In the Netherlands, three full-scale pilots have been in operation for approximately five years to understand the effects of leachate recirculation or aeration on waste stabilisation. This study employs the ratio of N2 to Ar in the landfill gas in comparison to the ratio in atmospheric air to derive the share of N2 that originates from denitrification. We collected samples from the three pilot landfills from different gas wells, gas collector systems and from the total bulk extracted gas and measured its composition using gas chromatography. We estimated the aeration efficiency of the two landfills under in situ aeration based on the CO2/CH4 ratio as an indicator of aerobic processes. Denitrification dominated in the aerated landfills, with as much as 13% of N2 being explained by the net effect of denitrification, whereas the landfill under leachate recirculation showed a net ‘loss’ of N2, indicating N2 fixation to be a dominant mechanism. There was a seasonal variability of the balance between denitrification and N2 fixation in the aerated sites, likely caused by increased aeration efficiency and hence increased availability of NO3- for denitrification under summer conditions with lower moisture content in the cover soil, allowing for increased air ingress. No such variability was observed for the landfill under liquid recirculation. Future evaluation of the microbial community composition will further elucidate N transformation pathways in landfills under different in-situ stabilisation treatments.@en

This study quantifies the share of aerobically produced carbon (aeration efficiency) during six years of a full scale landfill aeration project using the balance between methane and carbon dioxide in the bulk extracted gas. Aeration was realized by overextraction. Aeration enhanced carbon release in comparison to the anaerobic ‘base case’, as predicted by the Afvalzorg multiphase model, by a factor of 3.7. Aeration efficiency, averaging around 44%, varied seasonally, and was lower in periods of low or no evapotranspiration and hence higher moisture content in the landfill cover soil (winter). Higher aeration efficiencies were observed when evapotranspiration enables increased cover soil permeability (summer). Correspondingly, aeration efficiency was linearly related to the concentration of N2 in the bulk extracted gas. To a lesser extent, condensate and its removal also affected flow and hence the aeration efficiency. Except for the modulation by seasonal effects, the cumulative amount of extracted ‘aerobic carbon’ increased linearly over time, independent of changes in the blower pressure and flow. This suggests that below the cover soil, within the waste body, flow is chanelled in preferential pathways, limiting the intrusion of oxygen into the bulk waste. Aeration can hence only be enhanced by reducing well spacing. The blower efficiency, assessed by the ratio of flow to pressure, decreased markedly over time, likely indicating diminishing waste permeability as a result of waste consolidation.@en

In-situ stabilization of waste bodies can be achieved by the infiltration of water or recirculation of leachate into the landfill, which is thought to enhance the microbial degradation of waste organics by (re-)moisturizing dry zones and flushing out metabolic products of organic matter decay. The success of in-situ stabilization should reflect in initially accelerated and thereafter reduced rates of anaerobic waste organic matter decay rates. This paper compares the methane generation that was modelled using the Afvalzorg multiphase model without the added effect of leachate recirculation with actually extracted methane in the landfill and gas generation on sampled wastes following five years of leachate recirculation on a Dutch landfill. Laboratory incubations revealed a methane potential between 0.03 kg CH4/t dw and 15.8 kg CH4/t dw for 365 days. Clear trends with respect to depth, moisture content, total organic carbon or share in hard plastics did not emerge as overall waste heterogeneity was high and likely obfuscated the correlation analysis. The results showed a recovery efficiency of 30.4% for 2021, with 0.07 kg CH4/t dw for the recovered methane and 0.23 kg CH4/t dw for the predicted methane in compartment 3. The average methane potential measured in the laboratory was almost twice as high as the remaining methane potential predicted for the period of 2021-2093. The discrepancy could be due to (i) enhanced waste degradability as a result of five years of recirculation, (ii) enhancing effects of material perturbation during sampling and/or (iii) impeded on-site methane generation and gas and water transport limitations due to presence of plastics. Overall, the laboratory incubations demonstrate a significant potential for waste biodegradation still residing in the waste.@en

In-situ aeration of landfills accelerates biodegradation of waste organic matter and hence advances waste stabilization. The spatial outreach of aeration greatly affects stabilization efficiency. This study analyzed the spatial variability of gas composition and flow in 230 wells spread over four compartments of a Dutch landfill which is under in situ aeration since 2017, as well as the carbon extraction efficiency, tem-perature, and settlement. Flow rates and gas composition in the extraction wells varied strongly. The highest variability was observed in the compartment with the highest water tables with submerged filter screens for most wells, with low flow rates, and elevated ratios of CH₄ to CO₂, indicating predominance of anaerobic processes (compartment 11Z). The compartment with the most uniform distribution of gas flow rates, composition and lower ratios of CH₄ to CO₂, suggesting a significant share of aerobic carbon mineralization, also showed higher temperatures, a carbon extraction efficiency, and larger cumulative settlement, all indicative of enhanced microbial activity (compartment 11N). In this compartment, the amount of extracted carbon exceeded the carbon generation predicted from landfill gas modeling by the factor of 2 over the hitherto four years aeration. The effect of water tables on gas flow and the correlation between the flow, and the ratio of CH₄ to CO₂ appeared weak, indicating that also other factors than water tables influence gas concentration and flow. Future work includes stable isotope probing to analyze the significance of microbial respiration and microbial CH₄ oxidation for the composition of the final extracted gas mixture.

@en

Within the framework of the Dutch sustainable landfill project iDS, four compartments of the Dutch landfill Braambergen have been treated by in-situ aeration since 2017. The aeration infrastructure comprises 230 wells with a spacing of 15 to 20 m, distrib-uted over an area of around 10 ha, intercepting a waste body of 1.2 × 10⁶ t of contam-inated soils, soil treatment residues, bottom ashes and construction and demolition waste. The wells, used in an alternating fashion for air injection and gas extraction, can also be used to monitor water tables within the waste body. In order to describe the spatial variability of waste hydraulics, design a larger scale leachate pumping test and, eventually, support model predictions of the site’s water balance and emission potential, analyses of leachate composition and pumping tests on individual wells have been conducted. The spatial variability of leachate quality and water tables is very high with no geospatial relationship between the sampling points. Each sampling point is representative of itself only. Large differences prevail not only between and across the compartments, but also between directly neighbouring wells. Both the small scale differences in leachate tables as well as in leachate quality indicate a spatial pattern of zones with low horizontal connectivity within the waste body. Recovery rates of drawdown in the wells yielded preliminary estimates of horizontal waste hydraulic conductivity in the order of 1×10^-7 to 6×10^-4 m/s.

@en