Aerated biofilters with multiple-level air injection configurations to enhance biological treatment of methane emissions

Exploring the potential of biofiltration for mitigating harmful gaseous emissions from small or old landfills: a review

Landfills are widely employed as the primary means of solid waste disposal. However, this practice generates landfill gas (LFG) which contains methane (CH4), a potent greenhouse gas, as well as various volatile organic compounds and volatile inorganic compounds. These emissions from landfills contribute to approximately 25% of the total atmospheric CH4, indicating the imperative need to valorize or treat LFG prior to its release into the atmosphere. This review first aims to outline landfills, waste disposal and valorization, conventional gas treatment techniques commonly employed for LFG treatment, such as flares and thermal oxidation. Furthermore, it explores biotechnological approaches as more technically and economically feasible alternatives for mitigating LFG emissions, especially in the case of small and aged landfills where CH4 concentrations are often below 3% v/v. Finally, this review highlights biofilters as the most suitable biotechnological solution for LFG treatment and discusses several advantages and challenges associated with their implementation in the landfill environment.

Quantification and control of gaseous emissions from solid waste landfill surfaces

Landfilling is the most common option for solid waste disposal worldwide. Landfill sites can emit significant quantities of greenhouse gases (GHGs; e.g., methane, carbon dioxide, and nitrous oxide) and release toxic and odorous compounds (e.g., sulfides). Due to the complex composition and characteristics of landfill surface gas emissions, the quantification and control of landfill emissions are challenging. This review attempts to comprehensively understand landfill emission quantification and control options by primarily focusing on GHGs and odor compounds. Landfill emission quantification was highlighted by combining different emissions monitoring approaches to improve the quality of landfill emission data. Also, landfill emission control requires a specific approach that targets emission compounds or a systematic approach that reduces overall emissions by combining different control methods since the diverse factors dominate the emissions of various compounds and their transformation. This integrated knowledge of emission quantification and control options for GHGs and odor compounds is beneficial for establishing field monitoring campaigns and incorporating mitigation strategies to quantify and control multiple landfill emissions.

Experimental analysis and modeling of the methane degradation in a three stage biofilter using composted sawdust as packing media

Methane gas is a very effective greenhouse gas and the second-largest contributor to global warming. Biofiltration is an effective technology that uses microorganisms to degrade the pollutant by oxidizing it. In this work, the performance of a biofilter with supporting filter media, consisting of composted sawdust, is evaluated at three different sampling ports. Furthermore, a transient model is developed to predict methane concentration at various heights and times. The developed model is validated with the experimental data and shows good agreement with the experimental data. The highest removal efficiency and elimination capacity was found to be 72% and 0.108 g m^-3 h^-1 respectively. The effect of parameters such as specific surface area, the reaction rate constant, biofilm thickness and airflow rate were studied on the outlet methane concentration. Under similar conditions, the simulations showed that the removal efficiency of 95% might be achieved for the height of 2 m.

Enhancement of the methane removal efficiency via aeration for biochar-amended landfill soil cover

Methane (CH₄) mitigation of biocovers or biofilters for landfills is influenced by the bed material and oxygen availability. The improvement of active aeration for the CH₄ oxidation efficiency of biochar-amended landfill soil cover was investigated over a period of 101 days. There were column 1 as the control group, column 2 with biochar amending the soil cover, and column 3 with daily active aeration besides the same biochar amendment. All groups were inoculated with enriched methane oxidation bacteria (MOB). The average CH₄ removal efficiency was up to 78.6%, 85.2% and 90.6% for column 1, 2, and 3, respectively. The depth profiles of CH₄ oxidation efficiencies over the whole period also showed that the stimulation of CH₄ oxidation by biochar amendment was apparent in the top 35 cm but became very faint after two months. This probably was due to the rapid depletion of nitrogen nutrition caused by enhanced methanotrophic activities. While through aeration, CH₄ oxidation efficiency was further improved for column 3 than column 2. This enhancement also lasted for the whole period with a reduced decline of CH₄ oxidation. Finally, the major MOB Methylocystis, commonly found in the three columns, were most abundant in the top 35 cm for column 3. A more balanced ratio of MOB and more homogeneous microbial community structures across different soil depths were also the results of active aeration.

