Enhancing nitrogen removal efficiency and reducing nitrate liquor recirculation ratio by improving simultaneous nitrification and denitrification in integrated fixed-film activated sludge (IFAS) process

An integrated fixed-film activated sludge (IFAS) process (G1) and an activated sludge anoxic–oxic process (G2) were operated at nitrate liquor recirculation ratio (R) of 100, 200 and 300% to investigate the feasibility of enhancing nitrogen removal efficiency (R_TN) and reducing R by improving simultaneous nitrification and denitrification (SND) in the IFAS process. The results showed that the effluent NH₄⁺-N and total nitrogen (TN) of G1 at R of 200% were less than 1.5 and 14.5 mg/L, satisfying the Chinese discharge standard (NH₄⁺-N < 5 mg/L; TN < 15 mg/L). However, the effluent NH₄⁺-N and TN of G2 at R of 300% were higher than 8.5 and 15.3 mg/L. It indicated that better R_TN could be achieved at a lower R in the IFAS process. The polymerase chain reaction–denaturing gradient gel electrophoresis results implied that nitrifiers and denitrifiers co-existed in one microbial community, facilitating the occurrence of SND in the aerobic reactor of G1, and the contribution of SND to TN removal efficiency ranged 15–19%, which was the main reason that the R_TN was improved in the IFAS process. Therefore, the IFAS process was an effective method for improving R_TN and reducing R. In practical application, this advantage of the IFAS process can decrease the electricity consumption for nitrate liquor recirculation flow, thereby saving operational costs.