Avaliação da redução de sílica em pré-tratamento de águas para sistemas de pequeno porte por osmose inversa.

Abstract

The presence of silica in natural waters constitutes one of the largest problems in terms of incrustation of the surfaces of membranes during the desalination process by reverse osmosis. There is a need for effective pretreatment since incrustation by silica limits the efficiency of the desalination process, compromises the effective life of the membranes and it is practically impossible to be removed. A survey of the literature reveals no really effective anti-incrustation reagent against silica due to its changeable nature in solution. In order to contributing to the prevention of this type of incrustation, a study and an evaluation were undertaken of chemical pretreatment based on sodium hydroxide and hydrochloric acid applied to a small desalination system. The work was performed in two phases, bench-scale and pilot-scale, using synthetic solutions and also natural waters from the localities of Uruçu, Marinho, Inga and Galante all in the State of Paraíba, Brazil. During the bench-scale, synthetic solutions were prepared of sodium metasilicate nono-hydrate, mixed with calcium and magnesium salts with a view to better understanding the silica incrustation processes. With this in mind, various parameters were evaluated such as the initial concentration of silica, total hardness, the relationship between the concentrations of calcium and magnesium and the settling time of the precipitate. Natural waters were also evaluated at bench-scale to optimize operational conditions to be subsequently used at a pilot-scale plant. During this phase the feed-water flow rate to the system and the dosing concentrations of sodium hydroxide and hydrochloric acid were analysed. The pilot-scale desalination system (SDP) was designed and constructed at LABDES, Federal University of Paraíba, Brazil which houses various small-scale desalination systems. From the results obtained the following conclusions could be made: i) The total hardness and relationship between calcium and magnesium had a major influence on the silica incrustation process. ii) The level of silica removal increased with the concentration of sodium hydroxide in the medium. iii) The physico-chemical characteristics of the feed water affected the process such that 88.7% silica removal was obtained with water from Uruçu and 48.5% with water from Galante for the same operational conditions. iv) The silica removal process with sodium hydroxide was shown to be effective for small desalination systems.