Remoção de Microcystis aeruginosa em água de manancial de abastecimento com a aplicação de coagulantes químico e natural

Abstract

The excess of nutrients in water bodies, mainly nitrogen and phosphorus, is an extremely important factor for increasing the density of algae and cyanobacteria in a water body. The presence of these organisms tends to reduce the efficiency of water treatment, requiring, among other things, the adoption of a higher dosage of coagulant, which are normally chemical coagulants. This research consisted of evaluating the performance of the polyaluminum chloride (PAC) chemical coagulant and a tanninderived natural coagulant (TANFLOC SG), in the removal of cyanobacterial cells of the species Microcystis aeruginosa, in natural waters, when used alone and in an association. For this, samples with an approximate cell density of 5,0×105 cells/mL were subjected to treatment using dissolved air flotation (DAF) as a clarification process followed by filtration. For the coagulation/flocculation process, chemical and natural coagulants were applied separately, each at its optimal dosage previously determined (PAC, 16 mg/L and Tanfloc SG, 8 mg/L). Then, tests were carried out using the associated coagulants, in proportions of 25%, 50% and 75% (Tanfloc:PAC = 100:0, 75:25, 50:50, 25:75 and 0:100). As a result, for the isolated application of coagulants, very similar removals were obtained, in the order of 87% for turbidity and 94% for cell density, both for PAC (16 mg/L) and for Tanfloc SG (8 mg/L), distancing slightly only in color removals, approximately 91% with PAC (16 mg/L) and 84% with Tanfloc (8 mg/L). As for the association of coagulants, a slight improvement was identified in relation to treatments with the isolated application of PAC and Tanfloc, with the best efficiency being obtained in the highest proportion of Tanfloc (75%, in 25% of PAC), with the removal of approximately 92% turbidity, 89% color and 97% relative to cell density. Thus, the use of DAF proved to be quite efficient in the treatment of samples containing M. aeruginosa cells, as well as the complementary filtration step, which was essential at the end of the treatment, in some cases contributing to an increase of 10 % in the removal level of the evaluated quality parameters. Therefore, depending on the efficiency of the treatment, the option is to use the natural coagulant studied as a partial or total substitute for the chemical coagulant, in the treatment of water containing M. aeruginosa cells by the processes of coagulation/flocculation/DAF/filtration.