Hydrochar-Facilitated Anaerobic Digestion: Evidence for Direct Interspecies Electron Transfer Mediated through Surface Oxygen-Containing Functional Groups

Coupling hydrothermal liquefaction and anaerobic digestion for waste biomass valorization: A review in context of circular economy

In light of the substantial global production of biomass waste, effective waste management and energy recovery solutions are of paramount importance. Hydrothermal liquefaction (HTL) and anaerobic digestion (AD) have emerged as innovative techniques for converting biomass waste into valuable resources. Their integration creates a synergistic framework that mitigates inherent limitations, leading to improved efficiency, enhanced product quality, and the comprehensive utilization of biomass. This review paper investigates the integration of HTL and AD, highlighting its significance and potential benefits as well as the optimal sequencing (HTL followed by AD and AD followed by HTL). The review encompasses experimental procedures, factors influencing both sequencing options, energy recovery characterizations, final product outcomes, as well as toxicological assessments and discussions on reduction. Additionally, it delves into the transition towards a circular bioeconomy and discusses the challenges and opportunities intrinsic to these processes. The findings presented in this review offer valuable insights to shape future research in this evolving field.

Hydrothermal-enhanced pyrolysis for efficient NOX reduction and biochar valorization from food waste digestate

Pyrolysis has emerged as a promising technology for valorizing digestate resulting from the anaerobic digestion of food waste. However, the high NOX emissions during pyrolysis limit its application. This study proposed a hydrothermal coupled pyrolysis process to control the element transfer in digestate during biochar production. The efficient reduction of NOX emissions and the improvement of biochar adsorbability were realized. The hydrothermal process reduced the nitrogen content in solid digestate by 49.10 %-81.79 %, thus reducing the NOX precursors in syngas and the N-containing substances in bio-oil. Additionally, the specific surface area and the total pore volume of biochar were enhanced from 25 m2/g to 60-73 m2/g and 0.06 cm3/g to 0.12-0.14 cm3/g, respectively. More defects, oxygen-containing functional groups, and doped Ca on the biochar resulted in a high phosphate removal efficiency of 94 %. The proposed technology provides an efficient and environmentally friendly way to utilize the digestate.

Magnetic porous microspheres altering interfacial thermodynamics of sewage sludge to drive metabolic cooperation for efficient methanogenesis

Controllable and recyclable magnetic porous microspheres (MPMs) have been proposed as a means for enhancing the anaerobic digestion (AD) of sludge, as they do not require continuous replenishment and can serve as carriers for anaerobes. However, the effects of MPMs on the interfacial thermodynamics of sludge and the biological responses triggered by abiotic effects in AD systems remain to be clarified. Herein, the underlying mechanisms by which MPMs alter the solid-liquid interface of sludge to drive methanogenesis were investigated. A significant increase in the contents of 13C and 2H (D) in methane molecules was observed in the presence of MPMs, suggesting that MPMs might enhance the CO2-reduction methanogenesis and participation of water in methane generation. Experimental results demonstrated that the addition of MPMs did not promote the anaerobic bioconversion of soluble organics for methanogenesis, suggesting that the enhanced methanogenesis and water participation were not achieved through promotion of the bioconversion of original liquid-state organics in sludge. Analyses of the capillary force, surface adhesion force, and interfacial proton-coupled electron transfer (PCET) of MPMs revealed that MPMs can enhance mass transfer, effective contact, and electron-proton transfer with sludge. These outcomes were confirmed by the statistical analyses of variations in the interfacial thermodynamics and PCET of sludge with and without MPMs during AD. It was thus proposed that the MPMs enhanced the PCET of sludge and PCET-driven release of protons from water by promoting the interfacial Lewis acid-base interactions of sludge, thereby resulting in the enrichment of free and attached methanogenic consortia and the high energy-conserving metabolic cooperation. This proposition was further confirmed by identifying the predominant syntrophic partners, suggesting that PCET-based efficient methanogenesis was attributable to the enrichment of genomes harbouring CO2-reducing pathway and genes encoding water-mediated proton transfer. These findings offer new insights into how substrate properties can be altered by exogenous materials to enable highly efficient methanogenesis.

Synergistic enhancement of microbes-to-pollutants and inter-microbes electron transfer by Fe, N modified ordered mesoporous biochar in anaerobic digestion

Extracellular electron transfer was essential for degrading recalcitrant pollutants by anaerobic digestion (AD). Therefore, existing studies improved AD efficiency by enhancing the electron transfer from microbes-to-pollutants or inter-microbes. This study synthesized a novel Fe, N co-doped biochar (Fe, N-BC), which could enhance both the microbes-to-pollutants and inter-microbes electron transfer in AD. Detailed characterization data indicated that Fe, N-BC has an ordered mesoporous structure, high specific surface area (463.46 m2/g), and abundant redox functional groups (Fe2+/Fe3+, pyrrolic-N), which translate into excellent biocompatibility and electrochemical properties of Fe, N-BC. By adding Fe, N-BC, the stability and efficiency of the medium-temperature AD system in the treatment of methyl orange (MO) wastewater were improved: obtained a high degradation efficiency of MO (96.8 %) and enhanced the methane (CH4) production by 65 % compared to the control group. Meanwhile, Fe, N-BC reduced the accumulation of volatile fatty acids in the AD system, and the activity of anaerobic granular sludge electron transport system and coenzyme F420 was enhanced. In addition, Fe, N-BC showed positive enrichment of azo dyes decolorization bacteria (Georgenia) and direct interspecies electron transfer (DIET) synergistic partners (Syntrophobacter, Methanosarcina). Overall, the rapid degradation of MO and enhanced CH4 production in AD systems by Fe, N-BC is associated with enhancing two electronic pathways, i.e., microbes to MO and DIET between syntrophic bacteria and methanogenic archaea. This study introduced an enhanced "two-pathways of electron transfer" theory, realized by Fe, N-BC. These findings provided new insights into the interactions within AD systems and offer strategies for enhancing their performance with recalcitrant pollutants.

Advances in microbial community, mechanisms and stimulation effects of direct interspecies electron transfer in anaerobic digestion

Anaerobic digestion (AD) has been proven to be an effective green technology for producing biomethane while reducing environmental pollution. The interspecies electron transfer (IET) processes in AD are critical for acetogenesis and methanogenesis, and these IET processes are carried out via mediated interspecies electron transfer (MIET) and direct interspecies electron transfer (DIET). The latter has recently become a topic of significant interest, considering its potential to allow diffusion-free electron transfer during the AD process steps. To date, different multi-heme c-type cytochromes, electrically conductive pili (e-pili), and other relevant accessories during DIET between microorganisms of different natures have been reported. Additionally, several studies have been carried out on metagenomics and metatranscriptomics for better detection of DIET, the role of DIET's stimulation in alleviating stressed conditions, such as high organic loading rates (OLR) and lower pH, and the stimulation mechanisms of DIET in mixed cultures and co-cultures by various conductive materials. Keeping in view this significant research progress, this study provides in-depth insights into the DIET-active microbial community, DIET mechanisms of different species, utilization of various approaches for stimulating DIET, characterization approaches for effectively detecting DIET, and potential future research directions. All these can help accelerate the field's research progress, enable a better understanding of DIET in complex microbial communities, and allow its utilization to alleviate various inhibitions in complex AD processes.

Heterogeneous g-C3N4/polyaniline composites enhanced the conversion of organics into methane during anaerobic wastewater treatment

In this study, g-C3N4/PANI was prepared by in situ oxidative polymerization. Graphite-phase carbon nitride (g-C3N4) with surface defects was deposited onto the surface of conductive polyaniline (PANI) to form a p-n heterojunction. This construction aimed to create an efficient heterogeneous catalyst, increasing the surface defect level and active sites of the composite, and augmenting its capability to capture and transfer extracellular electrons under anaerobic conditions. This addresses the challenge of low efficiency in direct interspecies electron transfer between bacteria and archaea during anaerobic digestion for methane production. The results showed that the prepared g-C3N4/PANI increased the CH4 yield and CH4 production rate by 82% and 96%, respectively. Notably, the conductivity and XPS test results showed that the ratio of g-C3N4 to PANI was 0.15, and the composite exhibited favorable conductivity, with a uniform distribution of pyrrolic nitrogen, pyridinic nitrogen, and graphitic nitrogen, each accounting for approximately 30%. Furthermore, g-C3N4/PANI effectively enhanced the metabolic efficiency of intermediate products such as acetate and butyrate. Analysis of the microbial community structure revealed that g-C3N4/PANI led to a significant increase in the abundance of hydrogenotrophic methanogen Methanolinea (from 48% to 64%) and enriched Clostridium (a rise of 1%) with direct interspecies electron transfer capability. Microbial community function analysis demonstrated that the addition of g-C3N4/PANI boosted the activities of key enzymes involved in anaerobic digestion, including phosphate transacetylase (PTA), phospho-butyryl transferase (PTB), and NAD-independent lactate dehydrogenase (NNLD), by 47%, 135%, and 153%, respectively. This acceleration in enzymatic activity promoted the metabolism of acetyl-CoA, butyryl-CoA, and pyruvate. Additionally, the function of ABC transporters was enhanced, thereby improving the efficiency of material and energy exchange among microorganisms.

