Wastewater treatment using Scenedesmus almeriensis: effect of operational conditions on the composition of the microalgae-bacteria consortia

Primary urban wastewater was treated in outdoor raceways using microalgae-bacteria consortia dominated by Scenedesmus almeriensis. The current study aimed at assessing the effect of operational conditions, namely, culture depth and dilution rate, on the following: (i) biomass productivity, (ii) the nutrient removal capacity and (iii) the composition of the microalgae-bacteria consortium and the presence of unwanted microorganisms. Optimum dilution rates to process large quantities of wastewater during summer and achieve high biomass productivities were 0.3-0.5 day(-1). Under the optimum operational conditions, nitrogen and phosphorus removal rates were higher than 90% while removal of chemical oxygen demand was 70%. Operating at different culture depths had a striking effect on the composition of the microalgae-bacteria consortium. The relative abundance of nitrifiers increased with culture depth and was minimised at 0.05 m: larger culture depths led to enhanced nitrifying activity and therefore to nitrate production and accumulation in the system. Results demonstrate the potential of microalgae-based wastewater treatment processes and the importance of selecting suitable operational conditions to maximise both, biomass production and nutrient removal by minimising the occurrence of nitrifying bacteria.