Bioremediation of drainwater from soilless cultivation by Tetradesmus obliquus and Raphidonema monicae: growth performance and biochemical composition from lab to industrial scale

To meet the demands of the growing population, agricultural practices have been increasing and putting a strain on land and freshwater usage. Soilless agriculture has emerged as a more sustainable practice to mitigate this issue but still generates nutrient-rich drainwater that can harm the environment if not properly managed. In this context, this study explores a circular economy approach to reuse the drainwater of soilless farming as a culture medium for microalgae production. For this, the growth performance of four strains, Chlorella sp., Nannochloropsis limnetica, Raphidonema monicae and Tetradesmus obliquus, was assessed using drainwater under summer and winter conditions at lab-scale. Based on productivity, protein and polyunsaturated fatty acids (PUFA) contents, T. obliquus and R. monicae were selected for comparison in pilot-scale flat-panel photobioreactors (FP-PBR) and raceway ponds (RW). T. obliquus presented significantly higher growth in FP-PBR (0.11 g L⁻¹ d⁻¹) compared to RW (0.09 g L⁻¹ d⁻¹), with complete nitrate removal in both systems. R. monicae showed similar growth across systems, removing 19 % of nitrate in FP-PBR and 56 % in RW. Principal component analysis indicated species-specific traits drive biochemical profiles, with limited influence from the cultivation system. Both species were cultivated in 19-m³ tubular photobioreactors, with improved productivities (T. obliquus with 0.23 g L⁻¹ d⁻¹ and R. monicae with 0.13 g L⁻¹ d⁻¹) until stationary phase or legal nitrate limits, yielding about 20 kg of dry weight each. The biomass produced in drainwater was biochemically characterized, showing it was rich in proteins (>30 %), PUFA (>55 %) and phenolics, highlighting their potential application in various sectors, including aquaculture and agriculture. Overall, this study demonstrates the potential of these strains for drainwater treatment, promoting a circular economy by converting waste into valuable biomass.