Hydrogen sulfide affects the performance of a methanogenic bioelectrochemical system used for biogas upgrading

Robust biogas upgrading process via homoacetogens against ammonia and sulfide toxicities

Using hydrogen derived from surplus green energy (e.g., solar and wind) to convert carbon dioxide to acetate via homoacetogens represents a promising technology for simultaneous biogas upgrading and biochemical production. However, effects of hydrogen sulfide and ammonia on activities of homoacetogens remain unknown, hindering their applications in biogas upgrading. This study investigated the impacts of ammonia and sulfide on homoacetogen-dominated microbial community for biogas upgrading process by combining short-term batch tests and long-term membrane biofilm reactor (MBfR) operation. Results showed that sulfide concentrations ≤ 2 mM TDS (total dissolved sulfide) increased H2 and CO2 uptake rates and acetate production both in the short-term and long-term tests. The relative abundance of Acetobacterium (typical homoacetogens) in the MBfR also increased from 30 % without TDS addition to 40 % with the addition of 2 mM TDS. These results suggest that sulfide addition (≤ 2 mM TDS) likely promoted the growth of homoacetogens, thereby enhancing the biogas upgrading efficiency. In terms of ammonia, results suggested that 0.5 g NH4+-N/L has negligible impacts on the homoacetogens' activities, while concentrations ≥ 1 g NH4+-N/L significantly inhibited homoacetogens' activities, resulting in negligible acetate production during the short-term tests. However, the long-term biogas upgrading performance remained unaffected by 1 g NH4+-N/L. Moreover, with the simultaneous additions of typical concentrations of sulfide (2 mM TDS, equivalent to the H2S concentration of 0.8 % in biogas) and ammonia (1 g NH4+-N/L, equivalent to the NH3 concentration of 0.1 % in biogas) in raw biogas, our MBfR still achieved high H2 and CO2 utilization efficiencies (95 % and 97 %, respectively) and acetate production rate (550 mg/L/d). These highlight the robustness of MBfR against ammonia and sulfide toxicities. Additionally, the injection of extra H2 could alleviate the ammonia and sulfide inhibitions on homoacetogens with acetate production increased by 13-80 times. This provides a new strategy to enhance the tolerance of homoacetogens against high concentrations of hydrogen sulfide and ammonia. Collectively, our findings advance the understanding of the response of homoacetogens to ammonia and sulfide stress and facilitate the development of a resilient and efficient homoacetogen-mediated bioprocess for upgrading biogas to biomethane and chemicals simultaneously.

Effect of aeration pretreatment on anaerobic digestion of swine manure

To investigate the effects of aeration pretreatment on the anaerobic digestion (AD) of swine manure, five pretreatment groups were established with dissolved oxygen (DO) in each group set to 0.0, 0.4, 0.8, 1.4, and 2.0 mg/L, respectively. The results demonstrated that compared to the non-aeration group, methane production increased to varying degrees with different aeration pretreatments (AP), with a maximum increase of 27.98% (DO = 2.0 mg/L). AP reduced the hydrogen sulfide (H2S) content of biogas. The H2S concentration in the DO = 2.0 mg/L was only 0.209%, and this represented an increased H2S removal rate of 49.27% compared to that of the DO = 0.0 mg/L (0.412%). Simultaneously, AP increases the hydrolysis rate. When the DO concentration reached 2.0 mg/L, the hydrolysis rate reached its maximum. An increase in the hydrolysis rate further enhanced the removal rate of organic matter. The organic matter removal rate was highest (36.96%) at DO = 2.0 mg/L. AP effectively prolonged the methane generation time and shortened the lag time of methane generation. AP creates a brief micro aerobic environment, accelerates substrate hydrolysis, and promotes the production and consumption of total volatile fatty acids, particularly acetic acid. Additionally, AP promoted the symbiotic relationship between Caldicoprobacter (20.93%-34.96%) and Metanosaeta (14.73%-18.45%).

