Desalination of geothermal waters using a hybrid UF-RO process. Part II: Membrane scaling after pilot-scale tests

Abstract This paper presents the results of membrane scaling studies obtained after eight months of desalinating geothermal water in an integrated system that combines iron removal, ultrafiltration and two independent reverse osmosis systems fitted with low-pressure brackish water reverse osmosis membranes that are connected in series. Scaling prediction conducted using geochemical modelling showed that at a temperature of 30 °C and for neutral-pH water from the geothermal intake (GT-1 well) there were good conditions for the precipitation of aragonite, barite, calcite, chalcedony, dolomite, hydroxyapatite and quartz on the reverse osmosis membranes. A theoretically selected phosphonate scaling inhibitor failed to protect the membranes against sediment precipitation. Adjusting the pH of the water to ca. 5 by treating it with hydrochloric acid had a positive effect on the desalination process. On the basis of a morphological evaluation of sediments in the electron scanning microscope image and through the analysis of the chemical composition of the sediments crystallised on the membrane, silicates (SiO2) and aluminosilicates (Al2SiO5) as well as barite (BaSO4) and copper sulphate (CuSO4) crystalline structures were found. Calcium phosphate as well as celestite (SrSO4) and strontianite (SrCO3) sediments were also observed, which points to significant concentration polarisation. Most grains were a few to a maximum of 30 μm in size. Sediment clusters were usually heterogeneous in character. Desalination was performed with a high permeate recovery ratio (75–78%) with favourable key indicator retention ratios.

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