Micro-aeration: an attractive strategy to facilitate anaerobic digestion

Sodium citrate enhances anaerobic fermentation of granular sludge: the multifaceted roles of structure disruption and metabolic regulation

Anaerobic fermentation is an efficient approach for recovering organic carbon and other valuable resources from waste sludge, yet its efficiency is constrained by the structural stability of extracellular polymeric substances (EPS), especially for aerobic granular sludge (AGS). Despite the abundant physical-chemical pre-treatment approaches for enhancing EPS dissolution, biocompatible strategies coordinating structural disruption with metabolic regulation remain unexplored. Herein, sodium citrate (SC) was used to enhance the performance of anaerobic fermentation of AGS. The results suggested that SC significantly enhanced the hydrolysis efficiency and volatile fatty acids (VFAs) production of AGS. Despite the direct conversion of SC to acetate, indirect enhancement played more important roles in AGS fermentation. Mechanism analysis indicated that SC disrupted granular sludge structure by chelating Ca2+ and facilitated the release of EPS and hydrolytic enzymes, which was conducive to sludge hydrolysis and acidification. At the level of microbial community, SC facilitated the accumulation of VFAs by enriching the acid-producing microorganisms and inhibiting the acid-consuming microorganisms. Furthermore, SC regulated the genes involved in the direct generation of acetate and pyruvate-centric metabolism, resulting in the massive accumulation of VFAs. Finally, the economic benefits resulting from increased VFA production versus SC costs. Overall, SC enhanced the anaerobic fermentation of AGS by simultaneously affecting EPS structure and regulating metabolism, and this study provided efficient methods for AGS anaerobic treatment.

In-situ intermittent micro-aeration in food waste and sewage sludge anaerobic co-digestion: Performance, stability, and microbial dynamics

Anaerobic co-digestion (AcoD) of sewage sludge (SS) with food waste (FW) is a practical approach in urban areas due to spatio-temporal availability of these co-substrates. While micro-aeration could enhance hydrolysis and control acidification during mono-digestion, the effects of micro-aeration on AcoD remains unclear. This study explored the influence of oxidation-reduction potential (ORP)-based in-situ micro-aeration on AcoD performance of SS and FW. Although mono-digestion of FW failed due to acidification and micro-aeration of SS was unstable due to high solids content and viscosity, micro-aeration improved specific biogas and methane yields by 21.1 % and 13.1 %, respectively, at the FW:SS ratio of 60:40 ((volatile solids (VS) basis). This improvement was observed at an organic loading rate of 2.25 g VS/L·d and alkalinity below 5000 mg CaCO3/L. The observed positive effect of micro-aeration was accompanied by changes in relative abundance of Aminicenantes. Functional analysis suggested that AcoD was driven by homoacetogenesis and acetoclastic methanogenesis.

Study on the growth and decline patterns and environmental drivers of pathogens during the stabilization process of simulated landfilling municipal solid waste

Waste and leachate in landfills are substantial reservoirs of pathogens, however information about the risk of pathogen contamination during the stabilization process under different landfill conditions is very limited. In this study, dynamic changes of culturable pathogens, bacteria community, and human bacterial pathogens (HBPs) during the stabilization process under different landfill conditions were investigated, and the environmental drivers were explored. Results showed that total coliforms, Enterococcus, and Staphylococcus aureus were the dominant pathogens detected in waste and leachate samples. During the landfill stabilization process, the concentration of culturable pathogens peaked at the hydrolysis-acidification stage (3.6 × 105 CFU·g-1) in the anaerobic condition, fluctuated from 4.18 × 104 to 5.35 × 105 CFU·g-1 in the anaerobic leachate-recirculation condition, and kept rising (from 4.18 × 104 to 2.12 × 106 CFU·g-1) in the micro-aerobic condition. Moreover, HBPs abundance and diversity in the waste and leachate under micro-aerobic conditions were higher than those under the other two conditions, suggesting a higher risk of pathogen contamination. Sulfate and pH were significantly (p < 0.05) correlated with the composition of bacterial communities and HBPs, likely serving as the major environmental driving factors. Additionally, the interactions between HBPs and functional bacterial groups tended towards cooperative symbiotic relationships, with hydrolytic-acidogenic bacteria promoting the growth and proliferation of most pathogens. These findings will help to understand the changes and environmental drivers of pathogens during landfill stabilization, which will provide a theoretical basis for the risk prevention and control of pathogens in waste disposal.

