Denitrification of low C/N landfill leachate in lab-scale landfill simulation bioreactors

Old landfill leachate can be characterized by high ammonia nitrogen concentrations and limited biodegradable carbon availability. A promising and cost-effective option for ammonia nitrogen removal involves ex situ nitrification and in situ denitrification. This study aimed to investigate the denitrification capacity of old MSW in six landfill bioreactors with very low COD/NO3--N mass ratios that ranged between 0.12 and 3.99 g/g. In particular, this study is novel in that it tested COD/NO3--N mass ratios lower than previous studies. The experiment lasted 83 days. The results showed that denitrification occurred in all bioreactors and even at considerably low concentrations of biodegradable organic matter (BOD5 <= 9 mg O-2/L). In all but one case, when nitrate removal stopped at 55% due to the absence of leachate recirculation, nitrate removal was higher than 95%. The average nitrate removal rates (ANRRs), calculated under significantly different conditions, ranged from 33 to 135 mg NO3--N/L/d. The initial COD concentration and COD/NO3--N ratio did not appear to affect the ANRRs, which were influenced by the initial nitrate concentration and leachate recirculation. The maximum ANRR (135 mg NO3--N/L/d) was measured with the highest initial nitrate concentration (4491 mg NO3--N/L) and the lowest COD/NO3--N mass ratio (0.12 g COD/g NO3--N). The lowest ANRR (33 mg NO3--N/L/d) was calculated for a bioreactor with no leachate recirculation. Sulphate production observed in some bioreactors may suggest that, together with the heterotrophic pathway, autotrophic denitrification contributed to the removal of nitrate, especially in bioreactors with low COD/NO3--N mass ratio. (C) 2020 Elsevier Ltd. All rights reserved.