This study evaluates the hydrodynamic and mass-transfer behaviour of a 20-L airlift photobioreactor, employed for secondary wastewater treatment of domestic effluent. Gas hold-up (e), as well as the volumetric mass transfer coefficient (k L a) was assessed as a function of superficial gas velocity (Ug) for both bi- and triphasic systems. In the experimental range of Ug investigated, the maximum k L a and e obtained were in the order of 0.0074 s −1 and 0.035 for water/ air system and 0.0097 s −1 and 0.026 for triphasic system respectively. The results indicate that this PBR is a good candidate for wastewater treatment, with lower power costs than other types of biological reactors.