Controlled pre-chlorination to enhance coagulation based on the released humic-like fluorescent substance for algae-laden source water

Sodium hypochlorite (NaClO) pre-chlorination enhanced coagulation represents a conventional approach for treating algae-laden water, yet excessive dosing can cause cell lysis and release of intracellular organic matter. This study developed a controlled pre-chlorination strategy guided by the release of humic-like fluorescent substances (biopterin) to optimize coagulation under varying water quality conditions. Experiments were conducted across a range of algal densities, growth phases, and natural organic matter (NOM) levels. Fluorescence excitation-emission matrix spectroscopy combined with parallel factor analysis was employed to track biopterin release as a real-time indicator of algal cell integrity. Results demonstrated that biopterin effectively guided moderate pre-chlorination at high algal densities (≥ 5 × 10 5 cells/mL), while the fluorescence intensity ratio of biopterin to NOM-associated components (C1/C3) provided a more reliable criterion under low algal density (≤ 10 5 cells/mL) or NOM-rich conditions. Optimal NaClO and polyaluminum chloride (PACl) dosages increased with algal density and NOM concentration. NOM concentration was the most influential factor, evidenced by the highest standardized coefficient (β = 0.839, p < 0.001). Based on this, a refined intelligent dosing control system was proposed that integrated fluorescence ratio evaluation with chemical agent dosage optimization. These advancements provide theoretical foundations and practical solutions for treating algae-laden water.