Effect of dissolved oxygen changes on activated sludge fungal bulking during lab-scale treatment of acidic industrial wastewater.

The cloning and sequencing of fungal 18S rRNA genes followed by the identification of filamentous fungal species by fluorescent in situ hybridization (FISH) and the enumeration of filamentous fungal cells by flow cytometry-FISH (FC-FISH) were used to investigate the effect of dissolved oxygen (DO) changes on activated sludge (AS) fungal bulking during a lab-scale treatment of acidic industrial wastewater. By increasing DO levels from < .5 to > 2 mg L⁻¹, bulking started to occur due to the outbreak of fungal filaments, whereas the chemical oxygen demand (COD) removals sharply increased from < 40 to > 70%. Clone library analyses revealed that all clonal fungal sequences were of yeast origin, and that only one and four yeast species were individually detected in AS at two DO levels. Subsequent FISH identification of filamentous yeast species within bulking sludge using self-designed oligonucleotide probes suggested that all probe-reactive cells of Trichosporon asahii had a filamentous morphology and were the dominating filamentous microorganism in the AS. The FC-FISH analyses of bacteria and two main yeast species showed that the DO shift resulted in a sharp increase of T. asahii, by a factor of 48-60, which caused filamentous yeast bulking. Subsequently, the restoration of DO levels to <0.5 mg L⁻¹ effectively restored the sludge settlement and yeast community, as well as unacceptable COD removals.