Anaerobic Biotransformation of Carbamazepine During Graphene Oxide Biological Reduction

Carbamazepine (CBZ) is well-known for its resistance to biological wastewater treatment. In the present study, the biological reduction of graphene oxide (GO) is investigated as the inducer of the co-metabolic biodegradation of CBZ under anaerobic conditions. The addition of 0.05 and 0.1 g GO g -1 VS resulted in a CBZ removal >75% and >90% within 48 hours, respectively. Moreover, the sequential GO dosing strategy greatly enhanced the anaerobic biotransformation of carbamazepine, achieving >95% removal. Structural elucidation of the major transformation products of carbamazepine revealed the epoxidation of the azepine ring after each dosage of GO, forming 10,11-epoxycarbamazepine, confirming biotransformation as a removal mechanism. In contrast, the formation of the biologically reduced GO (bio-rGO) negatively affected methane production, reducing the specific methane production and prolonging the lag phase. The present study demonstrates that, despite its persistence to biological degradation, carbamazepine can be effectively co-metabolically transformed under anaerobic conditions. This study demonstrates that exposure of the anaerobic community to oxidative stress may result in novel wastewater treatment strategies to remove persistent organic contaminants.

Intermittent Microaeration Enhanced Anaerobic Digestion: The Key Role of Fe(III)/Fe(II) Cycle and Reactive Oxygen Species

Microaeration has been reported to improve anaerobic digestion, which is generally attributed to increased microbial diversity, but a thorough explanation is lacking. In this study, it was found that intermittent microaeration (IMA) supplied to an anaerobic digester could oxidize Fe(II) produced by dissimilatory iron reduction (DIR) to form a Fe(III)/Fe(II) cycle and generate extracellular reactive oxygen species (ROS) to improve anaerobic treatment of phenol-containing wastewater. The results showed that compared to the control group without IMA, the removal rates of COD and phenol increased by 29.54 and 49.68 percentage points, respectively, and daily average methane production increased by 85.44%. The Fe(III)/Fe(II) cycle slowed down the loss of iron (13.97%) released from sludge due to the lower solubility of Fe(III) and facilitated •OH generation (1.22 ± 0.04 μM) via Fenton-like reactions. The DIR and the generation of •OH accelerated phenol degradation. Metagenomic analysis revealed that the abundance of methanogens and antioxidant enzymes-encoding genes in response to oxidative stress significantly increased in the IMA group compared to the control, enabling methanogenesis to proceed smoothly under microaeration. This study investigated the extracellular ROS generation induced by microaeration during anaerobic digestion and their roles in promoting anaerobic performance, thereby providing a new perspective for optimizing anaerobic systems with microaeration.

Comparative assessment of a restored and natural wetland using 13C-DNA SIP reveals a higher potential for methane production in the restored wetland

Wetlands are the largest natural source of methane (CH4), a potent greenhouse gas produced by methanogens. Methanogenesis rates are controlled by environmental factors such as redox potential, temperature, and carbon and electron acceptor availability and are presumably dependent on the composition of the active methanogen community. We collected intact soil cores from a restored and natural freshwater depressional wetland on Maryland's Delmarva Peninsula (USA) to assess the effects of wetland restoration and redox shifts on microbial processes. Intact soil cores were incubated under either saturated (anoxic) or unsaturated (oxic) conditions and amended with 13C-acetate for quantitative stable isotope probing (qSIP) of the 16S rRNA gene. Restored wetland cores supported a distinct community of methanogens compared to natural cores, and acetoclastic methanogens putatively identified in the genus Methanosarcina were among the most abundant taxa in restored anoxic and oxic cores. The active microbial communities in the restored wetland cores were also distinguished by the unique presence of facultatively anaerobic bacteria belonging to the orders Firmicutes and Bacteroidetes. In natural wetland incubations, methanogen populations were not among the most abundant taxa, and these communities were instead distinguished by the unique presence of aerobic bacteria in the phyla Acidobacteria, Actinobacteria, and class Alphaproteobacteria. Iron-reducing bacteria, in the genus Geobacter, were active across all redox conditions in both the restored and the natural cores, except the natural oxic-anoxic condition. These findings suggest an overall higher potential for methanogenesis in the restored wetland site compared to the natural wetland site, even when there is evidence of Fe reduction.IMPORTANCEMethane (CH4) is a potent greenhouse gas with an atmospheric half-life of ~10 years. Wetlands are the largest natural emitters of CH4, but CH4 dynamics are difficult to constrain due to high spatial and temporal variability. In the past, wetlands were drained for agriculture. Now, restoration is an important strategy to increase these ecosystems' potential for sequestering carbon. However, the consequences of wetland restoration on carbon biogeochemistry are under-evaluated, and a thorough assessment of the active microbial community as a driver of biogeochemical changes is needed. Particularly, the effects of seasonal flooding/drying cycles in geographically isolated wetlands might have implications for CH4 emissions in both natural and restored wetlands. Here, we found that active microbial communities in natural and restored wetlands responded differently to flooding and drying regimes, resulting in differences in CH4 production potentials. Restored wetlands had a higher potential for CH4 production compared to natural wetlands. Our results show that controls on CH4 production in a restored wetland are complex, and dynamics of active microbial communities are linked to seasonal dry-wet cycles.

