Study on enhancing sludge methanogenesis by adding acetylene black and effect on the characteristics & microbial community of anaerobic granular sludge

Comparative investigations on the incorporation of biogenic Fe products into anaerobic granular sludge of different sources: Fe loading capacity, physicochemical properties, microbial community and long-term methanogenesis performance

Anaerobic granular sludge (AGS) has been regarded as the core of lots of advanced anaerobic reactors. Formation of biogenic Fe products and their incorporation into AGS could influence interspecies electron transfer and methanogenesis performance. In this study, with anaerobic granular sludge (AGS) from different sources (brewery, chemical plant, paper mill, citric acid factory, and food factory) as the research targets, the formation of biogenic iron products in AGS through the biologically induced mineralization process was studied. Furthermore, the influences of physicochemical properties and microbial community on methanogenesis were investigated. Results showed that all the AGS of different sources possessed the capacity to form biogenic Fe products through dissimilatory iron-reduction process, and diverse Fe minerals including magnetite (Fe3O4), hematite (Fe2O3), goethite (FeOOH), siderite (FeCO3) and wustite (FeO) were incorporated into AGS. The AGS loaded with Fe minerals (Fe-AGS) showed increased conductivity, magnetism and zeta-potential comparing to the control. Those Fe-AGS of different sources demonstrated different methanogenesis performance during the long-term operation (50 days). Methane production was increased for the Fe-AGS of citric acid (6.99-32.50%), food (8.33-37.46%), chemical (2.81-7.22%) and brewery plants (2.27-2.81%), but decreased for the Fe-AGS of paper mill (54.81-72.2%). The changes of microbial community and microbial correlations in AGS as a response to Fe minerals incorporation were investigated. For the Fe-AGS samples with enhanced methane production capability, it was widely to find the enriched populations of fermentative and dissimilatory iron reducing bacteria Clostridium_sensu_stricto_6, Bacteroidetes_vadinHA17 and acetoclastic methanogens Methanosaeta, and positive correlations between them. This study provides comprehensive understanding on the effects of incorporation biogenic Fe products on AGS from different sources.

Optimization of the Ex Situ Biomethanation of Hydrogen and Carbon Dioxide in a Novel Meandering Plug Flow Reactor: Start-Up Phase and Flexible Operation

With the increasing use of renewable energy resources for the power grid, the need for long-term storage technologies, such as power-to-gas systems, is growing. Biomethanation provides the opportunity to store energy in the form of the natural gas-equivalent biomethane. This study investigates a novel plug flow reactor that employs a helical static mixer for the biological methanation of hydrogen and carbon dioxide. In tests, the reactor achieved an average methane production rate of 2.5 LCH4LR∗d (methane production [LCH4] per liter of reactor volume [LR] per day [d]) with a maximum methane content of 94%. It demonstrated good flexibilization properties, as repeated 12 h downtimes did not negatively impact the process. The genera Methanothermobacter and Methanobacterium were predominant during the initial phase, along with volatile organic acid-producing, hydrogenotrophic, and proteolytic bacteria. The average ratio of volatile organic acid to total inorganic carbon increased to 0.52 ± 0.04, while the pH remained stable at an average of pH 8.1 ± 0.25 from day 32 to 98, spanning stable and flexible operation modes. This study contributes to the development of efficient flexible biological methanation systems for sustainable energy storage and management.

Feasibility of anaerobic co-digestion of biodegradable plastics with food waste, investigation of microbial diversity and digestate phytotoxicity

The effects of biodegradable plastics of different thicknesses (30 and 40 μm) and sizes (20 × 20, 2 × 2, and 1 × 1 mm) on anaerobic digestion of food waste and digestate phytotoxicity were investigated. Methane productions (38 days) for the groups with 20 × 20, 2 × 2, and 1 × 1 mm of 30 μm plastics were 92.46, 138.27, and 259.95 mL/gVSremoval, respectively which are nearly 58 % higher than the control group (58.86 mL/gVSremoval). Methane production in 40 μm plastics groups was lower than in 30 μm groups of equal size. All sizes of 30 µm plastics promoted substrate hydrolysis, acidification, and relative abundance of key hydrolytic bacteria and methanogens. Phytotoxicity tests results showed that seed root elongation was inhibited in groups with 40 μm plastics. In conclusion, 30 μm biodegradable plastics were more suitable for anaerobic digestion with food waste than 40 μm.

