Impact of nitrate-enhanced leachate recirculation on gaseous releases from a landfill bioreactor cell

摘要

This study evaluates the impact of nitrate injection on a full scale landfill bioreactor through the monitoring of gaseous releases and particularly N_2O emissions. During several weeks, we monitored gas concentrations in the landfill gas collection system as well as surface gas releases with a series of seven static chambers. These devices were directly connected to a gas chromatograph coupled to a flame ionisation detector and an electron capture detector (GC-FID/ECD) placed directly on the field. Measurements were performed before, during and after recirculation of raw leachate and nitrate-enhanced leachate. Raw leachate recirculation did not have a significant effect on the biogas concentrations (CO_2, CH_4 and N_2O) in the gas extraction network. However, nitrate-enhanced leachate recirculation induced a marked increase of the N_2O concentrations in the gas collected from the recirculation trench (100-fold increase from 0.2 ppm to 23 ppm). In the common gas collection system however, this N_2O increase was no more detectable because of dilution by gas coming from other cells or ambient air intrusion. Surface releases through the temporary cover were characterized by a large spatial and temporal variability. One automated chamber gave limited standard errors over each experimental period for N_2O releases: 8.1 ± 0.16 mg m~(-2) d~(-1) (n = 384), 4.2 ± 0.14 mg ~(-2) d~(-1) (n = 132) and 1.9 ± 0.10 mg ~(-2) d~(-1) (n = 49), during, after raw leachate and nitrate-enhanced leachate recirculation, respectively. No clear correlation between N_2O gaseous surface releases and recirculation events were evidenced. Estimated N_2O fluxes remained in the lower range of what is reported in the literature for landfill covers, even after nitrate injection.