Improving methane production and 4-chlorophenol removal in anaerobic digestion of corn straw by adding Phanerochaete chrysosporium and biochar under microaerobic conditions

The stable lignocellulose structure in the straw is the main obstacle for methane production during its anaerobic digestion, and the residual chlorophenols in the straw further increase the difficulty. In this study, the anaerobic digestion of corn straw containing 4-chlorophenol was enhanced by the addition of Phanerochaete chrysosporium and biochar. The results revealed that P. chrysosporium significantly increased the soluble COD concentration and total COD removal efficiency in the anaerobic digestion of corn straw, which initially contained a small amount of residual oxygen (4.1-4.5 mg/L). The accumulative methane production of the P. chrysosporium-coupled biochar (PC-BC) group and the PC group with P. chrysosporium alone were 232.9 ± 3.0 mL and 201.7 ± 5.1 mL, respectively, which were significantly higher than the control group (19.4 ± 1.0 mL) with the sterilized P. chrysosporium. The presence of biochar increased 4-CP removal rate to 93.3 %, which was 15.2 % higher than the control. Additionally, FTIR analysis indicated that the addition of P. chrysosporium and biochar enhanced the decomposition of lignocellulose structure. Moreover, the sludge capacitance and electron transfer capacity were highest in the PC-BC group. Also, microbial community analysis showed that biochar could enrich dechlorinating bacteria (e.g., Sedimentibacter) and electroactive microorganisms, which further enhanced dechlorination and methanogensis.

Groundwater denitrification using electro-assisted autotrophic processes: exploring bacterial community dynamics in a single-chamber reactor

Nitrate, a major groundwater pollutant from anthropogenic activities, poses serious health risks when present in drinking water. Denitrification using bio-electrochemical reactors (BER) offers an innovative technology, eco-friendly solution for nitrate removal from groundwater. BER use electroactive bacteria to reduce inorganic compounds like nitrate and bicarbonate by transferring electrons directly from the cathode. In our work, two batch BER were implemented at 1V and 2V, using anaerobic digestate from a full-scale wastewater treatment plant as inoculum. Nitrate, nitrite, sulfate, total ammoniacal nitrogen, and 16S rRNA analysis of bacterial community, were monitored during BER operation. The results showed effective nitrate removal in all BERs, with denitrification rate at 1V and 2V higher than the Control system, where endogenous respiration drove the process. At 1V, complete nitrate conversion to N2 occurred in 4 days, while at 2V, it took 14 days. The slower rate at 2V was likely due to O2 production from water electrolysis, which competed with nitrate as final electron acceptor. Bacterial community analysis confirmed the electroactive bacteria selection like the genus Desulfosporosinus and Leptolinea, confirming electrons transfer without an electroactive biofilm. Besides, Hydrogenophaga was enhanced at 2V likely due to electrolytically produced H2. Sulfate was not reduced, and total ammoniacal nitrogen remained constant indicating no dissimilatory nitrite reduction of ammonia. These results provide a significant contribution to the scaling up of electro-assisted autotrophic denitrification and its application in groundwater remediation, utilizing a simple reactor configuration-a single-chamber, membrane-free design- and a conventional power source instead of a potentiostat.

Microbiome regulation for sustainable wastewater treatment

Sustainable wastewater treatment is essential for attaining clean water and sanitation, aligning with UN Sustainable Development Goals. Wastewater treatment plants (WWTPs) have utilized environmental microbiomes in biological treatment processes in this effort for over a century. However, the inherent complexity and redundancy of microbial communities, and emerging chemical and biological contaminants, challenge the biotechnology applications. Over the past decades, understanding and utilization of microbial energy metabolism and interaction relationships have revolutionized the biological system. In this review, we discuss how microbiome regulation strategies are being used to generate actionable performance for low-carbon pollutant removal and resource recovery in WWTPs. The engineering application cases also highlight the real feasibility and promising prospects of the microbiome regulation approaches. In conclusion, we recommend identifying environmental risks associated with chemical and biological contaminants transformation as a prerequisite. We propose the integration of gene editing and enzyme design to precisely regulate microbiomes for the synergistic control of both chemical and biological risks. Additionally, the development of integrated technologies and engineering equipment is crucial in addressing the ongoing water crisis. This review advocates for the innovation of conventional wastewater treatment biotechnology to ensure sustainable wastewater treatment.

