Primary urban wastewater was processed using the microalga Scenedesmus sp. in an outdoor pilot-scale raceway reactor connected to an ultrafiltration membrane. The goal was to separate cellular retention time from hydraulic retention time. This strategy led to a 129.3% increase in the daily volume of wastewater treated per square meter, and to a 48.7% increase in biomass productivity to a final value of 22.2 ± 1.9 g·m-2·day-1. Nutrient removal was highly influenced by permeate rate, allowing to remove up to 0.65 mg·m-2·day-1 of phosphates. Over 99% of ammonia was removed when the ultrafiltration membrane was used, although this was partially due to nitrate production by nitrifying bacteria: higher permeate rates led to higher relative abundance of the nitrifying bacterial. The amplification and sequencing of the microalgae-bacteria samples led to the detection of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, such as Bradyrhizobiaceae, Nitrospiraceae, Nitrosomonadaceae, and Chromatiaceae. The most abundant families detected in the microalgae-bacteria biomass were Rhodobacteraceae and Comamonadaceae
