Desenvolvimento e aplicação de unidades de desfluoretação para tratamento de água

Abstract

Fluoride is an element present in groundwater and is associated with health benefits, however, in concentrations, generally above 1.5 mg L-1, these benefits can become risks. So that water with excess fluoride can be used, there are several consolidated technologies that can efficiently fulfill this defluoridation step. However, these treatments can result in costs that are incompatible with the operation of a water treatment plant, requiring a precise assessment of the characteristics of the environment which one intends to use. Thus, this study is directed to the evaluation of different defluoridation mechanisms in order to obtain efficient equipment that can be used in the production of water. The first treatment group was made with the use of coagulants. This is usually used in industrial effluents with a high amount of fluoride, in which high dosages of the coagulant are applied. Thus, studies with the application of coagulants sought to evaluate this application for the defluoridation of groundwater with a fluoride concentration of approximately 3.5 mg L-1. For this, hydrodynamic and efficiency hydraulic studies were carried out from the hydraulic mixture in a Helical Tubular Flocculator (HTF), through continuous treatment in the idealized treatment unit. The experimental investigation was divided into the following stages: (I) evaluation of the type and dosage of aluminum sulfate and calcium chloride coagulants; (II) determination of mixing conditions in the (HTF); (III) comparison with mechanical mixing; (IV) application of coagulative bubbles in the HTF. Regarding the first group, (I) the lowest dosages required for fluoride reduction, in accordance with the legislation, were obtained with Al (approximately 30 mg L-1), while removal with Ca was effective at dosages greater than 160 mg L-1; (II) the HTF sizing parameters revealed that the unit without distance between the rings generated removals close to 70% of the initial fluoride concentration, from a concentration of 30 mg L-1 of Al; (III) the mechanical mixture needed more agitation energy. While the G.t required by the HTF for 65% fluorine removal was 8500, the mechanical system required a G.t of 18000 and resulted in 45% F- removal. The required Al dose is also a significant advantage of HTF. From 20 mg L-1, the hydraulic mixture removed 55% of the fluorine. As for the mechanical mixture, there was a greater demand for coagulants; (IV) coagulative bubbles were generated from a mixed system composed of amphoteric surfactant (Cocamidopropyl betaine, CAPB) and inorganic polymeric coagulants (aluminum sulfate and calcium chloride). The critical micellar concentration (CMC) of CAPB was 0.26 mmol L-1. Coagulating bubbles (BC) were generated by pressurizing the coagulant solution in distilled water and continuously applied to the HTF. The CB treatment method provided greater fluoride removal compared to conventional treatment with simple coagulant application. BCs generated with Al enabled the removal of approximately 98% of the initial amount of fluorine (3.5 mg L-1) with a dosage of 31 mg L-1 of Al; feed flow of 20 L min-1; pH 6±0.5; and saturation pressure of 3 atm. The second treatment was carried out by adsorption with activated alumina (V); in addition to a comparative study with application of Al in FTH (VI), both procedures on a pilot scale. This step was also intended to fulfill a partnership between the Federal University of Santa Maria and the Riograndense Sanitation Company (CORSAN). There was a need to develop an adsorption equipment that could be compact and efficient. The built unit has three filters in series and pH and fluoride control devices. In the filters, 40 kg of alumina were added and the ideal flow condition of 500 L h-1 (V). The results showed that the use of coagulation-precipitation presents better application conditions in different fluoride concentrations. This denotes that a careful evaluation of the adsorbent must be carried out, in view of its rapid saturation and, therefore, a demand for regeneration/replacement, which may result in excessive costs (VI).