Metagenomic analysis reveals metabolic mechanism of enhancing lignocellulosic anaerobic digestion mediated by CO2/O2-nanobubble water

Nanobubble water (NW) has been reported to enhance anaerobic digestion (AD), but its influence on the metabolic pathways of microorganisms remains unclear. In this study, the specific methane yields of rice straw in the CO2NW and O2NW treatments increased by 6.9% and 18.3%, respectively. The electron transport system (ETS) and coenzyme F420 activities were enhanced by the addition of NW. Metagenomic analysis showed that the abundances of most enzymes in the acidification were significantly increased by both CO2NW and O2NW. Regarding methanogenesis, CO2NW promoted the expression of genes encoding enzymes of hydrogenotrophic methanogenesis, while O2NW stimulated both the acetoclastic and hydrogenotrophic methanogenesis. With the addition of O2NW, the expressions of modules related to the tricarboxylic acid (TCA) cycle and oxidative phosphorylation were enhanced, resulting in increased ATP production. This study provided fundamental evidence of the metabolic pathways of microorganisms mediated by NW at each stage of AD.

Positive effects of appropriate micro-aeration on landfill stabilization: Mitigating ammonia and VFAs accumulation

The slow stabilization process of landfill had brought obstacles to urbanization. The paper investigated the efficacy and mechanism of micro-aeration intensity for landfill stabilization. The micro-aeration intensity of 0.05 L/(h·kg) resulted in a significant increase of volatile fatty acids (VFAs) in the hydrolysis stage, and the NH4+-N concentration was reduced by 22.1 %. At the end of landfill, VFAs were rapidly degraded and organic matter was reduced from 36 % to 16 %, which was 55.5 % more efficient than the control group. In addition, the community succession and structure of bacteria and archaea were analyzed. The micro-aeration intensity of 0.05 L/(h·kg) increased the abundance of hydrolyzing functional bacteria such as Pseudomonas and Bacillus, and allowed methanogenic bacteria such as Methanobacterium and Methanothrix to gradually establish oxygen tolerance in the microaerobic environment. The appropriate micro-aeration intensity can accelerate the stabilization process of landfill, which has environmental and economic benefits.

Optimizing intermittent micro-aeration as a strategy for enhancing aniline anaerobic biodegradation: kinetic, ecotoxicity, and microbial community dynamics analyses

Groundwater and soil contamination by aromatic amines (AAs), used in the production of polymers, plastics, and pesticides, often results from improper waste disposal and accidental leaks. These compounds are resistant to anaerobic degradation; however, micro-aeration can enhance this process by promoting microbial interactions. In batch assays, anaerobic degradation of aniline (0.14 mM), a model AA, was tested under three micro-aeration conditions: T30, T15, and T10 (30, 15, and 10 min of micro-aeration every 2 h, respectively). Aniline degradation occurred in all conditions, producing both aerobic (catechol) and anaerobic (benzoic acid) byproducts. The main genera involved in T30 and T15 were Comamonas, Clostridium, Longilinea, Petrimonas, Phenylobacterium, Pseudoxanthomonas, and Thiobacillus. In contrast, in T10 were Pseudomonas, Delftia, Leucobacter, and Thermomonas. While T30 and T15 promoted microbial cooperation for anaerobic degradation and facultative respiration, T10 resulted in a competitive environment due to dominance and oxygen scarcity. Despite aniline degradation in 9.4 h under T10, this condition was toxic to Allium cepa seeds and exhibited cytogenotoxic effects. Therefore, T15 emerged as the optimal condition, effectively promoting anaerobic degradation without accumulating toxic byproducts. Intermittent micro-aeration emerges as a promising strategy for enhancing the anaerobic degradation of AA-contaminated effluents.

