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22 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT 2.4.2 Measurement Parameters and Methods
Commonly used parameters that can be used to predict source water potential to form mineral scale of calcium carbonate are the Langelier Saturation Index (LSI) and the Stiff and Davis Saturation Index (SDSI). These indexes are function of the source seawater pH, calcium concentration, alkalinity, temperature, and TDS concentration/ionic strength.
Langelier Saturation Index can be calculated using the following equation:
LSI 1⁄4 pH  pHs (2.2)
where: pH is the actual pH of the saline source water;
pHs 1⁄4 ð9:30þAþBÞðCþDÞ (2.3)
A 1⁄4 (Log10 (TDS  1)/10), where TDS is in mg/L; B 1⁄4 13.2  Log10(Temperature þ 273) þ 34.55, where Temperature is in C; C 1⁄4 Log10 (Ca2þ)  0.4, where Ca2þ concentration is expressed in mg/L of CaCO3; D 1⁄4 Log10 (Alkalinity), where Alkalinity is expressed in mg/L of CaCO3.
The value of LSI is indicative of the ability of the source water to form calcium carbonate scale only and is not reflective of formation of other scaling compounds. If LSI is higher than 0.2, the source water is likely to cause slight scaling and if it is above 1.0, the source water would cause severe scaling on the membranes. If LSI is negative, then the water has tendency to dissolve scale.
The LSI index predicts scaling potential of the source water only if its TDS is lower than 4000 mg/L. For saline source waters of higher TDS, the Stiff-Davis Saturation Index (SDSI) is applied. This index can be calculated as follows:
SDSI 1⁄4 pHpCapalk K (2.4)
where: pCa 1⁄4 Log (Ca2þ), where Ca2þ is in mg/L as CaCO3; palk: Log10 (Total Alkalinity), where Alkalinity is in mg/L as CaCO3; K: constant that is function of total ionic strength and temperature.
Values of K, and nomographs and examples for calculation of LSI and SDSI indexes are presented elsewhere (AWWA, 2007).
2.4.3 Threshold Levels of Scaling Foulants
Table 2.4 presents the threshold levels of common mineral scalants in concentrate above which these compounds will begin to accumulate on the membrane surface and form mineral scales (Hydranautics, 2008). The mineral-scaling potential of the saline source water can be determined using proprietary software available from the manufacturers of antiscalants.