Enhancing sustainable waste management: Hydrothermal carbonization of polyethylene terephthalate and polystyrene plastics for energy recovery

Hydrothermal carbonization (HTC) of single plastic polymers such as polyethylene terephthalate (PET) and polystyrene (PS) has not yet been explored on a large scale, particularly their thermal behavior, chemical transformations under subcritical conditions, and the energy properties of the resultant hydrochar. This study investigated these aspects by employing techniques, such as thermogravimetric analysis (TGA), Fourier transformed infrared spectroscopy (FTIR), elemental and calorific analysis. The results show that PET hydrochar has a superior energy densification (1.37) and energy yield (89 %) compared to PS hydrochar (1.13, 54 %). Hydrothermal carbonization modifies the chemical structure of the polymers by increasing the number of carbonyl groups (CO) in PET and forming new ones in PS, and by enhancing hydroxyl groups (OH) in PET while retaining them in PS. Both materials preserve their aromatic and aliphatic structures, with the introduction of alkenes groups (CC) in the PET hydrochar. PET hydrochar begins to decompose at lower temperatures (150-270 °C) than PS hydrochar (242-283 °C) but reaches higher peak temperatures (420-585 °C vs. 390-470 °C), with both types achieving similar burnout temperatures (650-800 °C). PET hydrochar recorded a higher activation energy (121-126 kJ/mol) than PS hydrochar (67-74 kJ/mol) with the Mampel first-order reaction model as the best fit.

Different regulation strategies of anaerobic digestion by AC/CaO2 and Fe3O4/CaO2: Reactive oxygen species induction, methanogenic performance, and microbial response

This study examined the combination of activated carbon and magnetite with calcium peroxide in enhancing the anaerobic digestion (AD) performance of food waste (FW). The individual mechanisms of these two approaches were also clarified. The results indicated that AC/CaO2 achieved the highest specific methane yield of 434.4 mL/g VS, followed by Fe3O4/CaO2 (416.9 mL/g VS). Both were significantly higher than other groups (control, AC, Fe3O4, and CaO2 were 330.1, 341.4, 342.8, and 373.2 mL/g VS, respectively). Additionally, compared to Fe3O4/CaO2, AC/CaO2 further increased reactive oxygen species (ROS), thereby enhancing the hydrolytic acidification process. Simultaneously, the higher ROS levels of Fe3O4/CaO2 and AC/CaO2 promoted the formation of microbial aggregates and established a more robust enzymatic defense system and unique damage repair strategy. The research comparatively analyzed the synergistic mechanism of iron-based and carbon-based conductive materials with CaO2, providing new perspectives for optimizing the AD of FW.

Coupling physiochemical adsorption with biodegradation for enhanced removal of microcystin-LR in water

To innovate the design of water treatment technology for algal toxin removal, this research investigated the mechanisms of cyanotoxin microcystin-LR (MC-LR) removal by a coupled adsorption-biodegradation. Eight types of woody carbonaceous adsorbents with and without Sphingopyxis sp. m6, a MC-LR degrading bacterium, were tested for MC-LR removal in water. All adsorbents showed good adsorption capability, removing 40 % to almost 100 % of the MC-LR (4.5 mg/L) within 48 h in batch experiments. Adding Sphingopyxis sp. m6 continuously promoted MC-LR biological removal, and successfully broke the barrier of adsorption capacity of tested adsorbents, removing >90 % of the MC-LR in most of the coupled adsorption-biodegradation tests, especially for those adsorbents had low physiochemical adsorption capacity. Variance partitioning analysis indicated that mesopore was the dominant contributor to adsorption capacity of MC-LR in pure adsorption treatments, which acted synergistically with electrical conductivity, polarity and total functional groups on the absorbent. Pore structure was the key factor beneficial for the growth of Sphingopyxis sp. m6 (51% contribution) and subsequent MC-LR biological removal rate (80 % contribution). Overall, pinewood-based carbonaceous adsorbents (especially pinewood activated carbon) exhibited the highest adsorption capacity towards MC-LR and provided the most favorable conditions for biological removal of MC-LR, largely because of their high mesopore volume, total functional groups and electric conductivity. The research outcomes not only deepened the quantitative understanding of mechanisms for MC-LR removal by the coupled process, but also provided theoretical basis for future materials' selection and modification during the practical application of coupled process.

Exploring nature's filters: Peat-mineral mix for low and high-strength oilfield produced water reclamation

Nature-based solutions are encouraged for treating oilfield produced water from oil and gas extraction, a crucial undertaking that aligns with the Canadian oil sands industry's ambitious goal of zero waste, and the globally recognized Sustainable Development Goals (SDGs) pertaining to water conservation and ecosystem preservation. This study explored the use of peat-mineral mix (PMM), a leftover of inevitable oil sands mining, for treating low and high-strength wastewaters during biofiltration, which contained large molecular weight (44.3 kDa), which include alcohols, aliphatics, aromatics, and ketones, and can impart high toxicity to both fauna and flora (MicroTox: 99 %). The breakthrough curve indicated an effective initial adsorption phase driven by advection within the column dynamics. For complete organics removal and mechanistic insights, the wastewater was re-circulated in a continuous mode for up to 42 days. Here, we found that chemical oxygen demand was reduced from ∼85,000 mg/L to ∼965 mg/L). Kinetics investigations along with physicochemical characterization of PMM and wastewater suggested that chemisorption and anaerobic digestion contributed to the overall removal of contaminants. Chemisorption, led by hydrogen bonding and hydrophobic interactions, was the dominant mechanism, with a limited contribution from physical adsorption (surface area: 2.85 m2/g). The microbial community within the PMM bed was rich/diverse (Shannon > 6.0; Chao1 > 600), with ∼ 50 % unclassified phylotypes representing 'microbial dark matter'. High electric conductivity (332.1 μS cm-1) of PMM and the presence of Geobacter, syntrophs, and Methanosaeta suggest that direct interspecies electron transfer was likely occurring during anaerobic digestion. Both low and high-strength wastewaters showed effective removal of dissolved organics (e.g., naphthenic acids, acid extractable fraction, oil and grease content), nutrients, and potentially toxic metals. The successful use of PMM in treating oilfield produced water offers promising avenues for embracing nature-based remediation solutions at oil refining sites.

Material and microbial perspectives on understanding the role of biochar in mitigating ammonia inhibition during anaerobic digestion

With the increasing adoption of carbon-based strategies to enhance methanogenic processes, there is a growing concern regarding the correlation between biochar properties and its stimulating effects on anaerobic digestion (AD) under ammonia inhibition. This study delves into the relevant characteristics and potential mechanisms of biochar in the context of AD system under ammonia inhibition. The introduction of optimized biochar, distinguished by rich CO bond, abundant defect density, and high electronic capacity, resulted in a significant reduction in the lag period of anaerobic digestion system under 5.0 g/L ammonia stress, approximately by around 63 % compared to the control one. Biochar helps regulate the community structure, promotes the accumulation of acetate-consuming bacteria, in the AD system under ammonia inhibition. More examinations show that biochar promotes direct interspecies electron transfer in AD system under ammonia inhibition, as evidenced by diminished levels of bound electroactive extracellular polymeric substances, increased abundance of electroactive bacteria, and notably, the up-regulation of direct interspecies electron transfer associated genes, including the conductive pili and Cytochrome C genes, as revealed by meta-transcriptomic analysis. Additionally, gene expression related to proteins associated with ammonium detoxification were found to be up-regulated in systems supplemented with biochar. These findings provide essential evidence and insights for the selection and potential engineering of effective biochar to enhance AD performance under ammonia inhibition.

