Simulação em instalação piloto da adequação de ETA convencional para dupla filtração

Abstract

There are water treatment plants (ETA) that operate with hydraulic overload, for example ETA Extremoz, located in the state of Rio Grande do Norte, operates with hydraulic overload of 30% in relation to the project flow. Although the project indicates full-cycle technology, ETA Extremoz operates with direct filtration technology and the structures of the flocculators and decanters act as junction boxes and in this scenario, the construction of the second module in full cycle technology is not feasible economically and technically. The reform of structures currently inoperative to function as pre-oxidation chambers (flocculators) and upflow gravel filters (FAPs), ie, adapting it from full-cycle to double filtration (DF) technology with gravel pre-filter and rapid sand filter (FDA), is an alternative that results in significant financial and spatial savings for a possible expansion of ETA. The objective of this work is to evaluate the operational conditions for the adaptation of ETA Extremoz to double filtration technology in a pilot plant, maintaining and taking advantage of the existing ETA structure and verify the treatment technology for current and future water flow. The research consisted of two phases, the first one evaluated the treatment behavior by the DF technology with flow rates of 190, 210, 230 and 255 m3 .m-2 .dia-1 for FAP and 250, 280, 310 and 340 m3 .m-2 .dia-1 for the rapid sand filters in pilot-plant trials with eight-hour trials. The results obtained in the first phase indicated the FAP 3 as the one with the best performance and that the filtration rates studied did not significantly affect the quality of the filtered effluent. From then on, a more detailed investigation was carried out on the behavior of FAP 3 and fast sand filters, in pilot tests with a filtration rate of 230 and 255 m³.m-².dia-1 for FAP and 310 e 340 m3 .m-2 .dia-1 for the FDA and 24-hour trials. From the analysis of the obtained data, it was concluded that despite the performance similar to the other FAP, the pre-filter FAP3 showed better efficiency and the lower averages of turbidity and apparent color remaining and the FAP1 was the worse performance. The performance of the FDA was statically equivalent, however, because of having a filtration career superior to the others, the FDA3 was the filter with granulometry more appropriate to the treatment of raw water. Turbidity peaks were recorded in the prefiltered water after the intermediate bottom discharges (DFI) and in the filtered water after washing the sand filters. In all tests, the final load loss of the FAP was less than 0.60 m (maximum available load of 240 cm), a result indicating that the hydraulic load expected for the pre-filter could be lower than that indicated in the literature. The elevation of the filtration rate did not influence the performance of the prefilters, however filtration rates between 200 and 300 m3 .m-2 .dia-1 were the chosen range. The major influencer in the total charge loss of FAP3 is the false bottom. The lower layers of FAP3 did not contribute to the removal of turbidity and color. The FAP proved to be an equalizer in the quality of the influent water of the sand filters. For the studied flow rates the grain size of the FDA does not generate different effluents from each other.