Hydroponic Nature-Based Wastewater Treatment: Changes in Algal Communities and the Limitations of Laser Granulometry for Taxonomic Identification

The increasing need for sustainable wastewater treatment technologies has accelerated the development of Nature-Based Solutions (NBS), including hydroponic systems applied as tertiary treatment. This study aimed to assess changes in algal species composition in hydroponically treated municipal wastewater and to evaluate whether laser granulometry can be used as a rapid tool for preliminary identification of algal taxa. The experiment was conducted in a static hydroponic system with three macrophyte species (Pistia stratiotes, Limnobium laevigatum, and Myriophyllum verticillatum) under white and red–blue light conditions. Microscopic identification was compared with indirect indicators such as chlorophyll a concentration and particle size distribution (D-values) obtained using laser granulometry. The results showed a substantial reduction in cyanobacteria and a shift towards diatoms and green algae, demonstrating the ecological benefits of hydroponic NBS. However, regression analysis revealed no significant correlation between algal cell volume and D(3.0) or D(4.3) values (R2 < 0.06, p > 0.38), excluding the use of granulometric data for taxonomic purposes. This limitation complicates monitoring of potentially harmful cyanobacteria in effluent and may necessitate additional algal removal before discharge