Applying the 15N labelling technique to material derived from a landfill simulation experiment to understand nitrogen cycle processes under aerobic and anaerobic conditions.

Reactive nitrogen (N) species, such as ammonium (NH4+), nitrate (NO3) and gaseous nitrous oxide (N2O), are released into the environment during the degradation of municipal solid waste (MSW), causing persistent environmental problems. Landfill remediation measures, such as in-situ aeration, may accelerate the degradation of organic compounds and reduce the discharge of ammonium via leachate. Nonetheless, the actual amount of N in the waste material remains relatively constant and a coherent explanation for the decline in leachate ammonium concentrations is still lacking. Hence, the present study aimed to elucidate the dynamics of N and its transformation processes during waste degradation. To this end, the gross rates of organic N mineralization and nitrification were measured using 15N pool dilution in waste material derived from a landfill simulation reactor (LSR) experiment. The results revealed a high potential for N mineralization and nitrification, the latter of which declined with the diminishing amount of extractable ammonium (after aeration). The analysis of the concentration and isotopic composition of N2O formed confirmed incomplete denitrification as the main source for N2O. Moreover, the natural abundance of 15N was investigated in various waste N pools to verify the conclusions drawn from the 15N tracing experiment. δ15N values of total waste N increased during aeration, indicating that nitrification is the major driver for N losses from aerated waste. The application of stable isotopes thereby allowed unprecedented insights into the complex N dynamics in decomposing landfill waste, of their response to aeration and their effect on hydrological versus gaseous loss pathways.

The microbial metabolic activity on carbohydrates and polymers impact the biodegradability of landfilled solid waste.

Biological waste degradation is the main driving factor for landfill emissions. In a 2-year laboratory experiment simulating different landfill in-situ aeration scenarios, the microbial degradation of solid waste under different oxygen conditions (treatments) was investigated. Nine landfill simulation reactors were operated in triplicates under three distinct treatments. Three were kept anaerobic, three were aerated for 706 days after an initial anaerobic phase and three were aerated for 244 days in between two anaerobic phases. In total, 36 solid and 36 leachate samples were taken. Biolog® EcoPlates™ were used to assess the functional diversity of the microbial community. It was possible to directly relate the functional diversity to the biodegradability of MSW (municipal solid waste), measured as RI4 (respiration index after 4 days). The differences between the treatments in RI4 as well as in carbon and polymer degradation potential were small. Initially, a RI4 of about 6.5 to 8 mg O2 kg-1 DW was reduced to less than 1 mg O2 kg-1 DW within 114 days of treatment. After the termination of aeration, an increase 3 mg O2 kg-1 DW was observed. By calculating the integral of the Gompertz equation based on spline interpolation of the Biolog® EcoPlates™ results after 96 h two substrate groups mainly contributing to the biodegradability were identified: carbohydrates and polymers. The microbial activity of the respective microbial consortium could thus be related to the biodegradability with a multilinear regression model.

Enduring reduction of carbon and nitrogen emissions from landfills due to aeration?

The objective of the present work is to investigate to what extent emission reductions observed during landfill aeration are permanent. To do so, lab-scale degradation experiments using waste from an old landfill have been conducted under different conditions (anaerobic, (partly) aerobic returning to anaerobic, aerobic) and balances for carbon and nitrogen have been established. For the latter, all emissions of C and N (except N2) and their pools at the start and end of the experiment have been determined. In addition, the chloroform fumigation-extraction method (biocidal treatment) has been applied to determine microbially bound carbon and to estimate nitrogen in microbial biomass accordingly. The results reveal that 18 g TOC·kg DM-1 of the waste material were mineralized during aerobic treatment for 699 days, which is equivalent to about 14% of the initial TOC content. For the anaerobic treatment, only 10 g TOC·kg DM-1 were released. For the aerobic-anaerobic reactors, a slight increase in methane emissions approximately 10 months after termination of aeration was observed. With respect to leachate emissions, the results indicate significantly lower emission levels (factor 1.5 for TOC and factor 4 for TN) for the reactors, which were aerated at least sometimes. The biocidal treatment highlights that this emission reduction is rather based on an increased sorption capacity of aerated waste (higher ion exchange capacity) than a lower overall pollutant potential. It is shown that regardless of the operation mode, most nitrogen remained in solids (83.1-92.6%) and is subject to internal recycling during waste degradation.