## 4 Discussion
A slight decrease in pH (\(\approx\)0.3 units) may occur between coagulation and sand filtration as CO\({}_{2}\) is degassed from the newly formed flocs. This shift has increased the neutral fraction of sulfamethazine and weakened its electrostatic affinity for the sand surface, so previously adsorbed molecules could desorb or pass through as fine colloids, producing the observed rebound [53]. Conversely, sulfamethazine showed a notable resurgence to 438.9 \(\mathrm{\SIUnitSymbolMicro g}\)/L in the final effluent, despite being the subject of 99.9% removal in the ozone reactors. This observation points to problems such as adsorption saturation in the activated carbon, competitive adsorption among multiple organics, or potential recontamination during handling. Despite high removal efficiencies at earlier stages, the reemergence of specific antibiotics post-ozonation highlights the complexity caused by pollutant transformation, intermediate byproducts, and the various operational factors involved in a full-scale treatment system. Another plausible reason for the rebound concerns the persistence and regrowth of antibiotic-resistant bacteria. As qPCR amplifies both intracellular genes and cell-free DNA, these residual signals do not necessarily indicate viable resistant bacteria. They may also derive from extracellular plasmid or chromosomal fragments that survive oxidative stress [54]. Additionally, several studies have demonstrated that sub-lethally damaged antibiotic-resistant bacteria can regrow within 48-72 h after ozonation; specifically, nutrient-rich effluents can induce rapid recovery of _E. coli_ carrying sulfonamide and tetracycline genes, while continuous-flow ozonation tests have shown a similar rebound for urban wastewater [55; 56]. These findings suggest that, although ozone markedly reduces ARGs, complete and lasting removal may require either a polishing step that degrades extracellular DNA (e.g., photo-Fenton) or post-ozone storage conditions that preclude bacterial regrowth. Ozone treatment considerably suppressed ARG concentrations to the 10\({}^{7}\)-10\({}^{8}\) copies/L range, indicating that ozonation might inactivate resistant cells and inhibit HGT in addition to directly oxidizing antibiotic compounds. Nonetheless, _tet_B, _tet_M, and _sul_1 persisted at levels of 10\({}^{4}\)-10\({}^{6}\) copies/L even after activated carbon filtration and deaeration, suggesting incomplete elimination or reintroduction of these genes. The _sul_1 gene exhibits a propensity for rapid resurgence over time. This rebound phenomenon may be attributable to the selective pressure exerted by residual sulfamethazine or the release of harbored ARGs [57; 58]. Furthermore, the involvement of mobile genetic elements such as _int_1, which facilitate HGT, likely contributes to this complex dynamic [59]. Similarly, the fact that _tet_B was not detected immediately after coagulation but appeared from the sedimentation process onward can be attributed to several interacting factors. Storing coagulated effluent for four days increased total antibiotic-resistant bacteria by 6-138 times, since densely packed flocs create micro-environments where conjugation accelerates, while shifts in the microbial community lead to a rapid rise in tetracycline-resistance genes [60; 61; 62]. [PERSON]'s correlation revealed strong positive associations between SS and certain antibiotics, suggesting that elevated levels of suspended particulates tend to coincide with higher antibiotic concentrations [63]. The high correlation between limomycin and tiamulin (\(r=0.97\), \(p<0.0001\)) indicates similar transport or transformation behaviors in the treatment process. Lincomycin and tiamulin are used in swine production facilities to prevent Gram-positive bacterial infections associated with respiratory diseases. Their concurrent administration is often employed due to the expectation of a synergistic effect [64; 65]. Among ARGs, _tet_M, _sul_1, and _sul_2 displayed particularly high inter-correlations; _emr_B also strongly correlated to these genes.