Biological nitrous oxide production from landfill operation

Abstract

Landfill leachate recirculation is efficient in reducing the quantity of leachate that needs to be treated by a leachate treatment plant in a landfill site. However, after landfill leachate is applied on land, nitrous oxide (N2O), an important greenhouse gas, is mainly produced in soils by nitrification and denitrification. Field studies were conducted in Likang Landfill, Guangzhou, PRC, where leachate recirculation has been practiced for 8 years. Monthly productions and fluxes of N2O were studied from June 2000 to July 2001. The ammoniacal-nitrogen level in Likang landfill leachate was high, about 1,535-4,220 mg L-1. A high emission rate of N2O at 187 mg m-2 h-1 was detected from a leachate pond purposely formed on landfill topsoil after leachate irrigation. A high level of N2O (10.2 ng ml-1) was detected in gas emitted from topsoil 1 m from the leachate pond. Less than 19% of total soil N2O was produced by autotrophic nitrification; the highest percentage was determined in June 2001. The relative net N2O production from denitrification ranged from 18 to 100%. Thus, the major source of N2O production was from denitrification, rather than from nitrification, in leachate-contaminated soil, as quantified by acetylene inhibition tests. The correlations between key environmental and chemical factors and N2O levels are determined. The quantity of N2O flux (ng of N cm-2 h-1) due to leachate recirculation was compared with figures reported in different ecosystems and it was 215-1,652 times higher. Toxic substances such as heavy metals in landfill leachate exert a significant impact on the performance of nitrifying and denitrifying bacteria. They affect the compound nitrogen mineralization and also the N2O production in a landfill site. Since copper (Cu) and zinc (Zn) are metals commonly found in municipal solid waste, acetylene was again used as an inhibitor to study their effects on the contributions of autotrophic nitrification and denitrification N2O production. The effects of Cu and Zn on N2O production, nitrifying and denitrifying activities under aerobic and anaerobic atmospheres were also studied. For a given metal concentration of 250-1,000 mg kg-1, the largest ammonium accumulation (by dissimilatory nitrate reduction) and highest N2O production were observed for Cu followed by the Zn. Same order was also observed in the largest net mineral N lost from landfill soil, especially in higher metal concentrations. The inhibitory effects of Cu and Zn at 250-1,000 mg kg-1 on nitrification were obvious, since no N2O was produced from this biological process. Furthermore, the relative net N2O production from denitrification ranged from 22% to more than 100% after Cu or Zn addition. Although both Cu and Zn solutions could enhance the denitrification N2O production, Cu contributed more than Zn in this aspect. High concentrations of soil Cu (up to 1000 mg kg-1) could greatly promote denitrification N2O production under anaerobiosis. Moreover, the results of the effects of these two metals in landfill soils incubated in aerobic and aerobic atmospheres provided evidence that denitrification was the most probable source of N2O in Likang Landfill soils.