Question 1

Why use 1.46 instead of 1.0 for sodium bisulfite?

Accepted Answer

The molecular weight ratio of NaHSO₃ (104 g/mol) to Cl₂ (71 g/mol) is 1.46, so 1.46 mg of bisulfite is needed per mg of chlorine removed.

Question 2

How much SO₂ does it take to remove chlorine?

Accepted Answer

Theoretically 0.9 mg SO₂ per mg Cl₂. Most plants dose 1.0–1.1 mg/mg to ensure complete dechlorination.

Question 3

Will dechlorination remove chloramines?

Accepted Answer

Yes. SO₂ and bisulfite reduce both free chlorine and chloramines, though chloramine reduction is slightly slower and may need extra contact time.

Question 4

What is the NPDES dechlorination limit?

Accepted Answer

Most NPDES permits cap total chlorine residual at 0.1–0.5 mg/L at the outfall. Some sensitive waters require non-detect (<0.02 mg/L).

Dechlorination Calculator – SO₂ & Sodium Bisulfite