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 (RTN) 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 RTN 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 RTN was improved in the IFAS process. Therefore, the IFAS process was an effective method for improving RTN 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.