Activated sludge morphology significantly impacts oxygen transfer at the air-liquid boundary

Oxygen transfer is a key process determining the energy use of a biological wastewater treatment process. In this research, we investigated the effect of sludge morphology, especially the role of filamentous microorganisms, on oxygen transfer using bench-scale complete-mix activated sludge reactors with solids retention times (SRTs) of 10-, 20-, and 40-days, respectively. Results indicated 5%-10% reduced aeration need in the 40-day SRT reactor, compared with 10- and 20-day SRT reactors to maintain the same dissolved oxygen level, due to the improvement in sludge settleability and oxygen transfer efficiency (OTE). Filamentous microorganisms adversely impacted OTE via an increase in apparent viscosity of the mixed liquor, which resulted in an increase in the air bubble size and liquid film thickness and, therefore, limited oxygen transfer at the air-liquid boundary. A statistical analysis also confirmed that the mixed liquor viscosity is a statistically significant parameter links to OTE. PRACTITIONER POINTS: Filamentous organisms reduce oxygen transfer via increasing mixed liquor viscosity, which increases air bubble size and liquid film thickness at the air-liquid boundary. Increasing solids retention time reduces filament density. As a result, very long solids retention time promotes oxygen transfer.