Avaliação em escala de bancada do emprego de carvão ativado granular na remoção de microcistina-LR na potabilização de águas eutrofizadas do semiárido nordestino

Abstract

Usually, conventional water purification technology with eutrophic waters (high density of cyanobacteria and dissolved toxins) does not guarantee that the water produced is within the maximum allowable concentration of 1.0 µg/L of microcystin (Decree 2914/2011). Additional processes are needed, and the use of activated carbon is considered effective to reduced or eliminate those toxins. The present study aimed to evaluate in a bench-scale the removal of microcystin-LR in water supplies using conventional treatment followed by adsorption with granular activated carbon. The experiment was conducted in two stages. In the first stage coagulation assays were performed with the evaluation of five coagulants (with and without the use of polymers as auxiliaries) in order to select the most appropriate coagulant and its concentration to the water under investigation. The second stage were coagulation / flocculation tests and sedimentation using the conditions defined in the first step with raw water with an addition of a defined concentration of microcystin-LR extract. The extracted was obtained from the culture of a toxigenic strain of Microcystis aeruginosa. The final effluent was decanted in the jar test and stored. Two peristaltic pumps adduced the reserved water to the sand filters; the sand filter effluent was distributed to two columns of granular activated carbon with different particle size (0.42 to 1.40 and 0.60 to 2.36 mm). The results of coagulation assays showed the best results of removal of apparent color, turbidity, phytoplankton and particular true color, which is a strong competitor for the adsorption sites of activated carbon, with the polyaluminum chloride (dosage of 9.92 mg L -1 of Al 3 + and pH between 5 and 6, without the addition of alkalizing or acidifying). The conventional treatment steps were not efficient for microcystin-LR removal while the granular activated carbon from coconut showed high percentages of removal (80 to 100%). Although no statistically significant differences were observed in the removal of MC-LR between the different particle size, the had breakthrough coal to the smaller particle size (CC2) occurred in a shorter contact time (2 hours) which eventually reflected in a lower qe (2.1 µg/g) and a higher rate of use (8.09 gCAG / Lwater) when compared to the smaller particle size (CC1) that showed the best performance both in terms of qe (8.9 µg/g) as the rate use of (1.93 gCAG / Lwater) confirming its use in actual scale, since it ensures an effluent concentration below the Decree 2914/11 for a longer time and using a smaller quantity of coal.