Current Environmental Engineering (v.3, #2)

Meet Our Associate Editor by Jimmy (C.M.) Kao (77-77).

Optimized Landfill Biocover for CH4 Oxidation I: Experimental Design and Oxidation Performance by Inga Ute Röwer, Heijo Scharff, Eva-Maria Pfeiffer, Julia Gebert (80-93).
Background: Methane oxidation in landfill biocovers is a recognized technology for the mitigation of greenhouse gas emission.
Objective and Method: A test cell was designed to investigate the combination of minimizing water infiltration into the waste and the oxidation of methane in an optimized landfill cover under Dutch climatic conditions. Limitation of infiltration was intended by means of a capillary barrier. With respect to methane oxidation it was intended to achieve a cover with a relatively high air filled porosity.
Results and Conclusion: This paper reports on the design of the landfill biocover test cell, presents a novel, spatially more representative chamber measurement method, assesses its applicability, and presents the established carbon balance and the associated restrictions. An asset was the controlled gas influx and the coverage of the entire test cell with chamber measurements, by means of which CH4 oxidation rates (12.0 ± 1.7 g CH4 m-2 d-1) and efficiencies (at least 77 ± 5%) could be derived.

Optimized Landfill Biocover for CH4 Oxidation II: Implications of Spatially Heterogeneous Fluxes for Monitoring and Emission Prediction by Inga Ute-Röwer, Jan Streese-Kleeberg, Heijo Scharff, Eva-Maria Pfeiffer, Julia Gebert (94-106).
Landfill emission monitoring is a prerequisite for the assessment of the efficiency of methane oxidation systems. Monitoring methods have limitations related to the spatial representativeness in small scale methods such as chambers, and separability of emission sources for large scale measurements. In the present study, the reliability of the chamber method was combined with the requirement of spatial representativeness using a novel large scale chamber with a base area of 17.6 m2. The emission for the entire test cell was used as a baseline to test spatial representativeness of the widely used monitoring technique of surface FID screening. Even on the scale of the investigated 953 m2 test cell the variability of emissions was so high that the number of measurements required to capture the total emission would not have been feasible with chambers of conventional size. The main part of emissions occurred upslope. This was attributed to the capillary barrier, which served both the reduction of leachate infiltration and the spatial harmonization of the landfill gas load in the capillary block. A campaign-specific correlation was observed between surface methane concentrations and actual emissions. However, the error of the slope and intercept of the data fit produced an overall error of 94% for emissions predicted from surface concentrations in the same campaign. Moreover, the nature of the correlation changed from campaign to campaign, reflecting the changing specific combination of environmental conditions on the very measurement day, so that a generalized relationship between surface concentrations and fluxes could not be established.

Evaluating Aeration Configurations in Aerobic Landfills by E.M. Bartholameuz, J.P.A. Hettiaratchi (107-117).
Operation of waste cells in aerobic mode could be a viable option to control the production of waste associated greenhouse gases. Due to the lack of data in literature, this study focuses on evaluating three different landfill active aeration methods: aeration using a vertical piping system, aeration using a horizontal piping system, and aeration through the leachate collection system. Evaluation of these configurations at bench scale is conducted using scaled-down experiments based on air dynamics similitude. A theoretical analysis is used to determine the similitude parameters of the system. Based on the similitude analysis, a two-stage lab-scale lysimeter experiment was conducted to identify the gas migration patterns and to assess the impact on biodegradation. The air delivery system with vertical piping showed better gas migration and waste degradation characteristics than others. Aeration using a leachate collection system leads to channeling effects. The horizontal distribution system resulted in localized anaerobic regions.

Innovations in Measuring Field Scale Biological Methane Oxidation at Two Soil-Covered Closed Landfills by Tarek Abichou, Haykel Melaouhia, Bentley Higgs, Jeff Chanton, Roger Green (118-130).
One of the most promising and cost-effective options for control of low-level methane (CH4) emissions is the use of engineered bio-based systems. It has been proved that in landfill cover soil, micro-organisms remove CH4 from Landfill Gas (LFG) as the gas migrates through aerobic regions above the buried waste. However, one of the main issues delaying the field implementation of such techniques capable of reducing CH4 emissions from landfills is the lack of a proper field technique to assess the level of CH4 oxidation under field conditions. Only specialized Stable Isotope (SI) based methods are capable of such a task. Gas Push Pull Test (GPPT) method was developed and used to assess in-situ methane oxidation in two low-emitting closed landfills. Simple interpretations of the GPPT data was developed. Stable carbon isotope data was also obtained in conjunction with the GPPTs which provides an independent verification of CH4 oxidation. The GPPT analysis yielded an average percent oxidation of 40 and 52% for the two closed sites. The GPPT methane oxidation results agreed with the SI based values obtained from 20 above ground air samples from each of the closed sites. The study suggests that GPPT can be easily used to as-sess methane oxidation in in the field. Additionally, the results of the GPPT and the Isotopes analysis were lower than the overall oxidation rates estimated from the modeled generation and the measured tracer-based total emissions testing performed at the two landfills.

Methane Oxidation Prediction Curves of Soil at Different Organic Contents by Dinesh Pokhrel, J.P.A. Hettiaratchi, Matthew Steele (131-143).
The methane oxidation capacities of soil, compost, and various soilcompost mixtures at different moisture contents and temperatures were determined through a series of batch incubation experiments. The maximum methane oxidation capacities of compost and soil-compost mixtures were almost an order magnitude greater than mineral soil, as a result of increasing organic content of the media. Media with higher organic content also required much higher optimum moisture contents to achieve their maximum methane oxidation rate, with optimum moisture content for maximum methane oxidation capacity ranging between 35 and 50% of water holding capacity. The effect of temperature changes between 10°C and 30°C was found to be greater at higher organic contents. Moisture content and temperature were found to be equally important parameters for methane oxidation rate, but these effects only significantly influence oxidation rates within a range that is close to their optimum. Outside this range, methane oxidation rates fall well below their maximum and changes to moisture and temperature do not considerably affect the methane oxidation rate. The relationship between organic content and moisture content at a given temperature was developed. This curve, when combined with temperature effect can estimate methane oxidation rate at a given organic content, moisture content and temperature.