The role of superoxide anion to Cr(VI) reduction by pine biochar

It has been reported that Cr(VI) can be reduced by biochar because of its redox activity. Considering the anionic form of Cr(VI), we hypothesize that the reduction in aqueous phase is significant. However, the contribution of different reactive oxygen species in the biochar-Cr(VI) reaction system has not been distinguished. Herein, we quantitatively identified Cr(VI) adsorption and reduction in biochar systems. The reduction content of Cr(VI) was 1.5 times higher in untreated conditions than in anaerobic conditions. The disappearance of·O2- under anaerobic conditions illustrated that·O2- may be involved in the reduction of Cr(VI). Quenching of·O2- resulted in a decrease of Cr(VI) reduction by 34%, while 1O2 was negligible, probably due to the stronger electron-donating capacity of·O2-. The degradation of nitrotetrazolium blue chloride (quenching agent of·O2-) confirmed that the reduction process of·O2- mainly occurred in the liquid-phase. Boehm titration and quantification of·O2- further elucidated the significant correlation (P < 0.05) between phenolic groups and the formation of·O2-, which implied that phenolic groups acted as the primary electron donors in generating·O2-. This study highlights the importance of the liquid-phase reduction process in removing Cr(VI), which provides theoretical support for biochar conversion of Cr(VI).

Facilitating effects of the synergy with zero-valent iron and peroxysulfate on the sludge anaerobic fermentation system: Combined biological enzyme, microbial community and fermentation mechanism assessment

This study evaluated a synergetic waste activated sludge treatment strategy with environmentally friendly zero-valent iron nanoparticles (Fe0) and peroxysulfate. To verify the feasibility of the synergistic treatment, Fe0, peroxysulfate, and the mixture of peroxysulfate and Fe0 (synergy treatment) were added to different sludge fermentation systems. The study demonstrated that the synergy treatment fermentation system displayed remarkable hydrolysis performance with 435.50 mg COD/L of protein and 197.67 mg COD/L of polysaccharide, which increased 1.13-2.85 times (protein) and 1.12-1.49 times (polysaccharide) for other three fermentation system. Additionally, the synergy treatment fermentation system (754.52 mg COD/L) exhibited a well acidification performance which was 1.35-41.73 times for other systems (18.08-557.27 mg COD/L). The synergy treatment fermentation system had a facilitating effect on the activity of protease, dehydrogenase, and alkaline phosphatase, which guaranteed the transformation of organic matter. Results also indicated that Comamonas, Soehngenia, Pseudomonas, and Fusibacter were enriched in synergy treatment, which was beneficial to produce SCFAs. The activation of Fe0 on peroxysulfate promoting electron transfer, improving the active groups, and increasing the enrichment of functional microorganisms showed the advanced nature of synergy treatment. These results proved the feasibility of synergy treatment with Fe0 and peroxysulfate to enhance waste activated sludge anaerobic fermentation.

Magnetic porous microspheres enhancing the anaerobic digestion of sewage sludge: Synergistic free and attached methanogenic consortia

The addition of exogenous materials is a commonly reported method for promoting the anaerobic digestion (AD) of sludge. However, most exogenous materials are nano-sized and their use encounters problems relating to a need for continuous replenishment, uncontrollability and non-recyclability. Here, magnetic porous microspheres (MPMs), which can be controlled by magnetic forces, were prepared and used to enhance the methanogenesis of sludge. It was observed that the MPMs were spherical particles with diameters of approximately 100 µm and had a stable macroporous hybrid structure of magnetic cores and polymeric shells. Furthermore, the MPMs had good magnetic properties and a strong solid-liquid interfacial electron transfer ability, suggesting that MPMs are excellent carriers for methanogenic consortia. Experimental results showed that the addition of MPMs increased methane production and the proportion of methane in biogas from AD by 100.0 % and 21.2 %, respectively, indicating the MPMs notably enhanced the methanogenesis of sludge. Analyses of variations in key enzyme activities and electron transfer in sludge samples with and without MPMs in AD revealed that the MPMs significantly enhanced the activities of key enzymes involved in hydrolysis, acidification and methanation. This was achieved mainly by enhancing the extracellular electron transfer to strengthen the proton motive force on the cell membrane, which provides more energy generation for methanogenic metabolism. A careful examination of the variations in the morphology, pore structure and magnetism of the MPMs before and after AD revealed that the MPMs increased the prevalence of many highly active anaerobes, and that this did not weaken the magnetic performance. The microbial community structure and metatranscriptomic analysis further indicated that the acetotrophic methanogens (i.e., Methanosaeta) were mainly in a free state and that CO2-reducing methanogens (i.e., Methanolinea and Methanobacterium) mainly adhered to the MPMs. The above synergistic metabolism led to efficient methanogenesis, which indicates that the MPMs optimised the spatial ecological niche of methanogenic consortia. These findings provide an important reference for the development of magnetic porous materials promoting AD.

Responses of methane production and methanogenic pathways to polystyrene nanoplastics exposure in paddy soil

Nanoplastics (NPs) have attracted increasing attention within terrestrial ecosystems. However, our understanding of their impacts on the intricate anaerobic methanogenesis processes occurring in paddy soils microbial communities remains limited with respect to nanoplastics shape, function, and metabolic effects. Herein, we explored the effects of polystyrene nanoplastics (PS-NPs) and microplastics (PS-MPs) on anaerobic methanogenesis in a typical paddy soil. The results show that PS-NPs delayed methane production and the time to reach peak acetate content in incubation process of paddy soils, and the methanogenic rate increased rapidly after 13 days, with a maximum increase of 87.97%. However, PS-MPs had no marked effect on CH4, CO2 and acetate production. In addition, PS-NPs affected soil physicochemical properties by reducing pH and increasing electrical conductivity. Acetoclastic methanogens were enriched and the relative abundance of the genes ackA, pta, ACSS, cdhC, cdhD and cdhE in the acetoclastic pathways were significantly increased under PS-NPs exposure. In addition, PS-MPs had significant effect on the microbial community structure but no effect on methanogenic pathways of the paddy soils. This study provides important insights into the response of key microorganisms, functional genes and methanogenesis pathways to NPs during anaerobic methanogenesis in paddy soils.

Metagenomic analysis towards understanding the effects of ammonia on chain elongation process for medium chain fatty acids production

The production of medium chain fatty acids (MCFAs) through chain elongation (CE) from organic wastes/wastewater has attracted much attention, while the effects of a common inhibitor-ammonia has not been elucidated. The mechanism of ammonia affecting CE was studied by metagenomic. The lag phase duration of caproate production was increased, and the maximum caproate production rate was decreased by 43.4 % at 4 g-N/L, as compared to 0 g-N/L. And hydrochar (HC) alleviated the inhibition of ammonia at 4 g-N/L. Metagenomic analysis indicated that ammonia induced UBA4085 sp.FDU78 as the dominant microorganism, and metabolic reconstruction revealed its potential CE ability. Furthermore, ammonia inhibited the reverse β oxidation pathway and Acetyl-CoA production pathway. The tolerance of UBA4085 sp.FDU78 to ammonia was associated with the uptake of inorganic ions, energy conservation, and synthesis of osmoprotectants. The present study provided a deep-insight on the ammonia tolerance mechanism on the CE process.

A review on photocatalytic attribution and process of pyrolytic biochar in environment

Biochar has attracted significant attention due to its excellent environmental benefits and extensive applications. Recently, a consensus has been accepted that biochar can act as a photocatalyst and trigger effective photocatalytic reactions in the environment, which is important to energy conversion and the cycle of elements. However, its photocatalytic processes and the corresponding environmental impacts need to receive more and due attention. In this review, we provide a comprehensive summary of the underlying correlations among the pyrolytic evolution of biomass, the structure characteristic of biochar, and the resultant photocatalytic performance. Moreover, the photocatalytic processes and the influence of environmental factors were elaborately investigated on biochar. Finally, future tendencies and challenges in the photocatalysis of biochar have been prospected in the environmental field. This review has offered innovative insights into the photocatalytic essential of biochar and highly enhanced the understanding of its environmental impact.

