Page 38 - ro membanes

2.4.1 Description
Source Water Factor
Source water TDS concentration increase
Ca, Mg, Ba, Sr, Si, F, SO4, CO3, HCO3 concentration increase
pH increase
Temperature increase
RO system recovery increase
Impact on Scaling Potential of Source Water
Decrease in scaling potential
Increase in scaling potential
Increase in scaling potential
Increase in scaling potential
Increase in scaling potential
Note
Seawater typically does not scale at ambient pH and temperature <35C if plant recovery is <45%.
CaCO3 is common scale in BWRO plants while CaSO4 and MgSO4 are common scales in SWRO plants.
Increase of seawater above 8.6 (pH > 8.6) accelerates scaling.
Scaling accelerates significantly when temperature exceeds 35C.
Scaling potential is low if recovery <45%.
2.4 MINERAL-SCALING FOULANTS 21 2.4 MINERAL-SCALING FOULANTS
All minerals contained in the saline source water are concentrated during the process of membrane salt separation. As their concentration increases during the desalination process, ions of calcium, magnesium, barium, strontium, sulfate, and carbonate can form salts, which could precipitate on the RO-membrane surface. The mineral scales that typically form during desalination are those of calcium carbonate, calcium and magnesium sulfate, and barium and strontium sulfate.
The potential for formation of these compounds (e.g., the scaling potential of source water) depends on their content in the concentrate flow stream, water temperature, pH, desalination plant recovery, and other factors. Table 2.3 indicates the impact of the key source water qual- ity and operational factors on the scaling potential of the saline source water.
Formation of mineral scales on the membrane surface is balanced by the high salinity of the source water, which tends to increase the solubility of all salts. This means that the higher the salinity of the source water, the less likely mineral scale would form on the membrane surface at typical saline water pH of 7.6e8.3 and desalination system recovery of 45%e50%.
In brackish water desalination systems that typically operate at much higher recoveries (65%e85%) and use source waters that have relatively lower ionic strength, mineral scaling is a frequent problem.
In typical seawater desalination systems operating at recovery in a range of 40%e50%, mineral-scale fouling is usually not a challenge, unless the seawater pH has to be increased to 8.6 or more in order to enhance boron removal by the RO membranes.
Calcium carbonate is the most commonly encountered mineral foulant in brackish water desalination plants. Calcium sulfate and magnesium hydroxide are the most frequent causes of SWRO membrane scaling. Scale formation can be prevented by addition of antiscalant/ dispersant to the source water.
TABLE 2.3 Factors Impacting Scaling Potential of Saline Source Water