Potencial de utilização de vinhaça como meio para o cultivo da microalga Chlorella vulgaris : aspectos toxicológicos e microbiológicos

Abstract

Microalgae have been cultivated worldwide to obtain biomass for various applications. Nonetheless the production costs of microalgae is still high due to factors such as photobioreactors, water and nutrients to grow the organisms. In recent years the use of wastewaters of several origins for microalgal cultures have been studied. However wastewater generally are more complex than synthetic microalgae culture media. Their characteristics can vary over time and have associated microorganisms such as bacteria, fungi and yeasts. Vinasse is a wastewater generated in large volumes by the ethanol industry of Brazil and it is rich in nutrients and organic materials. The use of this effluent to support microalgae growth can reduce the microalgae production costs at the same time that it decreases the contaminant potential of the effluent through the incorporation of nutrients into biomass. The aim of this research was to evaluate the potential of the use of sugarcane vinasse as culture medium for the growth of the freshwater microalgae Chlorella vulgaris. C. vulgaris is a robust species commonly used in the treatment of effluents. Laboratory tests were initially conducted with two distinct vinasses, a conventional and an originating from a agroecological processing, at different concentrations. Based on these results the conventional vinasse was selected for more detailed studies that aimed at treating the vinasse in defined procedures known as Toxicity Identification Evaluation (TIE). Through such procedure, we attempted to answer why it is so difficult to grow microalgae in sugarcane vinasse and how could the microalgal production be increased in such a complex residue. The results showed that pH adjustment resulted in the highest Chlorella vulgaris biomass in vinasse, whereas the addition of EDTA, sodium thiosulfate and XAD-8 resin to remove specific compounds from the wastewater limited the microalgal final yield. The growth of C. vulgaris was higher at intermediate treated vinasse concentrations (30 - 70%). This demonstrated the possibility of using of this residue as a medium for microalgal cultivation. Once selected the ideal condition for C. vulgaris cultivation (30% vinasse and TIE pH=3.0), there was a screening of the microorganisms present in the vinasse and in non axenic cultures of microalgae after its inoculation with C. vulgaris. The microrganisms were isolated and characterized for indole acetic acid (IAA) production, nitrogen fixation and inorganic phosphate solubilization. The presence of heterotrophic microorganisms in the vinasse did not affect the microalgal production. We obtained promising results about future applications of these microorganisms as growth promoters in microalgal cultures. The biomass produced showed protein productivity of 15.57 mg L-1 day-1 and a decrease of nitrogen, sulfate, biochemical oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC) and total nitrogen (TN) contents in comparison with the initial vinasse was obtained. The results suggest the possibility of using vinasse as a medium for the cultivation of C. vulgaris, with decrease of the potential contaminant of the effluent and increase of biomass production when compared to synthetic culture medium.