Abstract:The use of microorganisms to clean up wastewater provides a cheaper alternative to the conventional treatment plant. The efficiency of this method can be improved by the choice of microorganism with the potential of removing contaminants. One such group is photosynthetic bacteria. Rhodobacter capsulatus is a purple non-sulfur bacterium (PNSB) found to be capable of different metabolic activities depending on the environmental conditions. Cell growth in different media and conditions was tested, obtaining a concentration of about 10 8 CFU/mL under aerobic-dark and 10 9 CFU/mL under anaerobic-light conditions. The biomass was then used as a bioremediation agent for denitrification and nitrification of municipal wastewater to evaluate the potential to be employed as an additive in biological wastewater treatment. Inoculating a sample of mixed liquor withdrawn from the municipal wastewater treatment plant with R. capsulatus grown in aerobic-dark and anaerobic-light conditions caused a significant decrease of N-NO 3 (>95%), N-NH 3 (70%) and SCOD (soluble chemical oxygen demand) (>69%), independent of the growth conditions. A preliminary evaluation of costs indicated that R. capsulatus grown in aerobic-dark conditions could be more convenient for industrial application.