Clarificação de águas de lavagem de filtros em estações de tratamento de água por coagulação, floculação e flotação por ar dissolvido

Abstract

This paper presents results of coagulation, flocculation and dissolved air flotation studies that were conducted from bench experiments, validated in a pilot-unit. The experiments were conducted with the purpose of promoting the clarification of filter backwash water (FBW) of a Water Treatment Plant (WTP). The coagulation parameters were investigated from pH adjustments, aluminum chloride dosage and mixing time. The effectiveness of the coagulation was analyzed from the buoyancy of the aggregates. The flotation tests were investigated from the bench saturation efficiency and recirculation rate. The results indicated that the pH and coagulant concentration represented critical conditions in the coagulation step. The best responses were obtained with aluminum 8 mg.L-1, by the use of polyaluminium chloride (PAC). The analysis of the null load point of the ALF microflocs generated after addition of CPA allowed to identify which mechanisms additive to the charge neutralization acted in the coagulation-flocculation system. The pH variation of ALF conditioned with 8 mg.L-1 of Al+3, highlighted the formation of hydrolyzable species and the high activity of colloid charges indicated the occurrence of different mechanisms of the charge neutralization in the coagulation. The maximum clarification in the pilot unit occurred with a dosage of 12 mg.L-1, recirculation ratio of 10%, flocculation time of 15 min and pH 6.5. Flotation presented a significant potential for the FBW, especially regarding the issue of solids removal from the system. More than 11 kilograms of solids can be removed from the treatment system in each backwash cycle compared to sedimentation, which would imply a 33% increase in the time between backwashes and the economy of the station. These benefits can impact water treatment plants in Brazil and worldwide.