Pollutants removal from municipal sewage by means of microalgae

Abstract

Microalgae are microorganisms able to photosynthesize, namely transforming inorganic substrates and sun light into organic compounds and chemical energy. The industry of microalgae has expanded in the last decades and several applications are now developed, making their biomass interesting under an economic perspective. Nannochlopsis gaditana is one of the most interesting species already employed in industry because of its high content in lipids that could be employed as source for biodiesel synthesis but also in other fields such as cosmetic and pharmaceutic. One of the most promising application is the exploitation of microalgal grow for bioremediating wastewaters polluted with inorganic nutrients such as nitrates and phosphates that microalgae are able to employ as nutrients. Bio-treatment of wastewaters by using microalgae has the advantage to reclassify the water and preserve it from wasting while producing a valuable biomass. In this work, a microalgal strain, Nannochloropsis gaditana, was employed for testing its performance in the bioremediation of municipal sewages. The wastewater was taken from a municipal plant, after the primary treatment, and the algae processing was aimed at replacing the secondary treatment. Algal growth in its growth medium and in the sewage was compared and algal biomass was characterized. Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), total nitrogen and total phosphorous levels of the sewage before and after algae treatment were also determined in order to evaluate the efficiency of this microalgal strain on wastewater bioremediation. Our results showed that N. gaditana grows better in wastewater than in the control growth medium and it is able to efficiently remove nutrients from the sewage. However, COD and BOD values did not decrease after algal treatment. These results suggest that the use of selected bacteria and/or yeast strains (together with microalgae) could improve the efficiency of wastewater treatments decreasing BOD and COD values.