The microalga Scenedesmus sp. was grown in artificial wastewater with 30 and 50% daily replacements of the culture volume and under continuous lighting. The biomass per unit volume was higher with the 30% dilution, but daily yields were better with 50%. In 24 h, phosphate removal was more than 50% in both cases. The total nitrogen removed, most of which was ammonia, was approximately 50 and 66% of the initial concentration for the 30 and 50% dilutions, respectively, and a nitrogen budget calculated using the total daily removal and the average algal protein harvested gave an imbalance of close to 65 and 75%, showing that ammonia stripping due to high pH values and aeration might play an important role in the biological tertiary treatment of wastewater with photosynthetic organisms.