Revalorization of microalgae cultivation effluent: a circular strategy for long-term sustainable production of Tetraselmis striata

Energy and fertilizers constitute the main economic and environmental costs for microalgae cultivation. This study aims to optimize the production of Tetraselmis striata by recirculating the whole untreated culture medium. Firstly, T. striata was cultivated inside a greenhouse ( IN ) to assess process viability. Subsequently, 10-m 3 cultures were operated outside ( OUT ) semicontinuously to assess the effect of medium recirculation on biomass productivity, composition and quality. Additionally, T. striata was cultivated under nutrient depletion to identify changes in biomass composition according to nutrient status. Medium was successfully recirculated for two months. Productivities were similar in all conditions ( OUT recirculation: 0.097 ± 0.024 g/L/day; 43.7 t/ha/year). Biomass protein and carbohydrate content did not differ between fresh (F) and recirculated (R) medium, whereas higher protein and lower carbohydrate levels were obtained OUT than IN (%AFDW protein: F IN 46.1 ± 0.5, R IN 47.6 ± 1.4, F OUT 53.6 ± 1.5, R OUT 54.3 ± 1.5; carbohydrates: F IN 43.2 ± 0.5, R IN 41.1 ± 2.5, F OUT 35.9 ± 2.9, R OUT 34.8 ± 2.4). Lipid and ash content were similar across conditions. Heavy metals and microbiological analyses complied with EU feed and food standards. Recirculation reduced energy for cultivation by 7.5 % and effluent volume by 84 %, saving 3 tons/year of nitrate and 1 ton/year of phosphate for a 10-ha facility. A final effluent with a limited load in nutrient was generated obtaining a protein-rich biomass apt for feed, while low-nitrogen biomass suitable for biofuel was obtained under nutrient depletion. Thus, medium revalorization enables a sustainable strategy for T. striata cultivation, reducing costs and impacts while providing high-value biomass for multiple industrial applications within a circular economy framework. • T. striata was grown for 2 mo. recirculating the whole untreated culture medium. • Biomass productivity, composition and quality was the same as in the fresh medium. • Energy for whole cultivation process and effluent volume are 7.5 % and 84 % lower using this strategy. • Annual savings of 3 tons of nitrate and 1 ton of phosphate can be achieved for a 10-ha facility. • A final effluent with low nutrient load and a protein-rich biomass can be obtained.