Effects of long-term exposure to ciprofloxacin on the performance of an enhanced biological phosphorus removal (EBPR) and its microbial structure

In this study, the long-term impacts of ciprofloxacin (CIP) on the performance as well as the microorganisms of an EBPR system were assessed in a sequencing batch reactor (SBR). The results showed that nitrogen and phosphorus removal performance was significantly inhibited (P < 0.01) by CIP. Semi-inhibitory concentration (IC50) of anaerobic phosphate release, aerobic phosphate uptake and ammonia oxidation (1.9, 1.5 and 1.2 mg·L−1) suggested that the inhibition of CIP on ammonia oxidation was the strongest. Meanwhile, the diversity and evenness of microorganisms were reduced greatly (P < 0.01) due to the toxicity of CIP. Lysobacter, however, gradually dominated because of its strong resistance to CIP. The tolerance ability of functional microorganisms related to phosphorus and nitrogen removal to CIP was as follows: nitrite-oxidizing bacteria (NOB) > phosphorus-accumulating bacteria (PAOs) > Zoogloea > glycogen-accumulating bacteria (GAOs). It was also found that tightly bound EPS (TB-EPS) concentration decreased companying with a decrease of sludge activity as CIP addition increased. Moreover, an obvious sludge granulation was observed with protein (PN) proportion increased and polysaccharide (PS) proportion decreased. Nevertheless, the results of recovery experiment showed that the deterioration of the EBPR caused by long-term exposure to CIP could be replenished within 10 days, implying the inhibitory effect of CIP on EBPR was reversible.