A Detail Analysis of the Performance of Different Antiscalants in Calcium Sulfate Supersaturated Brackish Water

A Detail Analysis of the Performance of Different Antiscalants in CaSO₄ Supersaturated Brackish Water

Anthony Tarquin, PhD.; Guillermo Delgado, PhD Candidate

University of Texas at El Paso

Reverse osmosis (RO) desalination is an economic method to remove dissolved solids from brackish water.  However, the presence of sparingly soluble salts in the water (e.g. CaSO₄, CaCO₃) reduces the recovery of these systems.  If the solubility limit of these salts is reached, precipitation is likely to occur causing fouling and scaling of the membranes.  It is at this point where the use of antiscalants greatly improves the recovery of desalination systems. 

This research presents the evaluation of different antiscalants in brackish water supersaturated with calcium sulfate.  A total of eight different antiscalants from five different manufacturers were tested.  Each antiscalant was tested at different calcium sulfate concentrations and different antiscalants dosages.  Brackish water from Alamogordo, New Mexico was used for this project.  The brackish water was saturated using 1 Molar solutions of sodium sulfate and calcium chloride.  The efficiency of the antiscalants was measured by the extent that the antiscalants increased the induction time of calcium sulfate precipitation in the water.  The turbidity of the water was monitored in time intervals to identify the time of calcium sulfate precipitation.  A saturated solution without antiscalant served as a control.  The results showed that the type of antiscalant is an important factor that significantly affects the induction time of calcium sulfate precipitation.  Figure one shows the results from one test done using different types of antiscalants.

Figure 1: Antiscalant Performance at 0.03 M CaSO₄ Concentration and 0.8 ppm of Antiscalant

Additionally, the results showed that the dosage of the antiscalant is a factor for increasing induction time.  However, the results showed that antiscalants are not efficient at high supersaturation concentrations (0.1 M) of calcium sulfate, no matter the dose.  In the same way, the results showed that antiscalants only increase the induction time of calcium sulfate precipitation, but they do not increase the solubility of calcium sulfate.  Figure 2 shows the results of a test done using different concentrations of the same antiscalant.

Figure 2: Different Concentrations of the Same Antiscalant at 0.05 M CaSO₄ Concentration.

The selection of the best antiscalant is a very important step during the design of a membrane desalination system, especially batch processes such as the CERRO process.  This research showed that the selection process should include factors such as type and dosage of antiscalant.  Recovery rates can be increased and operation costs can be reduced by optimizing system design.