From Outpatient Consumption to Receiving Waters: A Retrospective Screening Assessment of Antibiotic Loads, Resistance Indicators, and Implications for the Recast Urban Wastewater Treatment Directive

Background/Objectives: Wastewater treatment plants (WWTPs) are recognized point sources of pharmaceutical residues and antimicrobial-resistant bacteria. This challenge is explicitly addressed in the 2024 recast of the Urban Wastewater Treatment Directive (UWWTD), which introduces risk-based obligations for micropollutant removal. Methods: Here, five antibiotics (azithromycin, clarithromycin, erythromycin, ciprofloxacin and sulfamethoxazole) were retrospectively (2017–2018) assessed in influent and effluent of four WWTPs and in their receiving waters (riverine and marine outfalls), including the Vistula Estuary. Measured incoming load (MIL) was compared with predicted incoming load (PIL), a cost-efficient proxy derived from outpatient consumption data. Results: The screening-level PIL–MIL consistency check showed closer agreement in catchments without major hospital inputs. In WWTPs, ciprofloxacin showed consistently high apparent aqueous-phase removal in both campaigns (>95%), sulfamethoxazole showed moderate removal (64–85%), while macrolides exhibited highly variable and sometimes apparent negative removal, yielding effluent concentrations up to 3989 ng L−1 (azithromycin) and 2866 ng L−1 (clarithromycin). Receiving waters generally contained < 10 ng L−1 of the investigated antibiotics, but a low-dilution-capacity river showed a clear WWTP influence. Presumptive cefotaxime-resistant Escherichia coli, used as proxies for emerging ESBL/AmpC-associated resistance, reached ~5% and 20% of CFU downstream of WWTP discharges. Environmental (ERA) and AMR-related (AMR-RA) risk quotients (RQs) were mostly <0.1. However, RQs for azithromycin approached or exceeded unity in Gdańsk and Puck Bay. Coastal recipients are more vulnerable despite high dilution, since marine predicted no-effect concentrations (PNECs) for antimicrobials are lower than those for freshwater. Conclusions: The study supports the value of retrospective baseline data and site-specific screening frameworks for risk-based prioritization under the recast UWWTD.