Stimulation of methanogenesis in anaerobic digesters treating leachate from a municipal solid waste incineration plant with carbon cloth

Stimulation of the anaerobic digestion of the dry organic fraction of municipal solid waste (OFMSW) with carbon-based conductive materials

Growth of bacterial and archaeal species capable of interspecies electron exchange was stimulated by addition of conductive materials (carbon cloth or granular activated carbon (GAC)) to anaerobic digesters treating dog food (a substitute for the dry-organic fraction of municipal solid waste (OFMSW)). Methane production (772-1428mmol vs <80mmol), volatile solids removal (78%-81% vs 54%-64%) and COD removal efficiencies (∼80% vs 20%-30%) were all significantly higher in reactors amended with GAC or carbon cloth than controls. OFMSW degradation was also significantly accelerated and VFA concentrations were substantially lower in reactors amended with conductive materials. These results suggest that both conductive materials (carbon cloth and GAC) can promote conversion of OFMSW to methane even in the presence of extremely high VFA concentrations (∼500mM).

Carbon nanotubes accelerate methane production in pure cultures of methanogens and in a syntrophic coculture

Carbon materials have been reported to facilitate direct interspecies electron transfer (DIET) between bacteria and methanogens improving methane production in anaerobic processes. In this work, the effect of increasing concentrations of carbon nanotubes (CNT) on the activity of pure cultures of methanogens and on typical fatty acid-degrading syntrophic methanogenic coculture was evaluated. CNT affected methane production by methanogenic cultures, although acceleration was higher for hydrogenotrophic methanogens than for acetoclastic methanogens or syntrophic coculture. Interestingly, the initial methane production rate (IMPR) by Methanobacterium formicicum cultures increased 17 times with 5 g·L^-1 CNT. Butyrate conversion to methane by Syntrophomonas wolfei and Methanospirillum hungatei was enhanced (∼1.5 times) in the presence of CNT (5 g·L^-1 ), but indications of DIET were not obtained. Increasing CNT concentrations resulted in more negative redox potentials in the anaerobic microcosms. Remarkably, without a reducing agent but in the presence of CNT, the IMPR was higher than in incubations with reducing agent. No growth was observed without reducing agent and without CNT. This finding is important to re-frame discussions and re-interpret data on the role of conductive materials as mediators of DIET in anaerobic communities. It also opens new challenges to improve methane production in engineered methanogenic processes.

Towards engineering application: Potential mechanism for enhancing anaerobic digestion of complex organic waste with different types of conductive materials

Conductive materials have been widely investigated to accelerate and stabilize the conversion of organic wastes to methane. However, the potential mechanisms involved with different types of conductive materials are still unclear. In this study, magnetite (Fe₃O₄) and granular activated carbon (GAC), as the two typical conductive materials, were respectively supplemented to acidogenesis and methanogenesis of a two-phase anaerobic digestion (AD) system in an attempt to explore their different mechanisms. The results showed that, magnetite supplemented to the acidogenic phase could enhance the decomposition of complex organics into simples, but significantly raise the hydrogen partial pressure as well as enrich the hydrogen-utilizing methanogens, which were not expected for aceticlastic methanogenesis known as a mainstream of methanogenesis in most of traditional digesters. GAC supplemented to the methanogenic phase had less influences on syntrophic metabolism of alcohols and fatty acids when acidogenesis was ineffective or out of work. Microbial community analysis suggested that direct interspecies electron transfer (DIET) had been established on the GAC, though the insignificant improvement of performances. Once magnetite was supplemented to the acidogenesis to improve the acidification efficiency, the syntrophic conversion of alcohols and fatty acids to methane in the GAC-supplemented methanogenic phase was significantly improved. These results suggested that, DIET was unlikely to participate in the direct decomposition of complex organics, even in the presence of GAC, but it could work effectively once acidogenesis functioned well.

Enhancing biomethanogenic treatment of fresh incineration leachate using single chambered microbial electrolysis cells

Methanogenic treatment of municipal solid waste (MSW) incineration leachate can be hindered by high concentrations of refractory organic matter and humification of compounds in the leachate. In an attempt to overcome some of these impediments, microbial electrolysis cells (MECs) were incorporated into anaerobic digesters (ADMECs). COD removal efficiencies and methane production were 8.7% and 44.3% higher in ADMECs than in controls, and ADMEC reactors recovered more readily from souring caused by high organic loading rates. The degradation rate of large macromolecules was substantially higher (96% vs 81%) in ADMEC than control effluent, suggesting that MECs stimulated degradation of refractory organic matter and reduced humification. Exoelectrogenic bacteria and microorganisms known to form syntrophic partnerships were enriched in ADMECs. These results show that ADMECs were more effective at treatment of MSW incineration leachate, and should be taken into consideration when designing future treatment facilities.

On the Edge of Research and Technological Application: A Critical Review of Electromethanogenesis

The conversion of electrical current into methane (electromethanogenesis) by microbes represents one of the most promising applications of bioelectrochemical systems (BES). Electromethanogenesis provides a novel approach to waste treatment, carbon dioxide fixation and renewable energy storage into a chemically stable compound, such as methane. This has become an important area of research since it was first described, attracting different research groups worldwide. Basics of the process such as microorganisms involved and main reactions are now much better understood, and recent advances in BES configuration and electrode materials in lab-scale enhance the interest in this technology. However, there are still some gaps that need to be filled to move towards its application. Side reactions or scaling-up issues are clearly among the main challenges that need to be overcome to its further development. This review summarizes the recent advances made in the field of electromethanogenesis to address the main future challenges and opportunities of this novel process. In addition, the present fundamental knowledge is critically reviewed and some insights are provided to identify potential niche applications and help researchers to overcome current technological boundaries.

Biologically Produced Methane as a Renewable Energy Source

Methanogens are a unique group of strictly anaerobic archaea that are more metabolically diverse than previously thought. Traditionally, it was thought that methanogens could only generate methane by coupling the oxidation of products formed by fermentative bacteria with the reduction of CO₂. However, it has recently been observed that many methanogens can also use electrons extruded from metal-respiring bacteria, biocathodes, or insoluble electron shuttles as energy sources. Methanogens are found in both human-made and natural environments and are responsible for the production of ∼71% of the global atmospheric methane. Their habitats range from the human digestive tract to hydrothermal vents. Although biologically produced methane can negatively impact the environment if released into the atmosphere, when captured, it can serve as a potent fuel source. The anaerobic digestion of wastes such as animal manure, human sewage, or food waste produces biogas which is composed of ∼60% methane. Methane from biogas can be cleaned to yield purified methane (biomethane) that can be readily incorporated into natural gas pipelines making it a promising renewable energy source. Conventional anaerobic digestion is limited by long retention times, low organics removal efficiencies, and low biogas production rates. Therefore, many studies are being conducted to improve the anaerobic digestion process. Researchers have found that addition of conductive materials and/or electrically active cathodes to anaerobic digesters can stimulate the digestion process and increase methane content of biogas. It is hoped that optimization of anaerobic digesters will make biogas more readily accessible to the average person.