Rapid establishment of thermophilic anaerobic microbial community during the one-step startup of thermophilic anaerobic digestion from a mesophilic digester

Porous polyurethane biocarriers could enhance system nitrification resilience under high organic loading by retaining key functional bacteria

Resilience to increasing organic loading rates (OLRs) is the key to maintaining stable performance in treating industrial wastewater. First, this study compared the stability, particularly the nitrification performance, of two lab-scale moving bed biofilm reactors (MBBRs) filled with porous polyurethane biocarriers with two conventional activated sludge reactors (ASRs) in the treatment of synthetic coking wastewater under OLRs increasing from 0.3 kg to 1.5 kg COD m-3 day-1. In comparison with the ASRs, which could only achieve complete nitrification (99.31 % ± 0.43 %) at an OLR of 0.7 kg COD m-3 day-1, the MBBRs could achieve efficient NH4+-N removal (99.45 % ± 0.21 %) at an OLR as high as 1.3 kg COD m-3 day-1. Even at an OLR of 1.5 kg COD m-3 day-1 where nitrification was inhibited, the porous polyurethane biocarriers in the MBBRs still maintained a highly diversified bacterial community (Shannon index, 4.34 ± 0.31) by retaining the slow-growing nitrifying bacteria and phenol-degrading bacteria, including Methyloversatilis and Acinetobacter, whose phenol degradation functions were confirmed by metagenome-assembled genome extraction and analysis, while the ASRs lost diversity (Shannon index, 1.41 ± 0.45) due to the sequential occurrence of filamentous and viscous sludge bulking. The advantage of the MBBR was further verified in a full-scale coking wastewater treatment system, where a reactor series filled with 4.35 % porous polyurethane biocarriers exhibited better NH4+-N removal of 99.57 % ± 0.34 % compared to 96.85 % ± 2.56 % for a conventional one under an OLR of 0.54 ± 0.12 kg COD m-3 day-1. The results could contribute to the development of more effective and resilient treatment systems for industrial wastewater.

Insights into kinetic and regression models developed to estimate the abundance of antibiotic-resistant genes during biological digestion of wastewater sludge

Wastewater treatment plants are hubs of antibiotic-resistant genes (ARGs). During wastewater treatment, ARGs accumulate in wastewater sludge and some survive biological digestion. After land application of digested sludge, ARGs are transported to soil, water, and air, and may encounter humans and animals. ARGs are typically quantified by quantitative polymerase chain reaction (qPCR) on isolated DNA. Nevertheless, DNA isolation and qPCR are time-consuming, expensive, and prone to contamination. Therefore, there is a need to estimate ARGs quantities via methods that can be readily employed. Such estimation would help to protect public health via modifying biological digestion to maximize the removal of ARGs. Two approaches that make such estimation are kinetic and regression modeling. The kinetic models have been mainly of the first order. This review examines the application of the kinetic models to estimate the abundance of ARGs during biological sludge digestion. It also discusses how biological sludge digesters can be designed using kinetic models. The literature provides single and multiple regression models, from which an ARGs -Solids -Nutrients nexus, a focal point of this review, is inferred. This review demonstrates that regression models are mathematical expressions of that nexus. Also, existing challenges are highlighted and suggestions for future are provided.

Mesophilic and thermophilic fermentation of activated sludge for volatile fatty acids production: focusing on anaerobic degradation of carbohydrate and protein

The volatile fatty acids (VFAs) productions, as well as particulate organics decomposition, soluble chemical oxygen demand (SCOD) yield, and the VFAs production pathways from mesophilic and thermophilic anaerobic fermentation in waste activated sludge were investigated. Batch experiments showed that the decomposition rate of volatile suspended solids (VSS), particulate carbohydrate (P-C) and particulate protein (P-P) followed the first-order kinetic model at different temperatures. However, the intermediates, accumulated in the process of protein or carbohydrate digestion had a more significant inhibitory effect on the production of VFAs during the mesophilic anaerobic acidification process. The production of VFAs by thermophilic anaerobic fermentation is 2086.05 mg COD/L, which is about twice the production under mesophilic conditions. Among them, the concentration and proportion of high molecular weight organic acids such as isobutyric acid (320.29 mgCOD/L) and isovaleric acid (745.75 mgCOD/L) are relatively high. Then 13C stable isotope labelling experiment demonstrated that, the decomposition of carbohydrates yields 77% acetic acid and 86% butyric acid, while protein breakdown produces 85% propionic acid and 99% valeric acid. This confirms that carbohydrates are more favourable for the formation of even-carbon organic acids, while proteins tend to yield odd-carbon organic acids. Additionally, this helps refine the pathway for valeric acid formation during anaerobic acidogenesis.

Sludge bound-EPS solubilization enhance CH4 bioconversion and membrane fouling mitigation in electrochemical anaerobic membrane bioreactor: Insights from continuous operation and interpretable machine learning algorithms

Bound extracellular polymeric substances (EPS) are complex, high-molecular-weight polymer mixtures that play a critical role in pore clogging, foulants adhesion, and fouling layer formation during membrane filtration, owing to their adhesive properties and gelation tendencies. In this study, a novel electrochemical anaerobic membrane bioreactor (EC-AnMBR) was constructed to investigate the effect of sludge bound-EPS solubilization on methane bioconversion and membrane fouling mitigation. During the 150-days' operation, the EC-AnMBR demonstrated remarkable performance, characterized by an exceptionally low fouling rate (transmembrane pressure (TMP) < 4.0 kPa) and high-quality effluent (COD removal > 98.2 %, protein removal > 97.7 %, and polysaccharide removal > 98.5 %). The highest methane productivity was up to 38.0 ± 3.1 mL/Lreactor/d at the applied voltage of 0.8 V with bound-EPS solubilization, 107.6 % higher than that of the control stage (18.3 ± 2.4 mL/Lreactor/d). Morphological and multiplex fluorescence labeling analyses revealed higher fluorescence intensities of proteins, polysaccharides, total cells and lipids on the surface of the fouling layer. In contrast, the interior exhibited increased compression density and reduced activity, likely attributable to compression effect. Under the synergistic influence of the electric field and bound-EPS solubilization, biomass characteristics exhibited a reduced propensity for membrane fouling. Furthermore, the bio-electrochemical regulation enhanced the electroactivity of microbial aggregates and enriched functional microorganisms, thereby promoting biofilm growth and direct interspecies electron transfer. Additionally, the potential hydrogenotrophic and methylotrophic methanogenesis pathways were enhanced at the cathode and anode surfaces, thereby increasing CH₄ productivity. The random forest-based machine learning model analyzed the nonlinear contributions of EPS characteristics on methane productivity and TMP values, achieving R² values of 0.879 and 0.848, respectively. Shapley additive explanations (SHAP) analysis indicated that S-EPSPS and S-EPSPN were the most critical factors affecting CH₄ productivity and membrane fouling, respectively. Partial dependence plot analysis further verified the marginal and interaction effects of different EPS layers on these outcomes. By combining continuous operation with interpretable machine learning algorithms, this study unveils the intricate impacts of EPS characteristics on methane productivity and membrane fouling behaviors, and provides new insights into sludge bound-EPS solubilization in EC-AnMBR.

Optimizing anaerobic digestion: Benefits of mild temperature transition from thermophilic to mesophilic conditions

Anaerobic digestion (AD) plays a significant role in renewable energy recovery. Upgrading AD from thermophilic (50-57 °C) to mesophilic (30-38 °C) conditions to enhance process stability and reduce energy input remains challenging due to the high sensitivity of thermophilic microbiomes to temperature fluctuations. Here we compare the effects of two decreasing-temperature modes from 55 to 35 °C on cell viability, microbial dynamics, and interspecies interactions. A sharp transition (ST) is a one-step transition by 20 °C d-1, while a mild transition (MT) is a stepwise transition by 1 °C d-1. We find a greater decrease in methane production with ST (88.8%) compared to MT (38.9%) during the transition period. ST mode overproduced reactive oxygen species by 1.6-fold, increased membrane permeability by 2.2-fold, and downregulated microbial energy metabolism by 25.1%, leading to increased apoptosis of anaerobes by 1.9-fold and release of intracellular substances by 2.9-fold, further constraining methanogenesis. The higher (1.6 vs. 1.1 copies per gyrA) metabolic activity of acetate-dependent methanogenesis implied more efficient methane production in a steady mesophilic, MT-mediated system. Metagenomic binning and network analyses indicated that ST induced dysbiosis in keystone species and greatly enhanced microbial functional redundancy, causing loss of microbial syntrophic interactions and redundant metabolic pathways. In contrast, the greater microbial interconnections (average degrees 44.9 vs. 22.1) in MT at a steady mesophilic state suggested that MT could better maintain necessary system functionality and stability through microbial syntrophy or specialized pathways. Adopting MT to transform thermophilic digesters into mesophilic digesters is feasible and could potentially enhance the further optimization and broader application of practical anaerobic engineering.

