Optimization of the abatement of landfill gas diffuse emissions in cover soils treated with sewage sludge and leachate: A laboratory experiment

Landfilling represents the most common option for municipal waste (MW) management worldwide, also in expected temporal projection. The degradation process of deposited MW generates two potential contaminating outputs that may cause severe environmental impacts on both local and global scales: (i) landfill gas (LFG), mainly consisting of CH4, CO2 and a minor fraction of non-methane volatile organic compounds (VOCs), and (ii) landfill leachate (LL). Nowadays, several mitigation strategies are deployed to minimize the uncontrolled release of LFG into the atmosphere, including bio-cover systems, which degrade CH4 and a variety of VOCs by the action of microbial population consortia, and reduce meteoric water infiltration. However, the current methods to attenuate LFG diffuse emissions are not fully effective, and a significant part of generated LFG is emitted into the atmosphere. The purpose of this study was to identify and investigate potential low-cost and sustainable approaches to effectively manage the LFG in combination with LL and dewatered sewage sludge (SS), according to the concept of circular economy aimed at utilizing complementary waste categories (i.e. LL and SS) as resources to favor the abatement of LFG diffuse emissions in soils. Two experimental runs were conducted using lab-scale landfill prototypes developed and tested in a previous study. Particularly, the individual prototype consisted of a maceration chamber (i.e. landfill body filled with degrading waste that generated the LFG) connected to a soil column (i.e. landfill cover soil). The first experimental run was carried out using two prototypes, with the purpose to investigate the LFG abatement potential in a new bio-cover system, realized modifying a natural soil with addition of dewatered SS and LL recirculation. The second experimental run, using four prototypes, focused on investigating the individual contributions of dewatered SS addition and LL recirculation, also reducing the amounts of dewatered SS and LL used in the treatment of the natural soil. In both experimental runs, gas samples were analyzed by (i) gas chromatography using a thermal conductivity detector (GC-TCD) for main gaseous components, (ii) gas chromatography using a flame ionization detector (GC-FID) for light hydrocarbons (C2-C4) and CH4 at concentrations < 0.05%, (iii) cavity ring-down spectroscopy (CRDS) for carbon isotopic signatures in CH4 and CO2, and (iv) gas chromatography coupled with a quadrupole mass spectrometry (GC-MS), after solid-phase micro extraction (SPME), for C4+ VOCs. The first experimental run highlighted the enhanced efficiency of the soil treatment with LL and dewatered SS in degrading CH4 and selected VOCs, pertaining to alkanes and highly odorous terpenes, with respect to non-treated soil. The second experimental run showed how the efficiency in LFG abatement by the treated soil was influenced to a larger extent by the addition with dewatered SS as compared with LL recirculation.