Revealing the effect of Al2O3 on sulfur transformation in anaerobic sludge process

Al2O3, one multifunctional adsorbent and dehydrator, was widely recognized as a practical and environmentally friendly additive that enhances fermentation efficiency and facilitates the recovery of resources from waste-activated sludge (WAS). However, its potential harmful effects on WAS fermentation, such as the generation of hydrogen sulfide (H2S), have been previously overlooked. This study found that with the increase of Al2O3 dosage from 0 to 60 mg/g VSS, the maximum production of H2S decreased from 371.60 ± 3.72 × 10-4 to 303.36 ± 3.03 × 10-4 mg/g VSS. The study on the transformation of sulfur-containing compounds has identified that the primary cause for lowering the formation of hydrogen sulfide (H2S) is the inhibitory effect of aluminium oxide (Al2O3) on sulfate reduction. The mechanism analysis discovered that Al2O3 initially stimulated the functional groups and hydrogen bonding networks present in sludge EPS. This resulted in a 2.04 % rise in the content of C-C groups, a 7.78 % increase in the content of C-O-C groups, and a 4.24 % increase in the content of β-turn and α-Helix structures. This resulted in the fracturing of sludge EPS and the release of soluble metal ions such as aluminium, magnesium, and iron. The liberated metal ions facilitated the conversion of H2S gas and dissolved sulfide into metal sulfide, hence contributing significantly to the reduction of H2S gas emissions. Microbial community research revealed that the inclusion of Al2O3 enhanced the performance of methanogens (e.g., Methanothrix), but inhibited sulfate reducing bacteria (e.g., unclassified_c__Deltaproteobacteria). Additional examination of functional genes demonstrated that Al2O3 decreases the amount of functional genes involved in the hydrolysis of organic sulfur (such as MetQ, pepD, CDO1, yhdR, etc.). and sulfate reduction processes (sat, cysC, aprAB, dsrAB, etc.). These findings offer novel perspectives on the treatment of sludge using Al2O3 and could have substantial consequences for sludge treatment.

Biofilm application for anaerobic digestion: a systematic review and an industrial scale case

Biofilm is a syntrophic community of microorganisms enveloped by extracellular polymeric substances and displays remarkable adaptability to dynamic environments. Implementing biofilm in anaerobic digestion has been widely investigated and applied as it promotes microbial retention time and enhances the efficiency. Previous studies on anaerobic biofilm primarily focused on application in wastewater treatment, while its role has been significantly extended to accelerate the degradation of lignocellulosic biomass, improve gas-liquid mass transfer for biogas upgrading, or enhance resistance to inhibitors or toxic pollutants. This work comprehensively reviewed the current applications of biofilm in anaerobic digestion and focused on impacting factors, optimization strategies, reactor set-up, and microbial communities. Moreover, a full-scale biofilm reactor case from Norway is also reported. This review provides a state of-the- art insight on the role of biofilm in anaerobic digestion.

Application of an anaerobic reactor for the treatment of sulfide-rich wastewater using biogas for H2S removal

Anaerobic treatment of sulfur-rich wastewater is challenging because sulfide greatly inhibits the activity of anaerobic microorganisms, especially methanogenic archaea. We developed an internal phase-separated reactor (IPSR) that removed sulfide prior to methanogenesis by gas stripping using biogas produced in the reactor. The IPSR was fed with synthetic wastewater containing a very high sulfide concentration of up to 6,000 mg S L-1 with a chemical oxygen demand (COD) of 30,000 mg L-1. The IPSR was operated at an organic loading rate of 5-12 kg COD m-3 day-1 at 35 °C. The results show that the sulfide concentration was reduced from 6,000 mg S L-1 in the influent to <700 mg S L-1 in the first-stage effluent. The second-stage effluent contained <400 mg S L-1. As a result of effective sulfide removal by its gas stripping function, the IPSR had a COD removal efficiency of >90% over the entire experimental period. High-throughput 16S rRNA gene sequencing revealed that the major anaerobic archaea were Methanobacterium and Methanosaeta, which are frequently found in high-rate anaerobic reactors. Thus, the IPSR maintains these microorganisms and achieves high-process performance even when fed wastewater with very high sulfide concentrations.

Enhancing assimilatory sulfate reduction with ferrihydrite-humic acid coprecipitate in anaerobic sulfate-containing wastewater treatment

Sulfide produced from dissimilatory sulfate reduction can combine with hydrogen to form hydrogen sulfide, causing odor issues and environmental pollution. To address this problem, ferrihydrite-humic acid coprecipitate was added to improve assimilatory sulfate reduction (ASR), resulting in a decrease in sulfide production (190.2 ± 14.6 mg/L in the Fh-HA group vs. 246.3 ± 8.1 mg/L in the Fh group) with high sulfate removal. Humic acid, adsorbed on the surface of ferrihydrite, delayed secondary mineralization of ferrihydrite under sulfate reduction condition. Therefore, more iron-reducing species (e.g. Trichococcus, Geobacter) were enriched with ferrihydrite-humic acid coprecipitate to transfer more electrons to other species, which led to more COD reduction, an increase in electron transfer capacity, and a decrease in the NADH/NAD+ ratio. Metagenomic analysis also indicated that functional genes related to ASR was enhanced with ferrihydrite-humic acid coprecipitate. Thus, the addition of ferrihydrite-humic acid coprecipitate can be considered as a promising candidate for anaerobic sulfate wastewater treatment.

