Effective carbon and nitrogen removal with reduced sulfur oxidation in an anaerobic baffled reactor for fresh leachate treatment

- Nitrified effluent recycled to the middle of ABR avoided inhibition on methanogenesis.
- ABR created distinct zones along the length of reactor for different processes.
- SCOD and TN removal was enhanced by increasing recirculation ratio.
- Insufficient carbon for denitrification can be supplied by step-feeding.
- Various reduced sulfur could be involved in autotrophic denitrification.

The application of an anaerobic baffled reactor (ABR) with four compartments was investigated for the simultaneous removal of carbon and nitrogen from leachate. The nitrified effluent was recycled to compartment 3 of the ABR, thereby avoiding the adverse influence of nitrogen oxides on anaerobic methanogenesis in compartment 1. Nitrified effluent recirculation not only enhanced chemical oxygen demand removal (>95.6%) but also improved the total nitrogen removal efficiency from 12.7% to 67.4% with increasing recirculation ratio from 0.25 to 2. The challenge of insufficient carbon sources for heterotrophic denitrification in compartment 3 with a high recirculation ratio could be overcome by step feeding of leachate. Moreover, various reduced sulfurs (e.g., sulfide, elemental sulfur, and organic sulfur) were involved in nitrate reduction via sulfur-based autotrophic denitrification. The addition of sulfide to compartment 3 further confirmed nitrate reduction using reduced sulfur as an electron donor. The interaction of organic carbon, reduced sulfur, and nitrate in leachate treatment needs further study.