Syntrophic microbes involved in the oxidation of short-chain fatty acids in continuous-flow anaerobic digesters treating waste activated sludge with hydrochar

The rapid degradation of short-chain fatty acids (SCFAs) is an essential issue of anaerobic digestion (AD), in which SCFA oxidizers could generally metabolize in syntrophy with methanogens. The dynamic responses of active metagenome-assembled genomes to low concentrations of propionate and acetate were analyzed to identify specific syntrophic SCFA oxidizers and their metabolic characteristics in continuous-flow AD systems treating waste activated sludge with and without hydrochar. In this study, hydrochar increased methane production by 19%, possibly due to hydrochar enhancing acidification and methanogenesis processes. A putative syntrophic propionate oxidizer and two acetate oxidizers contributed substantially to the syntrophic degradation of SCFAs, and hydrochar positively regulated their functional gene expressions. A significant relationship was established between the replication rate of SCFA oxidizers and their stimulation-related transcriptional activity. Acetate was degraded in the hydrochar group, which might be mainly through the syntrophic acetate oxidizer from the genus Desulfallas and methanogens from the genus Methanosarcina.IMPORTANCEShort-chain fatty acid (SCFA) degradation is an important process in the methanogenic ecosystem. However, current knowledge of this microbial mechanism is mainly based on studies on a few model organisms incubated as mono- or co-cultures or in enrichments, which cannot provide appropriate evidence in complex environments. Here, this study revealed the microbial mechanism of a hydrochar-mediated anaerobic digestion (AD) system promoting SCFA degradation at the species level and identified key SCFA oxidizing bacteria. Our analysis provided new insights into the SCFA oxidizers involved in the AD of waste activated sludge facilitated by hydrochar.

Fe loading 3D micro-meso-porous carbon sphere derived from natural cellulose of sawdust activating peroxymonosulfate for degradation of enrofloxacin

Fe loading 3D micro-meso-porous carbon sphere (Fe@3C-2N) was derived from natural cellulose of sawdust and melamine through sodium alginate and ferric chloride cross-linking followed by carbonization processes, which served as peroxymonosulfate (PMS) activators for enrofloxacin (ENR) degradation. The cellulose was produced by the delignification of sawdust with sodium chlorite. The delignification of sawdust and the addition of melamine increased the porosity and electron transport capacity of Fe@3C-2N. When the dosages of Fe@3C-2N and PMS were 0.60 g L-1 and 0.20 g L-1 respectively, the degradation rate of ENR (20 mg L-1) reached 92.17 % within 80 min, suggesting the satisfactory activation performance of PMS. The good structural stability of Fe@3C-2N makes it suitable for use as packing in continuous flow reactors for wastewater treatment. Quenching experiments and electron paramagnetic resonance (EPR) suggested that SO4•- and 1O2 were the dominant reactive oxygen species (ROSs) in Fe@3C-2N/PMS system. X-ray photoelectron spectroscopy (XPS) revealed that Fe3C, pyrrolic N and graphitic N were the potential active sites.

Overview of enhancing biological treatment of coal chemical wastewater: New strategies and future directions

Coal chemical wastewater (CCW) is a type of refractory industrial wastewater, and its treatment has become the main bottleneck restricting the sustainable development of novel coal chemical industry. Biological treatment is considered as an economical, effective and environmentally friendly technology for CCW treatment. However, conventional biological process is difficult to achieve the efficient removal of refractory organics because of CCW with the characteristics of composition complexity and high toxicity. Therefore, seeking the novel enhancement strategy appears to be a favorable solution for enhancing biological treatment efficiency of CCW. This review focuses on presenting a comprehensive picture about the exogenous enhancement strategies for CCW biological treatment. The performance and potential application of exogenous enhancement strategies, including co-metabolic substrate enhancement, biofilm filler enhancement, adsorption material enhancement and conductive mediator enhancement, were expounded. Meanwhile, the enhancing mechanisms of different strategies were comprehensively discussed from a biological perspective. Furthermore, the prospects of enhancement strategies based on the engineering performance, economic cost and environmental impact (3E) evaluation were introduced. And novel enhancement strategy based on "low carbon emissions", "resource recycling" and "water environment security" in the context of carbon neutrality was proposed. Taken together, this review provides technical reference and new direction to facilitate the regulation and optimization of typical industrial wastewater biological treatment.

Performance and mechanism of chromium reduction in denitrification biofilm system with different carbon sources

In the process of biological reduction of Cr(VI), the type of carbon sources affects the rate and effect of Cr(VI) reduction, but its specific performance and influencing mechanism have not yet been explored. In this study, four denitrification biofilm reactors were operated under four common carbon sources (C6H12O6, CH3COONa, CH3OH, CH3COONa:C6H12O6 1:1) to reveal the impact of carbon sources on Cr(VI) reduction. Through preliminary experimental concentration research, 75 mg/L Cr(VI) was selected as the dosing concentration. In long-term operation, the composite carbon sources of CH3COONa and C6H12O6 demonstrated excellent stability and achieved an impressive Cr(VI) removal efficiency of 99.5 %. The following sequence was C6H12O6, CH3COONa, and CH3OH. Among them, CH3OH was less competitive and the system was severely unbalanced with lowest Cr(VI) reduction efficiency. The toxicity reactions, changes in EPS and its functional groups, and electron transfer revealed the reduction and fixation mechanism of chromium on denitrification biofilm. The changes in microbial communities indicated that microbial communities in composite carbon sources can quickly adapt to the high toxic environment. The proportion of Trichococcus reached 43.6 %, which played an important role in denitrification and Cr(VI) reduction. Meanwhile, the prediction of microbial COG function reflected its excellent metabolic ability and defense mechanism.

Functional biochar in enhanced anaerobic digestion: Synthesis, performances, and mechanisms

Anaerobic digestion technology is crucial in bioenergy recovery and organic waste management. At the same time, it often encounters challenges such as low organic digestibility and inhibition of toxic substances, resulting in low biomethane yields. Biochar has recently been used in anaerobic digestion to alleviate toxicity inhibition, improve the stability of anaerobic digestion processes, and increase methane yields. However, the practical application of biochar is limited, for the properties of pristine biochar significantly affect its application in anaerobic digestion. Although much research focuses on understanding original biochar's fundamental properties and functionalization, there are few reviews on the applications of functional biochar and the effects of critical properties of pristine biochar on anaerobic digestion. This review systematically reviewed functionalization strategies, key performances, and applications of functional biochar in anaerobic digestion. The properties determining the role of biochar were reviewed, the synthesis methods of functional biochar were summarized and compared, the mechanism of functional biochar was discussed, and the factors affecting the function of functional biochar were reviewed. This review provided a comprehensive understanding of functional biochar in anaerobic digestion processes, which would be helpful for the development and applications of engineered biochar.

Ruminal content biochar supplementation for enhanced biomethanation of rice straw: Focusing on biochar characterization and dose optimization

Anaerobic digestion (AD) of agricultural wastes is a promising approach for energy recovery and crop residue management. However, its recalcitrant chemical structure hinders microbial hydrolysis and reduces biomethane production under AD. Biochar supplementation has been proven to promote the digestibility and biomethanation of lignocellulosic substrates. Therefore, this study investigated the influence of different pyrolysis temperatures (450 °C, 550 °C, and 650 °C) on the physicochemical properties of biochar. Furthermore, the impact of ruminal content biochar supplementation (1 %, 2 %, and 3 %) on the AD of rice straw with rumen fluid as inoculum has been investigated. The ruminal content biochar (RUCB) supplemented reactors showed an increment in biomethane yield and the highest cumulative biomethane yield 243.11 mL/g volatile solids (VS)) was recorded at 2 % RUCB supplementation, followed by 227.12 mL/g VS at 1 % RUCB supplementation and 162.86 mL/g VS at 3 % RUCB supplementation (P > 0.05). Compared to the control reactors (128.68 mL/g VS), RUCB supplemented reactors exhibited 1.88-fold, 1.76-fold, and 1.26-fold increments in biomethane yield due to pH stabilization and facilitation of microbial biofilm formation on the biochar. The correlation analysis showed that biomethane production is positively correlated with VS reduction (R2 = 0.9852). This study proposed a potential strategy to utilize ruminal content waste as a feedstock for biochar production and its application in AD for accelerating the biomethanation of rice straw.

From feedstock to digestion: Unraveling the impact of humic acid composition on anaerobic digestion

In the anaerobic digestion (AD) process, the effects of humic acid (HA) derived from different feedstocks on AD are influenced by the variations in their structural composition and oxygen-containing functional groups. Thus, clarifying the structural differences of HA obtained from different feedstocks is crucial for understanding their impact on AD. In this study, the structure of five humic acids (HAs) derived from liquid digestate, food waste, silage corn straw, lignite and commercial HA, and their effects on AD were investigated. The study found that HA from food waste had more carboxyl groups, while straw-derived HA had more phenolic hydroxyl groups. Both types of HA had higher aromaticity and humification degree and showed significant inhibition effect on AD. HA from food waste had an average methanogenic inhibition rate of 43.5 % with 1 g/L HA added. In addition, commercial HA and HA derived from lignite had similar functional group types and aromaticity, with an average methanogenic inhibition rate of about 20 %. The study revealed that HAs with more carboxyl groups exhibited greater effectiveness in inhibiting AD, thereby confirming the influence of HA structures derived from different feedstocks on AD. In conclusion, this study provides valuable insights into the mechanism of HA effect on AD and offers guidance for future research focused on enhancing AD efficiency.

