Effects of oxidants on the removal of algae and manganese by coagulation/sedimentation
Prechlorination with NaOCl is usually used to improve the algae removal by coagulation/sedimentation process for water treatment plants (WTPs), but prechlorination may cause cell lysis and increase disinfection by-products such as trihalomethanes. In addition, the majority of soluble manganese forms in the raw water, it would increase the dosage of prechlorination. When the raw water contains a great amount of algae in the presence of soluble manganese, NaOCl chlorination is not effectively to oxidize soluble manganese. Therefore, ClO2 and KMnO4 are used instead to improve the formation of oxidized manganese. The oxidants in oxidation process prior to coagulation could strongly affect the algae and Mn oxides removal through coagulation/sedimentation process. This study aims to investigate the effect of different oxidants (NaOCl, ClO2 and KMnO4) on the removal of algae and Mn oxides by coagulation/sedimentation. The raw water collected from Bao-San reservoir was first concentrated through ultrafiltration process, and then the prepared soluble manganese was added into the concentrated water to a desire concentration of 0.3 mg/L. After that, preoxidation with NaOCl, ClO2 and KMnO4 oxidants were conducted at various oxidant dosage and mixing time. After oxidation, algae viability was determined by flow cytometer, and residual algae and soluble manganese (i.e., Mn ions and complexed Mn) were analyzed. The addition of various oxidants in preoxidation process was carried out to evaluate the removal in algae and manganese through coagulation/sedimentation process. The results showed that NaOCl and ClO2 oxidation lower the viability of algae cells due to the occurrence of intact cells. However, KMnO4 oxidation is not effectively to reduce the viability of algae cells, and extracellular organic matter (EOM) releases from cells. ClO2 oxidation has a higher rate constants (k_(t=30s)) in lowering the cell viability more than NaOCl and KMnO4 oxidation. In the presence of soluble manganese, NaOCl oxidation is effectively to reduce the viability of algae cells instead of oxidizing Mn ions and complexed Mn. The majority of particulate manganese was formed with KMnO4 oxidation, accompanying the formation of Mn ions. ClO2 oxidation is effectively to simultaneously deform algae and oxidize Mn ions and complexed Mn. For alum coagulation, the optimum algae destabilization can be achieved when KMnO4 preoxidation is performed. However, ClO2 preoxidation has a better ability to improve the removal in algae and manganese by coagulation/sedimentation.