Limits and Perspectives of Established Technologies for Urban Wastewater Depuration

After more than thirty years, the Directive 91/271/EEC on urban wastewater is currently under revision. In the new directive, the clear specification of the concept of domestic wastewater, with inclusion in this category of sewages from human excretions alone, reflects the particular attention recently paid to the pollution of anthropic origin, mainly accounted for by contaminants of emerging concern (CECs) such as pharmaceutical compounds and personal care products. To this aim, by explicitly introducing the term “quaternary treatment”, the new directive intends to emphasize the need to remove a broad spectrum of micropollutants from urban wastewater through specific treatment processes, which indeed are not currently routinely integrated into wastewater treatment plants, unless in the presence of peculiar pollution from industrial activity. This chapter gives an insight into the established technologies for urban wastewater depuration, evaluates their performance towards CEC removal, and highlights critical situations, such as the increased concentrations of some CECs along the treatment chain. To this purpose, more than one thousand CEC removal data in articles published since 2000, identified through Google Scholar and Scopus databases, were reviewed. The data covered various geographical regions, to consider the different technologies used in several countries, according to specific conditions of technological development and financial resources. Technologies used as secondary, tertiary, and quaternary treatment stages of urban wastewater were considered, highlighting, where possible, the specific contributions of each treatment to the overall removal. The heterogeneity in the removal data presentation (i.e. the aggregation of classes of CECs, rather than individual CEC, the grouping of removal efficiency within the same technology) prevented, in many cases, a complete interpretation of data. Nevertheless, careful extraction of removal data for selected CECs from the reviewed literature allowed us to gather information on the efficiency of secondary and tertiary/quaternary treatments through a meta-analysis approach, which is not biased by the high level of data heterogeneity. Our analysis suggests higher removal efficiency of combined anaerobic-anoxic–oxic stages towards CECs and a statistically significant impact of disinfection/oxidation process.