Natural zeolite enhances anaerobic digestion of waste activated sludge: Insights into the performance and the role of biofilm

Anaerobic digestion is widely employed for the treatment of waste activated sludge (WAS) due to its advantages like simultaneous energy recovery and sludge stabilization, promoting carbon-neutral operation of wastewater treatment plants. Natural zeolite, a low-cost and eco-friendly additive, has the potential to improve methane production from anaerobic digestion. This study investigated the effects of natural zeolite on anaerobic digestion when the substrate was WAS. It was found that methane production potential in response to natural zeolite was dosage-dependent. The optimal dosage was 0.1 g zeolite/g volatile suspended solids (VSS), with a methane yield of 181.89 ± 6.75 mL/g VSS, which increased by 20.1% compared to that of the control. Although the methane yields with other dosages of natural zeolite were higher than that of control, they were lesser than that with 0.1 g zeolite/g VSS. Natural zeolite affected transfer and conversion of proteins much more than polysaccharides in liquid phase and extracellular polymeric substances. In anaerobic digestion, natural zeolite had with little effects on WAS solubilization, while it improved hydrolysis, acidification, and methanogenesis. The dosages of natural zeolite did have significant effects on bacterial communities in biofilm rather than suspension, while the archaeal communities in biofilm and suspension were all greatly related to natural zeolite dosages. The developed biofilms promoted richness and functionality of microbial communities. The syntrophic metabolism relationships between methanogens and bacteria were improved, which was proved by selective enrichment of Methanosarcina, Syntrophomonas, and Petrimonas. The findings of this work provided some new solutions for promoting methane production from WAS, and the roles of natural zeolite in anaerobic digestion.

Metagenomic approach reveals the mechanism of calcium oxide improving kitchen waste dry anaerobic digestion

In light of the characteristics of excessive acidification and low biogas yield during kitchen waste (KW) dry digestion, the impact of the calcium oxide (CaO) on KW mesophilic dry digestion was investigated, and the enhanced mechanism was revealed through metagenomic approach. The results showed that CaO increased the biogas production, when the CaO dosage was 0.07 g/g (based on total solid), the biogas production reached 656.84 mL/g suspended solids (VS), approximately 8.38 times of that in the control. CaO promoted the leaching and hydrolysis of key organic matter in KW. CaO effectively promoted the conversion of volatile fatty acid (VFA) and mitigated over-acidification. Macrogenome analysis revealed that CaO increased the microbial diversity in KW dry digestion and upregulated the abundance of genes related to amino acid and carbohydrates metabolism. This study provides an effective strategy with potential economic benefits to improve the bioconversion efficiency of organic matter in KW.

Multi-omics analysis unravels effects of salt and oil on substance transformation, microbial community, and transcriptional activity in food waste anaerobic digestion

In this study, through quantitative detection of key substances and enzyme activities, an integrated analysis of 16S rRNA sequencing and metatranscriptomics revealed the mechanisms by which salt and oil influence the biotransformation process during anaerobic digestion (AD). The results demonstrated that a salt concentration of 6 g/L promoted lipid metabolism and hydrogenotrophic methanogenesis, while inhibiting the acetoclastic pathway. An oil concentration of 5 g/L facilitated the expression of key enzyme-encoding genes involved in β-oxidation of long-chain fatty acids, transcription, and acetoclastic methanogenesis. It also promoted the enrichment of syntrophic propionate/butyrate oxidation bacteria (Syntrophomonas and DMER64). Salt/oil co-addition enhanced the expression of genes related to glucose metabolism, amino acid metabolism, organic acid synthesis, and quorum sensing. Furthermore, salt/oil co-addition inhibited the secretion of key enzymes related to methanogens by impeding the transcription process. Collectively, these findings provide systematic insights into how salt and oil affect the biochemical metabolic mechanisms of AD.

Evaluating effects of biochar on anaerobic digestion of dewatered waste activated sludge: Digester performance, microbial co-metabolism and underlying mechanism

Biochar has been proven to be capable of improving the performance of anaerobic digestion (AD). However, the effect of biochar on microbial communities remains ambiguous. In this study, the influence of pH was excluded in a semi-continuous anaerobic digestor for the treatment of dewatered waste activated sludge (WAS) to determine the effect of biochar on microbes. Compared with the control group, the average methane production increased by 24.5% and 23.2% at the organic loading rates (OLRs) of 1.56 and 3.00 gTS/L/d, respectively, in the presence of biochar. This study innovatively found biochar accelerated the enrichment of Methanofastidiosaceae, which competed with Methanobacteriaceae for H2, and its abundance increased from 0.99% at the OLR of 1.56 g TS/L/d to 16.57% and 38.11% at the OLR of 3.00 and 5.60 gTS/L/d, respectively. The efficient metabolic network of f__norank_o__Aminicenantales, syntrophic bacteria, Methanofastidiosaceae and Methanosaetaceae promoted the conversion of WAS to CH4 in the biochar group. In addition, metagenome analysis revealed that biochar optimized the metabolites related to energy conservation and electron transfer, particularly for hydrogenase (frhABG, mbhLHK and hndA-D), confirming that biochar changed the way H2 was involved in methanogenesis. These findings provide novel insights into the direct effect of biochar on microbial evolution and facilitate the reduction of WAS to achieve higher economic benefits in biogas production.

Biochar symbiosis in anaerobic digestion to enhance biogas production: A comprehensive review

In recent years, anaerobic digestion (AD) has gained popularity as a practical method for generating clean energy and efficiently managing organic waste. However, the effectiveness of the reactor is compromised by the accumulation of ammonia, acids, and nutrients, leading to inhibition and instability. Because of its adaptability, biochar (BC) has sparked a substantial interest in biogas production and can be created by charring biomass and waste materials. Adding BC to the AD process could yield the following benefits: mitigating toxic inhibition, reducing the duration of the methanogenic lag phase, immobilising functional bacteria, and enhancing the rate of electron transfer between methanogenic and acetogenic microorganisms. Nonetheless, there remains to be more comprehensive knowledge regarding the multifaceted function of BC and its intricate mechanisms in the generation of biogas in AD. The research summarises scattered information from the literature on BC production from various feedstocks and factors affecting its characteristics. Additionally, a comprehensive analysis of the utilisation of BC as an additive within AD is presented here, emphasising how BC characteristics impact AD processes and how they effectively engage key challenges.

Influence of Pre-Incubation of Inoculum with Biochar on Anaerobic Digestion Performance

The application of biochar as an additive to enhance the anaerobic digestion (AD) of biomass has been extensively studied from various perspectives. This study reported, for the first time, the influence of biochar incubation in the inoculum on the anaerobic fermentation of glucose in a batch-type reactor over 20 days. Three groups of inoculum with the same characteristics were pre-mixed once with biochar for different durations: 21 days (D21), 10 days (D10), and 0 days (D0). The BC was mixed in the inoculum at a concentration of 8.0 g/L. The proportion of the inoculum and substrate was adjusted to an inoculum-to-substrate ratio of 2.0 based on the volatile solids. The results of the experiment revealed that D21 had the highest cumulative methane yield, of 348.98 mL, compared to 322.66, 290.05, and 25.15 mL obtained from D10, D0, and the control, respectively. Three models-modified Gompertz, first-order, and Autoregressive Integrated Moving Average (ARIMA)-were used to interpret the biomethane production. All models showed promising fitting of the cumulative biomethane production, as indicated by high R2 and low RMSE values. Among these models, the ARIMA model exhibited the closest fit to the actual data. The biomethane production rate, derived from the modified Gompertz Model, increased as the incubation period increased, with D21 yielding the highest rate of 31.13 mL/gVS. This study suggests that the application of biochar in the anaerobic fermentation of glucose, particularly considering the short incubation period, holds significant potential for improving the overall performance of anaerobic digestion.