Biogas Upgrading Using a Single-Membrane System: A Review

In recent years, the use of biogas as a natural gas substitute has gained great attention. Typically, in addition to methane (CH4), biogas contains carbon dioxide (CO2), as well as small amounts of impurities, e.g., hydrogen sulfide (H2S), nitrogen (N2), oxygen (O2) and volatile organic compounds (VOCs). One of the latest trends in biogas purification is the application of membrane processes. However, literature reports are ambiguous regarding the specific requirement for biogas pretreatment prior to its upgrading using membranes. Therefore, the main aim of the present study was to comprehensively examine and discuss the most recent achievements in the use of single-membrane separation units for biogas upgrading. Performing a literature review allowed to indicate that, in recent years, considerable progress has been made on the use of polymeric membranes for this purpose. For instance, it has been documented that the application of thin-film composite (TFC) membranes with a swollen polyamide (PA) layer ensures the successful upgrading of raw biogas and eliminates the need for its pretreatment. The importance of the performed literature review is the inference drawn that biogas enrichment performed in a single step allows to obtain upgraded biogas that could be employed for household uses. Nevertheless, this solution may not be sufficient for obtaining high-purity gas at high recovery efficiency. Hence, in order to obtain biogas that could be used for applications designed for natural gas, a membrane cascade may be required. Moreover, it has been documented that a significant number of experimental studies have been focused on the upgrading of synthetic biogas; meanwhile, the data on the raw biogas are very limited. In addition, it has been noted that, although ceramic membranes demonstrate several advantages, experimental studies on their applications in single-membrane systems have been neglected. Summarizing the literature data, it can be concluded that, in order to thoroughly evaluate the presented issue, the long-term experimental studies on the upgrading of raw biogas with the use of polymeric and ceramic membranes in pilot-scale systems are required. The presented literature review has practical implications as it would be beneficial in supporting the development of membrane processes used for biogas upgrading.

Prospect of denitrifying anaerobic methane oxidation (DAMO) application on wastewater treatment and biogas recycling utilization

Old-fashioned wastewater treatments for nitrogen depend on heterotrophic denitrification process. It would utilize extra organic carbon source as electron donors when the C/N of domestic wastewater was too low to ensure heterotrophic denitrification process. It would lead to non-compliance with carbon reduction targets and impose an economic burden on wastewater treatment. Denitrifying anaerobic methane oxidation (DAMO), which could utilize methane serving as electron donors to replace traditional organic carbon (methanol or sodium acetate), supplies a novel approach for wastewater treatment. As the primary component of biogas, methane is an inexpensive carbon source. With anaerobic digestion becoming increasingly popular for sludge reduction in wastewater treatment plants (WWTPs), efficient biogas utilization through DAMO can offer an environmentally friendly option for in-situ biogas recycling. Here, we reviewed the metabolic principle and relevant research for DAMO and biogas recycling utilization, outlining the prospect of employing DAMO for wastewater treatment and biogas recycling utilization in WWTPs. The application of DAMO provides a new focal point for enhancing efficiency and sustainability in WWTPs.

Biohydrogen production potential with sulfate and nitrate removal by heat-pretreated enriched sulfate-reducing microorganisms-based bioelectrochemical system

In this study, heat-pretreated sulfate-reducing bacteria (SRBs) were evaluated for simultaneous sulfate and nitrate removal in a bioelectrochemical system (BES). The effect of the applied potential of 20 mV to SRBs was evaluated at a sulfate concentration of 3 g/L and/or nitrate concentration of 0.5 g/L supplemented before heat pretreatment for sulfate and nitrate removal. The highest H₂ production of 2.24 ± 0.04 mM/L in heat-pretreated culture was observed in the presence of sulfate at an applied potential of 20 mV (BHE-S). Simultaneous reduction of sulfate and nitrate was significant in BESs supplemented with either sulfate or nitrate during heat-shock pretreatment of the culture. The highest SO₄^2- removal of 88.91 ± 0.8% was found in culture heat pretreated with NO₃^- and applied with 20 mV potential (BHE-N). The kinetics of heat-pretreated culture showed higher R² and ultimate potential for H₂ on the continuous application of 20 mV potential.

