This project aims to investigate the potential use of thermally treated seagrass P. Oceanica to recover phosphate from wastewater and the production of a fertilizer substitute. Posidonia Oceanica residues were thermally treated at 550o C for 15 minutes and were tested as an adsorbent of phosphate for water collected from Finikaria dam. Results shows that thermally treated seagrass could be applied as adsorbents for Phosphate from synthetic and waster from dam (high in phosphorous), with 98% and 97% adsorption efficiency, respectively. Finally, by using thermally treated P. Oceanica, which was exposed to phosphate, as a potential fertilizer to lens it seems to positively affect their growth.