Temperature drives microbial communities in anaerobic digestion during biogas production from food waste

Resource depletion and climate changes due to human activities and excessive burning of fossil fuels are the driving forces to explore alternatives clean energy resources. The objective of this study was to investigate the potential of potato peel waste (PPW) at various temperatures T15 (15 °C), T25 (25 °C), and T35 (35 °C) in anaerobic digestion (AD) for biogas generation. The highest biogas and CH4 production (117 mL VS-g and 74 mL VS-g) was observed by applying 35 °C (T35) as compared with T25 (65 mL VS-g and 22 mL VS-g) on day 6. Changes in microbial diversity associated with different temperatures were also explored. The Shannon index of bacterial community was not significantly affected, while there was a positive correlation of archaeal community with the applied temperatures. The bacterial phyla Firmicutes were strongly affected by T35 (39%), whereas Lactobacillus was the dominant genera at T15 (27%). Methanobacterium and Methanosarcina, as archaeal genera, dominated in T35 temperature reactors. In brief, at T35, Proteiniphilum and Methanosarcina were positively correlated with volatile fatty acids (VFAs) concentration. Spearman correlation revealed dynamic interspecies interactions among bacterial and archaeal genera; facilitating the AD system. This study revealed that temperature variations can enhance the microbial community of the AD system, leading to increased biogas production. It is recommended for optimizing the AD of food wastes.

Correlations and impact of anaerobic digestion operating parameters on the start-up duration: Database construction for robust start-up guidelines

Anaerobic digestion (AD) has become a popular technique for organic waste management while offering economic and environmental advantages. As AD becomes increasingly prevalent worldwide, research efforts are primarily focused on optimizing its processes. During the operation of AD systems, the occurrence of unstable events is inevitable. So far, numerous conclusions have been drawn from full and lab-scale studies regarding the driving factors of start-up perturbations. However, the lack of standardized practices reported in start-up studies raises concerns about the comparability and reliability of obtained data. This study aims to develop a knowledge database and investigate the possibility of applying machine learning techniques on experimentation-extracted data to assist start-up planning and monitoring. Thus, a standardized database referencing 75 cases of start-up of one-stage wet continuously-stirred tank reactors (CSTR) processing agricultural, industrial, or municipal organic effluent in mono-digestion from 31 studies was constructed. 10 % of the total observations included in this database concern failed start-up experiments. Then, correlations between the parameters and their impacts on the start-up duration were studied using multivariate analysis and a model-based ranking methodology. Insights into trends of choices were highlighted through the correlation analysis of the database. As such, scenarios favoring short start-up duration were found to involve relatively low retention times (average initial and final hydraulic retention times, (HRTi) and (HRTf) of 26.25 and 20.6 days, respectively), high mean organic loading rates (average OLRmean of 5.24 g VS·d-1·L -1) and the processing of highly fermentable substrates (average feed volatile solids (VSfeed) of 81.35 g L-1). The model-based ranking of AD parameters demonstrated that the HRTf, the VSfeed, and the target temperature (Tf) have the strongest impact on the start-up duration, receiving the highest relative scores among the evaluated AD parameters. The database could serve as a reference for comparison purposes of future start-up studies allowing the identification of factors that should be closely controlled.

Co-occurrence of dominant bacteria and methanogenic archaea and their metabolic traits in a thermophilic anaerobic digester

Thermophilic anaerobic digestion (TAD) represents a promising biotechnology for both methane energy production and waste stream treatment. However, numerous critical microorganisms and their metabolic characteristics involved in this process remain unidentified due to the limitations of culturable isolates. This study investigated the phylogenetic composition and potential metabolic traits of bacteria and methanogenic archaea in a TAD system using culture-independent metagenomics. Predominant microorganisms identified in the stable phase of TAD included hydrogenotrophic methanogens (Methanothermobacter and Methanosarcina) and hydrogen-producing bacteria (Coprothermobacter, Acetomicrobium, and Defluviitoga). Nine major metagenome-assembled genomes (MAGs) associated with the dominant genera were selected to infer their metabolic potentials. Genes related to thermal resistance were widely found in all nine major MAGs, such as the molecular chaperone genes, Clp protease gene, and RNA polymerase genes, which may contribute to their predominance under thermophilic condition. Thermophilic temperatures may increase the hydrogen partial pressure of Coprothermobacter, Acetomicrobium, and Defluviitoga, subsequently altering the primary methanogenesis pathway from acetoclastic pathway to hydrogenotrophic pathway in the TAD. Consequently, genes encoding the hydrogenotrophic methanogenesis pathway were the most abundant in the recovered archaeal MAGs. The potential interaction between hydrogen-producing bacteria and hydrogenotrophic methanogens may play critical roles in TAD processes.

Performance of sewage sludge treatment for the removal of antibiotic resistance genes: Status and prospects

Wastewater treatment plants (WWTPs) receive wastewater containing antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs), which are predominant contributors to environmental pollution in water and soil. Of these sources, sludge is a more significant contributor than effluent. Knowing how sludge treatment affects the fate of ARGs is vital for managing the risk of these genes in both human and natural environments. This review therefore discusses the sources and transmission of ARGs in the environment and highlights the risks of ARGs in sludge. The effects of co-existing constituents (heavy metals, microplastics, etc.) on sludge and ARGs during treatment are collated to highlight the difficulty of treating sludge with complex constituents in ARGs. The effects of various sludge treatment methods on the abundances of ARGs in sludge and in soil from land application of treated sludge are discussed, pointing out that the choice of sludge treatment method should take into account various potential factors, such as soil and soil biology in subsequent land application. This review offers significant insights and explores the abundances of ARGs throughout the process of sludge treatment and disposal. Unintentional addition of antibiotic residues, heavy metals, microplastics and organic matter in sludge could significantly increase the abundance and reduce the removal efficiency of ARGs during treatment, which undoubtedly adds a barrier to the removal of ARGs from sludge treatment. The complexity of the sludge composition and the diversities of ARGs have led to the fact that no effective sludge treatment method has so far been able to completely eliminate the ecological risk of ARGs. In order to reduce risks resulting by transmission of ARGs, technical and management measures need to be implemented.

Acclimatizing waste activated sludge in a thermophilic anaerobic fixed-bed biofilm reactor to maximize biogas production for food waste treatment at high organic loading rates

Thermophilic anaerobic digestion (TAD) provides a promising solution for sustainable high-strength waste treatment due to its enhanced methane-rich biogas recovery. However, high organic loading rates (OLR) exceeding 3.0 kgCOD/m3/day and short hydraulic retention times (HRT) below 10 days pose challenges in waste-to-energy conversion during TAD, stemming from volatile fatty acids (VFAs) accumulation and methanogenesis failure. In this study, we implemented a stepwise strategy for acclimatizing waste activated sludge (WAS) in a thermophilic anaerobic fixed-bed biofilm reactor (TA-FBBR) to optimize methanogen populations, thereby enhancing waste-to-energy efficiencies under elevated OLRs in food waste treatment. Results showed that following stepwise acclimatization, the TA-FBBR achieved stable methane production of approximately 5.8 L/L-reactor/day at an ultrahigh OLR of ∼20 kgCOD/m3/day and ∼15 kgVS/m3/day at 6-day HRT in food waste treatment. The average methane yield reached 0.45 m3/kgCODremoval, attaining the theoretical production in TAD. Moreover, VFA concentrations were stabilized below 1000 mg/L at the ultrahigh OLR under 6-day HRT, while maintaining an acetate/propionate ratio of > 1.8 and a VFA/TAK ratio of < 0.3 serving as effective indicators of system stability and methane yield potential. The microbial community analysis revealed that the WAS acclimatization strategy fostered the microbial diversity and abundance of Methanothermobacter and Methanosarcina. Methanosarcina in the biofilm were observed to be twice as abundant as Methanothermobacter, indicating a potential preference for biofilm existence among methanogens. The findings demonstrated an effective strategy, specifically the stepwise acclimatization of WAS in a thermophilic fixed-bed biofilm reactor, to enhance the food waste treatment performance at high OLRs, contributing valuable mechanistic and technical insights for future sustainable high-strength waste management.

