Guidance on effective exploitation of spectroscopic techniques for assessing wastewater discharges in water bodies

Environmental protection is increasingly critical in urban drainage management. While treated wastewater discharges are routinely monitored, the detection of unexpected or unauthorized drainage inflows into surface waters remains a challenge - especially as discharge points are often undocumented, irregular in time and space, occur under highly variable conditions, and monitoring conditions are far from standard. This is one of the emerging challenges of the 2024 European Directive on Urban Wastewater Treatment. Spectrophotometric techniques, more common in wastewater treatment plants, but less common in drainage systems and water bodies, allow for real-time, in-situ water quality monitoring, even for low-pollution discharges like treated effluents. This paper presents a novel, stepwise methodology for using spectroscopic techniques to locate and characterize urban discharges into water bodies, proposes a new Water Quality Label index, and comprehensively guides their use. Based on UV-Vis absorbance and Fluorescence spectroscopy, the application of visual data mining, differential spectra, isosbestic points, PCA, PLS and PARAFAC was demonstrated for this purpose. The methodology was applied in a case study of a treated wastewater discharge, with lower contaminant concentrations than those typically associated with unregistered illicit drainage discharges. By detecting subtle changes in river water composition, spectrophotometry helps to locate unknown discharges and support treatment plant operation. This approach improves the understanding of the interactions between urban drainage dynamics, treated wastewater discharges and receiving water quality and supports monitoring and control efforts and integrated urban water management, in line with the new EU Directive and paving the way to discharge limits based on receiving water quality.