Removal of Antimicrobial-Resistant Bacteria of Clinical Relevance in a Wastewater Treatment Plant with a UASB–Trickling Filter and Its Impact on the Receiving Waterbody

Wastewater treatment plants (WWTPs) are recognized hotspots for the dissemination of antibiotic resistance genes (ARGs) from antimicrobial-resistant bacteria (ARB), potentially shaping the resistomes of microbial communities in receiving aquatic ecosystems. The combination of an upflow anaerobic sludge blanket (UASB) reactor with trickling filters and settling tanks has been widely adopted, mainly in developing countries, due to its cost-effectiveness and operational simplicity. This study aimed to evaluate the removal dynamics of clinically relevant ARB within a full-scale WWTP employing this configuration and to assess the impact of treated wastewater discharge on the receiving water body. Sampling campaigns were conducted during the dry season and the transition to the rainy season to investigate the seasonal effect on ARB removal and environmental dissemination. Treatment resulted in significant reductions of chemical oxygen demand (COD, p = 0.01), biochemical oxygen demand (BOD, p = 0.02), and electrical conductivity (EC, p = 0.04); pH and total suspended solids (TSS) remained unchanged. Microbial loads decreased by 0.42 log CFU/mL for total heterotrophic bacteria (THB) and 0.48 log CFU/mL for ARB along the treatment process. However, the discharge of treated effluent increased downstream THB and ARB by 0.66 and 0.32 log CFU/mL, respectively, evidencing environmental release and dissemination. Of all isolates, 19.50% were identified as members of the clinically relevant ESKAPEE group. Most of these isolates remained susceptible to the antibiotics tested, though sporadic resistance was detected to key classes, including β-lactams, carbapenems, colistin, and glycopeptides, highlighting the persistence of clinically important resistance even after treatment. Only one Escherichia coli isolate was classified as multidrug-resistant, exhibiting resistance to at least three antibiotic categories, while several Enterobacter spp. isolates displayed elevated colistin MICs (4–12 μg/mL), higher than values frequently reported for clinical strains. Seasonal variation was associated with reduced ARB removal and greater downstream contamination. Altogether, these findings show that while UASB–trickling filter WWTPs reduce organic and bacterial loads, they fail to fully prevent the release of ARB, including clinically relevant and multidrug-resistant strains, underscoring the urgent need for improved treatment strategies, continuous resistance monitoring, and integration of effluent quality into regulatory frameworks to protect aquatic environments and public health.