Drinking Water

Desalination of brackish water - separation of anti-soluble salts through crystallisation in order to prevent incrustation

The procedure developed in this project enables an almost water-free removal of the process concentrate from brackish water desalination plants. The pilot plant consisting of a combination of membrane and evaporation processes serves to characterise the occurring inorganic precipitate. The previous precipitation of calcium sulphate dihydrate and brucite using sodium hydroxide solution proves to be suitable for preventing such precipitate in membrane units.

Summary

In the interior of arid regions, the water supply is mostly based on ground water resources, which mostly consists of brackish water and is thus salty. A major problem in desalinating this water is the cost-intensive removal of residual concentrate and thereby preventing harmful environmental influences.
Therefore, a process was established in this project that enables a nearly water-free removal of process concentrate from desalination plants for brackish water with low energy costs.
For this purpose, a pilot plant was developed with a combined membrane process for water desalination and an evaporation process for reducing the volume of the concentrate. Further, the concentrate from individual process steps was examined considering the solubility of minerals and calculating mineral saturation.
The occurring inorganic precipitates in the membrane units of the pilot plant were characterised, and the causes and possibilities for preventing such malfunctions were developed as well.

Results:

  • Calcium sulphate dihydrate and brucite are the main precipitate products on the membrane of the electro-dialysis (ED) unit on whose surface silicic acid is adsorbed.
  • At pH 2, the ED-concentrate does not exceed the saturation of calcium sulphate dihydrate and brucite. The interlocking of membranes through calcium sulphate dihydrate and brucite scaling is the result of a dysfunction in operational process. The resulting need to exchange membranes increases costs significantly. Therefore, an interlocking of membranes should definitely be prevented.
  • Before discharging in the ED-unit, Mg²⁺ and Ca²⁺-concentrations in the salt concentrate need to be reduced to such an extent that exceeding the solubility product of calcium sulphate dihydrate and brucite is not possible. For this purpose, the specific precipitation of calcium sulphate dihydrate and brucite with sodium hydroxide solution was successfully carried out in laboratory tests. Further, parameters like electrolyte concentration and flow velocity were examined to develop a database for the cooperation partner.
  • Using evaporation tests, a database that contains information on evaporation rates, changes in pH-values, changes in thickness, and changes in electrolyte concentration during the evaporation of ED-concentrates was developed.

More Project Information

Project title: Near zero liquid discharge (ZLD)-Strategie zur Aufbereitung von Brackwässern mittels kombinierter Verfahren (RO-ED-WAIV). TP: Abtrennung schwerlöslicher Salze mittels Kristallisation, Vermeiden der Inkrustation der Membranen (Deutsch-Israelische Wassertechnologie-Kooperation)

Project number: 02WT0775

Project period: 2006 - 2009

Project region: Germany (Baden-Württemberg), Israel

Project contact:

Herr Dr. Weidler
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Source: German National Library of Science and Technology Hannover (TIB)