A critical review of the contradictory roles of algal organic matter in microalgae coagulation-flocculation: effects of composition, properties, and mechanisms

Coagulation is a widely used pretreatment for algae-water separation, offering significant potential to enhance removal efficiency and reduce downstream processing costs. However, algal organic matter (AOM), a complex mixture of algae-derived organic compounds, plays contradictory roles in coagulation. Depending on its properties, AOM can either promote or deteriorate coagulation, posing challenges for process optimization. This review critically evaluated how AOM influences coagulant demand and floc characteristics, with a particular focus on its composition and properties. Evidence shows that low concentrations of high-molecular weight (MW) proteins and polysaccharides enhance coagulation through bridging mechanisms. This enhances the capture of algal cells, forming larger flocs while reducing coagulant demand. In contrast, low-MW AOM components, or excessive AOM concentrations, compete with algal cells for coagulant binding sites. This compromises coagulation efficiency, resulting in smaller flocs that exhibit reduced removal performance. Furthermore, AOM derived from different cellular compartments exerts distinct effects on algal coagulation. Soluble organic matter from intracellular (IOM) and extracellular (EOM) sources typically impair coagulation efficiency and increase coagulant consumption. In contrast, organic matter bound to the algal cell surface, specifically extracellular polymeric substances (EPS), enhances coagulation efficiency and reduces coagulant requirements. Strategies to mitigate AOM’s negative effects are discussed in detail, including optimizing coagulation parameters, developing novel coagulants to enhance flocculation, and integrating pretreatment techniques (e.g., pre-oxidation and adsorption) aimed at reducing AOM concentration or altering its characteristics. Key challenges remain, including inconsistent AOM analytical approaches and the lack of mechanistic clarity. Future research should focus on improving AOM extraction methods, identifying species- and stage-specific components, and elucidating key mechanisms underlying AOM behavior during coagulation. By linking AOM properties to its functional role, this review provides a foundation for improving algae coagulation strategies and supporting effective algal bloom control.