Overfeeding SO2 causes DO?

• A. increase
• B. lower
• C. stay the same
• D. fluctuate

Answer: (B)

Excess sulfur dioxide (as sulfite) acts as a reducing agent and is oxidized by the dissolved oxygen in the water, forming sulfate. That reaction uses up dissolved oxygen, so the DO level decreases when SO2 is fed in too much. In dechlorination, SO2 is used to neutralize residual chlorine, but overfeeding it pulls oxygen out of the water rather than adding it, leading to lower DO. The other options don’t fit because the chemical process does not release oxygen or stabilize DO; it consumes it, so DO drops rather than increases or staying the same.