Emerging contaminants in sludge treatment reed beds: Removal, persistence, or accumulation?

Sludge treatment reed beds (STRBs) are a sustainable and cost-effective approach to dewatering and mineralizing sewage sludge. While these nature-based solutions are robust and efficient in dewatering and stabilizing excess sludge, it is still unclear the fate of micropollutants in such technology and consequently, the quality of the final product biosolids that can be used as fertilizer, when legislation allows for it. The STRBs consisted of an area of 2.5 m² planted with Miscanthus x giganteus, which was tested in this test site for the first time. The average annual sludge loading rate (SLR) was 16 kg of Dry Matter (DM)·m^-2, and sludge was loaded every 40 days. The feeding sludge had a 0.2 % averaged DM and 78.3 % Volatile Solid (VS) content. This study investigated the fate of 49 organic micropollutants for almost 16 months in pilot-scale STRBs under three aeration regimes to disclose whether enhanced STRB operation improves micropollutant removal. The aeration types consisted of active aeration (air blower 100 L·min^-1), passive aeration (4 m vertical pipe), and non-aerated control (without enhanced aeration). A subset of 25 compounds, measured at least 10-fold above their respective detection limits, was selected for detailed analysis. Twenty-one of these 25 compounds degraded up to 99 % removal regardless of aeration type, indicating that other environmental factors influence removal more than aeration intensity. However, some compounds, including benzalkonium chlorides (BACs), carbamazepine, and propranolol, persisted or accumulated, posing potential environmental risks. These findings highlight the potential of STRB for micropollutant removal, while also identifying recalcitrant compounds in these systems. Although STRBs improve sludge management, the presence of persistent compounds requires further evaluation and detailed risk assessment. A preliminary risk assessment considering residual sludge application in soil showed that trimethoprim (RQ = 8.25) and azithromycin (RQ up to 4.50) posed the highest risk to soil microorganisms among the measured compounds. In contrast, most other compounds exhibited low risk (RQ < 1), suggesting limited ecological concern under current exposure levels, including for carbamazepine, which was recalcitrant. Safe and sustainable sludge reuse demands a better understanding of removal pathways and the long-term ecological impacts of STRBs.