Regulating microbial redox reactions towards enhanced removal of refractory organic nitrogen from wastewater

Biotechnology for wastewater treatment is mainstream and effective depending upon microbial redox reactions to eliminate diverse contaminants and ensure aquatic ecological health. However, refractory organic nitrogen compounds (RONCs, e.g., nitro-, azo-, amide-, and N-heterocyclic compounds) with complex structures and high toxicity inhibit microbial metabolic activity and limit the transformation of organic nitrogen to inorganic nitrogen. This will eventually result in non-compliance with nitrogen discharge standards. Numerous efforts suggested that applying exogenous electron donors or acceptors, such as solid electrodes (electrostimulation) and limited oxygen (micro-aeration), could potentially regulate microbial redox reactions and catabolic pathways, and facilitate the biotransformation of RONCs. This review provides comprehensive insights into the microbial regulation mechanisms and applications of electrostimulation and micro-aeration strategies to accelerate the biotransformation of RONCs to organic amine (amination) and inorganic ammonia (ammonification), respectively. Furthermore, a promising approach involving in-situ hybrid anaerobic biological units, coupled with electrostimulation and micro-aeration, is proposed towards engineering applications. Finally, employing cutting-edge methods including multi-omics analysis, data science driven machine learning, technology-economic analysis, and life-cycle assessment would contribute to optimizing the process design and engineering implementation. This review offers a fundamental understanding and inspiration for novel research in the enhanced biotechnology towards RONCs elimination.

A novel microbial community restructuring strategy for enhanced hydrogen production using multiple pretreatments and CSTR operation

To achieve rapid enrichment of the targeted hydrogen-producing bacterial population and reconstruction of the microbial community in the biological hydrogen-producing reactor, the activated sludge underwent multiple pretreatments using micro-aeration, alkaline treatment, and heat treatment. The activated sludge obtained from the multiple pretreatments was inoculated into the continuous stirred tank reactor (CSTR) for continuous operations. The community structure alteration and hydrogen-producing capability of the activated sludge were analyzed throughout the operation of the reactor. We found that the primary phyla in the activated sludge population shifted to Proteobacteria, Firmicutes, and Bacteroidetes, which collectively accounted for 96.69% after undergoing several pretreatments. This suggests that the multiple pretreatments facilitated in achieving the selective enrichment of the fermentation hydrogen-producing microorganisms in the activated sludge. The CSTR start-up and continuous operation of the biological hydrogen production reactor resulted in the reactor entering a highly efficient hydrogen production stage at influent COD concentrations of 4000 mg/L and 5000 mg/L, with the highest hydrogen production rate reaching 8.19 L/d and 9.33 L/d, respectively. The main genus present during the efficient hydrogen production stage in the reactor was Ethanoligenens, accounting for up to 33% of the total population. Ethanoligenens exhibited autoaggregation capabilities and a superior capacity for hydrogen production, leading to its prevalence in the reactor and contribution to efficient hydrogen production. During high-efficiency hydrogen production, flora associated with hydrogen production exhibited up to 46.95% total relative abundance. In addition, redundancy analysis (RDA) indicated that effluent pH and COD influenced the distribution of the primary hydrogen-producing bacteria, including Ethanoligenens, Raoultella, and Pectinatus, as well as other low abundant hydrogen-producing bacteria in the activated sludge. The data indicates that the multiple pretreatments and reactor's operation has successfully enriched the hydrogen-producing genera and changed the community structure of microbial hydrogen production.

Rice-fish coculture without phosphorus addition improves paddy soil nitrogen availability by shaping ammonia-oxidizing archaea and bacteria in subtropical regions of South China

Rice-fish coculture (RFC), as a traditional agricultural strategy in China, can optimally utilize the scarce resource, especially in subtropical regions where phosphorus (P) deficiency limits agricultural production. However, ammonia-oxidizing archaea (AOA) and bacteria (AOB) are involved in the ammonia oxidation, but it remains uncertain whether their community compositions are related to the RFC combined with and without P addition that improves soil nitrogen (N) use efficiency. Here, a microcosm experiment was conducted to assess the impacts of RFC combined with and without inorganic P (0 and 50 mg P kg-1 as KH2PO4) addition on AOA and AOB community diversities, enzyme activities and N availability. The results showed that RFC significantly increased available N content without P addition compared with P addition. Moreover, RFC significantly increased urease activity and AOA shannon diversity, and reduced NAG activity and AOB shannon diversity without P addition, respectively. Higher diversity of AOA compared with that of AOB causes greater competition for resources and energy within their habitats, thereby resulting in lower network complexity. Our findings indicated that the abundances of AOA and AOB are influenced through the introduction of fish and/or P availability, of which AOB is linked to N availability. Overall, RFC could improve paddy soil N availability without P addition in subtropical region, which provides a scientific reference for promoting the practices that reduce N fertilizer application in RFC.

