Enhanced nitrogen removal by a combination of anammox and heterotrophic denitrification

This study examines the removal of nitrogen from nitrate, which accumulates in the anammox process, during the heterotrophic denitrification (HD) reaction using organic compounds that were originally contained in the influent as electron donors. The experimental system comprises a single partial-nitritation/anammox (PNA) reactor and HD reactor for biological oxygen demand (BOD) reduction that is placed in series before the PNA reactor. A portion of the effluent from the PNA reactor was allowed to flow back to the HD reactor. The nitrate concentration in the effluent from the PNA reactor reduced as the recirculation ratio increased and the maximum nitrogen removal efficiency (NRE) was optimized at a hydraulic retention time (HRT) of 3.0 hours in the HD reactor out of a total HRT of 36 hours at 4 backflow/influent, demonstrating an NRE of 95.4% that represented an enhancement of 1.24 times the NRE without recirculation in the flow path.