Produção de biomassa de microalgas utilizando águas residuárias como fonte de nutrientes em meios salobros, para análise da viabilidade de produção de biodiesel.

Abstract

Microalgae biomass has shown great potential for the production of biodiesel, with intracellular lipids being the main raw material. The lipid accumulation by the microalgae may increase depending on the characteristics of the culture media, which may be fresh, brackish, saline or wastewater. It is known that factors such as the salinity of the medium and the concentration of nutrients cause physiological changes, which can potentiate the production of lipids from microalgae biomass. The present work studied the production of biomass, as well as the levels of lipids in microalgae species isolated from the semiarid region of northeastern Brazil, analyzing the potential for the production of biodiesel in this region. As a means of cultivation, it is proposed to use water from wells drilled in the region, with brackish characteristics and, as a source of nutrients, the use of effluents from septic tanks, in order to reduce production costs. The microalgae species Chlorella sp., Scenedesmus acuminatus, Monoraphidium contortum and Pediastrum tetras and cyanobacteria Nannochloropsis sp. in closed crops, in brackish synthetic media and in the mixture of well water and effluent from a septic tank. The species that showed the best growth rates, combined with the highest lipid content in crops using synthetic brackish media, were the microalgae Scenedesmus acuminatus and the cyanobacterium Nannochloropsis sp., Reaching lipid concentrations of 41.96% and 62.04% of dry weight of biomass, respectively. In crops that used brackish well water and septic tank effluent as a source of nutrients in different proportions, the species that showed the highest accumulation of lipids were Chlorella sp., with a lipid content of 81.20% and Nannochloropsis sp. with 73.68%. From the results obtained in closed crops, the production of biomass was scaled on a pilot scale, with open crops, in which Chlorella sp. reached percentages of lipid accumulation between 24% and 51%. In addition to the biomass production potential, nutrient removal percentages were studied in crops that used septic tank effluents, where significant decreases in nitrogen levels were observed, with removal percentages greater than 90%, in addition to the reduction of phosphorus and BOD5. In addition to the removal of nutrients, significant decreases were found in the number of Escherichia coli bacteria in the media, the main indicator of faecal contamination in water bodies. With regard to the biomass separation process, the study studied the feasibility of using microfiltration by ceramic membranes, produced for this research, with low-cost material, found in the northeastern semi-arid region, also aiming to reduce the costs of biomass production. The tests and cultivations performed were quite satisfactory when showing the great feasibility of large-scale production of microalgae in the mixture of brackish groundwater and septic tank effluents, aiming at the production of biodiesel.