Energetic utilization of corn stalk and elimination of methyl orange in ECMO-like integrated reactor: Co-occurrence of anaerobic digestion and aerobic treatment

For the simultaneous energetic utilization of corn stalk and azo-dye contaminated wastewater, an ECMO-like integrated reactor was come up to achieve the biogas production and azo-dye degradation during anaerobic digestion (AD). Methyl orange (MO) was selected as the model compound for azo-dye. The ECMO-like reactor included AD main reactor with a spray device and solid-liquid separation components, integrated with an aeration reactor for biogas slurry. Methane yields of corn stalks (100.82 mL/g VS) were highest in the ECMO-like reactor, compared with reactors without aeration. As a stable metabolite, 4-aminobenzenesulfonic acid (4-ABA) was detected in AD, while it was assumed that the metabolites can be further transformed in the ECMO-like reactor (R3), due to the 4-ABA removal efficiency as 92.87 % after 35 days' digestion. Class Alphaproteobacteria and Clostridia were assumed as functional microbes responding to aeration. Overall, this ECMO-like integrated reactor provided a novel biotechnology strategy for agricultural and azo dye waste treatment.

Enhanced anaerobic digestion performance and sludge filterability by membrane microaeration for anaerobic membrane bioreactor application

Slow dissolution/hydrolysis of insoluble/macromolecular organics and poor sludge filterability restrict the application potential of anaerobic membrane bioreactor (AnMBR). Bubble-free membrane microaeration was firstly proposed to overcome these obstacles in this study. The batch anaerobic digestion tests feeding insoluble starch and soluble peptone with and without microaeration showed that microaeration led to a 65.7-144.8% increase in methane production and increased critical flux of microfiltration membrane via driving the formation of large sludge flocs and the resultant improvement of sludge settleability. The metagenomic and bioinformatic analyses showed that microaeration significantly enriched the functional genes and bacteria for polysaccharide and protein hydrolysis, microaeration showed little negative effects on the functional genes involved in anaerobic metabolisms, and substrate transfer from starch to peptone significantly affected the functional genes and microbial community. This study demonstrates the dual synergism of microaeration to enhance the dissolution/hydrolysis/acidification of insoluble/macromolecular organics and sludge filterability for AnMBR application.

The Selenoproteome as a Dynamic Response Mechanism to Oxidative Stress in Hydrogenotrophic Methanogenic Communities

Methanogenesis is a critical process in the carbon cycle that is applied industrially in anaerobic digestion and biogas production. While naturally occurring in diverse environments, methanogenesis requires anaerobic and reduced conditions, although varying degrees of oxygen tolerance have been described. Microaeration is suggested as the next step to increase methane production and improve hydrolysis in digestion processes; therefore, a deeper understanding of the methanogenic response to oxygen stress is needed. To explore the drivers of oxygen tolerance in methanogenesis, two parallel enrichments were performed under the addition of H2/CO2 in an environment without reducing agents and in a redox-buffered environment by adding redox mediator 9,10-anthraquinone-2,7-disulfonate disodium. The cellular response to oxidative conditions is mapped using proteomic analysis. The resulting community showed remarkable tolerance to high-redox environments and was unperturbed in its methane production. Next to the expression of pathways to mitigate reactive oxygen species, the higher redox potential environment showed an increased presence of selenocysteine and selenium-associated pathways. By including sulfur-to-selenium mass shifts in a proteomic database search, we provide the first evidence of the dynamic and large-scale incorporation of selenocysteine as a response to oxidative stress in hydrogenotrophic methanogenesis and the presence of a dynamic selenoproteome.

Comparison of different sewage sludge pretreatment technologies for improving sludge solubilization and anaerobic digestion efficiency: A comprehensive review

