Enhancing drinking water treatment plant performances by the application of robustness indices and chemical-physical processes

Surface water quality is declining due to climate change, leading to increased concentrations of soluble metals, natural organic matter (NOM), turbidity, and algal blooms. These changes pose challenges to traditional water treatment plants, needing innovative approaches. 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.In this regard, the initial focus of the study lies on the development and application of a new turbidity robustness index (TRI95B) to be used as a warning tool for detecting deviations from water quality standards. 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. 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. In addition, the index has been correlated to the characteristics of the raw water at the inlet of plant, in order to identify for which particular load conditions and events, in terms of magnitude and duration, the plant loses robustness. The same approach has been applied for the development and validation of a manganese robustness index (MRI95B).The second part of the research explored the use of potassium ferrate(VI) as a potential solution to address the increasing concentration of pollutants in raw water and climate change-related water emergencies and to meet the more stringent regulations of drinking water and, in particular, to cope with losses of robustness of drinking water treatment plants. Conventional chemicals might not be able to ensure compliance with drinking water quality standards; in this light Fe(VI) has been investigated to be used either alone or in combination with conventional reagents to producing potable water.Results showed that the proposed RIs can identify short-duration and low magnitude failures, thus coping with the purpose of the new regulation for drinking water. Overall, TRI95B and MRI95B showed a great capability of synthetically and effectively describing the variability of plant performance, allowing highlighting process failures. Additionally, by linking TRI95B to the occurrence of turbidity peak events it was observed that there is a threshold value of such events that leads to a serious compromission of the plant’s robustness.Moreover, results showed that Fe(VI) was more effective to remove soluble manganese compared to a conventional oxidant (permanganate), while requiring a lower stoichiometric dosage (<2 mol of Fe(VI) per 3 mol of Mn(II)) as the reaction byproducts of Fe(VI) reduction (eg., Fe3+) contributed to manganese removal. A slightly alkaline environment (pH >8.5) was crucial to maximize manganese oxidation since pH closer to neutral caused the reduction of Fe(VI) to Fe(III), thus decreasing its reduction potential. Fe(VI) was also effective toward NOM although its activation was necessary to provide noticeable effects. In combination to conventional coagulant/flocculant agents, Fe(VI) was able to provide noticeable increases in removal of turbidity (<0.30 NTU) while involving a simultaneous decrease in other chemicals requirement (>50%). Besides, Fe(VI) was also capable of providing algae removal of approximately 80% higher than conventional oxidizing agent, through simultaneous oxidation and flocculation. In conclusion, this study demonstrated that, in a context of climate change, which leads to the occurrence of extreme and short-duration events, the indices can assess the potential vulnerability of the plant and can be used to identify for which magnitude of an event the plant loses robustness, requiring emerging treatment to comply with regulations. In this regard employing Fe(VI) as a treatment method for drinking water is very promising as it can serve as an alternative or complement conventional approaches in tackling the challenges presented by climate change and sustaining high-quality standard potable water.