Effects of biochar addition towards the treatment of blackwater in anaerobic dynamic membrane bioreactor (AnDMBR): Comparison among room temperature, mesophilic and thermophilic conditions

Effects of biochar addition on the performance of anaerobic dynamic membrane bioreactor (AnDMBR) under different temperatures for blackwater treatment was investigated. When the organic load ratios (OLR) was 1.0 g COD/L·d, the specific methane yield for the three biochar-amended reactors were 125.7, 148.0 and 182.3 mLCH₄/g COD_removed, respectively. Compare to those digesters without biochar participation, the methane production in the thermophilic reactor with biochar increased by 12% while the other two digesters increased by 6-10%. An analysis of membrane filtration resistance showed a reduction in total resistance (R_t) of 6.2 × 10¹¹-7.3 × 10¹¹ m^-1 when biochar was added to the three reactors. The thermophilic reactors with biochar increased the relative abundance of Methanothermobacter and promoted gene expression of metabolic pathways related to hydrolysis, acid production and methane production. Overall, biochar showed great potential as an inexpensive conductive material to increase methane production with reduced membrane fouling in AnDMBR systems.

Effect of Temperature on Anaerobic Fermentation of Poplar Ethanol Wastewater: Performance and Microbial Communities

Temperature plays an important role in anaerobic digestion (AD), and different substrates have different optimum temperatures in AD. However, the effect of temperature on the performance of AD when cellulosic ethanol wastewater was used as a substrate was rarely reported. Therefore, the digestion characteristics of cellulosic ethanol wastewater at 25, 35, 45, and 55 °C were investigated, and the microbial communities of the sludge sample were analyzed after fermentation. The results showed that the cumulative methane production was the highest at 55 °C, 906.40 ± 50.67 mL/g VS, which was 81.06, 72.42, and 13.33% higher than that at 25, 35, and 45 °C, respectively. The content of methane was 68.13, 49.26, 70.46, and 85.84% at the terminal period of fermentation at temperatures of 25, 35, 45, and 55 °C, respectively. The testing of volatile fatty acids (VFAs) indicated that the accumulation of VFAs did not occur when the fermentation was carried out at 25, 35, and 45 °C; however, the VFA content at 55 °C was much larger than that in the three groups (25, 35, and 45 °C), and the ratio of propionic acid to acetic acid was larger than 1.4 at the late stage of fermentation, so it inhibited the fermentation. The diversity of the microbial community indicated that the floral structure and metabolic pathway of fermentation were alike at 25 and 35 °C. Firmicutes and Proteobacteria were the main flora covering the 25-55 °C-based phylum or below it. The relative abundance of Methanosaeta was the highest when fermentation temperatures were 25 and 35 °C; however, its relative abundance decreased sharply and the relative abundance of Methanosarcina increased substantially when the temperature increased from 35 to 45 °C, which indicated that Methanosarcina can exist in higher temperatures. At the same time, hydrogenotrophic methanogens such as Methanoculleus and Methanothermobacter were dominant when fermentation temperatures were 45 and 55 °C, which indicated that the metabolic pathway changed from acetoclastic methanogenesis to hydrogenotrophic methanogenesis.

Effects of antibiotics on anaerobic digestion of sewage sludge: Performance of anaerobic digestion and structure of the microbial community

As a common biological engineering technology, anaerobic digestion can stabilize sewage sludge and convert the carbon compounds into renewable energy (i.e., methane). However, anaerobic digestion of sewage sludge is severely affected by antibiotics. This review summarizes the effects of different antibiotics on anaerobic digestion of sewage sludge, including production of methane and volatile fatty acids (VFAs), and discusses the impact of antibiotics on biotransformation processes (solubilization, hydrolysis, acidification, acetogenesis and methanogenesis). Moreover, the effects of different antibiotics on microbial community structure (bacteria and archaea) were determined. Most of the research results showed that antibiotics at environmentally relevant concentrations can reduce biogas production mainly by inhibiting methanogenic processes, that is, methanogenic archaea activity, while a few antibiotics can improve biogas production. Moreover, the combination of multiple environmental concentrations of antibiotics inhibited the efficiency of methane production from sludge anaerobic digestion. In addition, some lab-scale pretreatment methods (e.g., ozone, ultrasonic combined ozone, zero-valent iron, Fe³⁺ and magnetite) can promote the performance of anaerobic digestion of sewage sludge inhibited by antibiotics.

Effect of sulfamethazine on anaerobic digestion of manure mediated by biochar

Antibiotic contamination from animal production and wastewater treatment process will release antibiotic resistant genes to the environment and potentially threaten human health. Meanwhile, the residual antibiotic in manure could have inactive impacts on anaerobic digestion (AD). This study explores the effect of sulfamethazine on manure AD mediated by biochar. The results show that biochar weakens the adverse effects of sulfamethazine on AD by adsorption sulfamethazine during the initial stage (0-3 days) of AD and promoting the growth of hydrolytic bacteria (especially Firmicutes and Bacteroidetes) and methanogens (especially Methanothrix and Methanosarcina). Besides, the presence of biochar improves the biogas production capacity of AD and promotes microbial diversity and community richness. Thus, the addition of biochar greatly reduces sulfamethazine and is testified to be a desirable strategy to mitigate the inhibition of sulfamethazine on AD.

Nonlinear Optimization-Based Robust Control Approach for a Two-Stage Anaerobic Digestion Process

A two-stage anaerobic digestion (AD) process has been applied to improve the efficiency of methane production from various organic materials. However, the performance of traditional process controllers may be limited by differences in the rate of biochemical reactions, process uncertainties, and the consequences of interconnection between the two bioreactors. In this work, a nonlinear optimization-based control strategy that applies an analytical model predictive control (AMPC) scheme with an adaptive optimal set-point is proposed for the control of the two-stage AD system. The objectives of the proposed control system are to stabilize the system under uncertain operating conditions and maximize biomethane production. The optimal set-points for the controller are adapted in real-time operation, and then the control system is performed to manipulate the controlled output to the optimal trajectories. Compensators and nonlinear state observers are applied to handle the process/model mismatch and estimate unmeasured variables. The proposed control system is applied to the process with disturbances, fluctuations of inlet stream concentrations, and changes in the bacterial growth rate, and the control performance is investigated. Simulation results show that the developed control scheme automatically adjusts the optimal set-points and provides adequate control actions to maintain the maximum rate of methane production. The results of this investigation demonstrate that the control strategy promotes different biochemical reactions, avoids the inhibition effect, and handles the mutual effects between acidogenic and methanogenic bioreactors for methane production effectively.

Study of the Behavior of Alkalinities Predicted by the AM2 Model

Anaerobic digestion (AD) is an efficient wastewater bioprocess, suitable for treating agroindustrial residues with high organic loads and characterized by both a low environmental impact and energy generation. This process is conformed by several chemical and biological reactions in an oxygen free atmosphere, that degrades high molecular weight organic compounds into carbon dioxide and methane mainly but also into traces of hydrogen and ammonia. This process is potentially unstable to volatile fatty acids (VFA), and the alkalinity. variations and is satisfactorily described by the non-linear AM2 model. In this contribution, the AM2 model is modified to include a more general expression for the pH, a cheap and continuous measurement, and also to add more detail in the interactions of the VFA, bicarbonates, and the alkalinity, key factors in the process stability. The stability of the AM2 modified model is explored through a rigorous bifurcation analysis that identifies unstable operation zones and viability of operation trajectories as a function of the dilution rate. Finally, an experimental validation is carried out to show the feasibility and accuracy of the proposed modifications.