Direct interspecies electron transfer mechanisms of a biochar-amended anaerobic digestion: a review

This paper explores the mechanisms of biochar that facilitate direct interspecies electron transfer (DIET) among syntrophic microorganisms leading to improved anaerobic digestion. Properties such as specific surface area (SSA), cation exchange capacity (CEC), presence of functional groups (FG), and electrical conductivity (EC) were found favorable for increased methane production, reduction of lag phase, and adsorption of inhibitors. It is revealed that these properties can be modified and are greatly affected by the synthesizing temperature, biomass types, and residence time. Additionally, suitable biochar concentration has to be observed since dosage beyond the optimal range can create inhibitions. High organic loading rate (OLR), pH shocks, quick accumulation and relatively low degradation of VFAs, and the presence of heavy metals and toxins are the major inhibitors identified. Summaries of microbial community analysis show fermentative bacteria and methanogens that are known to participate in DIET. These are Methanosaeta, Methanobacterium, Methanospirillum, and Methanosarcina for the archaeal community; whereas, Firmicutes, Proteobacteria, Synergistetes, Spirochetes, and Bacteroidetes are relatively for bacterial analyses. However, the number of defined cocultures promoting DIET is very limited, and there is still a large percentage of unknown bacteria that are believed to support DIET. Moreover, the instantaneous growth of participating microorganisms has to be validated throughout the process.

Revealing the mechanism of novel nitrogen-doped biochar supported magnetite (NBM) enhancing anaerobic digestion of waste-activated sludge by sludge characteristics

Anaerobic digestion (AD) is a promising technology in waste treatment and energy recovery. However, it suffers from long retention time and low biogas yield. In this study, novel nitrogen-doped biochar supported magnetite (NBM) was synthesized and applied to enhance the AD of waste-activated sludge. Results showed that NBM increased cumulative methane production and SCOD removal efficiency by up to 1.75 times and 15% respectively at 5 g/L compared with the blank. NBM enhanced both hydrolysis and methanogenesis process during AD and the activities of α-glucosidase, protease, coenzyme F₄₂₀ and electron transport system were increased by 19%, 163%, 104% and 160% respectively at 5 g/L NBM compared with the blank. NBM also facilitated the secretion of conductive protein in extracellular polymeric substances as well as the formation of conductive pili, leading to 3.18-7.59 times higher sludge electrical conductivity. Microbial community analysis revealed that bacteria Clostridia and archaea Methanosarcina and Methanosaeta were enriched by the addition of NBM, and direct interspecies electron transfer might be promoted between them. This study provides a practical reference for future material synthesis and its application.

Insights into the roles of humic acids in facilitating the anaerobic digestion process

Humic acids (HAs) are important byproducts of anaerobic digestion (AD), which have complex structures and dynamic electrochemical activities. However, the effects of HAs on AD process were usually misestimated due to the neglect of the in situ generated HAs and the interaction between HAs and metal ions. This study explored the effects of HAs on AD performance using corn straw as typical "clean" substrate (rare in metals content) via commercial HAs (C-HAs) addition and in-situ-generated HAs (In-HAs) removal. Results showed that C-HAs (1 g/L) addition promoted the maximum methane production rate (Rm) by 20.6%, while In-HAs removal decreased the Rm by 42.7%. Meanwhile, C-HAs showed little effect on the acidification of corn straw but increased the Rm during the methanation of ethanol by 41.6%. Both the C-HAs and In-HAs were rich in surface oxygen-containing functional groups, which enabled them to act as electron shuttles and facilitate the syntrophic methanogenesis. HAs also acted in regulation of syntrophic microorganisms. For instance, C-HAs addition enriched the relative abundances of Cloacimonadia, Spirochaetia, Synergistia and Methanosarcina, while the removal of In-HAs reduced the relative abundances of Spirochaetia and Synergistia. In conclusion, HAs addition to the AD process could be a feasible approach to improve methane production by enhancing direct interspecies electron transfer during AD of lignocellulosic biomass.

Microbial response behavior to powdered activated carbon in high-solids anaerobic digestion of kitchen waste: Metabolism and functional prediction analysis

Anaerobic digestion (AD) can not only treat organic waste, but also recycle energy. However, high-solids AD of kitchen waste usually failed due to excessive acidification. In this study, the effect of activated carbon (AC) on kitchen waste AD performance was investigated under high-solids conditions (total solids contents = 15%). The results showed that efficiencies of acidogenesis and methanogenesis were promoted in presence of moderate concentration (50 g/L > AC >5 g/L), but high concentration (AC >70 g/L) weakened AD performance. Moreover, AC addition enhanced the methane production rate from 66.0 mL/g VS to 231.50 mL/g VS, i.e., up to 250.7%. High-throughput sequencing results demonstrated that the abundance of electroactive DMER64 increased from less than 1%-29.7% (20 g/L AC). As AC gradually increased，aceticlastic methanogenesis changed to hydrogenotrophic pathway. Predicted functional analysis indicated that AC can enhance abundances of energy and inorganic ion metabolism, resulting in high methane production.

Co-digestion of food waste and hydrothermal liquid digestate: Promotion effect of self-generated hydrochars

Hydrothermal treatment (HTT) can efficiently valorize the digestate after anaerobic digestion. However, the disposal of the HTT liquid is challenging. This paper proposes a method to recover energy through the anaerobic co-digestion of food waste and HTT liquid fraction. The effect of HTT liquid recirculation on anaerobic co-digestion performance was investigated. This study focused on the self-generated hydrochars that remained in the HTT supernatant after centrifugation. The effect of the self-generated hydrochars on the methane (CH₄) yield and microbial communities were discussed. After adding HTT liquids treated at 140 and 180 °C, the maximum CH₄ production increased to 309.36 and 331.61 mL per g COD, respectively. The HTT liquid exhibited a pH buffering effect and kept a favorable pH for the anaerobic co-digestion. In addition, the self-generated hydrochars with higher carbon content and large oxygen-containing functional groups remained in HTT liquid. They increased the electron transferring rate of the anaerobic co-digestion. The increased relative abundance of Methanosarcina, Syntrophomonadaceae, and Synergistota was observed with adding HTT liquid. The results of the principal component analysis indicate that the electron transferring rate constant had positive correlationships with the relative abundance of Methanosarcina, Syntrophomonadaceae, and Synergistota. This study can provide a good reference for the disposal of the HTT liquid and a novel insight regarding the mechanism for the anaerobic co-digestion.

Cadmium-resistant phosphate-solubilizing bacteria immobilized on phosphoric acid-ball milling modified biochar enhances soil cadmium passivation and phosphorus bioavailability

Cadmium (Cd) can accumulate in agriculture soil from the regular application of phosphorus (P) fertilizer. Microbiological method is considered as a potentially effective strategy that can not only remediate the Cd-contaminated soil but also provide the phosphorus needed for crop growth. However, the toxicity of Cd may affect the activity of microorganisms. To solve this problem, Klebsiella variicola with excellent phosphate solubilization ability (155.30 mg L^-1 at 48 h) and Cd adsorption rate (90.84 % with 10 mg L^-1 Cd initial concentration) was firstly isolated and identified in this study. Then, a phosphoric acid and ball milling co-modified biochar (PBC) was selected as the carrier to promote the activities of K. variicola under Cd pollution. Surface characterization revealed that the promotion of K. variicola by PBC was mainly attributed to the large specific surface area and diverse functional groups. Compared to contaminated soil, microbial PBC (MPBC) significantly increased the pakchoi biomass and phosphorus (P) content, while the Cd content in leave and root of pakchoi (Brassica chinensis L.) decreased by 25.90-43.46 % (P < 0.05). The combined application also favored the transformation of the resistant P fractions to bioavailable P, and facilitated the immobilization of 20.12 % exchangeable Cd to reducible, oxidizable, and residual Cd in the treated soil. High-throughput sequencing revealed that the response of the soil microbial community to the MPBC was more beneficial than K. variicola or PBC alone. Therefore, the application of MPBC has the potential to act as an efficient, stable, and environmentally friendly sustainable product for Cd remediation and enhanced P bioavailability in agricultural production.

Microbial insights towards understanding the role of hydrochar in enhancing phenol degradation in anaerobic digestion

Anaerobic digestion (AD) of wastewater is the most promising bioprocess for organic conversion, however, phenol is toxic and resistant to anaerobic degradation. The current study compared the effect of hydrochar and granular activated carbon (GAC) on AD of phenol at four concentrations (100 mg/L, 250 mg/L, 500 mg/L and 1000 mg/L). Results demonstrated that hydrochar significantly improved the methane production rate and reduced the lag phase at all concentrations of phenol. The methane production rate was improved by about 50% at both 100 mg/L and 250 mg/L phenol, while it was raised by >160% at 500 mg/L and 1000 mg/L phenol by hydrochar. The GAC only increased the methane production rate at 500 mg/L and 1000 mg/L due to high adsorption capacity. Further, the adsorption of phenol by hydrochar had no apparent impact on the methane production rate, even though certain amounts of phenol were adsorbed. At 500 mg/L, the amount of methane produced significantly increased, so 16S rRNA transcripts sequencing and metabolomic analysis were conducted. 16S rRNA transcripts sequencing analysis indicated that hydrochar resulted in the enrichment of syntrophic bacteria (e.g., Syntrophorhabdus &Syntrophobacter) and Methanosaeta, which might be related with direct interspecies electron transfer. Further, it was noticed that the growth of Methanobacterium was repressed at 500 mg/L phenol, while hydrochar promoted its growth. Phenol was degraded into L-tyrosine and then followed the benzoate degradation pathway for methane production as revealed by metabolomic analysis. In addition, metabolomic analysis also revealed that hydrochar promoted the degradation of all metabolites and enhanced the phenol degradation into methane.

