Coupling granular activated carbon and exogenous hydrogen to enhance anaerobic digestion of phenol via predominant syntrophic acetate oxidation and hydrogenotrophic methanogenesis pathway

Influence of oxygen partial pressure on homoacetogenesis and promotion of acetic acid accumulation through low pH regulation under microaerobic conditions

Homoacetogenesis is an important pathway for bio-utilization of CO2; however, oxygen is a key environmental influencing factor. This study explored the impact of different initial oxygen partial pressures (OPPs) on homoacetogenesis, while implementing low pH regulation enhanced acetic acid (HAc) accumulation under microaerobic conditions. Results indicated that cumulative HAc production increased by 18.2% in 5% OPP group, whereas decreases of 31.3% and 56.0% were observed in 10% and 20% OPP groups, respectively, compared to the control group. However, hydrogenotrophic methanogens adapted to microaerobic environment and competed with homoacetogens for CO2, thus limiting homoacetogenesis. Controlling influent pH 5.0 per cycle increased cumulative HAc production by 18.3% and 18.2% in 5% and 10% OPP groups, respectively, compared with the control group. Consequently, regulating low pH effectively inhibited methanogenic activity under microaerobic conditions, thus increasing HAc production. This study was expected to expand the practical application of homoacetogenesis in bio-utilization of CO2.

Enhancement of hydrogenotrophic methanogenesis for methane production by nano zero-valent iron in soils

Nanoscale zero-valent iron (nZVI) is attracting increasing attention as the most commonly used environmental remediation material. However, given the high surface area and strong reducing capabilities of nZVI, there is a lack of understanding regarding its effects on the complex anaerobic methane production process in flooded soils. To elucidate the mechanism of CH4 production in soil exposed to nZVI, paddy soil was collected and subjected to anaerobic culture under continuous flooding conditions, with various dosages of nZVI applied. The results showed that the introduction of nZVI into anaerobic flooded rice paddy systems promoted microbial utilization of acetate and carbon dioxide as carbon sources for methane production, ultimately leading to increased methane production. Following the introduction of nZVI into the soil, there was a rapid increase in hydrogen levels in the headspace, surpassing that of the control group. The hydrogen levels in both the experimental and control groups were depleted by the 29th day of culture. These findings suggest that nZVI exposure facilitates the enrichment of hydrogenotrophic methanogens, providing them with a favorable environment for growth. Additionally, it affected soil physicochemical properties by increasing pH and electrical conductivity. The metagenomic analysis further indicates that under exposure to nZVI, hydrogenotrophic methanogens, particularly Methanobacteriaceae and Methanocellaceae, were enriched. The relative abundance of genes such as mcrA and mcrB associated with methane production was increased. This study provides important theoretical insights into the response of key microbes, functional genes, and methane production pathways to nZVI during anaerobic methane production in rice paddy soils, offering fundamental insights into the long-term fate and risks associated with the introduction of nZVI into soils.

Controlling carbon dioxide-to-hydrogen ratio to improve hydrogen utilization and denitrification rates of hydrogenotrophic autotrophic denitrification through homoacetogenesis-heterotrophic denitrification pathway

Hydrogenotrophic denitrification, an environment-friendly process for organic-free influents, is limited due to poor hydrogen mass transfer efficiency and significant pH fluctuations. In this study, we manipulated the carbon dioxide-to-hydrogen ratio to improve hydrogenotrophic denitrification. When carbon dioxide-to-hydrogen ratio was 1:1 (carbon dioxide, 200 ml: hydrogen, 200 ml), the hydrogen utilization and denitrification rates were 2.4 times and 3.0 times that when carbon dioxide-to-hydrogen ratio was 0:1 (carbon dioxide, 0 ml: hydrogen, 200 ml), respectively. The pH fluctuation decreased from 3.1±0.3 to 0.2±0.1. Furthermore, the hydrogenotrophic denitrification, acetoclastic denitrification, homoacetogenic, and electron transfer activities of the sludge were improved. A high carbon dioxide-to-hydrogen ratio augmented the acid-producing and heterotrophic denitrifying microorganism populations. By maintaining a high carbon dioxide-to-hydrogen ratio, the dominant hydrogenotrophic autotrophic denitrification pathway was transformed into a homoacetogenesis-heterotrophic denitrification pathway, thereby achieving higher hydrogen utilization and denitrification rates.