Effects of Stepwise Temperature Shifts in Anaerobic Digestion for Treating Municipal Wastewater Sludge: A Genomic Study

In wastewater treatment plants (WWTP), anaerobic digester (AD) units are commonly operated under mesophilic and thermophilic conditions. In some cases, during the dry season, maintaining a stable temperature in the digester requires additional power to operate a conditioning system. Without proper conditioning systems, methanogens are vulnerable to temperature shifts. This study investigated the effects of temperature shifts on CH₄ gas production and microbial diversity during anaerobic digestion of anaerobic sewage sludge using a metagenomic approach. The research was conducted in lab-scale AD under stepwise upshifted temperature from 42 to 48 °C. The results showed that significant methanogen population reduction during the temperature shift affected the CH₄ production. With 70 days of incubation each, CH₄ production decreased from 4.55 L·g⁻¹-chemical oxygen demand (COD) at 42 °C with methanogen/total population (M·TP⁻¹) ratio of 0.041 to 1.52 L·g⁻¹ COD (M·TP⁻¹ ratio 0.027) and then to 0.94 L·g⁻¹ COD ( M·TP⁻¹ ratio 0.026) after the temperature was shifted to 45 °C and 48 °C, respectively. Methanosaeta was the most prevalent methanogen during the thermal change. This finding suggests that the Methanosaeta genus was a thermotolerant archaea. Anaerobaculum, Fervidobacterium, and Tepidanaerobacter were bacterial genera and grew well in shifted-up temperatures, implying heat-resistant characteristics.

Unveiling the characterization and development of prokaryotic community during the start-up and long-term operation of a pilot-scale anaerobic membrane bioreactor for the treatment of real municipal wastewater

The anaerobic membrane bioreactor (AnMBR) is a promising sustainable process and technology for the treatment of municipal wastewater from the perspective of carbon neutrality. In this study, a large pilot-scale AnMBR was constructed and the microbial community development of the anaerobic digested sludge in the AnMBR was determined during the treatment of municipal wastewater. The AnMBR system was conducted for 217 days during a long-term operation with the feed of real municipal wastewater. The characterization and dynamics of the microorganisms revealed that a stable prokaryotic community was gradually achieved. In the community of methane-producing archaea (or methanogens), the acetotrophic methanogen Methanosaeta was significantly enriched at an ambient temperature of 25 °C with an overwhelming relative abundance in the entire community. The abundance of Methanosaeta was even higher than the most abundant bacterial phyla Chloroflexi, Firmicutes, Proteobacteria and Bacteroidetes. This phenomenon is quite different from that found in other typical anaerobic systems. The massive enrichment of methanogens is the key to maintaining stable methane production in the treatment of municipal wastewater by the AnMBR. The interspecies cooperation of major functional bacterial groups including protein/carbohydrate/cellulose-degrading (genera Anaerovorax, Aminomonas, Levilinea, Flexilinea and Ruminococcus etc.), sulfate-reducing (Desulfovibrio and Desulfomicrobium etc.) and syntrophic (Syntrophorhabdus and Syntrophus etc.) bacteria with acetotrophic and hydrogenotrophic archaea enhances the stability of reactor operation and help to acclimate the entire prokaryotic community to the characteristics of real municipal wastewater.

Key microbial clusters and environmental factors affecting the removal of antibiotics in an engineered anaerobic digestion system

To identify the key microbial clusters and influencing factors involved in antibiotic removal from engineered anaerobic digestion (AD) systems, the dynamic characteristics of antibiotics, physiochemical factors, microbial communities and functional genes were investigated by 16S rRNA and metagenome sequencing. The results showed that antibiotic removal occurred mainly in the first 21 days, and sulfonamides had the highest removal rate. The key microbial clusters related to the biodegradation of antibiotics consisted mainly of Firmicutes and Bacteroidetes. The key enzymes consisted of deaminases, peptidases, C-N ligases, decarboxylases and alkyl-aryl transferases. Structural equation modelling indicated that low concentrations of propionic acid promoted the biodegradation activities of key microbial clusters in the first 21 days, but their activities were inhibited by the accumulated propionic acid after 21 days. Thus, propionic acid should be regulated in engineered AD systems to prevent the adverse effect of acid inhibition on antibiotic-degrading bacteria.

Calcium hypochlorite pretreatment improves thermophilic digestion of waste activated sludge in an upflow anaerobic sludge blanket reactor

Anaerobic wasted activated sludge (WAS) digestion has been widely applied to reduce sludge volume and generate bioenergy in the form of methane. However, anaerobic WAS digestion performance is often challenged with poor hydrolysis of biomass cellular structures. In the present study, the feasibility of using calcium hypochlorite (Ca(ClO)₂) to improve the thermophilic digestion of WAS was studied. Two thermophilic upflow anaerobic sludge bed (UASB) reactors (one with and one without Ca(ClO)₂ pretreatment) were operated for 120 days under low and high organic loading rate (OLR) conditions, corresponding hydraulic retention time (HRT) of 10 days and 6 days, respectively. Both reactors achieved satisfied performance during the studied period. Under the low OLR condition, Ca(ClO)₂ pretreatment significantly improved WAS total volatile solids (VS) removal efficiency (from 48.06 ± 2.63% to 57.34 ± 3.54%) and methane yield (from 289.2 ± 27.6 to 362.2 ± 36.7 N mL/g VS). However, no significant improvement was observed under the high OLR condition. g_S1 and g_Fervidobacterium were predominant bacteria in the thermophilic UASB reactor fed with Ca(ClO)₂ pretreated WAS. Methanosarcina was dominant archaea in both reactors. The treatment mechanism and application potential of using Ca(ClO)₂ to enhance the WAS digestibility were further discussed.

Operational and biochemical aspects of co-digestion (co-AD) from sugarcane vinasse, filter cake, and deacetylation liquor

This work performed co-AD from the vinasse and filter cake (from 1G ethanol production) and deacetylation liquor (from the pretreatment of sugarcane straw for 2G ethanol production) in a semi-Continuous Stirred Tank Reactor (s-CSTR) aiming to provide optimum operational parameters for continuous CH₄ production. Using filter cake as co-substrate may allow the reactor to operate throughout the year, as it is available in the sugarcane off-season, unlike vinasse. A comparison was made from the microbial community of the seed sludge and the reactor sludge when CH₄ production stabilized. Lactate, butyrate, and propionate fermentation routes were denoted at the start-up of the s-CSTR, characterizing the acidogenic phase: the oxidation-reduction potential (ORP) values ranged from -800 to -100 mV. Once the methanogenesis was initiated, alkalizing addition was no longer needed as its demand by the microorganisms was supplied by the alkali characteristics of the deacetylation liquor. The gradual increase of the applied organic load rates (OLR) allowed stabilization of the methanogenesis from 3.20 g_VS L^-1 day^-1: the highest CH₄ yield (230 mL_NCH₄ g^-1_VS) and average organic matter removal efficiency (83% ± 13) was achieved at ORL of 4.16 g_VS L^-1 day^-1. The microbial community changed along with the reactor operation, presenting different metabolic routes mainly due to the used lignocellulosic substrates. Bacteria from the syntrophic acetate oxidation (SAO) process coupled to hydrogenotrophic methanogenesis were predominant (~ 90% Methanoculleus) during the CH₄ production stability. The overall results are useful as preliminary drivers in terms of visualizing the co-AD process in a sugarcane biorefinery integrated to scale. KEY POINTS: • Integration of 1G2G sugarcane ethanol biorefinery from co-digestion of its residues. • Biogas production from vinasse, filter cake, and deacetylation liquor in a semi-CSTR. • Lignocellulosic substrates affected the biochemical routes and microbial community. • Biomol confirmed the establishment of the thermophilic community from mesophilic sludge.

