(378d) 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.