Enhanced anaerobic biodegradation of typical phenolic compounds in coal gasification wastewater (CGW) using biochar: Focusing on the hydrolysis-acidification process and microbial community succession

The aim of this research is to investigate the effects of biochar (BC) on treatment performance (especially hydrolysis-acidification process) and microbial community shifts during anaerobic degradation of typical phenolic compounds in coal gasification wastewater. Compared to the control group, the removal of phenol, p-cresol and 3, 5-xylenol was gradually enhanced when increasing the BC addition within the test dosage (1-5 g/L). The biodegradation of phenol and p-cresol was significantly enhanced by BC addition while limited improvement for 3, 5-xylenol. The addition of BC significantly accelerated the hydrolysis-acidification process with the hydrolytic removal of phenol improved by 69.14%, the microbial activity was enhanced by 57.01%, and the key hydrolase bamA gene was enriched by 117.27%, respectively. Compared to 1-2 g/L dose, more protein-like and humic acid-like substances were secreted with 5 g/L BC, which probably contributed to higher extracellular electron transfer efficiency. In addition, phenol degrading bacteria (Syntrophorhabdus, Dysgonomonas, Holophaga, etc.) and electroactive microorganisms (Geobacter, Syntrophorhabdus, Methanospirillum, etc.) were enriched by BC addition. The functional genes related to carboxylation, benzoylation and ring cleavage processes of benzoyl-CoA pathway were potentially activated by BC.

Insights into high-solids anaerobic digestion of food waste concomitant with sorbate: Performance and mechanisms

High-solids anaerobic digestion (HS-AD) of food waste is increasingly applied commercially. Sorbate, a food preservative extensively used in the food industry, induces potential environmental risks. Results indicated sorbate at 0-10 mg/g VS slightly inhibited methane production, and the cumulative methane yield suggested a negative correlation with 25 mg/g VS sorbate, with a reduction of 15.0% compared to the control (from 285.7 to 253.6 mL CH₄/g VS). The reduction in methane yield could be ascribed to the promotion of solubilization and inhibition of acidogenesis and methanogenesis with sorbate addition. Excessive sorbate (25 mg/g VS) resulted in the inhibition of aceticlastic metabolism and the key enzymes activities (e.g., acetate kinase and coenzyme F₄₂₀). This study deeply elucidated the response mechanism of HS-AD to sorbate, supplemented the potential ecological risk assessment of sorbate, and could provide insights to further prevent the potential risk of sorbate in anaerobic digestion of FW.

Performance of anaerobic digestion of phenol using exogenous hydrogen and granular activated carbon and analysis of microbial community

Anaerobic conversion rate of phenol to methane was low due to its biological toxicity. In this study, the coupling of granular activated carbon (GAC) and exogenous hydrogen (EH) could enhance greatly methane production of phenol anaerobic digestion, and the metagenomic was firstly used to analyze its potential mechanism. The results indicated that a mass of syntrophic acetate-oxidizing bacteria and hydrogen-utilizing methanogens were enriched on the GAC surface, and SAO-HM pathway has become the dominant pathway. The energy transfer analysis implied that the abundance of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH) oxidase increased. Furthermore, direct interspecies electron transfer (DIET) was formed by promoting type IV e-pili between Methanobacterium and Syntrophus, thereby improving the interspecies electron transfer efficiency. The dominant SAO-HM pathway was induced and DIET was formed, which was the internal mechanism of the coupling of GAC and EH to enhance anaerobic biotransformation of phenol.

Mechanisms, performance, and the impact on microbial structure of direct interspecies electron transfer for enhancing anaerobic digestion-A review

Direct interspecies electron transfer (DIET) has been received tremendous attention, recently, due to the advantages of accelerating methane production via organics reduction during anaerobic digestion (AD) process. DIET-based syntrophic relationships not only occurred with the existence of pili and some proteins in the microorganism, but also can be conducted by conductive materials. Therefore, more researches into understanding and strengthening DIET-based syntrophy have been conducted with the aim of improving methanogenesis kinetics and further enhance methane productivity in AD systems. This study summarized the mechanisms, application and microbial structures of typical conductive materials (carbon-based materials and iron-based materials) during AD reactors operation. Meanwhile, detail analysis of studies on DIET (from substrates, dosage and effectiveness) via conductive materials was also presented in the study. Moreover, the challenges of applying conductive materials in boosting methane production were also proposed, which was supposed to provide a deep insight in DIET for full scale application.