Enrichment of thermophilic methanogenic microflora from mesophilic waste activated sludge for anaerobic digestion of garbage slurry

This study investigated a startup strategy for thermophilic methanogenic enrichment. Conventional waste activated sludge (WAS) was used as the seed. The WAS seed was incubated at 55 °C in a continuous-flow stirred tank reactor, with garbage slurry fed continuously as a substrate. One of the two reactors (termed reactor-high, RH) was fed with a high concentration of substrate (30 g-COD/L), while the other (reactor-low, RL) received a lower concentration of feed (15 g-COD/L). The specific organic loading rate was 0.2 g-COD/L/day initially, which was gradually increased by shortening the hydraulic retention time. The final OLR was 3.2 g-COD/L/day, after more than 90% of the initial WAS got washed out from the reactor and thermophilic microorganisms became dominant in the reactors. Biogas production rate and methane conversion ratio depended on substrate concentration, although total chemical oxygen demand removal and methane content were almost the same in RH and RL. Biogas production rate in RH was 3.2 times higher than that in RL, while the conversion ratio of RH was 1.6 times higher than that of RL. Quantitative polymerase chain reaction analysis using specific primers for the mcrA gene and high-throughput sequencing analysis of 16S rRNA gene amplicons demonstrated post enrichment differences in the microbial community, relative to that in the WAS. There was no significant difference in the enriched microbial community composition between RH and RL. In conclusion, thermophilic methanogenic microflora can be enriched from mesophilic seeds. Methanothermobacter, Methanosarcina, and other thermophilic bacteria were enriched in the community over time, with these thermophiles collectively accounting for ∼80% of the stable thermophilic community.

A mechanistic model of methane emission from animal slurry with a focus on microbial groups

Liquid manure (slurry) from livestock releases methane (CH4) that contributes significantly to global warming. Existing models for slurry CH4 production-used for mitigation and inventories-include effects of organic matter loading, temperature, and retention time but cannot predict important effects of management, or adequately capture essential temperature-driven dynamics. Here we present a new model that includes multiple methanogenic groups whose relative abundance shifts in response to changes in temperature or other environmental conditions. By default, the temperature responses of five groups correspond to those of four methanogenic species and one uncultured methanogen, although any number of groups could be defined. We argue that this simple mechanistic approach is able to describe both short- and long-term responses to temperature where other existing approaches fall short. The model is available in the open-source R package ABM (https://github.com/sashahafner/ABM) as a single flexible function that can include effects of slurry management (e.g., removal frequency and treatment methods) and changes in environmental conditions over time. Model simulations suggest that the reduction of CH4 emission by frequent emptying of slurry pits is due to washout of active methanogens. Application of the model to represent a full-scale slurry storage tank showed it can reproduce important trends, including a delayed response to temperature changes. However, the magnitude of predicted emission is uncertain, primarily as a result of sensitivity to the hydrolysis rate constant, due to a wide range in reported values. Results indicated that with additional work-particularly on the magnitude of hydrolysis rate-the model could be a tool for estimation of CH4 emissions for inventories.

Thermophilic rather than mesophilic sludge anaerobic digesters possess lower antibiotic resistant genes abundance

For exploring the impact of temperature on antibiotic resistant genes (ARGs) during sludge anaerobic digestion (AD), the dynamic variations of sludge ARGs, plasmid ARGs, and cell-free ARGs in mesophilic (25 °C and 35 °C) and thermophilic (55 °C) digesters were investigated. The study revealed that the abundance of sludge ARGs and plasmid ARGs in thermophilic sludge AD was significantly lower than that in mesophilic digesters, while the cell-free ARGs abundance of the thermophilic digesters was similar to mesophilic digesters. Higher archaea abundance, lower bacteria abundance, and different microbial community were found in thermophilic digesters compared to that of mesophilic ones. Firmicutes might be a main group of potential hosts of ARGs in sludge AD. The distinct microbial community was the main contributor to the low ARGs abundance in thermophilic sludge AD. Thermophilic operation at 55 °C rather than mesophilic operation is more conducive to control ARGs in sludge anaerobic digestion.

Rhamnolipid pretreatment enhances methane production from two-phase anaerobic digestion of waste activated sludge

In this work, a rhamnolipid (RL) pretreatment technology was proposed to promote methane production from two-phase anaerobic digestion of waste activated sludge. In the first phase (i.e., acidogenic phase), the WAS hydrolysis and acidogenesis were significantly enhanced after RL pretreatment for 4 day, under which the concentration of soluble protein and the short-chain fatty acids (SCFA) in the presence of RL at 0.04 g/g TSS was respectively 2.50 and 5.02 times higher than that without RL pretreatment. However, methane production was inhibited in the presence of RL. In the second phase (i.e., methanogenic phase), batch biochemical methane potential tests suggested that the addition of RL is effective in promoting anaerobic methane production. With an increase of RL dosage from 0 to 0.04 g/g TSS, the cumulative methane yield increased from 100.42 ± 3.01 to 168.90 ± 5.42 mL. Although the added RL could be utilized to produce methane, it was not the major contributor to the enhancement of methane yield. Further analysis revealed that total cumulative yield from the entire two-phase anaerobic digestion (sum of the yield of the acidogenic phase and methanogenic phase) increased from 113.42 ± 3.56 to 164.18 ± 5.20 mL when RL dosage increased from 0 to 0.03 g/g TSS, indicating that the addition of RL induced positive effect on the methane production of the entire two-phase anaerobic digestion. The enzyme activity analysis showed that although higher dosages of RL still inhibited the microorganisms related to methanogenesis to some extends in the methanogenic phase, the inhibitory effect was significantly weakened compared to the acidogenic phase. Microbial analysis revealed that RL reduced the abundance of Candidatus_Methanofastidiosum sp. while increased the abundance of Methanosaeta sp., causing the major methanogenesis pathway to change from hydrogenotrophic to aceticlastic. Moreover, the community of hydrolytic microbes and acidogens was shifted in the direction that is conducive to hydrolysis-acidogenesis. The findings reported not only expand the application field of RL, but also may provide supports for sustainable operation of wastewater treatment plants (WWTPs).

Prediction of the Long-Term Effect of Iron on Methane Yield in an Anaerobic Membrane Bioreactor Using Bayesian Network Meta-Analysis

A method for predicting the long-term effects of ferric on methane production was developed in an anaerobic membrane bioreactor treating food processing wastewater to provide management tools for maximizing methane recovery using ferric based on a batch test. The results demonstrated the accuracy of the predictions for both batch and long-term continuous operations using a Bayesian network meta-analysis based on the Gompertz model. The prediction bias of methane production for batch and continuous operations was minimized, from 11~19% to less than 0.5%. A biochemical methane potential-based Bayesian network meta-analysis suggested a maximum 2.55% ± 0.42% enhancement for Fe2.25. An anaerobic membrane bioreactor improved the methane yield by 2.27% and loading rate by 4.57% for Fe2.25, operating in the sequenced batch mode. The method allowed for a predictable methane yield enhancement based on the biochemical methane potential. Ferric enhanced the biochemical methane potential in batch tests and the methane yield in a continuously operated reactor by a maximum of 8.20% and 7.61% for Fe2.25, respectively. Copper demonstrated a higher methane (18.91%) and sludge yield (17.22%) in batch but faded in the continuous operation (0.32% of methane yield). The enhancement was primarily due to changing the kinetic patterns for the last period, i.e., increasing the second methane production peak (k₇₁), bringing forward the second peak (λ₇, λ₈), and prolonging the second period (k₆₂). The dual exponential function demonstrated a better fit in the last three stages (after the first peak), which implied that syntrophic methanogenesis with a ferric shuttle played a primary role in the last three methane production periods, in which long-term effects were sustained, as the Bayesian network meta-analysis predicted.

