Chlorella vulgaris in wastewater treatment: Dual-scale removal of siloxanes and conventional pollutants

Volatile methylsiloxanes (VMSs) are persistent and bioaccumulative contaminants of emerging concern that are commonly detected in wastewaters, posing both environmental risks and operational problems in wastewater treatment plants. This study investigates the potential of Chlorella vulgaris for the simultaneous removal of VMSs and conventional pollutants from primary urban wastewaters, combining laboratory- and pilot-scale experiments. It is also the first attempt to estimate the distribution of VMSs in the effluent, adsorbed to and accumulated in microalgal cells, using a novel QuEChERS extraction method for both intact and disrupted biomass. Laboratory-scale experiments indicated volatilisation as an important removal pathway in aerated photobioreactors (PBRs). C. vulgaris grew efficiently in wastewater in both conditions and contributed to total nitrogen and phosphorus removals up to 97 % and 87 % ( w /w), respectively. Decamethylcyclopentasiloxane (D5) was the dominant VMS in the wastewaters, and a mass balance analysis showed removals reaching 96 % ( w /w). Autochthonous wastewater microorganisms proliferated in the control experiments, also contributing to the removal of the target pollutants. The results suggest that D5, D6 (dodecamethylcyclohexasiloxane), and L5 (dodecamethylpentasiloxane) were adsorbed and accumulated inside the cells in biomass samples from all PBRs. D6 presented the highest overall accumulation inside the cells, up to 43 % ( w /w). Bioadsorption and bioaccumulation in microalgal biomass were identified as potential biological removal mechanisms at both scales. This work presents the first dual-scale evaluation of VMS fate in microalgal systems, highlighting C. vulgaris as a promising candidate for an integrated wastewater bioremediation and the removal of emerging contaminants. • First lab/pilot-scale study on siloxane fate in microalgal wastewater treatment; • Chlorella vulgaris removed up to 97 % of total nitrogen and 87 % total phosphorus; • Mass balance showed up to 96 % ( w /w) removal of decamethylcyclopentasiloxane; • The heavier siloxanes were adsorbed and accumulated inside the biomass cells; • Mass balances suggested siloxane volatilisation and bioadsorption/accumulation.