This work deals with steady-state optimization of a process formed by an algae-bacteria photo-bioreactor (PBR) in an activated sludge configuration. The optimization is done by considering the total PBR volume as two volumes in series, and aiming for the minimal nitrogen concentration in the effluent, for a given external light and carbon dioxide (CO2) injection. Results suggest that it is possible to obtain an optimum volume distribution that gives a lower effluent substrate concentration compared to a single volume, and this optimum volume depends on the CO2 applied.