Biophotoelectrochemical strategies for nutrient recovery from wastewater

Wastewater management is evolving from the sole removal of pollutants toward resource recovery within a circular economy framework. Purple phototrophic bacteria (PPB) represent a promising platform due to their metabolic versatility, i.e. their ability to use different electron donors and inorganic nitrogen sources, while generating a high-value biomass that can be exploited, for example, as a source of microbial protein. This thesis is part of the Spanish national project BEER4ALL, which aims to develop sustainable processes for nutrient recovery, particularly organic carbon and nitrogen, which can be valorized into bioplastics and fertilizers, respectively. A mixed PPB culture was investigated under illuminated anoxic conditions, evaluating the effect of ammonium and nitrate in combination with different electron donors: acetate, gaseous hydrogen, or a cathode under potentiostatic control at -0.6 V vs Ag/AgCl. In the presence of acetate, ammonium was observed to be mainly assimilated, resulting in higher biomass production and carbon consumption closely associated with growth. In contrast, nitrate enabled effective carbon and nitrogen removal, but led to lower biomass accumulation, suggesting a redistribution of metabolic fluxes toward dissimilatory pathways. In the absence of organic carbon, hydrogen supported photoautotrophic growth, although to a limited extent, plausibly due to gas-liquid mass transfer constraints and the need for an acclimation period. Under the same conditions, in the presence of a polarized cathode, sustained cathodic currents were recorded, whereas they were absent in abiotic controls; however, this did not result in biomass increase or significant nitrogen uptake. Future research should address electron transfer mechanisms, culture acclimation, and potential modulation. Nevertheless, the results highlight the potential of PPB for integrated strategies combining biomass valorization and nitrogen removal.