Pilot-scale nitrogen removal from leachate by ex situ nitrification and in situ denitrification in a landfill bioreactor

• Nitrogen removal performance of in situ landfill leachate treatment system was assessed.
• Strong positive correlation among nitrification, DO and related gene copy number.
• Total oxidizing nitrogen removal rate of 67.2 g N t−1 TSwaste d−1 was obtained in landfill refuse.
• Azoarcus tolulyticu was the primary denitrifier in the landfilled bioreactor.

A combined process consisting of ex situ nitrification and in situ denitrification in landfill refuse was studied in pilot scale for nitrogen removal from municipal landfill leachate. The results showed that above 80% of partial nitrification ratio and an average COD loading rate of 1.50 kg m−3 d−1 were steadily maintained under DO concentrations of 1.0–1.7 mg L−1 in the aerobic reactor. Quantitative PCR results indicated that nitrite-oxidizing bacteria being sensitive to DO fluctuations lead to partial nitrification when free ammonia inhibition was weak. Nitrified landfill leachate could be denitrified in the landfill bioreactor with maximum total oxidizing nitrogen removal rate of 67.2 g N t−1 TSwaste d−1. Clone and sequencing analysis of denitrifying bacterial nirS gene inferred that heterotrophic denitrifier Azoarcus tolulyticu was the primary nitrogen converter in the landfill bioreactor. The obtained results will provide valuable information for optimizing the design and operation of a landfill bioreactor.