting biogas production, which serves as a valuable source of heat, fuel and electricity. Additionally, the digestate can be processed through plasma pyrolysis to ensure the complete destruction of pathogens while promoting resource utilization. Plasma pyrolysis operates at extremely high temperatures, effectively sterilizing the digestate and eliminating both pathogens and harmful contaminants. This process not only guarantees the safety of the end products, but also transforms organic materials into valuable outputs such as syngas and slag. The syngas produced is a versatile energy carrier that can be utilized as a source of hydrogen, electricity, and heat, making it a valuable resource for various applications, including fuel cells and power generation. Furthermore, the slag has potential for reuse as an additive in the AD process or as a biofertilizer to enhance soil properties. This study aims to provide insights into the benefits of using modified BC as a co-substrate in AD systems. The findings will contribute to the development of more sustainable and efficient waste management strategies, addressing the challenges associated with VBW treatment while promoting renewable energy production.

Residual ciprofloxacin in chicken manure inhibits methane production in an anaerobic digestion system

Anaerobic digestion (AD) is commonly used to dispose of laying hen manure. However, veterinary antibiotic residues present in chicken manure may affect the AD process. Here, the effects of three types of veterinary antibiotics commonly used in laying hen breeding on AD were explored. Manures containing antibiotics at two different concentrations were continuously added during AD for 5 days: amoxicillin (HAMX: 145.06 mg/kg, LAMX: 57.88 mg/kg), doxycycline (HDOC: 183.61 mg/kg, LDOC: 98.00 mg/kg), and ciprofloxacin (HCIP: 96.34 mg/kg, LCIP: 40.43 mg/kg). Compared with a control with no veterinary antibiotics, the amoxicillin and doxycycline groups presented no significant effects on biogas production, methane production, VFA concentration, acetic acid concentration or the pH of the AD system (P > 0.05). However, compared with the control, the ciprofloxacin groups presented significantly inhibited biogas and methane production during AD (P < 0.05), and the HCIP and LCIP groups presented significantly decreased biogas (47.82% and 45.37%, respectively) and methane (58.24% and 52.55%, respectively) production (P < 0.05). Moreover, the VFA and acetic acid concentrations of the ciprofloxacin groups were significantly higher than those of control during the entire AD period (P < 0.05), and the pH value at the withdrawal stage was significantly lower than that of the control group (P < 0.01), with no significant difference between the HCIP and LCIP groups (P > 0.05). Our results suggest that ciprofloxacin causes VFA and acetic acid accumulation in AD systems, thereby reducing the pH of the systems and inhibiting methanogen growth, ultimately reducing methane production in the AD systems. These findings contribute to a deeper understanding of the impact of ciprofloxacin on methane production in AD systems and offer some considerations for the application of AD systems.

Characteristics of Phages and Their Interactions With Hosts in Anaerobic Reactors.

Anaerobic digestion (AD) of organic wastes relies on the interaction and cooperation of various microorganisms. Phages are crucial components of the microbial community in AD systems, but their diversity and interactions with the prokaryotic populations are still inadequately comprehended. In this study, 2121 viral operational taxonomic units (vOTUs) were recovered from 12 anaerobic fatty acid-fed reactors. Notably, 63.1% of these vOTUs could not be assigned to any known family, revealing a substantial presence of uncharted phages specifically associated with AD environments. Over half of the vOTUs associated with hosts had the capability to infect multiple hosts, ranging from 2 to 49, with a prevalent tendency to infect 2-5 hosts. In silico predictions of phage-host linkages uncovered that only a small fraction of vOTUs were shared across different functional groups, including fermentative bacteria, syntrophic fatty acid-oxidising bacteria (SFOB) and methanogens. Phages linked to hosts in all three groups primarily consisted of generalists and temperate species, especially those linked to SFOB. Additionally, metabolic reconstruction identified auxiliary metabolic genes participating in fatty acid degradation, methanogenesis and energy conservation. The present study provides insights into phage characteristics and their insitu interactions with prokaryotic hosts, highlighting their ecological role in AD systems.

