Problem 8.7: A tower uses sulfuric acid for pH control

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

Question

A tower uses sulfuric acid for pH control. Makeup sulfate = 50 mg/L. At 6 cycles, tower sulfate is measured at 480 mg/L - significantly higher than 50 × 6 = 300 mg/L. Why is the measured value higher than predicted?

Answer

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Sulfuric acid (H₂SO₄) adds sulfate to the system in addition to what enters with the makeup water. At each cycle, both the makeup sulfate and the acid-contributed sulfate concentrate. The measured value is higher because the acid feed is introducing sulfate ions not present in the makeup analysis. This is why CaSO₄ risk must be checked in any acid-fed system.