Hybrid bioreactor built-in with fixed bio-carriers for denitrification with low C/N ratio: Hydrodynamic optimization and microbial divergence

Hydrodynamics played an important role in the design and operation of bioreactors for wastewater treatment. In this work, an up-flow anaerobic hybrid bioreactor built-in with fixed bio-carriers was designed and optimized using computational fluid dynamics (CFD) simulation. The results indicated that the flow regime involving with vortex and dead zone was greatly affected by the positions of water inlet and bio-carrier modules. The ideal hydraulic features were obtained when the water inlet and bio-carrier modules located 9 cm and 60 cm above the bottom of reactor. Using the optimum hybrid system for nitrogen removal from wastewater with low carbon-to-nitrogen ratio (C/N = 3), the denitrification efficiency could reach 80.9 ± 0.4%. Illumina sequencing of 16S rRNA gene amplicons revealed that the microbial community divergence occurred among the biofilm on bio-carrier, the suspended sludge phase and the inoculum. Especially, the relative abundance of denitrifying genera Denitratisoma in the biofilm of bio-carrier reaches 5.73%, 6.2 times higher than that in the suspended sludge, implying the imbedded bio-carrier was conductive to enrich the specific denitrifiers to polish the denitrification performance with low carbon source. This work provided an effective method for the design optimization of bioreactor based on CFD simulation, and developed a hybrid reactor with fixed bio-carrier for nitrogen removal from wastewater with low C/N ratio.

Hydrochar mediated anaerobic digestion of bio-wastes: Advances, mechanisms and perspectives

Bio-wastes treatment and disposal has become a challenge because of their increasing output. Given the abundant organic matter in bio-wastes, its related resource treatment methods have received more and more attention. As a promising strategy, anaerobic digestion (AD) has been widely used in the treatment of bio-wastes, during which not only methane as energy can be recovered but also their reduction can be achieved. However, AD process is generally disturbed by some internal factors (e.g., low hydrolysis efficiency and accumulated ammonia) and external factors (e.g., input pollutants), resulting in unstable AD operation performance. Recently, hydrochar was wildly found to improve AD performance when added to AD systems. This review comprehensively summarizes the research progress on the performance of hydrochar-mediated AD, such as increased methane yield, improved operation efficiency and digestate dewatering, and reduced heavy metals in digestate. Subsequently, the underlying mechanisms of hydrochar promoting AD were systematically elucidated and discussed, including regulation of electron transfer (ET) mode, microbial community structure, bio-processes involved in AD, and reaction conditions. Moreover, the effects of properties of hydrochar (e.g., feedstock, hydrothermal carbonization (HTC) temperature, HTC time, modification and dosage) on the improvement of AD performance are systematically concluded. Finally, the relevant knowledge gaps and opportunities to be studied are presented to improve the progress and application of the hydrochar-mediated AD technology. This review aims to offer some references and directions for the hydrochar-mediated AD technology in improving bio-wastes resource recovery.

Proteomic profiling of robust acetoclastic methanogen in chrysene-altered anaerobic digestion: Global dissection of enzymes

Methanogen is a pivotal player in pollution treatment and energy recovery, and emerging pollutants (EPs) frequently occur in methanogen-applied biotechnology such as anaerobic digestion (AD). However, the direct effect and underlying mechanism of EPs on crucial methanogen involved in its application still remain unclear. The positive effect of chrysene (CH) on semi-continuous AD of sludge and the robust methanogen was dissected in this study. The methane yield in the digester with CH (100 mg/kg dry sludge) was 62.1 mL/g VS substrate, much higher than that in the control (46.1 mL/g VS substrate). Both methane production from acetoclastic methanogenesis (AM) and the AM proportion in the methanogenic pathway were improved in CH-shaped AD. Acetoclastic consortia, especially Methanosarcina and functional profiles of AM were enriched by CH in favor of the corresponding methanogenesis. Further, based on pure cultivation exposed to CH, the methanogenic performance, biomass, survivability and activity of typical Methanosarcina (M. barkeri) were boosted. Notably, iTRAQ proteomics revealed that the manufacturing (transcription and translation), expression and biocatalytic activity of acetoclastic metalloenzymes, particularly tetrahydromethanopterin S-methyltransferase and methyl-coenzyme M reductase with cobalt/nickel-cofactor (F₄₃₀ and cobalamin), and acetyl-CoA decarbonylase/synthase with cobalt/nickel-active site, of M. barkeri were upregulated significantly with fold changes in the range of 1.21-3.20 due to the CH presence. This study shed light on EPs-affecting industrially crucial methanogen at the molecular biology level during AD and had implications in the technical relevance of methanogens.

Tree-based machine learning model for visualizing complex relationships between biochar properties and anaerobic digestion

The ideal conditions for anaerobic digestion experiments with biochar addition are challenging to thoroughly study due to different experimental purposes. Therefore, three tree-based machine learning models were developed to depict the intricate connection between biochar properties and anaerobic digestion. For the methane yield and maximum methane production rate, the gradient boosting decision tree produced R² values of 0.84 and 0.69, respectively. According to feature analysis, digestion time and particle size had a substantial impact on the methane yield and production rate, respectively. When particle sizes were in the range of 0.3-0.5 mm and the specific surface area was approximately 290 m²/g, corresponding to a range of O content (>31%) and biochar addition (>20 g/L), the maximum promotion of methane yield and maximum methane production rate were attained. Therefore, this study presents new insights into the effects of biochar on anaerobic digestion through tree-based machine learning.

Physiological Responses of Methanosarcina barkeri under Ammonia Stress at the Molecular Level: The Unignorable Lipid Reprogramming

Acetotrophic methanogens' dysfunction in anaerobic digestion under ammonia pressure has been widely concerned. Lipids, the main cytomembrane structural biomolecules, normally play indispensable roles in guaranteeing cell functionality. However, no studies explored the effects of high ammonia on acetotrophic methanogens' lipids. Here, a high-throughput lipidomic interrogation deciphered lipid reprogramming in representative acetoclastic methanogen (Methanosarcina barkeri) upon high ammonia exposure. The results showed that high ammonia conspicuously reduced polyunsaturated lipids and longer-chain lipids, while accumulating lipids with shorter chains and/or more saturation. Also, the correlation network analysis visualized some sphingolipids as the most active participant in lipid-lipid communications, implying that the ammonia-induced enrichment in these sphingolipids triggered other lipid changes. In addition, we discovered the decreased integrity, elevated permeability, depolarization, and diminished fluidity of lipid-supported membranes under ammonia restraint, verifying the noxious ramifications of lipid abnormalities. Additional analysis revealed that high ammonia destabilized the structure of extracellular polymeric substances (EPSs) capable of protecting lipids, e.g., declining α-helix/(β-sheet + random coil) and 3-turn helix ratios. Furthermore, the abiotic impairment of critical EPS bonds, including C-OH, C═O-NH-, and S-S, and the biotic downregulation of functional proteins involved in transcription, translation, and EPS building blocks' supply were unraveled under ammonia stress and implied as the crucial mechanisms for EPS reshaping.