Enhancing sludge methanogenesis with changed micro-environment of anaerobic microorganisms by Fenton iron mud

Conductive materials have been demonstrated to enhance sludge methanogenesis, but few researches have concentrated on the interaction among conductive materials, microorganisms and their immediate living environment. In this study, Fenton iron mud with a high abundance of Fe(III) was recycled and applied in anaerobic reactors to promote anaerobic digestion (AD) process. The results show that the primary content of extracellular polymeric substances (EPS) such as polysaccharides and proteins increased significantly, possibly promoting microbial aggregation. Furthermore, with the increment of redox mediators including humic substances in EPS and Fe(III) introduced by Fenton iron mud, the direct interspecies electron transfer (DIET) between methanogens and interacting bacteria could be accelerated, which enhanced the rate of methanogenesis in anaerobic digestion (35.21 ± 4.53% increase compared to the control). The further analysis of the anaerobic microbial community confirmed the fact that Fenton iron mud enriched functional microorganisms, such as the abundance of CO2-reducing (e.g. Chloroflexi) and Fe(III)-reducing bacteria (e.g., Tepidimicrobium), thereby expediting the electron transfer reaction in the AD process via microbial DIET and dissimilatory iron reduction (DIR). This work will make it possible for using the recycled hazardous material - Fenton iron mud to improve the performance of anaerobic granular sludge during methanogenesis.

Enhancement of biomethane recovery from batch anaerobic digestion by exogenously adding an N-acyl homoserine lactone cocktail

Biomethane recovered through anaerobic digestion (AD) is a renewable, sustainable, and cost-effective alternative energy source that has the potential to help address rising energy demands. Efficient bioconversion during AD depends on the symbiotic relationship between hydrolytic bacteria and methanogenic archaea. Interactions between microorganisms occur in every biological system via a phenomenon known as quorum sensing (QS), in which signaling molecules are simultaneously transmitted and detected as a mode of cell-to-cell communication. However, there's still a lack of understanding on how QS works in the AD system, where diverse bacteria and archaea interact in a complex manner. In this study, different concentrations (0.5 and 5 μM) of signaling molecules in the form of an N-acyl homoserine lactone cocktail (C6-, C8-, C10-, and 3-oxo-C6-HSL) were prepared and introduced into anaerobic batch reactors to clearly assess how QS affects AD systems. It was observed that the methane yield increased with the addition of AHLs: a 5 μM AHL cocktail improved the methane yield (341.9 mL/g-COD) compared to the control without AHLs addition (285.9 mL/g-COD). Meanwhile, evidence of improved microbial growth and cell aggregation was noticed in AHLs-supplemented systems. Our findings also show that exogenously adding AHLs alters the microbial community structure by increasing the overall bacterial and archaeal population counts while favoring the growth of the methanogenic archaea group, which is essential in biomethane synthesis.

Resistance of anaerobic activated sludge acclimated by different feeding patterns: response to different stress shocks

Anaerobic activated sludge plays a key role in the anaerobic digestion (AD) treatment of wastewater. The ability of anaerobic activated sludge to endure stress shock determines the performance of AD. In this study, the resistance of anaerobic activated sludge acclimated by three feeding patterns (continuous, semi-continuous, and pulse) to four stress shocks, including low pH influent, high OLR (organic loading rate), high ammonium and high sulfate, was investigated respectively. The results showed that the anaerobic activated sludge acclimated by semi-continuous feeding had the best resistance to high OLR shock, followed by pulse feeding, and then continuous feeding. There was no significant difference in the resistance of the three activated sludge to the other stress shocks. Under stress shock, the microbial community structure and abundance of specific functional microorganisms in the activated sludge acclimated by different feeding patterns varied, while the relative abundance of Methanosarcinaceae in the anaerobic activated sludge increased. The variation in the relative abundance of specific functional microorganisms was in charge of the differences in the resistance of anaerobic activated sludge. Overall, the results presented herein provide reference for improving the stability and effectiveness of activated sludge under adverse conditions.

