Tratamento intermediário de concentrado de osmose inversa por precipitação para aumento da recuperação de água na dessalinização

Abstract

The membrane separation processes (MSP) have been widely used for water and wastewater desalination, due the inherent advantages of these processes. One of the challenges of these technologies application includes integrating treatment processes with the aim to reduce the effluent generation, the concentrate, and increase water recovery. In an oil refinery, the reverse osmosis (RO) unit, classified as a MSP, takes part of the high purity water treatment used in the steam generation (boiler water). The concentrate generated from this unit has no reuse, because of its high content of total dissolved solids and hardness. The objective of this work is to assess the technical feasibility of increasing RO unit water recovery. For this purpose, it was performed the concentrate physical-chemical characterization and its treatment by alkaline-induced precipitation. Firstly, the characterization of the concentrate was performed, weekly, for nine months. From the results, it was evaluated the limitations to the concentrated treatment by two stages of RO, through the Reverse Osmosis System Analysis software (ROSA). It was identified a high potential for fouling by the precipitation of calcium carbonate and silica. In this sense, the experimental design confronted the performance of the alkaline reagents, sodium hydroxide and calcium hydroxide, in the concentrate treatment by precipitation, in laboratory scale. It was used the statistical software Minitab as an evaluation tool of the results. In the first stage of testing, it was desired to define the experimental conditions and to prospect the use of precipitation and the selected alkalizing in the concentrate treatment. In the second step, the effect of the final pH precipitation and the use of seed in the fouling compounds removal were evaluated. In the third stage tests, it was investigated a broader range of final precipitation pH for the calcium hydroxide, in order to support the comparison of the alkaline reagents. It was conclude that sodium hydroxide has the best performance, since it provided the highest removal of Ca2+, Mg2+ and SiO2, with smaller dosage. Finally, treated concentrates by sodium hydroxide induced precipitation, at two final pH conditions, were evaluated as the feed of two stages RO desalination by ROSA simulation. It follows that the final pH control at 10.0 was sufficient to remove Ca2+ and SiO2 enough to provide the reduction of scale forming propensity to acceptable levels for the two new stages RO operation, with 68% water recovery. This finding suggests that may be possible to increase the RO unit global water recovery from 75% to 92%.