One-step acquirement of superior microbial communities from mesophilic digested sludge to upgrade anaerobic digestion

Anaerobic digestion is a promising waste-to-energy alternative technology. However, the efficiency upgrading for conventional mesophilic digestion of organic solid waste is always indispensable. Employing hyperthermophilic or thermophilic microbial community is one of the viable upgrading alternatives. Given the unavailability of the superior microbial communities, mesophilic digested sludge was used as inoculum, and instantly controlled at 70 °C and 55 °C for acclimation of hyperthermophilic and thermophilic inocula, respectively. Waste activated sludge was continuously and synchronously fed into two digesters. After one round, thermophilic digester achieved stable biogas production rate at 0.22 L L^-1 d^-1, with a methane proportion over 60%, whereas fluctuation was observed in the hyperthermophilic digester, and approximately triple time was needed to reach a relatively stable biogas production rate 0.12 L L^-1 d^-1. Nevertheless, higher hydrolysis ratio 24.4% was observed in the hyperthermophilic digester despite the lower biogas production. Therefore, methanogenesis step limited the whole anaerobic process for the hyperthermophilic digestion, and digestion at 70 °C was appropriate as a pre-fermentation stage to enhanced hydrolysis. The genus Methanothrix proportion in the thermophilic digester gradually decreased, while another acetoclastic genus Methanosarcina ultimately was acclimated to the dominant methanogen. In addition to Methanothrix, hydrogenotrophic archaea became competitive in the hyperthermophilic digester, with Methanothermobacter dominant at 22.6%. The genus Psychrobacter, affiliated to the phylum Proteobacteria could survive better than the others at 70 °C, with a final proportion of 62.5%.

Determining the composition of bacterial community and relative abundance of specific antibiotics resistance genes via thermophilic anaerobic digestion of sewage sludge

In this study, the effects of different temperature transitions on the dynamics of antibiotic resistance genes (ARGs) and bacterial community were investigated during start-up of thermophilic anaerobic digestion (AD) of sewage sludge. Although two thermophilic reactors showed dissimilar removal efficiencies of ARGs in batch mode, both the removal efficiency and reduction patterns of ARGs were similar in continuous stirred tank reactor (CSTR) mode, resulting in significant reduction of the total sum of the relative abundance of ARGs. Using network analysis to explore the correlation between bacterial community and some specific ARGs revealed that composition of the bacterial community played a vital role in the fluctuations in the relative abundance of the antibiotic resistome, demonstrating that shaping the development of ARGs was facilitated by vertical gene transfer. To facilitate eliminating ARGs, minimizing their hosts which persist even under long-term operations is vital in thermophilic AD.

Fate and driving factors of antibiotic resistance genes in an integrated swine wastewater treatment system: From wastewater to soil

Swine wastewater (SW) represents an important source of antibiotic resistance genes (ARGs) in the environment. However, few studies have assessed the occurrence and removal of ARGs in the whole wastewater treatment process followed by its farmland application. This study investigated the ARGs profiles in an integrated SW treatment system and its receiving soil, as well as their relationships with SW parameters and bacterial communities. Results revealed that sulfonamide, tetracycline and aminoglycoside resistance genes were dominant in SW. The relative abundance of total ARGs in SW was reduced by 84% after the treatments. Among the SW treatment units, anaerobic digestion, primary sedimentation and constructed wetland contributed to ARGs removal while secondary sedimentation increased the total ARGs abundance. Farmland irrigation of the treated SW resulted in enrichment of persistent ARGs in the receiving soil, which might be attributed to the propagation of potential bacterial hosts and high horizontal gene transferability. Redundancy analysis indicated that the relative abundance of total ARGs was significantly correlated with total nitrogen, total phosphorus, antibiotics and bacterial communities. The shift in bacterial community was the major driving factor for ARGs alteration during SW treatment process. Our results highlight the effect of treated SW irrigation on the antibiotic resistome in agricultural environment, and can contribute in improving SW treatment system for better antibiotic resistance control.

Dissecting methanogenesis for temperature-phased anaerobic digestion: Impact of temperature on community structure, correlation, and fate of methanogens

This study investigated the relationship between the temperature (35, 42, and 55 °C) used in temperature-phased anaerobic digestion (TPAD) and fate of methanogens between the two anaerobic digestion (AD) phases. Methanogens were profiled by using next generation sequencing (NGS) and droplet digital PCR approaches. The results showed that optimal combined temperatures for methane production were 55 °C during biological hydrolysis (BH) and 35 or 42 °C during AD. BH exhibited much lower archaeal population and was more susceptible to changes in temperature, compared to the AD phase. Additionally, we demonstrated, for the first time, that the BH step could affect the subsequent AD phase by altering AD methanogen composition and improve the stability of the process by enriching the rapidly growing Methanosarcina in the BH-AD process. These results are significant for understanding the mechanisms and stability of methane production in TPAD systems.

Continuous thermophilic hydrogen production from an enzymatic hydrolysate of agave bagasse: Inoculum origin, homoacetogenesis and microbial community analysis

In this research, the performance of two thermophilic inocula of different origin on continuous hydrogen production from an enzymatic hydrolysate of agave bagasse were compared; one of them was obtained from a thermophilic reactor and the second one was taken from a mesophilic reactor and acclimated to thermophilic conditions. The acclimation process in one-step quickly established a high-performance hydrogen producing community, obtaining a volumetric hydrogen production rate of 3811 ± 19 mL H2/L-d with an hydrogen yield of 121 L H2/kg bagasse compared to 1473 ± 6 mL H2/L-d and 26.6 L H2/kg obtained with the thermophilic-origin inoculum. The differences in the performance of both inocula were closely linked to the profile of volatile fatty acids produced, the homoacetogenic pathway and the microbial community, the latter being the determining factor. The use of mesophilic-origin inoculum acclimated to thermophilic conditions can significantly improve the hydrogen production from lignocellulosic bagasse.

Anaerobic treatment of glutamate-rich wastewater in a continuous UASB reactor: Effect of hydraulic retention time and methanogenic degradation pathway

To investigate the anaerobic treatment efficiency and degradation pathways of glutamate-rich wastewater under various hydraulic retention times (HRTs), a lab-scale upflow anaerobic sludge blanket (UASB) reactor was operated continuously for 180 days. Results showed that high chemical oxygen demand (COD) removal efficiencies of 95.5%-96.5% were achieved at HRTs of 4.5 h-6 h with a maximum methane yield of 0.31 L-CH₄/g-COD. When the HRT was shortened to less than 3 h, the removal performance of the reactor declined. There also was an excessive accumulation of volatile fatty acids, which implies that an appropriately small HRT is applicable to the UASB reactor treating glutamate-rich wastewater. Methanogenic degradation of glutamate in the UASB reactor depended on the HRT applied, and the typical methane-producing capability of the sludge at an HRT of 3 h, in descending order, was acetate > glutamate > butyrate > H₂/CO₂ > valerate > propionate. Clostridium and Methanosaeta were predominant in the glutamate-degrading sludge. At least three degradation pathways most likely existed in the UASB reactor, and the pathway via 3-methlaspartate by Clostridium pascui was expected to be dominant.

Temperature regulations impose positive influence on the biomethane potential versus digesting modes treating agricultural residues

Temperature regulations (mesophilic/thermophilic) and digesting modes (mono-/co-digestion) play key roles in the biomethane potential of anaerobic digestion, but limited research focus on the synergetic effects on microbial interconnections of the biomethane process. In this study, the pineapple and maize residues under different operations were monitored by batch biogas assays and 16S high-throughput sequencing to explore: 1) biomethane potential regarding different operations, 2) microbial communities in different treated reactors, and 3) significant factors determine microbial distribution. Results showed that the co-digestion had higher methanogenic abundance and biomethane production (~3300 mL_n) versus mono-digestion under mesophilic condition. To the thermophilic condition, the co-digestion had less methanogenic abundance but more biomethane production (~5000 mL_n). Statistical evidence uncovered that the Clostridiaceae and Thermoanaerobacteraceae dominated pathways linked closely with methanogenesis which may contribute the more biomethane production in the thermophilic condition. This study demonstrated the temperature regulations drove rare taxa as major contributors for biomethane production.

