Effect of cell integrity on algal destabilization by oxidation-assisted coagulation

• Preoxidation with NaOCl and ClO2 can enhance algae destabilization by coagulation.
• During oxidation, the degradation of algae cell by ClO2 is more severe than NaOCl.
• The degradation of humic-like substances is improved by ClO2 oxidation.
• With ClO2 preoxdiation, algae destabilization by alum coagulation is effectively improved.
• The residual algae count is inversely well-correlated to filterability.

A hybrid oxidation–coagulation process is commonly adopted to destabilize algae and subsequently improve the removal of algae from water through sedimentation and filtration in water treatment plants (WTPs). Preoxidation is crucial to improving algae removal by a coagulation–sedimentation process. The goal of this study was to investigate the effect of oxidation with NaOCl and ClO2 on the cell integrity of algae (i.e. diatoms) and the destabilization of algae by means of Alum coagulation. The effects of oxidation-assisted coagulation on the performance of sedimentation as well as filtration were evaluated. The results show that ClO2 reduces cell integrity more severely than NaOCl during oxidation. The degradation of chlorophyll a and humic-like substances generated by cells ruptured by ClO2 oxidation is stronger than that by NaOCl oxidation. During oxidation both NaOCl and ClO2 fail to cause significant cell lysis, while cell settleability can be improved markedly by using only ClO2. Preoxidation with ClO2 is more effective in destabilizing the particles and algae when applying Alum coagulation at low dosages. It was found that the residual algae counts in the supernatants are inversely well-correlated to its filterability instead of its residual turbidity. The reduced cell integrity resulting from ClO2 preoxidation effectively improves the performance of coagulation–sedimentation for algae (diatoms) removal and reduces the burden of filtration operation in the WTPs.

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