Livestock wastewater bioremediation through indigenous microalgae culturing as a circular bioeconomy approach as cattle feed

Growing human population demands the enhancement intensive livestock activities, which lead to severe environmental issues like water pollution. Cattle wastewater is rich in some inorganic pollutants, mainly nitrogen and phosphorous, which might constitute a risk to water bodies. Considering this scenario, microalgae's capacity to develop within manure wastewater is attracting interest, mainly for simultaneous wastewater bioremediation and biomass production. The aim of this work was evaluating the response of several microalgae strains to increasing percentages of unsterilized cattle farm wastewater. Moreover, this study also analyses the possibility of enhancing the development of indigenous native microalgal strains, to create a bioremediation system based on previously adapted species, which specific diversity was determined through taxonomic techniques. Two monospecific Chlorophyceae strains were used: Chlorella sp. and Coelastrella cogersae; and one Cyanobacteria strain: Arthrospira platensis. Experimental assays were conducted in 6 groups and triplicates with increasing percentages of Livestock wastewater (LW): control, 15, 25, 35, 50 and 70 %. All the strains tested showed that cattle wastewater represented a source of metabolic stress for microalgae development. Nevertheless, the autochthonous microalgal bloom showed better results in terms of tolerance to wastewater percentage and development under same conditions Laboratory scale results showed that microalgae could develop with optimum results in presence of 15 % of wastewater, reaching absorbance values close to 90 % of the achieved by control cultures. Moreover, indigenous microalgae cultures were scaled up to open type industrial systems (raceways) and influence of their growth on wastewater composition was evaluated. Results showed a severe reduction of the concentration of nitrates to 43.4 (mg center dot L-1) (20 % of initial measurements). Moreover, the obtained biomass showed a biochemical profile similar to traditional crops, in terms of proteins and carbohydrates (48 and 41 % respectively), suggesting that this biomass could be further valuated into cattle feed as a circular economy approach.