Quantifying drinking water treatment plant robustness with respect to turbidity: Examining raw water and treatment

With the emerging risks of climate change, it can be challenging for drinking water treatment plants (DWTPs) to maintain regulated water quality parameters to protect public health. Severe and more frequent precipitation events can increase suspended particles in surface water, which have been associated with water-borne pathogens, elevating public health risks. To manage such spikes, DWTPs should be robust. In the present study, the concept of operational robustness was advanced by tailoring general robustness framework to a turbidity-specific framework for two full-scale DWTPs: Plant A (river-based) and Plant B (lake-based) for normal raw water turbidity, and historical and future extreme turbidity events (Plant A only). The turbidity robustness index (TRI) was used to quantify the robustness of the critical treatment steps. Weekly TRIs revealed process-specific insights - robust filtration in both plants and comparatively higher robustness of coagulation and sand-ballasted clarification (CSBC) in Plant A compared to conventional coagulation-flocculation-sedimentation (CFS) in Plant B. A novel development of this study was to develop a procedure to distinguish turbidity events from normal turbidity to investigate the performance of individual treatment steps in periods of upsets. For the studied plants, higher TRIs were not correlated with historical turbidity events, indicating the need for further assessment of the DWTPs response to future extreme events. The overall robustness indices indicated that Plant A was more robust than Plant B. For Plant A, the response to an extreme turbidity event was assessed in a factorial experiment using modified jar tests, resulting in suggestions for improving the CSBC process. The robustness framework can be used as a simple and practical diagnostic tool in DWTPs to evaluate the operational regimes retroactively, identifying under-performing unit(s), enhance their robustness by implementing short-term operational changes and possibly establish raw water turbidity boundaries beyond which the operational adjustments may not work, and which can signal the need to initiate long-term capital planning to prepare for potential adverse effects of climate change.