Comparison on removal performance of virus, antibiotic-resistant bacteria, cell-associated and cell-free antibiotic resistance genes, and indicator chemicals by ozone in the filtrated secondary effluent of a sewage treatment plant

Abstract

Due to the widespread appearance of viruses, antibiotic-resistant bacteria (ARBs), and antibiotic resistance genes (ARGs) in the aquatic environment, more powerful oxidation processes such as ozonation are needed to enhance the efficiency of their inactivation and removal during wastewater treatment. However, information is lacking on the elimination rates of viruses, ARBs, cell-associated ARGs (ca-ARGs), and cell-free ARGs (cf-ARGs) during ozonation. This study examined the kinetics and dose-dependent inactivation of a virus (MS2 coliphage) and an ARB (Ampicillin-resistant [Amp^R] E. coli) and the removal of ca- and cf-ARGs (plasmid-encoded bla_TEM) by ozonation in a filtered secondary effluent (SE) of a municipal sewage treatment plant (STP). In addition, the ozonation kinetics of carbamazepine (CBZ) and metoprolol (MTP)-ubiquitous organic micropollutants with different removal rate constants-were also investigated in order to monitor their effectiveness as indicators for the abovementioned biological risk factors. Our results showed that ozonation was an efficient way to remove MS2, Amp^RE. coli, ARGs, CBZ, and MTP. We investigated the kinetics of their inactivation/removal with respect to exposure in terms of CT (dissolved ozone concentration C and contact time T) value, and found their inactivation/removal constants were in the following order: MS2 (8.66 ×10³ M^-1s^-1) ≈ Amp^RE. coli (8.19 ×10³ M^-1s^-1) > cf-ARG (3.95 ×10³ M^-1s^-1) > CBZ ⁽3.21 ×10³ M^-1s^-1) > ca-ARG (2.48^×10³ M^-1s^-1) > MTP (8.35 ×10² M^-1s^-1). In terms of specific ozone dose, > 5^-log inactivation of MS2 was observed at > 0.30 mg O3/mg DOC, while > 5-log inactivation of Amp^RE. coli was confirmed at 1.61-2.35 mg O3/mg DOC. Moreover, there was almost no removal of ca-ARG when the specific ozone dose was < 0.68 mg O3/mg DOC. However, 2.86-3.42-log removal of ca-ARG was observed at 1.27-1.31 mg O3/mg DOC, while 1.14-1.36-log removal of cf-ARG was confirmed at 3.60-4.30 mg O3/mg DOC. As alternative indicators, > 4-log removal of CBZ was observed at > 1.00 mg O3/mg DOC, while > 2-log removal of MTP was confirmed at > 2.00 mg O3/mg DOC. Thus, it was observed that inactivation of E. coli needs a greater ozone dose to achieve the same level of inactivation of Amp^RE. coli; for ARGs, cf-ARG can persist longer than ca-ARG if low dosages of ozone are applied in the filtrated SE, CBZ might act as an indicator with which to monitor the inactivation of viruses and ARBs, while MTP might act as an indicator with which to monitor removal of ARGs. Moreover, cf-ARG cannot be neglected even after ozonation due to the possibility that ca-ARGs can become cf-ARGs during ozonation and be discharged with the final effluent, posing a potential risk to the receiving environment.

Keywords: Antibiotic resistance genes; Antibiotic-resistant bacteria; Filtrated secondary effluent; Ozone; Virus.