Simultaneous biodegradation of methane and styrene in biofilters packed with inorganic supports: Experimental and macrokinetic study

Four upflow 0.018 m³ biofilters (3 beds), B-ME, B-200, B-500 and B-700, all packed with inorganic materials, were operated at a constant air flow rate of 0.18 m³ h^-1 to eliminate methane (CH₄), a harmful greenhouse gas (GHG), and styrene (C₈H₈), a carcinogenic volatile organic compound (VOC). The biofilters were irrigated with 0.001 m³ of recycled nutrient solution (NS) every day (flow rate of 60 × 10^-3 m³ h^-1). Styrene inlet load (IL) was kept constant in each biofilter. Different CH₄-ILs varying in the range of 7-60 gCH₄ m^-3 h^-1 were examined in B-ME (IL of 0 gC₈H₈ m^-3 h^-1), B-200 (IL of 9 gC₈H₈ m^-3 h^-1), B-500 (IL of 22 gC₈H₈ m^-3 h^-1) and B-700 (IL of 32 gC₈H₈ m^-3 h^-1). Finally, the effect of C₈H₈ on the macrokinetic parameters of CH₄ biofiltration was studied based on the Michaelis-Menten model. Average C₈H₈ removal efficiencies (RE) varying between 64 and 100% were obtained at CH₄-ILs increasing from 7 to 60 gCH₄ m^-3 h^-1 and for C₈H₈-ILs range of 0-32 gC₈H₈ m^-3 h^-1. More than 90% of C₈H₈ was removed in the bottom and middle beds of the biofilters. By increasing C₈H₈-IL from 0 to 32 gC₈H₈ m^-3 h^-1, maximal EC in Michaelis-Menten model and macrokinetic saturation constant declined from 311 to 39 g m^-3 h^-1 and from 19 to 2.3 g m^-3, respectively, which confirmed that an uncompetitive inhibition occurred during CH₄ biofiltration in the presence of C₈H₈.

Methane Elimination Using Biofiltration Packed With Fly Ash Ceramsite as Support Material

Methane is a greenhouse gas and significantly contributes to global warming. Methane biofiltration with immobilized methane-oxidizing bacteria (MOB) is an efficient and eco-friendly approach for methane elimination. To achieve high methane elimination capacity (EC), it is necessary to use an exceptional support material to immobilize MOB. The MOB consortium was inoculated in biofilters to continuusly eliminate 1% (v/v) of methane. Results showed that the immobilized MOB cells outperformed than the suspended MOB cells. The biofilter packed with fly ash ceramsite (FAC) held the highest average methane EC of 4.628 g h^–1 m^–3, which was 33.4% higher than that of the biofilter with the suspended MOB cells. The qPCR revealed that FAC surface presented the highest pmoA gene abundance, which inferred that FAC surface immobilized the most MOB biomass. The XPS and contact angle measurement indicated that the desirable surface elemental composition and stronger surface hydrophilicity of FAC might favor MOB immobilization and accordingly improve methane elimination.

Mitigation of Methane, NMVOCs and Odor Emissions in Active and Passive Biofiltration Systems at Municipal Solid Waste Landfills

Biofiltration systems are emerging technological solutions for the removal of methane and odors from landfill gas when flaring is no longer feasible. This work analyzed and compared two full-scale biofiltration systems: biofilter and biowindows. The emission mitigation of methane, non-methane volatile organic compounds (NMVOCs) and odors during a two-year management and monitoring period was studied. In addition to diluted methane, more than 50 NMVOCs have been detected in the inlet raw landfill gas and the sulfur compounds resulted in the highest odor activity value. Both systems, biofilter and biowindows, were effective for the oxidation of methane (58.1% and 88.05%, respectively), for the mitigation of NMVOCs (higher than 80%) and odor reduction (99.84% and 93.82% respectively). As for the biofilter monitoring, it was possible to define the oxidation efficiency trend and in fact to guarantee that for an oxidation efficiency of 80%, the methane load must be less than 6.5 g CH4/m2h with an oxidation rate of 5.2 g CH4/m2h.

