Quaternary treatments in the upgrading of wastewater treatment plants: micropollutant removal and operation reliability

The recent revised Urban Wastewater Treatment Directive (UWWTD) requires the implementation of quaternary treatments (QT) for large wastewater treatment plants (WWTPs) and for small ones placed in risk areas in order to reduce micropollutants (MPs) content. The current study compares the operation reliability and the removal efficiencies of the twelve MPs listed in the UWWTD of several consolidated technologies acting as a QT. The reliability evaluation is based on a risk assessment resulting from: (i) identification of the failure modes for each component of the different treatment trains and their related consequences; (ii) score assignment of the expected likelihood of occurrence (L) of each failure mode and the magnitude (M) of the corresponding effects to the final effluent quality, equipment, worker health and environment; (iii) estimation of the risk (R) of each failure mode (R = L x M); and (iv) ranking and identification of the most critical risks (highest R). Data on removal efficiencies are collected from literature. Ozonation followed by granular activated carbon is the QT with the highest number of very high risks, and the most difficult MPs to be removed are candesartan and irbesartan (slightly lower than 80%).