Uso de microalgas na remoção de nitrato em águas subterrâneas contaminadas

Abstract

Nitrate is the most frequent contaminant in aquifers around the world. Anthropogenic activity is the factor that most contributes to the contamination of water by the compost. Among the sources of contamination, the inappropriate discharge of domestic wastewater is one of the most relevant. In Brazil, sewage collection networks are absent and the widespread use of the septic tank and sinks as an alternative is an aggravating factor in terms of groundwater contamination. In this context, the present research aimed to evaluate the potential of using the microalgae Tetradesmus sp. in the remediation of groundwater contaminated by nitrate, nitrite, and ammonia, considering the influence of nutritional and environmental factors in the process. The experiments were set up in batch and semi-continuous, with natural and forced aeration (0.5vvm atmospheric air), light intensities between 50 and 200 μmol m-2 s-1, pH range 8.0 - 8.5, initial concentration of nitrate between 50 - 400 mg/L, nitrite between 0 - 16.8 mg/L, ammonia between 0.0 - 9.0 mg/L, total phosphate between 0 - 60 mg/L with synthetic effluent and real, and cultivation time between 3 and 15 days. The natural aeration, to provide CO2 to the system, did not get a satisfactory nitrate removal, showing the need to use forced aeration in the system. In the batch experiments, either with the modified BG-11 culture medium or with the real effluent, when forced aeration and 100 μmol m-2 s-1 of light intensity were used, the removal of nitrate content was up to 100%, and total phosphate of up to 94%, being below the maximum limit established by Brazilian legislation (45 mg/L of nitrate and 3.2 mg/L of phosphate), When conducted under forced aeration and light intensity of 100 μmol m-2 s-1 (also used in the semi- continuous process). For the experiment carried out in semi-continuous, volumetric replacement rates of 20, 40, 60, and 80% were tested at 3-day intervals and, until the third cycle, a satisfactory remotion of contaminants was obtained for rates of 40 and 60%. However, at a volumetric replacement rate of 20%, stability was not achieved for the production of biomass in the bioreactor, while using 80%, the removal of contaminants was not efficient. Thus, the results show the potential of Tetradesmus obliquus LCE-01 for remediation and adequacy of contaminated groundwater to current potability standards, mainly in terms of nitrate levels.