1-Butyl-3-methylimidazolium Chloride Affects Anaerobic Digestion through Altering Organics Transformation, Cell Viability, and Microbial Community

1-Butyl-3-methylimidazolium chloride (BmimCl), an imidazolium-based ionic liquid, is considered the representative emerging persistent aquatic pollutant, and its environmental toxicity has attracted a growing concern. However, most of the investigations focused on monocultures or a single organism, with little information available on the complex syntrophic consortium that dominates the complex and successional biochemical processes, such as anaerobic digestion. In this study, the effect of BmimCl at environmentally relevant levels on glucose anaerobic digestion was therefore investigated in several laboratory-scale mesophilic anaerobic digesters to provide such support. Experimental results showed that BmimCl at 1-20 mg/L inhibited the methane production rate by 3.50-31.03%, and 20 mg/L BmimCl inhibited butyrate, hydrogen, and acetate biotransformation by 14.29%, 36.36%, and 11.57%, respectively. Toxicological mechanism studies revealed that extracellular polymeric substances (EPSs) adsorbed and accumulated BmimCl through carboxyl, amino, and hydroxyl groups, which destroyed the EPSs' conformational structure, thereby leading to the inactivation of microbial cells. MiSeq sequencing data indicated that the abundance of Clostridium_sensu_stricto_1, Bacteroides, and Methanothrix decreased by 6.01%, 7.02%, and 18.45%, respectively, in response to 20 mg/L BmimCl. Molecular ecological network analysis showed that compared with the control, the lower network complexity, fewer keystone taxa, and fewer associations among microbial taxa were found in the BmimCl-present digester, indicating the reduced stability of the microbial community.

Nano magnetite-loaded biochar boosted methanogenesis through shifting microbial community composition and modulating electron transfer

A batch anaerobic fermentation system was employed to clarify how nano magnetite-loaded biochar can improve methanogenic performance of the propionate-degrading consortia (PDC). The nano magnetite-loaded biochar was prepared in a sequential hydrothermal and pyrolysis procedure using the household waste (HW), biogas residue (BR) and Fe (NO₃)₃ as pristine materials. Comprehensive characterization showed that the nano magnetite-loaded biochar ameliorated the biochar properties with large specific surface area, high electrochemical response and low electron transfer resistance. PDC supplemented with the magnetite/BR-originated biochar composites displayed excellent methanogenic performance, where the methane production rate was enhanced by 1.6-fold compared with the control. The nano magnetite-loaded biochar promoted methane production probably by promoting direct interspecies electron transfer between syntrophic bacteria (e.g., Syntrophobacter and Thauera) and their partners (e.g., Methanosaeta). In this process, magnetite might be responsible for triggering rapidly extracellular electron release, whereas both external functional groups and intrinsic graphitic matrices of biochar might work as electron bridges for electron transport.

Maximizing the energy recovery from rice straw through two-step conversion using eggshell-catalytic pyrolysis followed by enhanced anaerobic digestion using calcium-rich biochar

Anaerobic digestion of lignocelluloses for biogas production is greatly restricted by the poor biomass degradability. Herein, a novel approach is suggested to enhance the energy recovery from rice straw through a two-step conversion using eggshell-based catalytic pyrolysis followed by biochar-based anaerobic co-digestion. Pyrolysis with eggshell significantly enhanced the crude bio-oil yield by 4.6 %. Anaerobic digestion of rice straw using 4 g L^-1 of rice straw biochar (RB) showed the highest recorded biogas yield of 503.7 L kg^-1 VS, with 268.6 L kg^-1 VS biomethane yield. However, 4 g L^-1 of calcium-enriched eggshell rice straw biochar (ERB) enhanced the biomethane yield to 281.8 L kg^-1 VS, which represented 95.6 % higher than the control. It was attributed to enhancement of biomethanation, which resulted in 74.5 % maximum recorded biomethane content at the 7th day of anaerobic digestion. Microbial analysis confirmed that Methanosarciniales was the most dominant Archael group in the control (14.84 %), which increased sharply to 73.91 % and 91.66 % after addition of 4 g L^-1 RB and ERB, respectively. The suggested route enhanced the energy recovery in the form of bio-oil and biomethane by 41.6 %.

Novel nitrogen-doped biochar supported magnetite promotes anaerobic digestion: Material characterization and metagenomic analysis

Although different conductive materials have been applied to anaerobic digestion, there has not been a material that can really combine their merits and make up their shortcoming from each other. In this study, a novel nitrogen-doped biochar supported magnetite (Fe₃O₄@N-BC) was synthesized. Various material characterizations confirmed that nitrogen atoms were successful doped into the biochar and magnetite precipitated on its surface. 5 g/L Fe₃O₄@N-BC achieved the highest promotion of cumulative CH₄ production by 1.75 times compared with the blank group. Further metagenomic analysis revealed that Fe₃O₄@N-BC could increase the gene abundances of pilA, MmcA, Fpo, Rnf and HdrEd in bacteria Clotridium, Pseudomonas and Syntrophomonas and archaea Methanosarcina. Redundancy analysis showed that it was electrical conductivity and electron exchange capacity that were the key physicochemical characteristics for Fe₃O₄@N-BC to facilitating direct interspecies electron transfer. This study provides a reference for future conductive material synthesis and its application for anaerobic digestion.

Biochar-assisted anaerobic membrane bioreactor towards high-efficient energy recovery from swine wastewater: Performances and the potential mechanisms

A high-efficient energy recovery system of biochar-assisted anaerobic membrane bioreactor (BC-AnMBR) was established for swine wastewater treatment. Comparing with a conventional AnMBR, biochar addition accelerated volatile fatty acids (VFA) degradation during start-up stage, thereby shortened start-up duration by 44.0 %. Under a high organic loading rate (OLR) of 21.1 gCOD/L/d, BC-AnMBR promoted COD removal efficiency from 90.1 % to 95.2 %, and maintained a high methane production rate of 4.8L CH₄/L/d. The relative abundance of Methanosaeta declined from 53.9 % in conventional AnMBR to 21.0 % in BC-AnMBR, whereas that of Methanobrevibacter dramatically increased from 10.3 % to 70.9 %, respectively. Metabolic pathway analysis revealed that biochar not only strengthened hydrogenotrophic methanogenesis pathway, but also upregulated the genes encoding electron transfer carriers and riboflavin metabolism, suggesting the role of biochar facilitating direct interspecies electron transfer for syntrophic methanogenesis. The excellent energy yield performances under high OLR confirmed BC-AnMBR as an advanced system for high-strength swine wastewater treatment.

Enhanced anaerobic digestion of kitchen waste at different solids content by alkali pretreatment and bentonite addition: Methane production enhancement and microbial mechanism

High solid anaerobic digestion (AD) has been considered as a promising and sustainable technology for treating kitchen waste. To enhance AD of kitchen waste, alkali pretreatment and bentonite addition treatment (AP/Be) was performed on kitchen waste, and microbial community was investigated at different total solids (TS) content (10%, 13%, 19%, 22% and 25%). The results indicated that after AP/Be treatment, methane yield was as high as 198 mL CH₄/g volatile solid (VS), which increased by 236% as the control. Moreover, microbial community analysis revealed that AP/Be treatment enriched bacterial microbial diversity. At TS of 10%, AP/Be treatment enhanced the hydrogenotrophic methanogens (Methanobacterium) significantly. In addition, the dominant methanogenic pathways changed at different TS content. These results demonstrated AP/Be treatment had a positive effect on methanogenesis during kitchen waste anaerobic digestion process. This study threw new insights towards enhancing kitchen waste anaerobic digestion, as well as the microbial mechanism.

Enhancing Biodegradation of Pyridine with Trehalose Lipid in Rhodococcus pyridinivorans sp. Strain HR-1-Inoculated Microbial Fuel Cell

A Gram-positive exoelectrogen Rhodococcus pyridinivorans sp. strain HR-1 was cultivated from leachate-fed microbial fuel cell (MFC) to study the biodegradation effect of pyridine. In the comparison with mixed cultured MFC, HR-1 presented a remarkable electrical capacity with a maximum output of 4.33 W/m3 under 30 °C in neutral anolyte with 1 g/L acetate as a substrate. Further, HR-1 demonstrated the environmental resistance as a Gram-positive strain. Microbial metabolism was evident at pH between 5–9 and temperature in the range of 20–40 °C, whereas optimal condition for pyridine degradation was observed at 30 °C. This is the first study reporting the degradation of pyridine in the bio-electrochemical system that achieved a 42% ± 5% degradation rate in a full operation cycle at 2 g/L of the concentration. Considering the nonnegligible internal resistance of HR-1-inoculated MFC, trehalose lipid was also introduced as a bio-surfactant to reduce the charge transfer obstacle between the microbes and the electrodes. The surface morphology illustrated that the strain had a plump shape with a high specific area. Accordingly, bio-surfactant addition promoted the anode biomass (1.2 ± 0.1 mg/cm2 to 1.7 ± 0.2 mg/cm2) and achieved a higher degradation rate (68% ± 4%). The feasibility of electrochemical disposal on pyridine and eminent adaptability of strain sp. HR-1 as a Gram-positive exoelectrogen makes MFC a practical approach for real application.