Stress responses of partial denitrification system under long-term ciprofloxacin exposure in an anaerobic sequencing batch reactor

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This study evaluated the stress response of partial denitrification (PD) sludge system under ciprofloxacin (CIP) exposure. The results showed that the nitrite accumulation rate (NAR) decreased from 77.41 % to 28.39 % under CIP stress of 200 µg/L, and after adapting to CIP stress, the NAR of PD system recovered to 61.56 % even the CIP increased to 1000 µg/L. The average COD removal efficiency increased from 69.08 ± 2.01 % to 73.33 ± 2.33 %, and the average removal efficiency of CIP increased from 9.83 % ± 0.71 % in phase II (CIP=50 µg /L) to 30.15 % ± 1.63 % in phase V (CIP=1000 µg /L). Under CIP stress, the protein (PN) in Tightly Bound-extracellular polymeric substances (TB-EPS) increased from 77.60 mg/(g-SS) in phase I to 135.58 mg/(g-SS) in phase Ⅲ (influent CIP=200 µg /L). FTIR revealed that functional groups (CO, C-H, C-O-C) in EPS of PD sludge could bind to CIP to protect microbial cells from damage. The results of principal co-ordinates analysis (PCoA) showed that the addition of CIP had a significant effect on the microbial community structure, CIP exposure led to an increase in biodiversity of PD system. At genus level, Thauera genus abundance decreased, while OLB12 and Denitratisoma genus abundances increased under CIP stress. This was consistent with the result that the conversion of NO3--N to NO2--N was inhibited while NO2--N reduction was promoted. The findings of this study provided insight into the stress-responses of PD system under long-term CIP exposure and provided useful guidance for the risk assessment and control strategy of wastewater treatment plants (WWTPs).