Performance and microbial community variations of a upflow anaerobic sludge blanket (UASB) reactor for treating monosodium glutamate wastewater: Effects of organic loading rate

To investigate the effect of the organic loading rate (OLR) on anaerobic treatment of monosodium glutamate (MSG) wastewater, a lab-scale up-flow anaerobic blanket (UASB) reactor was continuously operated over a 222-day period. The overall performances of COD removal and methane recovery initially exhibited an increase and subsequently decreased when the OLR was increased from 1 g-COD/L/d to 24 g-COD/L/d. At the optimal OLR of 8 g-COD/L/d, superior performance was obtained with a maximum COD removal efficiency of 97%, a methane production rate of 2.3 L/L/d, and specific methanogenic activity of 86 mg-CH₄/g-VSS/d (feeding on glutamate), with superior characteristics of sludge in VSS concentration, average diameter of granules, and settling velocity. According to the results of the specific methanogenic activity, the methanogenic pathway was more inclined to pass through acetate than through hydrogen. Methanosarcina instead of Methanosaeta, with Methanobacterium and greatly increased Firmicutes, dominated in the UASB reactor after long term operation. These results support that the OLR had a substantial effect on both the treatment and energy recovery efficiency of MSG wastewater as well as on microbial community variations in the UASB reactor.

Evaluation of an anaerobic baffled reactor for pretreating black water: Potential application in rural China

Black water is highly concentrated human waste water but represents only a minor portion of domestic sewage. A modified type of anaerobic baffled reactor (ABR) was studied to assess its potential for pretreating black water in rural China. The classification of microbial structure was also investigated to confirm its potential in application. The structure of the ABR was modified according to demand for application in practice. A hydraulic retention time (HRT) of 48 h was chosen as the optimal HRT after comparison among 24 h, 36 h, 48 h, and 72 h. Under the 48 h HRT, the ABR achieved average removal efficiencies of 94.05% of chemical oxygen demand (COD), 28.78% of total nitrogen (TN), 14.21% of ammonium nitrogen (NH₄⁺-), and 32.54% of total phosphorus (TP) during 112 days of continuous operation. Samples from three different compartments were collected after 60-day continuous operation for bacterial and archaeal community investigation by 16S rRNA. Abundant degradation-related bacteria and methanogenic archaea were found in the ABR. The three samples had similar bacterial compositions at phylum, class, and genus levels, but the percentages of bacteria differed among the compartments. The distribution of archaea showed succession with the flow direction. In general, the ABR shows good performance under an HRT of 48 h and shows good potential for practical application.

Microbial consortia including methanotrophs: some benefits of living together

With the progress of biotechnological research and improvements made in bioprocessing with pure cultures, microbial consortia have gained recognition for accomplishing biological processes with improved effectiveness. Microbes are indispensable tool in developing bioprocesses for the production of bioenergy and biochemicals while utilizing renewable resources due to technical, economic and environmental advantages. They communicate with specific cohorts in close proximity to promote metabolic cooperation. Use of positive microbial associations has been recognized widely, especially in food industries and bioremediation of toxic compounds and waste materials. Role of microbial associations in developing sustainable energy sources and substitutes for conventional fuels is highly promising with many commercial prospects. Detoxification of chemical contaminants sourced from domestic, agricultural and industrial wastes has also been achieved through microbial catalysis in pure and co-culture systems. Methanotrophs, the sole biological sink of greenhouse gas methane, catalyze the methane monooxygenasemediated oxidation of methane to methanol, a high energy density liquid and key platform chemical to produce commodity chemical compounds and their derivatives. Constructed microbial consortia have positive effects, such as improved biomass, biocatalytic potential, stability etc. In a methanotroph-heterotroph consortium, non-methanotrophs provide key nutrient factors and alleviate the toxicity from the culture. Non-methanotrophic organisms biologically stimulate the growth and activity of methanotrophs via production of growth stimulators. However, methanotrophs in association with co-cultured microorganisms are in need of further exploration and thorough investigation to study their interaction mode and application with improved effectiveness.

Thermophilic anaerobic digestion: Effect of start-up strategies on performance and microbial community

Effects of two different start-up methods were compared during conversion from mesophilic to thermophilic anaerobic digestion of sewage sludge. During the batch operation, a transient increase in both total bacterial concentration and relative abundance of thermophilic bacteria in R1 (a one-step increase method) resulted in 34% higher volatile solids (VS) removal efficiency by R1 compared to R2 (a step-wise increase method). Meanwhile, higher total archaeal concentration and increased relative abundance of thermophilic archaea in R2 were attributed to 65% higher methane production by R2 compared to R1. The same trends for VS removal and methane production were observed during the subsequent continuous mode, although the microbial composition of the two reactors became similar. These findings may prove helpful for determining the preferred start-up method for thermophilic anaerobic digestion: a one-step method can be proposed for higher VS removal efficiency, or a step-wise method can be selected for enhanced methane production.

Microbial community composition and methanogens' biodiversity during a temperature shift in a methane fermentation chamber

More information on the connection between anaerobic digestion (AD) parameters and composition of the microbial community involved in the AD process is required to gain a better understanding of how a bioreactor functions. The aim of this study was to analyse the composition of microbial communities and the dynamics of methanogens' biodiversity changes during the shift from mesophilic (38°C) to thermophilic (55°C) conditions during biogas production. The total microbial composition was examined via the metagenomic approach based on 16S rRNA gene sequencing, whereas the methanogen communities were analysed using PCR-DGGE (Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis) of mcrA. Even though the temperature is one of the crucial parameters affecting microorganisms involved in the AD process, the results presented here revealed that there were no statistically significant differences in bacterial community composition between the mesophilic and thermophilic phases of the process. The most abundant phyla were found to be Proteobacteria, Firmicutes, Actinobacteria and Bacteroidetes. However, the methanogens' community genotypic structure as examined by the PCR-DGGE method changed under thermophilic conditions. The temperature had the strongest impact on the archaeal methanogens in the fermentation chamber directly after implementing the temperature shift. A relatively higher biogas yield and average content of CH₄ in the produced biogas were observed under thermophilic conditions.

Synergistic effects of key parameters on the fate of antibiotic resistance genes during swine manure composting

Livestock manure is a reservoir of antibiotic resistance genes (ARGs), posing a potential risk to environment and human health. However, there has been no optimization study about the comprehensive composting treatment for livestock manure ARGs based on multiple operation factors. In this study, anaerobic composting of swine manure in light was conducted under different combined conditions of composting time, temperature, water content, pH, heavy metal passivators and wheat straw. The diversity and relative abundance of ARGs in the compost were detected using high throughput quantitative real-time PCR, and the concentrations of antibiotics and heavy metals were determined. The results showed that under the optimized conditions (composting time, 30 d; temperature, 50 °C; water content, 50%; pH 9.0; heavy metal passivators and wheat straw), compared with the control, the detected number of ARGs and mobile genetic elements in the compost was reduced by 45% and 27.3%, and their relative abundance decreased by 33.9% and 36.9%, respectively. Moreover, the exchangeable heavy metal content of the compost declined by 34.7-57.1%, and the antibiotic level decreased by 28.8-77.8%. This study proposes that synergistic effects of key parameters can effectively mitigate the combined contamination of ARGs, antibiotics, and heavy metals in swine manure. MAIN FINDING: Optimized parameters (anaerobic composting time 30 d, temperature 50 °C, water content 50%, pH 9.0) effectively mitigated the combined pollution of ARGs, antibiotics, and heavy metals in swine manure.

Mesophilic and thermophilic anaerobic digestion of soybean curd residue for methane production: Characterizing bacterial and methanogen communities and their correlations with organic loading rate and operating temperature

To find the optimal operation parameters and provide an explanation of methanogenic pathway for methane production in mesophilic (35 °C) and thermophilic (55 °C) anaerobic digestion (MAD, TAD) of soybean curd residue (SCR), MAD and MAD were contrastively investigated for 95 days. The maximum available OLR was identified as 3.3 gVS/L for both MAD and TAD. Compared to MAD, TAD exhibited a 20% higher average methane yield (0.591 L/gVS) and a 7.5% higher volatile solids removal efficiency (74.1 ± 10.4%). Bacterial phyla Bacteroidetes, Firmicutes and Proteobacteria dominated in MAD digesters while genus Defluviitoga was selectively enriched in TAD digesters due to higher temperature and organic loading pressure. Principal coordinates analysis of methanogen community showed that both temperature and OLR were crucial environmental variables shifting the taxonomic patterns of the methanogens. The enriched methanogen genus Methanothermobacter (93%) with a hydrogenotrophic methanogenic pathway had a close correlation with the TAD performance.

