Ace the CWEA Grade 3 Lab Analyst Exam 2026 – Dive Into Your Future Success!

To determine the amount of sulfur dioxide that needs to be added, we must first understand the relationship between the chlorine residual and the dechlorination process using sulfur dioxide.

Chlorine and sulfur dioxide have a specific reaction ratio where one part of sulfur dioxide can neutralize a part of chlorine. The chlorine residual in this scenario is 4.0 mg/L, which indicates the amount of chlorine present in the water and thus, how much needs to be dechlorinated. The additional requirement of 0.5 mg/L of sulfur dioxide indicates the necessary excess to ensure complete reaction and effectiveness in the dechlorination process.

First, we find the total required dose of sulfur dioxide by adding both the chlorine residual (4.0 mg/L) and the excess requirement (0.5 mg/L), which equals 4.5 mg/L.

Next, we need to convert this mg/L requirement to lbs/day. This requires knowing the flow rate of the water being treated, which is typically provided in mgd (million gallons per day). The calculation would proceed as follows:

1. Convert mg/L to lbs/gallon:

- There are approximately 8.34 lbs per gallon in water.

- Therefore, 4.5 mg/L