Anaerobic digestion (AD) of sewage sludge reduces organic solids and produces methane, but the complex nature of sludge, especially the difficulty in solubilization, limits AD efficiency. Pretreatments, by destroying sludge structure and promoting disintegration and hydrolysis, are valuable strategies to enhance AD performance. There is a plethora of reviews on sludge pretreatments, however, quantitative comparisons from multiple perspectives across different pretreatments remain scarce. This review categorized various pretreatments into three groups: Physical (ultrasonic, microwave, thermal hydrolysis, electric decomposition, and high pressure homogenization), chemical (acid, alkali, Fenton, calcium peroxide, and ozone), and biological (microaeration, exogenous bacteria, and exogenous hydrolase) pretreatments. The optimal conditions of various pretreatments and their impacts on enhancing AD efficiency were summarized; the effects of different pretreatments on microbial community in the AD system were comprehensively compared. The quantitative comparison based on dissolution degree of COD (DDCOD) indicted that the sludge solubilization performance is in the order of physical, chemical, and biological pretreatments, although with each below 40 % DDCOD. Biological pretreatment, particularly microaeration and exogenous bacteria, excel in AD enhancement. Pretreatments alter microbial ecology, favoring Firmicutes and Methanosaeta (acetotrophic methanogens) over Proteobacteria and Methanobacterium (hydrogenotrophic methanogens). Most pretreatments have unfavorable energy and economic outcomes, with electric decomposition and microaeration being exceptions. On the basis of the overview of the above pretreatments, a full energy and economy assessment for sewage sludge treatment was suggested. Finally, challenges associated with sludge pretreatments and AD were analyzed, and future research directions were proposed. This review may broaden comprehension of sludge pretreatments and AD, and provide an objective basis for the selection of sludge pretreatment technologies.

Effect of micro-aeration on syntrophic and methanogenic activity in anaerobic sludge

Micro-aeration was shown to improve anaerobic digestion (AD) processes, although oxygen is known to inhibit obligate anaerobes, such as syntrophic communities of bacteria and methanogens. The effect of micro-aeration on the activity and microbial interaction in syntrophic communities, as well as on the potential establishment of synergetic relationships with facultative anaerobic bacteria (FAB) or aerobic bacteria (AB), was investigated. Anaerobic sludge was incubated with ethanol and increasing oxygen concentrations (0-5% in the headspace). Assays with acetate or H2/CO2 (direct substrates for methanogens) were also performed. When compared with the controls (0% O2), oxygen significantly decreased substrate consumption and initial methane production rate (MPR) from acetate or H2/CO2. At 0.5% O2, MPR from these substrates was inhibited 30-40%, and close to 100% at 5% O2. With ethanol, significant inhibition (>36%) was only observed for oxygen concentrations higher than 2.5%. Oxygen was consumed in the assays, pointing to the stimulation of AB/FAB by ethanol, which helped to protect the syntrophic consortia under micro-aerobic conditions. This highlights the importance of AB/FAB in maintaining functional and resilient syntrophic communities, which is relevant for real AD systems (in which vestigial O2 amounts are frequently present), as well as for AD systems using micro-aeration as a process strategy. KEY POINTS: •Micro-aeration impacts syntrophic communities of bacteria and methanogens. •Oxygen stimulates AB/FAB, maintaining functional and resilient consortia. •Micro-aeration studies are critical for systems using micro-aeration as a process strategy.

Optimization of biogas production from straw wastes by different pretreatments: Progress, challenges, and prospects

Lignocellulosic biomass (LCB) presents a promising feedstock for carbon management due to enormous potential for achieving carbon neutrality and delivering substantial environmental and economic benefit. Bioenergy derived from LCB accounts for about 10.3 % of the global total energy supply. The generation of bioenergy through anaerobic digestion (AD) in combination with carbon capture and storage, particularly for methane production, provides a cost-effective solution to mitigate greenhouse gas emissions, while concurrently facilitating bioenergy production and the recovery of high-value products during LCB conversion. However, the inherent recalcitrant polymer crystal structure of lignocellulose impedes the accessibility of anaerobic bacteria, necessitating lignocellulosic residue pretreatment before AD or microbial chain elongation. This paper seeks to explore recent advances in pretreatment methods for LCB biogas production, including pulsed electric field (PEF), electron beam irradiation (EBI), freezing-thawing pretreatment, microaerobic pretreatment, and nanomaterials-based pretreatment, and provide a comprehensive overview of the performance, benefits, and drawbacks of the traditional and improved treatment methods. In particular, physical-chemical pretreatment emerges as a flexible and effective option for methane production from straw wastes. The burgeoning field of nanomaterials has provoked progress in the development of artificial enzyme mimetics and enzyme immobilization techniques, compensating for the intrinsic defect of natural enzyme. However, various complex factors, such as economic effectiveness, environmental impact, and operational feasibility, influence the implementation of LCB pretreatment processes. Techno-economic analysis (TEA), life cycle assessment (LCA), and artificial intelligence technologies provide efficient means for evaluating and selecting pretreatment methods. This paper addresses current issues and development priorities for the achievement of the appropriate and sustainable utilization of LCB in light of evolving economic and environmentally friendly social development demands, thereby providing theoretical basis and technical guidance for improving LCB biogas production of AD systems.

