Microalgae wastewater treatment: Pharmaceutical removal and biomass valorization

The growing global challenges of wastewater treatment, resource recovery, and environmental pollution are critical in addressing the sustainability of both urban and agricultural systems. Conventional treatment methods often fail to remove pharmaceutical pollutants effectively, which can lead to serious environmental and health concerns. As an alternative, microalgae-based systems have shown potential for addressing wastewater treatment while recovering valuable nutrients. Thus, the present study aims to investigate (i) the removal of pharmaceuticals in microalgae-related wastewater treatment systems, (ii) the variation of biomass productivity and nutrient recovery as a function of the presence of these compounds, and (iii) the feasibility of using the produced biomass in agriculture as a biostimulant. Experiments were performed in pilot-scale thin-layer reactors fed in continuous mode by spiking the inlet wastewater with up to six pharmaceuticals (sulfamethoxazole, trimethoprim, metronidazole, ofloxacin, ciprofloxacin, and diclofenac), selected by its relevance. Results demonstrated that the presence of these compounds does not influence biomass productivity or the fluorescence of chlorophylls as an indicator of the status of the cells. Either, the presence of pharmaceuticals does not reduce the recovery of nutrients, with both biomass productivity and nutrient recovery being linearly related. On average, the removal of CECs was higher in summer (90 %) than in winter (74 %), with variations depending on the pharmaceutical tested. Sulfamethoxazole was the most effectively removed (>91 %), while trimethoprim had the lowest removal rate (>44 %). Most importantly, on average less than 3 % of the pharmaceuticals remain in the biomass, trimethoprim being the contaminant most retained in the biomass (up to 6 %), thus confirming that they are mainly degraded into the mixed liquor. Bioassays performed confirmed the biostimulant capacity of the microalga acting as plant-promoting agents. These results confirm the capacity of microalgae-related wastewater treatment processes to remove pharmaceuticals and to produce safe water and biomass for its final use in agriculture.