Emergency response technologies for Aulacoseira granulata blooms in drinking water resource: Pre-oxidation-Enhanced...

Eutrophication-induced winter drought blooms of Aulacoseira granulata pose a significant threat to drinking water treatment processes. A notable example is the bloom event that occurred in the Xijiang River in January 2024, where the peak algal density reached ∼8 × 10⁶ cells/L and chlorophyll-a (Chl-a) concentration attained 45 μg/L. This bloom severely disrupted the normal operation of the GM Waterworks in China, leading to a reduction in the backwash interval of V-type filters from 12-16 h to 8 h and thereby impairing the treatment efficiency of conventional coagulation-sedimentation processes. This study systematically evaluated pre-oxidation-coupled coagulation processes via batch experiments, integrating multi-scale characterizations (zeta potential, excitation-emission matrix fluorescence, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, floc imaging) to analyze the effects of algal density (<10⁵ to 10⁶-10⁷ cells/L) and initial pH (8.0-8.5). A critical breakthrough was the identification of the NaClO (1.5 mg/L)-PAC (6 mg/L)-xanthan gum (XG, 0.1 mg/L) ternary system as the optimal strategy. This biocompatible formulation (XG replacing toxic polyacrylamide, PAM) achieved residual algal density of 1408 cells/L (vs. 627,808 cells/L for PAC alone) with >95 % Chl-a removal. The system maintained >95 % algal removal at high densities (10⁶-10⁷ cells/L) (vs. 47.03% for PAC alone) while constraining trihalomethanes (THMs) to <8 μg/L (vs. 13.78 μg/L CHCl₃ for NaClO-PAC). Adjusting pH to 7.5-8.0 further improved PAC-only removal by 38.18 %, resolving the pH-induced coagulation bottleneck at alkaline conditions (pH > 8.2) caused by Al(OH)₃ precipitation. Mechanistically, NaClO-mediated lipid peroxidation disrupted algal cells and extracellular polymeric substances (EPS), reducing zeta potential from -29.16 mV to -15 mV (pH 7.5) and mitigating electrostatic repulsion. XG's hydroxyl/carboxyl groups formed hydrogen-bonded networks, enhancing floc compactness (average size: 459.9 μm vs. 289.6 μm for PAC + PAM) and settleability. This work resolves the long-standing trade-off between removal efficiency and THMs formation in algal bloom control, significantly mitigates PAM-related toxicity risks, and provides a mechanistically validated solution to enhance the resilience of drinking water supply systems against sudden A. granulata blooms in eutrophic sources.