Supplementation of CO2-nanobubble water to enhance the methane production from anaerobic digestion of corn straw

Nanobubble water (NBW) could improve methane production from anaerobic digestion (AD) of corn straw without secondary contamination. In this study, the effect of carbon dioxide nanobubble water (CO₂-NBW) volumes (0%, 25%, 50%, 75%, 100%) on methane production from corn straw was investigated. The results showed that addition of CO₂-NBW could improve methane production and promote substrate degradation in AD process. The highest cumulative methane production of 132.16 mL g^-1VS_added was obtained in the 100% CO₂-NBW added reactor, which was 17% higher than that in the control group. Additionally, the addition of CO₂-NBW could mitigate the sharp decrease in pH by acting as a buffer. CO₂-NBW could also enhance microorganism activity throughout the AD process. The electron transport system (ETS) activity was increased by 23%, while the β-glucosidase, dehydrogenase (DHA), and coenzyme F₄₂₀ activities were increased by 15%, 23%, and 11%, respectively, at optimum addition of CO₂-NBW. Meanwhile, addition of CO₂-NBW accelerated the production and consumption of reducing sugar and volatile fatty acids (VFAs), promoting the reduction rates of TS (Total solid) and VS (Volatile solid).

Temperature-regulated and starvation-induced refractory para-toluic acid anaerobic biotransformation

Little research was focused on the anerobic degradation of refractory para-toluic acid at present. Thus, temperature-regulated anaerobic system of para-toluic acid fed as sole substrate was built and investigated via microbiota, metabolism intermediates, and function prediction in this study. Results showed that low methane yield was produced in para-toluic acid anaerobic system at alkaline condition. And the causes were owing to anaerobic methane oxidation and potentially H2S production at 37 °C, N2 production by denitrification before starvation and propionic acid occurrence after starvation at 27 °C, and production of N2 and free ammonia, and accumulation of acetic acid at 52 °C. Simultaneously, hydrogenotrophic methanogenesis dependent on syntrophic acetate oxidation (SAO) was predominant, facilitating the removal of para-toluic acid at 52 °C. Moreover, the key intermediate changed from phthalic acid of 37 °C and 27 °C before starvation to terephthalic acid of 52 °C. Starvation promoted removal of para-toluic acid through benzoyl-CoA pathway by Syntrophorhabdus, enrichment of syntrophic propionate degraders of Bacteroidetes and Ignavibacteriaceae, and increase of methylotrophic methanogens.

Granular activated carbon and exogenous hydrogen enhanced anaerobic digestion of hypersaline phenolic wastewater via syntrophic acetate oxidation and hydrogenotrophic methanogenesis

High salinity in phenolic wastewater inhibited anaerobes' metabolic activity, thereby affecting the anaerobic biotransformation of phenol. In this study, granular activated carbon (GAC) coupled with exogenous hydrogen (H₂) was used to enhance the anaerobic digestion of phenol. The GAC/H₂ group's accumulative methane production, coenzyme F₄₂₀ concentration, and interspecies electron transfer system activity increased by 24 %, 53 %, and 16 %, respectively, compared with the control group. In the floc sludge of the GAC/H₂ group, the relative abundance of syntrophic acetate-oxidizing bacteria such as Syntrophus and Syntrophorhabdus were 18.7 % and 1.1 % at genus level, respectively, which were around 93.5 and 7.5 times of that of the controlgroup. Moreover, Acinetobacter (77.6 %), Methanobacterium (44.0 %), and Methanosarcina (34.2 %) were significantly enriched on the GAC surface in GAC/H₂ group. Therefore, the coupling of GAC and H₂ provided a novel attempt at anaerobic digestion of hypersaline phenolic wastewater via syntrophic acetate oxidation and hydrogenotrophic methanogenesis pathway.

