Toward a better understanding of coagulation for specific extracellular organic matter using polyferric sulfate and polydimethyl diallyl ammonium chloride

The impact of extracellular organic matter (EOM) on the coagulation behavior and harvest of algae remains unclear. In this study, bounded EOM (bEOM) and dissolved EOM (dEOM) extracted from laboratory-grown Microcystis aeruginosa were characterized in terms of specific ultraviolet absorbance, molecular weight (MW) distribution, Three dimensional excitation emission spectrum (3D-EEM), and component content. Three coagulants, namely, polyferric sulfate (PFS), polydimethyl diallyl ammonium chloride (PD), and PFS-PD, were used to determine the suitable coagulant and conditions for EOM removal. The coagulation behavior for EOM removal and the roles of EOM on algae water treatment were investigated. Results indicated that PFS-PD exhibited the highest efficiency in removing dissolved organic carbon. MW distribution and 3D-EEM analysis showed that the removal of high-MW protein-like substances in bEOM was acceptable and few low-MW microbial product-like and fulvic/humic-like substances in dEOM were coagulated. In addition to charge neutralization, the complex interaction mechanisms between N–H/C–N groups of bEOM and hydroxyl iron facilitated the formation of primary microflocs. High-MW fraction in bEOM promoted floc growth and sedimentation. EOM contributed to the removal of algae cells with its adherence to the surface of cell wall, and no positive impact was observed for the dissociative EOM.