Treatability of high-molecular-weight and ultra-hydrophilic haloacetic acid precursors from freshwater raphidophytes in water treatment

• HAAFPs of precursors from raphidophytes reduced after all unit operations. • Coagulation and sedimentation were highly effective, achieving up to 90% reduction. • PAC adsorption and ozonation were partially effective under realistic conditions. • Pre-chlorination enhanced PAC adsorption but inhibited coagulation. • Treatment process should be optimized based on raphidophyte growth levels. Freshwater raphidophytes in drinking water sources produce high concentrations of ultra-hydrophilic and high-molecular-weight haloacetic acid (HAA) precursors, posing a challenge to ensuring safe drinking water. Unlike other algae, raphidophytes lack cell walls, which makes them susceptible to disruption by minor physical effects. This makes complete cell removal complicated and necessitates the removal of dissolved HAA precursors to prevent HAA formation. This study evaluated the reduction in the HAA formation potentials (FPs) of precursors from lab-cultured raphidophytes and environmental samples collected during a raphidophyte bloom in water treatment. Several treatment methods have been evaluated, including coagulation and sedimentation, powdered activated carbon (PAC) adsorption, pre-chlorination combined with PAC adsorption, and ozonation. All unit operations removed HAA precursors, with coagulation and sedimentation being the most effective, achieving over a 90% reduction in HAAFPs. PAC adsorption, especially that of PAC with a high mesopore content, also effectively reduced HAAFPs, with higher doses yielding greater reductions. Ozonation, while reducing the HAAFP levels, was less effective under real-world treatment conditions. A combination of unit operations showed that pre-chlorination negatively affected the treatability by coagulation and sedimentation. The most successful process was a combination of coagulation and sedimentation with PAC adsorption and post-ozonation, which reduced trichloroacetic acid FPs from 1300 to 39 µg/L. Under certain conditions, such as high algal density or unsuitable treatment conditions, the HAA concentration after treatment may exceed the HAA regulatory standards. This emphasizes the need for flexible and adaptive treatment strategies to manage the elevated HAA concentrations in raphidophyte blooms.