Enhanced methane production in anaerobic digestion: A critical review on regulation based on electron transfer

Anaerobic digestion (AD) is a potential bioprocess for waste biomass utilization and energy conservation. Various iron/carbon-based CMs (e.g., magnetite, biochar, granular activated carbon (GAC), graphite and zero valent iron (ZVI)) have been supplemented in anaerobic digestors to improve AD performance. Generally, the supplementation of CMs has shown to improve methane production, shorten lag phase and alleviate environmental stress because they could serve as electron conduits and promote direct interspecies electron transfer (DIET). However, the CMs dosage varied greatly in previous studies and CMs wash out remains a challenge for its application in full-scale plants. Future work is recommended to standardize the CMs dosage and recover/reuse the CMs. Moreover, additional evidence is required to verify the electrotrophs involved in DIET.

Cometabolic biodegradation system employed subculturing photosynthetic bacteria: A new degradation pathway of 4-chlorophenol in hypersaline wastewater

4-chlorophenol (4-CP) as a toxic persistent pollutant is quite difficult treatment by using traditional biological processes. Herein, photosynthetic bacteria (PSB) driven cometabolic biodegradation system associated with exogeneous carbon sources (e.g., sodium acetate) has been demonstrated as an effective microbial technique. The biodegradation rate (r_i) can be at 0.041 d^-1 with degradation efficiency of 93% in 3094 lx. Through the study of subculturing PSB in absence of NaCl, it was found that 50% inoculation time can be saved but keeping a similar 4-CP biodegradation efficiency in scale-up salinity system. A new plausible biodegradation pathway for 4-CP in 4th G PSB cometabolic system is proposed based on the detected cyclohexanone generation followed by ring opening. It is probably ascribed to the increasement of Firmicutes and Bacteroidetes at phyla level classified based on microbial community. This study contributes to a new insight into cometabolic technology for chlorophenol treatment in industrial hypersaline wastewater.

Biological methane production coupled with sulfur oxidation in a microbial electrosynthesis system without organic substrates

Methane is produced in a microbial electrosynthesis system (MES) without organic substrates. However, a relatively high applied voltage is required for the bioelectrical reactions. In this study, we demonstrated that electrotrophic methane production at the biocathode was achieved even at a very low voltage of 0.1 V in an MES, in which abiotic HS^- oxidized to SO₄^2- at the anodic carbon-cloth surface coated with platinum powder. In addition, microbial community analysis revealed the most probable pathway for methane production from electrons. First, electrotrophic H₂ was produced by syntrophic bacteria, such as Syntrophorhabdus, Syntrophobacter, Syntrophus, Leptolinea, and Aminicenantales, with the direct acceptance of electrons at the biocathode. Subsequently, most of the produced H₂ was converted to acetate by homoacetogens, such as Clostridium and Spirochaeta 2. In conclusion, the majority of the methane was indirectly produced by a large population of acetoclastic methanogens, namely Methanosaeta, via acetate. Further, hydrogenotrophic methanogens, including Methanobacterium and Methanolinea, produced methane via H₂.

Enhanced treatment of low-temperature and low carbon/nitrogen ratio wastewater by corncob-based fixed bed bioreactor coupled sequencing batch reactor

In this study, a combined corncob-based fixed bed bioreactor and sequencing batch reactor system (CCF-SBR) was developed to treat low-temperature (3-12 °C) and low carbon/nitrogen ratio (C/N = 2) wastewater with a single SBR as the control. Results showed similarly low COD concentration of CCF-SBR (20.4 ± 3.7 mg·L^-1) and control SBR (24.9 ± 6.7 mg·L^-1) effluent. However, the total nitrogen (TN) removal rate of CCF-SBR was significantly higher than that of control SBR (29.6 ± 2.7% vs 8.6 ± 2.3%). According to the nitrification and denitrification activities and the analysis of microbial community, CCF mainly played the role of denitrification based on fermentation genera and denitrifying genera, and SBR mainly implemented nitrification with Nitrospira and Acinetobacter. This study explores a promising way for agricultural waste resource utilization and wastewater treatment under low-temperature and low C/N ratio.

