Impact of Landfill Gas Exposure on Vegetation in Engineered Landfill Biocover Systems Implemented to Minimize Fugitive Methane Emissions from Landfills.

Engineered landfill biocovers (LBCs) minimize the escape of methane into the atmosphere through biological oxidation. Vegetation plays a crucial role in LBCs and can suffer from hypoxia caused by the displacement of root-zone oxygen due to landfill gas and competition for oxygen with methanotrophic bacteria. To investigate the impact of methane gas on vegetation growth, we conducted an outdoor experiment using eight vegetated flow-through columns filled with a 45 cm mixture of 70% topsoil and 30% compost, planted with three types of vegetation: native grass blend, Japanese millet, and alfalfa. The experiment included three control columns and five columns exposed to methane, as loading rates gradually increased from 75 to 845 gCH4/m2/d over a period of 65 days. At the highest flux, we observed a reduction of 51%, 31%, and 19% in plant height, and 35%, 25%, and 17% in root length in native grass, Japanese millet, and alfalfa, respectively. The column gas profiles indicated that oxygen concentrations were below the levels required for healthy plant growth, which explains the stunted growth observed in the plants used in this experiment. Overall, the experimental results demonstrate that methane gas has a significant impact on the growth of vegetation used in LBCs.

The influence of biochar and compost mixtures, water content, and gas flow rate, on the continuous adsorption of methane in a fixed bed column.

Biofiltration is an excellent alternative for the treatment of diffuse emissions of methane (CH4) that cannot be treated by physical/chemical means or recovered for energy. Despite the advances on CH4 biological treatment technologies, they are limited by the low aqueous solubility of CH4 into the biofilm where CH4 mineralization occurs. In this study, the CH4 adsorption kinetics, adsorption capacity and transport behavior of CH4 was studied in batch experiments and in a fixed-bed column by varying the biochar and compost mixtures under 5-levels, 3 different water contents (dry, 15% and 30% water holding capacity), and 2 inlet flow rates. Experimental results were formally tested using analysis of variance (ANOVA) in order to draw objective conclusions based on statistical inference. As CH4 biofiltration requires water addition to maintain microbial activity, these results indicate adsorption capacity is not lost with water addition if biochar content is the dominant packing material. The Langmuir isotherm described the data best (R2 = 0.99). Maximum adsorption capacity by monolayer adsorption, or qmax, is relatively similar with or without the addition of water as long as the biochar component is the dominant material at 3.5 mg CH4/g medium for a 7:1 biochar: compost, v/v mixture. Empirical regression models for qo, kTh, (Thomas model) and τ and KYN (Yoon-Nelson model) were developed for the break through curves of CH4. The current work demonstrates the applicability of utilizing biochar, a relatively inexpensive adsorbent, can compensate for the low solubility of CH4 and overcome the rate-limiting step of mass transfer from the gas phase and into the methanotrophic biofilm. Further, biochar may be a reliable back-up system for CH4 storage especially for fluctuating inlet loads that may be encountered in industrial applications adsorbing up to 13 mg CH4/g biochar under dry conditions.

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 (KM(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.

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

Aiming to improve conventional methane biofilter performance, a multiple-level aeration biofilter design is proposed. Laboratory flow-through column experiments were conducted to evaluate three actively-aerated methane biofilter configurations. Columns were aerated at one, two, and three levels of the bed depth, with air introduced at flow rates calculated from methane oxidation reaction stoichiometry. Inlet methane loading rates were increased in five stages between 6 and 18mL/min. The effects of methane feeding rate, levels of aeration, and residence time on methane oxidation rates were determined. Samples collected after completion of flow-through experiments were used to determine methane oxidation kinetic parameters, Vmax, Km, and methanotrophic community distribution across biofilter columns. Results obtained from mixed variances analysis and response surfaces, as well as methanotrophic activity data, suggested that, biofilter column with two aeration levels has the most even performance over time, maintaining 85.1% average oxidation efficiency over 95days of experiments.

Rapid assessment of methanotrophic capacity of compost-based materials considering the effects of air-filled porosity, water content and dissolved organic carbon.

Since the global warming potential of CH4 is 25 times that of CO2 on a 100-year time horizon, the development of methanotrophic applications for the conversion of CH4 to CO2 is emerging as an area of interest for researchers and practicing engineers. Compost exhibits most of the characteristics required for methanotroph growth media and has been used in several projects. This paper presents results from a study that was undertaken to assess the influence of physical and chemical characteristics of compost-based materials on the biological oxidation of CH4 when used in methane biofilters. The results showed that easily-measurable parameters, such as air filled porosity, water content and dissolved organic carbon, are correlated with maximum CH4 removal rates. The results obtained were used to develop an empirical relationship that could be regarded as a rapid assessment tool for the estimation of the performance of compost-based materials in engineered methanotrophic applications.

Study of thin biocovers (TBC) for oxidizing uncaptured methane emissions in bioreactor landfills.

Bioreactor landfills are designed to accelerate municipal solid waste biodegradation and stabilization; however, the uncaptured methane gas escapes to the atmosphere during their filling. This research investigates the implementation of a novel methane emission control technique that involves thin biocovers (TBC) placed as intermediate waste covers to oxidize methane without affecting the operation of bioreactor landfills. Batch incubation experiments were conducted for selecting the optimum TBC materials, capable of oxidizing methane to carbon dioxide by methanotrophic bacteria. Column experiments were performed to investigate the TBC performance under varying moisture content, compost-to-sawdust ratio, methane flow rate, and biocover thickness. Overall, the optimum TBC is comprised of a 30-cm thick bed of 0-10mass% sawdust mixed with compost, having a moisture content of 52% ww, which showed 100% CH4 oxidation efficiency over an extended period of time even at a relatively high methane inlet load of 9.4gm(-3)h(-1).

WITHDRAWN: Study of thin biocovers (TBC) for oxidizing uncaptured methane emissions in bioreactor landfills.

The Publisher regrets that this article is an accidental duplication of an article that has already been published in Waste Management, volume 28 (2008) 1364 - 1374, doi:10.1016/j.wasman.2007.06.017. The duplicate article has therefore been withdrawn.

Relation of waste generation and composition to socio-economic factors: a case study.

To develop an effective waste management strategy for a given region, it is important to know the amount of waste generated and the composition of the waste stream. Past research has shown that the amount of waste generated is proportional to the population and the average mean living standards or the average income of the people. In addition, other factors may affect the amount and composition of waste. These are climate, living habits, level of education, religious and cultural beliefs, and social and public attitudes. This paper presents the findings of a study carried out in a suburban municipal area in Sri Lanka to determine the solid waste generation rate and waste composition based on field surveys and to determine the related socio-economic factors. A database was developed that included information on the quantity and composition of waste generated in a sample of households in the study area over a time period. The collected data was analysed to relate waste generation and composition data to various socio-economic factors. Over 400 sample households were selected for the study using a stratified random sampling methodology based on municipal wards and property values. A technique that considers both the number of households in a particular income group (property value range) and the standard deviation of property values within a given income group was used to determine the appropriate sample size for each municipal ward. Through category and regression analyses, the quantities of waste and waste composition were related to several socio-economic factors. The paper describes the basis for the sample selection, the methodology adopted for data collection, the socio-economic parameters used for the analysis, and the relationships developed from the analysis.