Wastewater Treatment Plants (WWTPs) are potential sources of microplastics (MPs) in the aquatic environment. This study aimed to investigate the potential of wastewater-native microalgae consortia to remove MPs from the effluent of two different types of WWTPs as a dual-purpose solution for MPs mitigation and biomass production. For that purpose, the occurrence of MPs from two types of WWTP effluents was analysed over one year. MPs were characterized in terms of morphology (microbead, foam, granule, irregular, filament and film), colour and size. The wastewater characterisation was followed by the removal of MP loads, using native microalgae consortia, pre-adapted to the wastewater effluent. Microalgae consortia evolved naturally through four mitigation assays, adapted to seasonal conditions, such as temperature, photoperiod, and wastewater composition. MPs were present in all the effluent samples, ranging from 52 to 233 MP L^-1. The characterisation of MPs indicated a predominance of white and transparent particles, with irregular and filament shapes, mainly under 500 μm in size. The μFTIR analysis revealed that 43% of the selected particles were plastic, with a prevalence of polypropylene (PP) (34%) and polyethylene terephthalate (PET) (30 %). In the mitigation experiments, substantial biomass production was achieved (maximum of 2.6 g L^-1 (d.w.)), with successful removal of MPs, ranging from 31 ± 25% to 82 ± 13%. These results show that microalgae growth in wastewater effluents efficiently promotes the removal of MPs, reducing this source of contamination in the aquatic environment, while generating valuable biomass. Additionally, the strategy employed, requires minimal control of culture conditions, simplifying the integration of these systems in real-world WWTP facilities for improved wastewater management.

The valorization of microalgal biomass produced during wastewater treatment has the potential to mitigate treatment costs. As contaminated biomass (e.g., with pharmaceuticals, toxic metals, etc.) is often generated, biogas production is considered an effective valorization option. The biomass was obtained from a pilot facility of photobioreactors for tertiary wastewater treatment. The pilots were run for one year with naturally formed microalgal consortia. The biogas was generated in 70 mL crimp-top vials at 35 °C, quantified with a manometer and the methane yield measured by gas chromatography. A maximum biogas production of 311 mL/g volatile solids (VS) with a methane yield of 252 mL/g VS was obtained with the spring samples. These rather low values were not improved using previous thermo-acidic hydrolysis, suggesting that the low intrinsic biodegradable organic matter content of the consortia might be the cause for low yield. Considering the total volume of wastewater treated by this plant and the average amount of methane produced in this study, the substitution of the current tertiary treatment with the one here proposed would reduce the energy consumption of the plant by 20% and create an energy surplus of 2.8%. The implementation of this system would therefore contribute towards meeting the ambitious decarbonization targets established by the EU.

Wastewater treatment plants play a key role in the release of pharmaceuticals and other contaminants into the aquatic environment, causing negative effects on the ecosystems of receiving water bodies. This work aimed to assess the removal efficiency of real loads of these contaminants by wastewater-native microalgae consortia acting on treated wastewater previously subjected to secondary treatment. Wastewater sampling and removal efficiency experiments were conducted over 1 year at three different sampling points. Nineteen pharmaceutical compounds of different drug classes (stimulants, anticonvulsants, antidepressants, antibiotics, β-blockers, lipid regulators, and nonsteroidal anti-inflammatory drugs/analgesics), as well as one herbicide/algicide (Diuron) were detected and quantified. Native blooming microalgae consortia were grown in treated wastewaters, and their potential to remove these compounds was quantified. The removal efficiency of these compounds by native microalgae consortia was variable, ranging from almost no removal for Clofibric acid or Ketoprofen to near complete removal for Fluoxetine, Venlafaxine, Atenolol or Diuron. These variations were influenced not only by the molecular nature of the compounds but also by the microbial composition variability of the microalgae consortia, especially among the prokaryotes present. Overall, microalgae consortia successfully removed between 40 % and 83 % of the total detected compounds, preventing a significant part of these from entering the aquatic environment, contributing to enhance treated wastewater quality. Significant biomass growth was observed, reaching dry-weight concentrations up to 2.6 g.L^-1, indicative of good capacity of the grown consortia to deal with the toxicity effects of the pollutants. In addition to what is now reported, microalgae treatment also removes other pollutants, such as nutrients, metals or microplastic particles, constituting a versatile tertiary treatment for polishing treated wastewaters. These findings demonstrate the potential of native microalgae consortia-based systems to improve wastewater treatment processes, mitigating the environmental impact of pharmaceutical compounds while producing potentially useful biomass.