Avaliação de técnicas convencionais de tratamento de água para consumo humano na remoção de cianobactérias e cianotoxinas e processos oxidativos para remoção de microcistinas

• Autor(a) principal: Morais, Anderson de Assis
• Data de Publicação: 2012
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/830
• topic: Diagrama de coagulação; Decantação; Filtração; Lodo; Oxidação; Sulfato de alumínio; Diagram coagulation; Decanting; Filtration; Sludge; Oxidation; Aluminum sulfate; CNPQ::ENGENHARIAS::ENGENHARIA SANITARIA::TRATAMENTO DE AGUAS DE ABASTECIMENTO E RESIDUARIAS

Bench scale and pilot scale assays were performed in order to simulate blooms of the cyanobacteria Microcystis aeruginosa NPLJ-4 and Cylindrospermopsis raciborskii T3 and study there removal during conventional water treatment processes. Removals on the order of 6 log units were observed in bench scale assays, with C. raciborskii removed more efficiently than M. aeruginosa, especially during filtration. M. aeruginosa showed similar removals during sedimentation and filtration, with removal of two log units at each step in bench scale assays. In pilot scale assays M. aeruginosa removals achieved 2-3 log units in water typical of the dry season and up to 4 log units in water typical of the rainy season. In all pilot scale assays C. raciborskii was removed to undetectable levels in filtered water, indicating again the better removal of this species than M. aeruginosa. These results indicate the removal of M.aeruginosa cells that are mostly retained in the clarifier and filter, where they may potentially release microcystin to the treated water. Thus, pilot-scale assays were performed with clarifier sludge and filter wash water inoculated with M.aeruginosa and stored in the dark to evaluate release of microcystin to the supernatant. It was found that toxin release began on the first day and reached its peak on the third day, after which it began to fall, reaching non-detectable levels after the tenth day. These results suggest that sludge must be removed daily and stored for at least ten days to minimize the risks due to microcystin. Another result obtained in pilot scale experiments was the low removal efficiency of dissolved toxins, and therefore tests were performed with different oxidative processes aimed at greater removal of dissolved microcystins. Chlorine, chlorine dioxide, ozone, ultraviolet light and heterogeneous photocatalysis were tested to evaluate whether microcystin degradation byproducts from such processes are toxic to the aquatic organism Daphnia similis. Ozone, ultraviolet light and heterogeneous photocatalysis presented high microcystin removal efficiencies, but toxicity was increased in tests with ozone and heterogeneous photocatalysis, indicating the formation of toxic byproducts that require further investigation. Ozone was the most efficient oxidant, with removals greater than 99%, with CT values of 10.5 mg min L-1. Removals above 99% were observed for chlorine with CT values of 128 mg min L-1. Ultraviolet light and heterogeneous photocatalysis showed similar removals, above 97%, in a contact time of 15 minutes. Chlorine dioxide, among the oxidants used, showed the least removal efficience, with removal of 89% in CT of 677 mg min L-1. In the former tests acute toxicity to Daphnia similis was observed, with preliminary results suggesting that chlorite, a byproduct of chlorine dioxide degradation, was the origin of this toxicity.