Stiff-Davis Saturation

Langelier Saturation Index is valid up to about 4,000 ppm TDS. At higher TDS concentrations, the Stiff-Davis Saturation Index (SDSI) is used, Equation 3.12 ... 

The program calculates total dissolved solids (TDS), conductivity, and the Langelier and Stiff-Davis Saturation Indexes (see Chapter 3.9). The screen allows the designer to use sodium, calcium, magnesium, chloride, sulfate, or bicarbonate to balance the water analysis. Water quality for up to five feed streams can be entered and blended together to make the total, combined feed water to the system. 

Other indices proposed that have tried to overcome the basic limitations of LSI include the Stiff and Davies Saturation Index, Larson and Buswell Index, Puckorius (Practical) Scale Index, Oddo-Tomson Index, and Larson-Skold Corrosivity Index, some of which are discussed briefly here. 

The Stijf-Davis index attempts to overcome the shortcomings of the LSI with respect to waters with high total dissolved solids and the impact of "common ion" effects on the scale formation driving force. Like the LSI, the Stiff-Davis index has its basis in the concept of saturation level. The solubility product used to predict the pH at saturation (pHs) for a water is empirically modified in the Stiff-Davis index. The Stiff-Davis index will predict that water is less scale forming than the LSI calculated for the same water chemistry and conditions. The deviation between the indices increases with ionic strength. Interpretation of the index is by the same scale as for the LSI. 

The calcium carbonate group includes calcite saturation level, the Langelier saturation index, the Stiff-Davis index, and the Oddo-Tomson index. 

The scaling tendency of a given feed water is often evaluated using the Langelier Saturation Index (LSI) for brackish water and the Stiff and Davis Stability Index (S DSI) for seawater (LSI is acceptable for seawater). LSI and S DSI are defined below ... 

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