Effects of different N-acyl-serine lactone signaling molecules on the performance of anaerobic granular sludge

Exogenous addition of acyl-homoserine lactone (AHL) signaling molecules can improve or inhibit the methane production performance of anaerobic granular sludge (AnGS) by quorum sensing (QS). To explore the specific effect of AHLs on AnGS, 2 μM of signal molecules were added to the reactor and we analyzed their effects on AnGS biodiversity, extracellular polymeric substance (EPS), specific methanogenic activity (SMA) and chemical oxygen demand (COD) removal rate of AnGS. The results indicated that the four types of AHLs improve the COD removal rate, SMA and organic composition of AnGS. The addition of N-(β-ketocaproyl)-dl-homoserine lactone (3O-C6-HSL) yielded the greatest increase in methanogenic activity, reaching a maximum of 30.83%. The four types of AHLs stimulate the secretion of EPS in AnGS by group sensing regulation. The addition of N-hexanoyl-l-homoserine lactone (C6-HSL), N-octanoyl-dl-lactone (C8-HSL) and 3O-C6-HSL induced the enrichment of Actinobacteria. Thus, the process of hydrolysis and acidification of AnGS is accelerated. The addition of N-butyryl-dl-homoserine lactone (C4-HSL), C6-HSL and 3O-C6-HSL promote the potential methanogenic metabolic pathway of AnGS. The addition of all AHLs directly or indirectly enhanced the methane metabolism pathway of sludge and improved the specific methane generation activity of AnGS. These results are expected to provide preliminary research data for enhancing the methane production efficiency of reactors and enriching the biological activity of AnGS.

Sludge Derived Carbon Modified Anode in Microbial Fuel Cell for Performance Improvement and Microbial Community Dynamics

The conversion of activated sludge into high value-added materials, such as sludge carbon (SC), has attracted increasing attention because of its potential for various applications. In this study, the effect of SC carbonized at temperatures of 600, 800, 1000, and 1200 °C on the anode performance of microbial fuel cells and its mechanism are discussed. A pyrolysis temperature of 1000 °C for the loaded electrode (SC1000/CC) generated a maximum areal power density of 2.165 ± 0.021 W·m-2 and a current density of 5.985 ± 0.015 A·m-2, which is 3.017- and 2.992-fold that of the CC anode. The addition of SC improves microbial activity, optimizes microbial community structure, promotes the expression of c-type cytochromes, and is conducive to the formation of electroactive biofilms. This study not only describes a technique for the preparation of high-performance and low-cost anodes, but also sheds some light on the rational utilization of waste resources such as aerobic activated sludge.

Study on the mechanism of inhibiting the calcification of anaerobic granular sludge induced by the addition of trace signal molecule (3O-C6-HSL)

Microbiota quorum sensing (QS) induced by 3O-C6-HSL (N-(β-ketocaproyl)-DL-homoserine lactone) inhibited the calcification of anaerobic granular sludge (AnGS), and the mechanism of promoting the activity recovery of calcified AnGS was studied in this paper. Through research, it was speculated that 3O-C6-HSL acted on calcified AnGS residual microorganisms to trigger QS. It enriched many functional microorganisms. For example, it promoted the growth of Methanospirillum. The CO₂ could be consumed quickly by Methanospirillum and reduced the calcium carbonate formation. The increase of microbial biomass would promote the activity of sludge. What's more, the pore size and porosity of sludge would increase, so the mass transfer channel will be broadened at same time. All those, could help the calcified AnGS quickly restore the activity and anaerobic system recover normal, which provided a new idea for the emergency rescue of anaerobic system in the future.