Biofiltration of diluted landfill gas in an active loaded open-bed compost filter

Microbial oxidation in a biofilter is a treatment solution for diluted landfill gas (LFG), for instance at old landfills, where LFG recovery is no longer feasible, or from remediation systems designed to cut off laterally migrating LFG. In this study, an actively loaded open-bed compost filter, designed for the treatment of diluted LFG, was tested at an old landfill in Denmark. An 18 m³ biofilter was constructed in a 30 m³ container loaded with LFG mixed with air, in order to obtain diluted LFG. The inlet concentration of methane (CH₄) fluctuated between 4.4 and 9.2 vol% during the five tested flow campaigns, resulting in CH₄ loads of 106-794 g CH₄ m^-2 d^-1. The maximum identified CH₄ oxidation rate was 460 g m^-2 d^-1, with an overall CH₄ oxidation efficiency of 58%. Due to preferential flows, especially along the edges of the filter at the transition points between the compost and the container wall, an overall CH₄ oxidation efficiency of 100% was never achieved. However, pore gas profiles in selected areas in the filter material showed oxidation efficiencies close to 100%. The results were supported by tracer gas tests showing average oxidation efficiency in the nine measuring points of 89% at a CH₄ load of 487 ± 64 g CH₄ m^-2 d^-1.

Treatment of landfill gas with low methane content by biocover systems

Landfills are significant sources of anthropogenic atmospheric methane (CH₄), which contributes to climate change. Large amounts of CH₄ are emitted from landfills in dilute form due to mixing with air in leachate collection systems, or during lateral migration away from landfills. The objective of this study was to investigate the CH₄ oxidation efficiency of a compost material subject to LFG diluted with atmospheric air resulting in CH₄ concentrations of 5-10% v/v. CH₄ oxidation rates and carbon dioxide (CO₂) production were measured through batch and dynamic column experiments where two laboratory scale biofilters were constructed. The columns were run at increasing flow rates. Column gas concentration profiles for each of five flow campaigns were compared to each other. This showed that oxygen (O₂) was present through the entire column and elevated CO₂ concentrations throughout the biofilters were found. Moreover, the oxidation process tended to be centred in the lower parts of both columns. It was observed that the biofilters performed better once they had adapted to the increasing loads of CH₄. In both columns, the maximum removal rate of CH₄ was found to be 98-100%. Using CH₄ mass balances the maximum oxidation rate was 238 g CH₄ m^-2 d^-1 in Column 1 and 483 g CH₄ m^-2 d^-1 in Column 2 (equal to the load). None of the biofilters reached their maximum CH₄ oxidation capacity, hence they could have been exposed to a larger CH₄ load. It was found that the retention time in the columns was not a factor limiting the oxidation process. High O₂ consumption and carbon mass balances underlined the strong microbial activity in the biofilters and it was not suspected that the methane oxidising bacteria were O₂ limited. The results of this study suggest that biofilters have great potential for reducing CH₄ in diluted LFG.

A comparative evaluation of the performance of full-scale high-rate methane biofilter (HMBF) systems and flow-through laboratory columns

Methane biofilter (MBF) technology, a cost effective method to control atmospheric emission of CH₄, is usually developed as a passively aerated system to control low-volume point-source emissions such as those from landfills with gas collection systems. Actively aerated high-rate methane biofilter (HMBF) systems are designed to overcome the shortcomings of passively aerated systems by ensuring the entire filter bed is utilized for CH₄ oxidation. Flow-through column experiments point to the fact that CH₄ oxidation rates of actively aerated systems could be several times higher than that of passively aerated systems. However, reports of the performance of field HMBF systems are not available in literature. Furthermore, there are no studies that demonstrate the possibility of using laboratory data in the design and operation of field systems. The current study was conducted to fill this research gap and involve a comparative study of the performance of laboratory columns to field performance of a HMBF system using solution gas produced at an oil battery site as the CH₄ source. The actively aerated column studies confirmed past results with high CH₄ oxidation rates; one column received air at two injection points and achieved an oxidation rate of 1417 g/m³/d, which is the highest reported value to date for compost-filled columns. Subsequent studies at a specially designed field HMBF filled with compost showed a higher oxidation rate of 1919 g/m³/d, indicating the possibility of exceeding the high CH₄ oxidation rates observed in the laboratory. The achievement of observed field oxidation rates being higher than those in the laboratory is attributed to the capability of maintaining higher temperatures in field HMBFs. Furthermore, results show that field HMBFs could operate at lower than stoichiometric air to CH₄ ratios, and lower retention times than that of laboratory columns. Results indicated that laboratory columns may not truly represent field behavior, and said results could only be used in the preliminary design of field HMBFs.