In-situ biogas upgrading by a stepwise addition of ash additives: Methanogen adaption and CO2 sequestration

Biogas from anaerobic digestion (AD) of waste activated sludge (WAS) limited its utilization due to low value-added. In this study, an innovative addition mode for ash known as stepwise addition was developed to enhance methane production and improve CO₂ scavenge from AD of sludge. Experimental results confirmed stepwise addition of ash improved methane content to 79.4%, compared to control group (69.1%). Compared to Pulse addition and Control, the cumulative CH₄ production was promoted by 39.2% and 35.4%, respectively. Investigation of the mechanism indicated that stepwise addition of ash could decrease hydrolytic and acidifying enzyme activities but increase activity of coenzyme F420, compared to pulse addition group. Furthermore, stepwise addition of ash not only increased the abundance of Methanomassiliicoccus (34.48%), but also promoted amounts of CO₂ capture. This method ameliorate utilization availability of sludge ash for sludge anaerobic digestion through promoting cumulative methane production and increasing CO₂ storage capacity.

Thermophilic anaerobic digestion reduces ARGs in excess sludge even under high oxytetracycline concentrations

The feasibility of thermophilic anaerobic digestion (AD) for the attenuation of antibiotic resistance genes (ARGs) in biomass wastes under high antibiotic concentrations remains unclear. In this study, a thermophilic completely stirred digester (55 °C) was fed with municipal excess sludge spiked with increasing concentrations of oxytetracycline (OTC) (0-1000 mg/L) over a period of 280 days. Results showed that thermophilic AD could maintain stable methane production (338.40 ± 26.26 mL/d/gVS) even at an OTC dose of 1000 mg/L with the sludge phase OTC concentration reaching around 24,000 mg/kg. More important, the abundance of resistome detected by high-throughput quantitative PCR in the substrate was reduced (p < 0.01) by 55.54%-86.27% by thermophilic AD over the whole period. Partial canonical correspondence and network analyses showed that the reduction of ARGs was achieved mainly through two ways: eliminating the original hosts of ARGs in the substrate (from 41.74% ± 2.60% in the substrate to 12.08% ± 1.02% in digested sludge), and blocking the horizontal proliferation of ARGs in the digested sludge by reducing the abundance of mobile genetic elements and restricting their horizontal exchange within a small number of thermophilic genera. This study showed that thermophilic AD is feasible for the attenuation of ARGs in biomass even containing high level of OTC.

Response of antibiotic resistance genes abundance by graphene oxide during the anaerobic digestion of swine manure with copper pollution

The pollution of various environments with antibiotic resistance genes (ARGs) is an urgent problem that needs to be addressed, especially in heavy metal-polluted environments. This study investigated the responses of ARGs and mobile genetic elements (MGEs) to the addition of graphene oxide (GO) to swine manure containing a high concentration copper during anaerobic digestion. The total copy numbers of ARGs and MGEs were significantly enhanced by the pressure due to Cu. GO significantly decreased the ARG and MGE copy numbers, where the low GO concentration performed better than the high GO concentration. Network analysis showed that most of the ARGs and MGEs co-occurred and they shared the same major potential host bacteria. The contributions of different factors to ARG abundances were assessed by redundancy analysis and MGEs had the most important effect on the fate of ARGs. Thus, GO may reduce the abundance of ARGs mainly by removing MGEs.

Feasibility of dry anaerobic digestion of beer lees for methane production and biochar enhanced performance at mesophilic and thermophilic temperature

This study investigated the feasibility of dry anaerobic digestion using beer lees as substrate and the effect of cow manure-derived biochar addition on dry anaerobic digestion performance at mesophilic and thermophilic temperature, respectively. With TS content of 25%, maximum cumulative methane production and yield were achieved to be 5230 ± 91 mL d^-1 and 220.1 ± 7.7 mL g^-1 VS at mesophilic condition and 7386 ± 134 mL d^-1 and 310.4 ± 9.2 mL g^-1 VS at thermophilic condition in the control cultures. The biochar addition has a positive effect in improving dry anaerobic digestion performance. The maximum cumulative methane production and yield in the cultures with 10 g L^-1 biochar were substantially improved by 82.9% and 82.6% at mesophilic condition and 47.2% and 46.8% at mesophilic condition when compared to the control.

A comprehensive review of anaerobic digestion of organic solid wastes in relation to microbial community and enhancement process

Organic solid wastes (OSWs) should be regarded as valuable resources rather than dead-end landfill waste that causes public health and odor concerns. Anaerobic digestion (AD) is an ideal approach for managing organic solid waste issues and involves using a group of anaerobic microorganisms to transform OSWs into useful products. In this review, over 100 publications related to AD of OSWs have been compiled, discussed, and analyzed. A comprehensive analysis of the environmental and safety impacts of AD, its key environmental factors, co-digestion, and pretreatment, as well as the AD of OSWs by various anaerobic microbes uncovered by high throughput sequencing-based approaches, is presented. The purpose of this review is to provide an outline of the current knowledge of AD processes from a multi-angle perspective. A comprehensive understanding of AD of OSWs and genome-enabled biology development could be helpful for providing up-to-date knowledge of AD, developing it, overcoming its drawbacks and, ultimately, improving global waste control for more efficient environmental management. © 2018 Society of Chemical Industry.

Pretreatment of spiramycin fermentation residue using hyperthermophilic digestion: quick startup and performance

This study aimed to evaluate the feasibility of hyperthermophilic anaerobic digestion at 70 °C in the pretreatment of spiramycin fermentation residue. By feeding municipal excess sludge under a solid retention time of 5 days, the hyperthermophilic digester was successfully started up within 3 days from mesophilic digestion by a one-step temperature increase from 35 to 70 °C. MiSeq sequencing showed the fast establishment of thermophilic fermenting bacterial communities in 3 days immediately after the temperature increase, with increases in abundance of Coprothermobacter, Spirochaetaceae_uncultured and Fervidobacterium from <0.001%, 1.06% and <0.001% to 33.77%, 11.65% and 3.42%, respectively. The feasibility of hyperthermophilic digestion for spiramycin residue was evaluated in batch experiments for 7 days. Hyperthermophilic digestion considerably reduced antibiotic concentrations, with removal efficiencies of 55.3% and 99.0% for the spiramycin residue alone and its mixture with hyperthermophilic sludge, respectively. At the same time, the abundances of four macrolide-lincosamide-streptogramin resistance genes were also reduced within 7 days, due to the decrease of their corresponding hosts. These results suggest that hyperthermophilic digestion could easily be started up from mesophilic digestion and might be a suitable pretreatment approach for spiramycin residue.

Variations in the denitrifying microbial community and functional genes during mesophilic and thermophilic anaerobic digestion of cattle manure

In this study, the anaerobic digestion (AD) of cattle manure was conducted at two temperatures (mesophilic: 35 °C; thermophilic: 55 °C) to analyze the dynamics of the denitrifying functional microbial community and functional genes. The cumulative N₂O production under thermophilic conditions was 130.3% higher than that under mesophilic conditions. Thermophilic AD decreased the abundance of nosZ, which was more functional than other denitrifying genes. Firmicutes, Proteobacteria, and Bacteroidetes were the main phyla, and they were also related to denitrification during AD. Redundancy analysis indicated that pH, temperature, and NH₄⁺-N mainly affected the functional bacterial community. Temperature altered the co-occurrence patterns of the bacterial community and the keystone genera in AD. Desulfovibrio in mesophilic AD and Thiobacillus in thermophilic AD were closely related to nitrogen transformation among the keystone genera. The variations in the abundances of members of the denitrifying microbial community and functional genes during AD suggest that thermophilic AD may have caused greater nitrogen losses.