Dual-chamber differs from single-chamber microbial electrosynthesis in biogas production performance under low temperature (15℃)

In this study, single-chamber and dual-chamber Microbial electrosynthesis (MES) with carbon fiber brushes as electrodes were operated at 15°C to compare and analyze the difference in methanogenic performance. Metatranscriptomic analysis showed that the relative abundance of electroactive microorganisms Syntrophomonas, Pseudomonas and Bacteroides in each group exceeded 90%, while the abundance of Geobacter was less than 4%. Acetoclastic methanogens Methahnosarcina was more enriched in dual-chamber MES (61.74%~70.42%), and Methanothrix showed higher abundance in single-chamber MES (33.44%~51.71%). Methahnosarcina and Methanothrix could interact with electroactive microorganisms to improve the electron transfer efficiency through direct interspecies electron transfer (DIET). Analysis of the methane metabolic pathways of low-temperature MES found acetoclastic pathway was domination, and single-chamber MES achieved acetate to acetyl-CoA through acetate-CoA ligase (EC: 6.2.1.1), whereas dual-chamber MES was by acetate kinase (EC: 2.7.2.1) and phosphate acetyltransferase (EC: 2.3.1.8). These results are beneficial to further research on the treatment of low-temperature wastewater.

Direct interspecies electron transfer mechanism in enhanced methanogenesis: A mini-review

The role of direct interspecies electron transfer (DIET) on enhancement of methanogenesis has been studied. This mini-review updated the current researches on the potential role of DIET on enhanced performance for anaerobic digestion of organic substrates with effective strategies implemented. Since most experimental observations correlated with the DIET mechanism are yet to be consolidated, this article categorized and discussed the current experimental observations supporting DIET mechanism for methanogenesis, mainly based on those with supplement of carbon materials, from which the prospects and challenges for further studies to confirm the role of DIET in anaerobic digestion processes were highlighted.

Electrocoagulation pre-treatment to simultaneously remove dissolved and colloidal substances and Ca2+ in old corrugated container wastewater

An effective electrocoagulation pre-treatment (ECP) method was proposed to simultaneously solve the problems of the micro stickies deposition and high Ca²⁺ content in old corrugated container (OCC) papermaking wastewater during the recycling process. The optimal ECP condition was investigated. The results indicated that the effect of an Al electrode on wastewater treatment was superior to that of a Fe or Mg electrode. The optimal treatment conditions of the current density, electrode distance and reaction time were 115 A m^-2, 5 cm and 60 min, respectively. After the ECP, the chemical oxygen demand (COD) and Ca²⁺ removal rates were 75.33% and 64.53%, respectively, and the turbidity and dissolved and colloidal substance (DCS) content decreased by 97.1% and 43.68%, respectively. The particle size of flocs in the liquid increased from 1.675 μm to 31.97 μm, and the floc content was 0.78 g L^-1 after ECP. The anode material and energy consumption were 0.1846 kg m^-3 and 4.56 kWh m^-3, respectively, and the cost of treatment was estimated to be 1.11 $ m^-3. The results demonstrate that ECP can effectively remove the micro stickies, COD, and Ca²⁺ in the OCC wastewater, which is conducive to reducing the cost of wastewater treatment and conform to the requirements of sustainable development.