Microbial electrolysis cell simultaneously enhancing methanization and reducing hydrogen sulfide production in anaerobic digestion of sewage sludge

The effects of microbial electrolysis cells (MECs) at three applied voltages (0.8, 1.3, and 1.6 V) on simultaneously enhancing methanization and reducing hydrogen sulfide (H2S) production in the anaerobic digestion (AD) of sewage sludge were studied. The results showed that the MECs at 1.3 V and 1.6 V simultaneously enhanced the methane production by 57.02 and 12.70% and organic matter removal by 38.77 and 11.13%, and reduced H2S production by 94.8 and 98.2%, respectively. MECs at 1.3 V and 1.6 V created a micro-aerobic conditions for the digesters with oxidation-reduction potential as -178∼-232 mv, which enhanced methanization and reduced H2S production. Sulfur reduction, H2S and elemental sulfur oxidation occurred simultaneously in the ADs at 1.3 V and 1.6 V. The relative abundances of sulfur-oxidizing bacteria increased from 0.11% to 0.42% and those of sulfur-reducing bacteria decreased from 1.24% to 0.33% when the applied voltage of MEC increased from 0 V to 1.6 V. Hydrogen produced by electrolysis enhanced the abundance of Methanobacterium and changed the methanogenesis pathway.

Microbiome Diversity of Anaerobic Digesters Is Enhanced by Microaeration and Low Frequency Sound

Biogas is produced by a consortium of bacteria and archaea. We studied how the microbiome of poultry litter digestate was affected by time and treatments that enhanced biogas production. The microbiome was analyzed at six, 23, and 42 weeks of incubation. Starting at week seven, the digesters underwent four treatments: control, microaeration with 6 mL air L-1 digestate per day, treatment with a 1000 Hz sine wave, or treatment with the sound wave and microaeration. Both microaeration and sound enhanced biogas production relative to the control, while their combination was not as effective as microaeration alone. At week six, over 80% of the microbiome of the four digesters was composed of the three phyla Actinobacteria, Proteobacteria, and Firmicutes, with less than 10% Euryarchaeota and Bacteroidetes. At week 23, the digester microbiomes were more diverse with the phyla Spirochaetes, Synergistetes, and Verrucomicrobia increasing in proportion and the abundance of Actinobacteria decreasing. At week 42, Firmicutes, Bacteroidetes, Euryarchaeota, and Actinobacteria were the most dominant phyla, comprising 27.8%, 21.4%, 17.6%, and 12.3% of the microbiome. Other than the relative proportions of Firmicutes being increased and proportions of Bacteroidetes being decreased by the treatments, no systematic shifts in the microbiomes were observed due to treatment. Rather, microbial diversity was enhanced relative to the control. Given that both air and sound treatment increased biogas production, it is likely that they improved poultry litter breakdown to promote microbial growth.

Microaerobic condition as pretreatment for improving anaerobic digestion: A review

Pretreatment of waste before anaerobic digestion (AD) has been extensively studied during the last decades. One of the biological pretreatments studied is the microaeration. This review examines this process, including parameters and applications to different substrates at the lab, pilot and industrial scales, to guide further improvement in large-scale applications. The underlying mechanisms of accelerating hydrolysis and its effects on microbial diversity and enzymatic production were reviewed. In addition, modelling of the process and energetic and financial analysis is presented, showing that microaerobic pretreatment is commercially attractive under certain conditions. Finally, challenges and future perspectives were also highlighted to promote the development of microaeration as a pretreatment before AD.

Limited dissolved oxygen facilitated nitrogen removal at biocathode during the hydrogenotrophic denitrification process using bioelectrochemical system

Effects of limited dissolved oxygen (DO) on hydrogenotrophic denitrification at biocathode was investigated using bioelectrochemical system. It was found that total nitrogen removal increased by 5.9%, as DO reached about 0.24 mg/L with the cathodic chamber unplugged (group R_Exposure). With the presence of limited DO, not only the nitrogen metabolic pathway was influenced, but the composition of microbial communities of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria were enriched accordingly. After metagenomic analysis, enriched genes in R_Exposure were found to be associated with nearly each of nitrogen removal steps as denitrification, nitrification, DNRA, nitrate assimilation and even nitrogen fixation. Moreover, genes encoding both Complexes III and IV constituted the electron transfer chain were significantly enriched, indicating that more electrons would be orientated to the reduction of NO₂^--N, NO-N and oxygen. Therefore, enhanced nitrogen removal could be attained through the co-respiration of nitrate and oxygen by means of NH₄⁺-N oxidation.