Geographical distribution and risk of antibiotic resistance genes in sludge anaerobic digestion process across China.

Anaerobic digestion (AD) is gaining increasing attention as the central reservoir of antibiotic resistance genes (ARGs), while the geographical distribution of ARGs in AD is neglected. Accordingly, a sampling scheme on full-scale AD plants across China was implemented, and the resistome therein was excavated. The abundance of ARGs in AD sludge ranged from 0.198 to 0.574 copies/cell. Some of the frequently reported and emergent ARGs were detected in our AD system. Both the abundance and composition of ARGs presented significant differences between the south and north regions of China, hinting the physical/economic factors may function in the formation of ARG profiles. The risk scores of AD samples were in middle of domestic and hospital wastewater. Risk scores were significantly higher in the north. Besides, the proportion of Rank I and Rank II ARGs was also higher in north, which explained the regional difference of ARG composition in a micro-perspective. This study provides a fundamental survey on the of ARG level and profile in AD process across China, reveals the biogeography of ARGs and inspires the control strategies of antibiotic resistance.

Metagenomic analysis reveals the effects of potassium ferrate and steel slag on fate of ARGs in anaerobic sludge digestion system.

Waste activated sludge (WAS) pose a potential risk for the spread of antibiotic resistance genes (ARGs). This study estimated the effect of sludge on antibiotic resistance genes (ARGs) in anaerobic sludge digestion process. Metagenomic analysis revealed anaerobic sludge with potassium ferrate (PF) and the modified PF loaded steel slag (MPF-SS) brought an increase of ARGs during digestion process. PF was found to effectively reduce most of the high-risk ARGs (i.e., acrB and mexW). Furthermore, network and correlation analysis among ARGs and genera verified that PF significantly increased the potential ARGs hosts. Mechanistic analysis revealed that PF induced oxidative stress behavior of anaerobic digestion microorganisms, and observably upregulated the relative genes about SOS response-related. These findings provide insights into the mechanism underlining PF for ARGs fate and its risk during anaerobic sludge digestion, which could offer practical guidance on the sustainable management of WAS.

A Critical Review on Phosphorus Recovery from Source-diverted Blackwater.

Source-diverted blackwater refers to toilet wastewater, sometimes including kitchen wastewater, which is rich in essential nutrients such as nitrogen (N) and phosphorus (P). Blackwater, especially concentrated blackwater collected from low-flush vacuum toilet systems, represents a valuable source for P recovery, addressing future P scarcity and mitigating environmental issues like eutrophication. While there has been growing interest in technologies for P recovery from blackwater, these technologies are still in the early stages of development. This study provides a comprehensive review of the mechanisms behind phosphate salt formation, the technologies for P recovery from blackwater-particularly through struvite and calcium phosphate precipitation-and the safety concerns associated with the use of recovered products. Despite advancements, most research is limited to lab-scale experiments, leaving significant gaps in optimizing P recovery technologies for broader application. Future research will be likely to focus on integrating bioenergy recovery with P recovery in anaerobic digestion (AD) systems, aiming to create a more sustainable and zero-waste approach. Addressing current challenges and scaling up from lab research to real-world applications will be crucial for making P recovery more efficient and economically viable.

The metabolic redundancy relieving VFAs shocks in anaerobic digestion system exposed sequentially to increasing acetic acid loading

The metabolic redundancy relieving VFAs shocks in anaerobic digestion system exposed sequentially to increasing acetic acid loading

Deciphering the potential mechanisms and influencing factors of the effects of micro(nano)plastics on microbe in sludge anaerobic digestion system

Deciphering the potential mechanisms and influencing factors of the effects of micro(nano)plastics on microbe in sludge anaerobic digestion system

Carbon credits monetary value for anaerobic digestion systems and energy policy implication in the UK

Carbon credits monetary value for anaerobic digestion systems and energy policy implication in the UK