Using N-TiO2 to enhance the coagulation of Oscillatoria sp. and subsequently degrade cells and their metabolites in sludge under visible light

Abstract

Oscillatoria, a common genus of filamentous cyanobacteria, has unsatisfactory removal efficiency in drinking water treatment plants, and the cyanobacteria will accumulate and release cyanotoxins into the sludge. This study therefore used photocatalyst particles (nitrogen-doped TiO2 (N-TiO2)) as ballast to enhance the coagulation of Oscillatoria sp. and to degrade both the cells and the metabolites in sludge by visible-light photocatalysis. Results show that combining 100 mg/L N-TiO2 with different coagulants (polyaluminium ferric chloride (PAFC), FeCl3, or AlCl3) was able to improve the Oscillatoria sp. removal efficiency of coagulation to 95 %, and there were no floating flocs. The addition of N-TiO2 resulted in less-negative electrokinetic potentials of Oscillatoria sp. aggregates, enhanced the adsorption bridging effect, and increased the density and compactness of flocs. N-TiO2 had little effect on the integrity of Oscillatoria sp. cells and did not cause the release of cylindrospermopsin (CYN) during coagulation. In the photocatalysis of Oscillatoria-laden sludge 100 mg/L N-TiO2 effectively inactivated Oscillatoria sp. cells within 48 h, regardless of the type of coagulant. After photocatalytic treatment the cell wall was rough and deformed and most of the cell cytoplasm had been removed. Furthermore, the content of extracellular organic matter in sludge supernatant was significantly reduced. After the 48-h treatment 89.2 % of total CYN in the sludge was able to be degraded. This study therefore suggests that N-TiO2 enhances coagulation technology can not only improve the efficiency of removal of filamentous cyanobacteria, but also reduce the ecological risk of sludge discharge and facilitate the reuse of sludge supernatant.