Trace gas emission and mitigation at Danish landfills receiving low-organic waste

Abstract

Landfill gas (LFG) is produced during the anaerobic degradation of organic fractions in deposited waste. The main components in LFG are methane (CH4) and carbon dioxide (CO2), but it also contains a large number of trace compounds (combined ~ 1% in the total volume), which originate from waste degradation, direct volatilisation of chemicals in waste products or conversion/reactions between other compounds. Although trace compounds represent only a small portion of the overall emission from landfills, they can represent disproportionate hazards to the environment and human health. The emission of carcinogens such as benzene and vinyl chloride may pose health risks to workers and local inhabitants, whilst chlorofluorocarbons (CFCs) contribute to ozone depletion and global warming. In addition, many trace compounds are strong odorants and can cause odour pollution, whilst others can participate in atmospheric chemical reactions and form secondary air pollutants. Denmark has banned the landfilling of biodegradable and combustible waste since 1997, and currently, Danish landfills receive mostly non-combustible waste with a low-organic content. Previous studies have found significant gas production at modern Danish landfills, especially from waste disposal cells containing shredder waste. As the waste potentially contains high levels of pollutants, there is a need to document the trace gas composition and concentration levels in LFG at such sites, and to demonstrate if mitigation measures are needed. To facilitate the reliable monitoring of trace gases, and to provide guidance for field sample collection, storage and analysis, an analytical method was developed based on multi-sorbent tubes and thermal desorption (TD) gas chromatography (GC) mass spectrometry (MS). Ninety-two trace compounds were quantitatively measured, with detection limits between 0.01 and 2 ng. In addition, the method also showed good precision (< 20%) and accuracy (< 30%). When field samples are collected on sorbent tubes, they should be properly capped, stored at 4 °C in clean, air-tight containers and analysed within 3 days to avoid significant alteration. For the analysis of one-off samples with unknown concentrations, splitting the sample and re-collecting the desorbed portion back onto the sorbent tube can be employed to ensure replicate analysis and to avoid analytical failures due to inappropriate operating conditions or high water content. Over 70 compounds showed satisfactory recoveries (100 ± 20%) after being re-collected multiple times, and the single (outlet) split mode is recommended for re-collection when seeking to avoid increased uncertainties. However, re-collecting thermal labile, polar or reactive compounds is not recommended, as they might experience significant alterations during the re-collection process. In addition, bromochloromethane should not be used as an internal standard for the quantitative calculation of re-collected samples, due to its significant loss during re-collection. Landfill gas samples were collected from four Danish landfills (Odense Nord, Stige Ø, Glatved and AV Miljø) to determine trace gas compositions in different waste disposal cells containing shredder, mixed and aged waste. The highest trace gas concentrations (average 1,000 mg m-3) were measured in shredder waste cells, comparable with traditional municipal solid waste landfills receiving organic and combustible waste. Aliphatic hydrocarbons and aromatics, originating from direct chemical volatilisations in the waste product, were dominant in the shredder waste cells, with benzene, toluene, ethylbenzene and xylenes (BTEXs) contributing over 75% of all aromatics concentrations. At Odense Nord, large amounts of oxygenated compounds were found in the shredder waste cell, most likely produced by a higher content of organic waste at this site. The mixed waste cells exhibited variable trace gas concentrations ranging from 1.34 mg m-3 to 1,074 mg m-3, which were highly dependent on the deposited waste composition, and the lowest concentrations were generally measured in aged waste cells (< 100 mg m-3). H2S and several aliphatic hydrocarbons dominated the mixed and aged waste cells. A constant ratio was found between the concentration of aliphatic hydrocarbons and aromatics and CH4 in LFG in shredder waste cells, which was used in an LFG production model to estimate trace gas productions. The annual BTEXs production rates in 2020 at Odense Nord and Glatved were calculated at 272 and 73 kg yr-1, respectively, and they were not considered to pose significant risks to the environment or human health if they were collected and combusted. However, since the LFG produced from shredder waste cells in Glatved landfill cannot be utilised or flared, due to poor gas quality and high siloxanes content, control measures need to be established to reduce the fugitive trace gas emissions. A full-scale active biofilter system was installed at Glatved landfill to minimise CH4 and trace emissions from shredder waste cells. Three biofilters with a total area of 3,950 m2 were constructed on an existing cover, and LFG produced mainly from the shredder waste cells was mixed with ambient air and then pumped into the biofilters, resulting in an average load of 60-75 g CH4 m-2 d-1 and 0.15-0.21 g trace gas m-2 d-1. Surface screening of CH4 showed heterogeneous gas distribution, and elevated surface concentrations were mostly observed close to the gas inlet. Point measurements using flux chambers across the biofilter surface showed CH4 emissions ranged from -0.36 to 4.25 g m-2 d-1, with atmospheric uptake measured in 32% of tests. Trace gas fluxes from the biofilter surface measured at selected points were all very small with both positive and negative fluxes in the order of 10-8 to 10-3 g m-2 d-1. The gas profiles showed that CH4 oxidation and trace gas mitigation had already happened in the gas distribution layer, probably due to the growth of microbial biofilm on gravel surfaces with compost intrusion, as well as a steady supply of oxygen, moisture and nutrients. Inside the compost layer, two aerobic zones and one anaerobic zone were developed, and the oxidation of CH4 and some trace gases such as aromatics and aliphatic hydrocarbons occurred mainly in the aerobic zones, whilst CFCs were degraded within the anaerobic zone. Over 98% of the BTEXs and 80% of CFCs in the LFG were removed by the biofilter system. Overall, the CH4 oxidation efficiency of the whole biofilter was between 99.8% and 100%, while trace gas mitigation efficiency was around 98-99%, indicating that the biofilter system installed at Glatved landfill was very efficient in reducing CH4 and trace gas emissions at the landfill site.