Metagenomic insights into direct interspecies electron transfer and quorum sensing in blackwater anaerobic digestion reactors supplemented with granular activated carbon

The addition of granular activated carbon (GAC) enhanced the performance of up-flow anaerobic sludge blanket (UASB) reactor treating blackwater at 35 °C. DNA were extracted from the sludge and biofilms attached to GAC and submitted for shotgun sequencing. In addition, the acyl-homoserine lactones (AHLs) were quantified. Diverse partners for direct interspecies electron transfer (DIET) were enriched in the sludge or biofilm (GAC-biofilm) of GAC amended UASB. Pedosphaera parvula, Syntrophus aciditrophicus and Syntrophorhabus aromaticivorans were dominant syntrophs. The analysis for type IV pilus assembly genes further suggested DIET may be functioned through GAC for GAC-biofilm, while through conductive pili for sludge aggregates. AHLs quantification and the analysis for quorum sensing (QS) related genes indicated higher QS activity at the population level was induced by GAC. Overall, the work illustrated the different DIET patterns, and suggested that QS played an important role in controlling the performance in GAC amended USAB.

Enhancement of methane production by anaerobic digestion of corn straw with hydrogen-nanobubble water

Hydrogen-nanobubble water was proposed to enhance methane production by anaerobic digestion (AD) with corn straw. The effects of H₂-nanobubble water (H₂-NBW) amounts (0%, 20%, 40%, 60%, 80%, and 100%) on methane production characteristics of corn straw were explored. The results showed that the methane yields were increased by 11.54%∼25.29% compared with the control group(CK), and the maximum cumulative methane production reached to 254.36 mL·g-VS^-1 when the H₂-NBW addition was of 60%. Interestingly, the maximum methane concentration increased by 4.37% compared with CK. H₂-NBW addition can destroy the cellulose structure of corn straw, reduce the crystallinity of cellulose, and promote the hydrolysis. The degradation rate of cellulose and hemicellulose were increased by 20%∼33% and 13% ∼25.7% respectively, and the removal rate of TS and VS were increased by 6.82%-27.93% and 8.52%-21.47%, respectively. The modified Gompertz equation fitted the cumulative methane production curves very well, with high correlation coefficients (R² > 0.992).

Distribution patterns of functional microbial community in anaerobic digesters under different operational circumstances: A review

Anaerobic digestion (AD) processes are promising to effectively recover resources from organic wastes or wastewater. As a microbial-driven process, the functional anaerobic species played critical roles in AD. However, the lack of effective understanding of the correlations of varying microbial communities with different operational factors hinders the microbial regulation to improve the AD performance. In this paper, the main anaerobic functional microorganisms involved in different stages of AD processes were first demonstrated. Then, the response of anaerobic microbial community to different operating parameters, exogenous interfering substances and digestion substrates, as well as the digestion efficiency, were discussed. Finally, the research gaps and future directions on the understanding of functional microorganisms in AD were proposed. This review provides insightful knowledge of distribution patterns of functional microbial community in anaerobic digesters, and gives critical guidance to regulate and enrich specific functional microorganisms to accumulate certain AD products.

Promoting direct interspecies electron transfer and acetoclastic methanogenesis for enhancing anaerobic digestion of butanol octanol wastewater by coupling granular activated carbon and exogenous hydrogen

Butanol octanol wastewater (BOW) generated from syngas conversion of coal contained abundant toxic organic pollutants. Anaerobic digestion is a promising technology for BOW, but abundant toxic substances would inhibit the activity of anaerobic microorganisms. Granular activated carbon (GAC) and exogenous hydrogen (EH₂) were employed to enhance anaerobic digestion of BOW. The results indicated that methane production increased to 289.55 ± 17.43 mL CH₄/g COD in EH₂/GAC group, which was 1.07, 2.04, and 1.98 times of that in GAC, EH₂, and control groups, respectively. In EH₂/GAC group. The relative abundance of Geobacter and Methanosaeta increased rapidly to 25.36% and 52.81%, respectively, and the relative abundance of Clostridium was 9.78%. The underlying mechanism might be that GAC promoted the enrichment of Geobacter, and EH₂ changed metabolic mechanism of Clostridium, stimulating the enrichment of Methanosaeta. Direct interspecies electron transfer was promoted by EH₂/GAC, thus improving the methane production rate of BOW.