Biofiltration of methane

The on-going annual increase in global methane (CH₄) emissions can be largely attributed to anthropogenic activities. However, as more than half of these emissions are diffuse and possess a concentration less than 3% (v/v), physical-chemical treatments are inefficient as an abatement technology. In this regard, biotechnologies, such as biofiltration using methane-oxidizing bacteria, or methanotrophs, are a cost-effective and efficient means of combating diffuse CH₄ emissions. In this review, a number of abiotic factors including temperature, pH, water content, packing material, empty-bed residence time, inlet gas flow rate, CH₄ concentration, as well biotic factors, such as biomass development, are reviewed based on empirical findings on CH₄ biofiltration studies that have been performed in the last decades.

Biofiltration of methane using hybrid mixtures of biochar, lava rock and compost

Using hybrid packing materials in biofiltration systems takes advantage of both the inorganic and organic properties offered by the medium including structural stability and a source of available nutrients, respectively. In this study, hybrid mixtures of compost with either lava rock or biochar in four different mixture ratios were compared against 100% compost in a methane biofilter with active aeration at two ports along the height of the biofilter. Biochar outperformed lava rock as a packing material by providing the added benefit of participating in sorption reactions with CH₄. This study provides evidence that a 7:1 volumetric mixture of biochar and compost can successfully remove up to 877 g CH₄/m³·d with empty-bed residence times of 82.8 min. Low-affinity methanotrophs were responsible for the CH₄ removal in these systems (K_M(app) ranging from 5.7 to 42.7 µM CH₄). Sequencing of 16S rRNA gene amplicons indicated that Gammaproteobacteria methanotrophs, especially members of the genus Methylobacter, were responsible for most of the CH₄ removal. However, as the compost medium was replaced with more inert medium, there was a decline in CH₄ removal efficiency coinciding with an increased dominance of Alphaproteobacteria methanotrophs like Methylocystis and Methylocella. As a biologically-active material, compost served as the sole source of nutrients and inoculum for the biofilters which greatly simplified the operation of the system. Higher elimination capacities may be possible with higher compost content such as a 1:1 ratio of either biochar or lava rock, while maintaining the empty-bed residence time at 82.8 min.

Mitigating Methane: Emerging Technologies To Combat Climate Change's Second Leading Contributor

Methane (CH₄) is the second greatest contributor to anthropogenic climate change. Emissions have tripled since preindustrial times and continue to rise rapidly, given the fact that the key sources of food production, energy generation and waste management, are inexorably tied to population growth. Until recently, the pursuit of CH₄ mitigation approaches has tended to align with opportunities for easy energy recovery through gas capture and flaring. Consequently, effective abatement has been largely restricted to confined high-concentration sources such as landfills and anaerobic digesters, which do not represent a major share of CH₄'s emission profile. However, in more recent years we have witnessed a quantum leap in the sophistication, diversity and affordability of CH₄ mitigation technologies on the back of rapid advances in molecular analytical techniques, developments in material sciences and increasingly efficient engineering processes. Here, we present some of the latest concepts, designs and applications in CH₄ mitigation, identifying a number of abatement synergies across multiple industries and sectors. We also propose novel ways to manipulate cutting-edge technology approaches for even more effective mitigation potential. The goal of this review is to stimulate the ongoing quest for and uptake of practicable CH₄ mitigation options; supplementing established and proven approaches with immature yet potentially high-impact technologies. There has arguably never been, and if we do not act soon nor will there be, a better opportunity to combat climate change's second most significant greenhouse gas.

Inhibitory effects of acidic pH and confounding effects of moisture content on methane biofiltration

This study focussed on evaluating the effect of hydrogen sulfide (H₂S) on biological oxidation of waste methane (CH₄) gas in compost biofilters, Batch experiments were conducted to determine the dependency of maximum methane oxidation rate (V_max) on two main factors; pH and moisture content, as well as their interaction effects. The maximum V_max was observed at a pH of 7.2 with decreasing V_max values observed with decreasing pH, irrespective of moisture content. Flow-through columns operated at a pH of 4.5 oxidized CH₄ at a flux rate of 53g/m²/d compared to 146g/m²/d in columns operated at neutral pH. No oxidation activity was observed for columns operated at pH 2.5, and DNA sequencing analysis of samples led to the conclusion that highly acidic conditions were responsible for inhibiting the ability of methanotrophs to oxidize CH₄. Biofilter columns operated at pH 2.5 contained only 2% methanotrophs (type I) out of the total microbial population, compared to 55% in columns operated at pH 7.5. Overall, changes in the population of methanotrophs with acidification within the biofilters compromised its capacity to oxidize CH₄ which demonstrated that a compost biofilter could not operate efficiently in the presence of high levels of H₂S.