Algal removal from cyanobacteria-rich waters by preoxidation-assisted coagulation-flotation: Effect of algogenic organic matter release on algal removal and trihalomethane formation

The cyanobacteria-bloom in raw waters frequently causes an unpredictable chemical dosing of preoxidation and coagulation for an effective removal of algal cells in water treatment plants. This study investigated the effects of preoxidation with NaOCl and ClO_2 on the coagulation-flotation effectiveness in the removal of two commonly blooming cyanobacteria species, Microcystis aeruginosa（MA） and Cylindrospermopsis raciborskii（CR）, and their corresponding trihalomethane（THM） formation potential. The results showed that dual dosing with NaOCl plus ClO_2 was more effective in enhancing the deformation of cyanobacterial cells compared to single dosing with Na OCl, especially for CR-rich water.Both preoxidation approaches for CR-rich water effectively reduced the CR cell count with less remained dissolved organic carbon（DOC）, which benefited subsequent coagulation–flotation. However, preoxidation led to an adverse release of algogenic organic matter（AOM） in the case of MA-rich water. The release of AOM resulted in a poor removal in MA cells and a large amount of THM formation after oxidation-assisted coagulation-flotation process. The reduction in THM formation potential of CR-rich waters is responsible for effective algae and DOC removal by alum coagulation. It is concluded that the species-specific characteristic of cyanobacteria and their AOM released during chlorination significantly influences the performance of coagulation–flotation for AOM removal and corresponding THM formation.

Formation of Iodinated Trihalomethane Disinfection By-products by Co-oxidation of Natural Organic Matter with Sodium Hypochlorite and Lead Dioxide

Iodinated trihalomethanes(I-THMs)in drinking water pipelines have attracted wide attention due to their high toxicity.The coexistence of widely present lead dioxide(PbO_(2))with commonly used disinfectant sodium hypochlorite(NaClO)in drinking water might change the formation characteristics of I-THMs due to the strongly oxidizing properties of PbO_(2).This study investigated the formation of I-THMs during the co-oxidation of natural organic matter including humic acid(HA),extracellular organic matter(EOM),and intracellular organic matter(IOM)of algogenic organic matter by PbO_(2) and NaClO.Triiodomethane(CHI_(3))is the dominant product in the single oxidation system of PbO_(2),whereas trichloromethane(CHCl_(3)),chlorodiiodomethane(CHClI_(2)),and dichloroiodomethane(CHCl2I)are the major products in the single NaClO system.In the co-oxidation system,the dominant I-THMs are similar to those in the single NaClO system.However,the CHCl_(3) content decreased to 56.4%whereas I-THMs concentrations remained unchanged with the increase of PbO_(2) concentration.The main reason is attributed to the reduced residual chlorine content due to the reaction of PbO_(2) with NaClO.IOM is more prone to forming I-THMs than HA and EOM due to the specified structures.This study suggested that PbO_(2) in the drinking water supply pipelines might change the risk of THMs.