Characteristics of desalinated water in reverse osmosis plants in Spain

Abstract

In this study on desalinated water characteristics, the mean total dissolved salts content is 280 mg/L, of which approximately 72% is NaCl. The HCO3 – (<4 mg HCO3– /L), Ca2+ (2.5 mg Ca2+/L) and Mg2+ (<4 mg Mg2+/L) contents are all low. The Mg2+ content is approximately twice that of Ca2+, both expressed in mg/L. The pH at the outlet of the reverse osmosis racks ranges from 5.2 to 6.7, depending on the plant. CO2 content varies between 0.8 and 16 mg CO2 /L. Certain waters may move outside this range because of specific treatments. Permeates with high pH are related to the need to increase boron rejection. The data obtained corroborate the logarithmic ratio established in the literature between the pH of desalinated water and its CO2 content. Desalinated water has a Langelier Saturation Index (LSI) of less than –4, because of which it tends to dissolve calcium carbonate. This also implies the risk of problems of corrosion due to its higher chlorine (60–200 mg Cl/L) and sulfate content (2–5.7 mg SO4 2– /L) compared to its content of bicarbonates (2–4 mg HCO3– /L). The pH of desalinated water quickly bal-ances with the CO2 content of the atmosphere, so that it should be analyzed carefully and preferably in situ. In equilibrium with the atmosphere, the pH of desalinated water is between 6.9 and 7.0 and its CO2 content around 0.3 mg CO2 /L. The LSI remains at around –4. The turbidity of desalinated water is around 0.2 NTU, and higher values may be an indication of problems in the facilities. The sodium adsorption ratio of desalinated water is equal to or higher than 9, indicating that it may be harmful to the structure of agricultural soils and cause problems of toxicity for certain crops due to the excess of Na+ with respect to Ca2+ and Mg2+. Finally, the results suggest that desalinated water generally does not contain sufficient CO2 to permit adequate remineralization without extra input of CO2 .