Utilization of secondary-treated wastewater for the production of freshwater microalgae

In this work, we studied the potential use of secondary-treated wastewater as nutrient source in the production of freshwater microalgae strains. Experiments were performed indoors in a semicontinuous mode, at 0.3 day⁻¹, simulating outdoor conditions. We demonstrated that all the tested strains can be produced by using only secondary-treated wastewater as the nutrient source. The utilization of secondary-treated wastewater imposes nutrient-limiting conditions, with maximal biomass productivity dropping to 0.5 g l⁻¹ day⁻¹ and modifies the biochemical composition of the biomass by increasing the amount of lipids and carbohydrates while reducing the biomass protein content. We measured fatty acid content and productivity of up to 25 %d.wt. and 110 mg l⁻¹ day⁻¹, respectively. We demonstrated that all the tested strains were capable of completely removing the nitrogen and phosphorus contained in the secondary-treated wastewater, and while the use of this effluent reduced the cells’ photosynthetic efficiency, the nitrogen and phosphorus coefficient yield increased. Muriellopsis sp. and S. subpicatus were selected as the most promising strains for outdoor production using secondary-treated wastewater as the culture medium; this was not only because of their high productivity but also their photosynthetic efficiency, of up to 2.5 %, along with nutrient coefficient yields of up to 96 gbᵢₒₘₐₛₛ gN ⁻¹ and 166 gbᵢₒₘₐₛₛ gP ⁻¹. Coupling microalgae production processes to tertiary treatment in wastewater treatment plants make it possible to recover nutrients contained in the water and to produce valuable biomass, especially where nutrient removal is required prior to wastewater discharge.