Microbial electrosynthesis of methane and acetate—comparison of pure and mixed cultures

Abstract

The electrochemical process of microbial electrosynthesis (MES) is used to drive the metabolism of electroactive microorganisms for the production of valuable chemicals and fuels. MES combines the advantages of electrochemistry, engineering, and microbiology and offers alternative production processes based on renewable raw materials and regenerative energies. In addition to the reactor concept and electrode design, the biocatalysts used have a significant influence on the performance of MES. Thus, pure and mixed cultures can be used as biocatalysts. By using mixed cultures, interactions between organisms, such as the direct interspecies electron transfer (DIET) or syntrophic interactions, influence the performance in terms of productivity and the product range of MES. This review focuses on the comparison of pure and mixed cultures in microbial electrosynthesis. The performance indicators, such as productivities and coulombic efficiencies (CEs), for both procedural methods are discussed. Typical products in MES are methane and acetate, therefore these processes are the focus of this review. In general, most studies used mixed cultures as biocatalyst, as more advanced performance of mixed cultures has been seen for both products. When comparing pure and mixed cultures in equivalent experimental setups a 3-fold higher methane and a nearly 2-fold higher acetate production rate can be achieved in mixed cultures. However, studies of pure culture MES for methane production have shown some improvement through reactor optimization and operational mode reaching similar performance indicators as mixed culture MES. Overall, the review gives an overview of the advantages and disadvantages of using pure or mixed cultures in MES.

Key points

• Undefined mixed cultures dominate as inoculums for the MES of methane and acetate, which comprise a high potential of improvement

• Under similar conditions, mixed cultures outperform pure cultures in MES

• Understanding the role of single species in mixed culture MES is essential for future industrial applications

Hydrogen sulphide management in anaerobic digestion: A critical review on input control, process regulation, and post-treatment

Hydrogen sulphide (H₂S) in biogas is a problematic impurity that can inhibit methanogenesis and cause equipment corrosion. This review discusses technologies to remove H₂S during anaerobic digestion (AD) via: input control, process regulation, and post-treatment. Post-treatment technologies (e.g. biotrickling filters and scrubbers) are mature with >95% removal efficiency but they do not mitigate H₂S toxicity to methanogens within the AD. Input control (i.e. substrate pretreatment via chemical addition) reduces sulphur input into AD via sulphur precipitation. However, available results showed <75% of H₂S removal efficiency. Microaeration to regulate AD condition is a promising alternative for controlling H₂S formation. Microaeration, or the use of oxygen to regulate the redox potential at around -250 mV, has been demonstrated at pilot and full scale with >95% H₂S reduction, stable methane production, and low operational cost. Further adaptation of microaeration relies on a comprehensive design framework and exchange operational experience for eliminating the risk of over-aeration.

A Review of the State of the Art of Biomethane Production: Recent Advancements and Integration of Renewable Energies

Anaerobic Digestion (AD) is a well-established process that is becoming increasingly popular, especially as a technology for organic waste treatment; the process produces biogas, which can be upgraded to biomethane, which can be used in the transport sector or injected into the natural gas grid. Considering the sensitivity of Anaerobic Digestion to several process parameters, mathematical modeling and numerical simulations can be useful to improve both design and control of the process. Therefore, several different modeling approaches were presented in the literature, aiming at providing suitable tools for the design and simulation of these systems. The purpose of this study is to analyze the recent advancements in the biomethane production from different points of view. Special attention is paid to the integration of this technology with additional renewable energy sources, such as solar, geothermal and wind, aimed at achieving a fully renewable biomethane production. In this case, auxiliary heat may be provided by solar thermal or geothermal energy, while wind or photovoltaic plants can provide auxiliary electricity. Recent advancements in plants design, biomethane production and mathematical modeling are shown in the paper, and the main challenges that these fields must face with are discussed. Considering the increasing interest of industries, public policy makers and researchers in this field, the efficiency and profitability such hybrid renewable solutions for biomethane production are expected to significantly improve in the next future, provided that suitable subsidies and funding policies are implemented to support their development.