Recirculação de água de lavagem de filtros em escala piloto: uma contribuição para avaliação dos perigos associados à presença de protozoários / Recycling of filter backwash water in pilot scale: a contribution for evaluation of the hazards associated to the presence of protozoa

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

Characterization of the filter backwash water (FBW) from the Federal University of Viçosa water treatment plant (UFV WTP), as well as the results from experiments conducted in a pilot scale WTP, suggested that recycling FBW could be regarded as a hazard introduction in the treatment process. The average concentration of Giardia spp. and Cryptosporidium spp. (oo)cysts in FBW was 1.3 to 4.5 times higher than that found in raw water. Aiming at evaluating FBW treatment criteria, experiments using sedimentation columns were conducted with and without polymer application (cationic, anionic and non-ionic), employing different polymer doses and concentrations, as well as mixing times. The use of the cationic polymer - 1 mg . L-1, concentration of 0.1% and 10 minutes mixing time - provided the lowest final concentrations of the various physical, chemical and microbiological parameters analyzed; on the other hand the worst results were obtained when polymers were not applied. In the FBW pilot scale experiments , under variable conditions of raw water and FBW turbidity, recirculation ratios and previous treatment (clarification with and without chemical conditioning), protozoa were detected in the sedimentator effluent in the following situations: (i) in the trial without FBW recirculation, using the WTP design flow; (ii) in the trial with FBW clarification using polymer and recirculation, applying 15% flow excess over the design flow; in this trial both the raw water and the sedimentator effluent presented high turbidity, but FBW turbidity was low (<5 NTU); protozoa were detected both in raw water and clarified FBW.; (iii) in the trial with FBW recirculation without preclarification, 5% flow excess, FBW high turbidity (>60 NTU), presence of protozoa both in raw water and clarified FBW. In filtered water samples protozoa were detected only once, when FBW was recycled without pre-clarification, applying 20% flow excess; in this trial the FBW high turbidity (≈ 60 NTU) doubled the affluent water turbidity; protozoa were also detected in FBW but were absent in raw water samples. The results did not allow conclusive statements regarding FBW water quality (turbidity limits) and recycling flow ratio. Nevertheless, there are suggestions that low values of such parameters would be advisable.

ASSUNTO(S)

treatment recirculation tratamento recirculação tratamento de aguas de abastecimento e residuarias Água para consumo humano water for human consumption

ACESSO AO ARTIGO

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