Problem 8.5: A water treater says Our LSI is negative so we

From Chapter 8 Problem Set. Level 2: Applied Thinking.

Question

A water treater says, “Our LSI is negative, so we don’t have a scale problem.” The makeup water contains 80 mg/L silica and the tower is running at 5 cycles. Why might the treater be wrong?

Answer

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LSI measures only CaCO₃ scaling pressure. At 5 cycles, silica concentration reaches 80 × 5 = 400 mg/L - far above the ~150 mg/L practical solubility limit. Silica polymerization and deposition are likely, regardless of LSI. A negative LSI means the bulk water is not biased toward calcium carbonate precipitation. It does not mean the system is safe from all forms of scale.