A modified robustness index for assessing operational performance of drinking water treatment plants: a comparative study within a new regulatory framework

Drinking water treatment plants (DWTPs) are facing emerging challenges affecting raw water quality. In addition, the new regulatory framework (EU 2184/2020) sets stricter limits for turbidity and percentile statistics for continuous compliance, demanding greater robustness of the treatment processes. To achieve this aim, this study proposes a turbidity robustness index (TRI), named TRI95B, to be used as a warning tool for detecting deviations from water quality standards. TRI95B has been compared with the TRIs existing in the literature. Furthermore, the TRI95B validation has been performed by a three-year monitoring dataset of a full-scale DWTP. The proposed TRI95B index has two key novelties compared to the existing indices required for adapting to the new drinking water regulation: i. introduces the 95th percentile as a statistical indicator; ii. considers an additional term that sets an alert when a threshold value is exceeded. The comparison results suggest a better correspondence to the real plant performances of TRI95B than the other TRIs. Indeed, both the sensitivity and specificity of TRI95B were significantly higher than the other TRIs, indicating a better capacity to correctly classify both positive and negative cases. Moreover, while the previous TRIs identify a critical operating condition when the turbidity goal was significantly exceeded, TRI95B highlights a failure condition at a lower discrepancy. Therefore, TRI95B is also able to identify short-duration and low magnitude failures, thus coping with the purpose of the new regulation for drinking water.