The treatment of high ammonium strength wastewater was achieved using an algal-bacterial consortium in two photo-sequencing batch reactors (PSBRs). The nitrogen removal mechanisms were nitritation/denitritation, in this process, algae provide oxygen for nitritation during the light period, while denitritation takes place during the dark (anoxic) period, reducing overall energy and chemical requirements. The two PSBRs were operated at different solids retention times (SRTs) and the ammonium concentration in the wastewater fed was 264 mg NH4+ -N L−1, with a 12 hour on/12 hour off light cycle. The average surface light intensity was 84 μmol m−2s−1. The total inorganic nitrogen removal efficiencies for the two PSBRs was ~95%, and the biomass settleability was measured as SVI (53–58 mLg−1). Higher biomass density was observed at higher SRT, resulting in greater light attenuation and less oxygen production. A mathematical model was developed to describe the algal-bacterial interactions using the Activated Sludge Model No.3 (ASM3) as base, and including two algal processes. The results of the model predicted the experimental data closely. One of the most sensitive parameters was found to be the maximum growth rate of algae ( μmax,p ).