Towards Proactive Coagulant Dosing in Drinking Water Treatment Using an Optical Proxy of Dissolved Organic Matter Aromaticity

Licentiate thesis, 2026

Changes in surface‑water quality driven by climate change, hydrological variability, and altered land use increasingly challenge drinking water treatment utilities that were originally designed for stable and predictable raw water conditions. Traditional coagulant dosing strategies that rely on static guidelines, periodic jar tests, or operator experience are not equipped to accommodate new dynamic conditions. This highlights the need for online monitoring tools and algorithms capable of tracking real‑time changes in water composition to support proactive process control. This study evaluated an optical measurement (ARIX) for predicting coagulant demand and coagulation performance under dynamic water quality conditions. ARIX and specific ultraviolet absorbance at 254 nm (SUVA254) were used to track spatial and temporal variability in dissolved organic matter (DOM) quality across a complex 240‑km river system. ARIX showed a strong linear relationship with SUVA254 during three surveys conducted over a one-year period along the river (R2 = 0.74, slope = 3.8 ± 0.2, intercept = 0.18 ± 0.14) and captured both spatial and seasonal differences in DOM quality. Coagulation experiments on river samples with varying ARIX and SUVA but similar dissolved organic carbon (DOC) concentrations demonstrated that both parameters exerted a consistent and linear influence on DOC removal and coagulation efficiency, regardless of dose or coagulation mechanism. Achieving 70% DOC removal required ~2.5x higher coagulant doses for samples with lower ARIX values (~0.6) compared to those with higher ARIX values (~0.8). These results showed that even modest changes in DOM quality can substantially alter coagulant demand. Therefore, ARIX emerges as a promising parameter for measuring operationally significant changes in DOM reactivity to coagulants. Since it is feasible for sensors to measure ARIX in real time, these findings support the development of real‑time coagulant dosing strategies that respond to rapid changes in source‑water quality. This capability facilitates the adaptability, efficiency, and resilience of drinking water treatment systems.