Methane Oxidation in a Landfill Cover Soil under Conditions of Diffusive and Advective Flux, Assessed by In-Situ and Ex-Situ Methods by Julia Gebert, Ingke-Maren Rachor, Jan Streese-Kleeberg, Eva-Maria Pfeiffer (144-160).
The cover soil of a non-sanitary municipal solid waste landfill was analyzed with respect to methane oxi-dation efficiency, oxidation rates, bottom and surface fluxes, thereby comparing hotspots of gas emission with non-emitting locations. The range of methods included in-situ approaches (gas push-pull tests, carbon mass balance, stable isotope fractionation) and exsitu methods (batch testing in the laboratory). The nature of the gas exchange through the cover soil was most relevant for the observed oxidation efficiency. At hotspots, gas transport was predominantly advective, resulting in bottom fluxes as high as 1,028 g CH4 m-2 d-1, restricting methane oxidation to the upper crust of the soil due to impeded ingress of atmospheric oxygen. At these locations, significant emissions were observed, but also high oxidation rates of up to 240 g m-2 d-1. Contrastingly, over the greatest part of the landfill area gas transport through the cover was diffusive with usually complete methane oxidation and subsequently no emissions. In all cases, oxidation efficiency and the soil depths participating in the process were tightly related to the extent of soil aeration, suggesting the seasonal variation in soil moisture as a key variable. Batch testing in the laboratory reflected the previous exposure of the soil to landfill gas rather than the soil's inherent oxidation potential. Compared to the in situ potential, soil sampling, pre-treatment and incubation in the laboratory yielded significantly lower values for the oxidation potential. For the given landfill, the carbon mass balance seemed to provide the easiest and most correct assessment for the quantification of methane fluxes to, through and from the cover soil.

Methane Biofiltration in the Presence of Non-Methane Organic Compounds in Solution Gas by P.A. Jayasinghe, C.K. Haththotuwa, J.P.A. Hettiaratchi (161-169).
Solution gas is a natural gas consisting of primarily methane and small amounts of non-methane organic compounds. When the amount of solution gas released at individual locations is relatively small and the quality is low, it is not economically feasible to recover this gas. Therefore, environmentally acceptable methods are needed for their control. This research is focused on assessing the viability of using methane biofiltration technology to control point source, low volume solution gas emissions. Unlike the systems with which methane biofilters have already been tested, solution gas contains diverse pollutants in addition to methane, particularly hydrogen sulfide and non-methane organic compounds. A comprehensive set of laboratory experiments were undertaken and the results showed that methane oxidation is not affected by the presence of low concentrations of ethane and propane that could be present in solution gas. However, in flow-through column experiments, the methane oxidation efficiency was adversely affected by increasing the inlet loading rate of ethane.

The stable isotope technique is a methodology that allows for the quantification of biologically oxidized methane (CH4) in a landfill cover. In this methodology, the oxidized fraction of methane is calculated based on the isotopic signature of the emitted CH4, the preference degree of methanotrophic bacteria in consuming methane with lighter isotopes and the isotopic signature of methane within anaerobic zone of the landfill. This study was conducted in four separate operational phases of the Vancouver landfill (VLF). These four phases were grouped into two areas based on the characteristics of the soil used as interim and/or final cover. Methane emission rates were measured using a flux chamber technique. The methane samples, which was collected from the landfill's anaerobic zones, the flux chambers, and aerobic soil incubations test, were analysed for isotopic signature. ?13C values for methane generated in the VLF ranged between -54‰ and -58‰. All CH4 samples collected from flux chambers were enriched in 13C with ?13C values ranging between -35‰ to -56‰. The overall average oxidation fractionation factor (?ox) at the VLF was 1.0266 ± 0.0037 at 25 °C. Methane oxidation ranged from 3 to 73% with mean values of 28% and 34% for two different areas of the landfill. Results suggested that the overall methane oxidation percentage, for a region with similar climatic conditions to Vancouver, may be considered more or less constant throughout the year. As our objective was to compare different sections and cover soil types across the landfill, the use of chambers to capture emitted methane was deemed appropriate.

A Comprehensive Anaerobic Degradation Model using both Mass and Energy Balance for Bio-Reactor Landfills by J.N. Meegoda, S. Bhuvaneshwari, J.P.A. Hettiaratchi, H. Hettiarachchi (181-193).
Landfills settle due to its weight and biodegradation of waste. Biodegradation-induced settlement is a direct result of rearrangement of waste skeleton in response to the conversion of waste mass into landfill gases. In this manuscript, a Comprehensive Anaerobic Degradation Model using both Mass and Energy Balance for Bio-Reactor Landfills was developed and compared model predations with measured data from Calgary Biocell. The model solves the mass and energy balance of waste decay, and computes the consequential rate of gas generation, and gas flux through the system. The decomposing biomass is represented as cellulose for energy balance computation. The microbial degradation of biomass generates methane, carbon dioxide and water in a exothermic reaction. The heat released due to anaerobic decay is used to calculate the increase in Biocell temperature. Then the decay constant for this Biocell temperature is used to calculate the decomposition of waste for the next time step. The above computation is continued in order to obtain the landfill settlement, temperature and the movement of landfill gas and leachate. The thermal profile predicted by the model matched reasonably well with the field values measured at the Calgary Biocell.