Scaling-up of wastewater bioremediation by Tetradesmus obliquus, sequential bio-treatments of nutrients and metals

Microalgae may be exploited in water or wastewater treatment facilities to reduce excess concentrations of nutrients and metals to comply with regulatory limits. In this study, we characterized the growth and phosphorus (P) removal capacity of an isolated strain of Tetradesmus obliquus VRUC280. Investigations were carried out from laboratory scale (50 mL) up to a 100 L outdoor photobioreactor (PBR). After 10 days, batch cultures removed up to 74% of the media P, while in the PBR, 95% removal was achieved within five days. The harvested biomass was then inactivated (freeze-dried) and used for metal adsorption tests, employing solutions containing 6.0 mg Cu L-1 or 4.8 mg Ni L-1. Metal removal rates were evaluated after 15, 30, 60 and 120 min by the analysis of liquid and biomass metal contents. For the latter, a specific biomass digestion method was developed. Cu removal ranged between 50% and 65%, while for Ni, removal varied between 30% and 50%. 300-400 mg Cu Kg DW-1 and 130-250 mg Ni Kg DW-1 were rapidly adsorbed on the cell surface of T. obliquus (ca. 15-30 min incubations). This study demonstrates the potential of microalgae, in this case T. obliquus, to remove sequentially P and metals from aqueous media.