Improvement in calcified anaerobic granular sludge performance by exogenous acyl-homoserine lactones

Given the high content of Ca²⁺ in waste paper recycling wastewater, the anaerobic granular sludge (AnGS) undergoes calcification during wastewater treatment and affects the treatment efficiency. To restore the activity of calcified AnGS and improve the performance of AnGS, four types of N-acyl-homoserine lactones (AHLs) were added to the AnGS system while papermaking wastewater treatment. The addition of N-butyryl-DL-homoserine lactone(C4-HSL) and N-octanoyl-DL-homoserine lactone (C8-HSL) had an inhibitory affect the COD removal efficiency and SMA of sludge at the inception. The addition of N-hexanoyl-L-homoserine lactone (C6-HSL) has no obvious effect on the COD removal efficiency, but can improve the SMA of sludge more obviously. The addition of N-(β-ketocaproyl)-DL-homoserine lactone (3O-C6-HSL) can increased COD removal efficiency and promoted SMA together obviously. The addition of C6-HSL and 3O-C6-HSL can increase volatile suspended solid (VSS)/total suspended solid (TSS), and regulate extracellular polymeric substance (EPS) secretion in AnGS. Analysis of microbial sequencing revealed changes in the microbial community structure following AHL addition, which enhanced the methane metabolism pathway in sludge. The addition of C6-HSL, C8-HSL, and 3O-C6-HSL increased Methanosaeta population, thus increasing the aceticlastic pathway in sludge. Thus, exogenous AHLs can play an important role in regulating microbial community structure, and in improving the performance of AnGS.

Explore the difference between the single-chamber and dual-chamber microbial electrosynthesis for biogas production performance

Microbial electrosynthesis (MES) is an advanced technology for efficient treatment of organic wastewater and recovery of new energy, with the advantages and disadvantages of single-chamber and dual-chamber MES reactors being less understood. Therefore, we explored the effects of single-chamber and dual-chamber structures on the methane production performance and microbial community structure of MES. Results indicated that methane concentration and current density of single-chamber MES were higher than those of dual-chamber MES, and the system stability was better, while chemical oxygen demand (COD) removal rate and cumulative methane production were not significantly different. Analysis of microbial community structure showed the abundance of acidogens and H₂-producing bacteria was higher in single-chamber MES, while fermentation bacteria and methanogens was lower. The abundance of methanogens of dual-chamber MES (21.74-24.70%) was superior to the single-chamber MES (8.23-10.10%). Moreover, in dual-chamber MES, methane was produced primarily through acetoclastic methanogenic pathway, while in single-chamber MES cathode, methane production was mainly by hydrogenotrophic methanogenic pathway. Information provided will be useful to select suitable reactors and optimize reaction design.

Comparison of long-term energy efficiency and microbial community dynamics of different reactors in response to increased loadings of water hyacinth juice

Water hyacinth is considered to be among the worst invasive weed species globally, causing detrimental environmental and social problems worldwide. It rapidly grows, and therefore has significant potential as a resource. Due to its high moisture content (approximately 95%), the by-product obtained by dehydrating water hyacinth yields a considerable amount of water hyacinth juice (WHJ). In this study, we performed a comparative assessment of long-term energy efficiency, maximum treatment capacity limits, and microbial community dynamics of modified internal circulation (MIC) and up-flow anaerobic sludge blanket (UASB) reactors in response to increasing loadings of WHJ. The MIC reactor exhibited a higher energy recovery rate and stronger performance compared with the UASB reactor. The optimal organic loading rates of the MIC and UASB reactors were 17.93 and 8.85 kg chemical oxygen demand (COD)/m³/d, with methane conversion rates of 0.21 and 0.15 m³ CH₄/kg COD, respectively. Furthermore, the engineering costs and project floor space required by the MIC reactor are less than those in the case of the UASB reactor. The high-throughput sequencing analysis indicated that the dominant phyla (e.g. Firmicutes and Bacteroidetes) were more abundant using the MIC reactor than with the UASB reactor, which may indicate WHJ degradation efficiency. Both reactors had similar predominant methanogens, suggesting that acetoclastic methanogenesis was the predominant metabolic pathway of methane formation. The results of this study provide new insights into the sustainable management of water hyacinth as a resource by establishing a regional ecosystem with biogas engineering applications.