Anybody here familiar with water purification calculations?

I do online tutoring. I just got a student who presented this problem from a high school course on water filtering and I realized that I had no idea what it was talking about. What the heck is meant by residual and what does it have to do with this problem?

1.You can estimate the quantity of SO2 necessary to dechlorinate your effluent if you know the initial chlorine dose and the chlorine demand. That way you can find the chlorine residual. In reality, you would know the chlorine residual because you have an expensive analyzer sampling the effluent chlorine residual continuously.

But, for this question we’ll assume you only have the initial dose and the demand. We know that it takes one part of SO2 to react with one part of chlorine. No example here. Use Spellman to find the residual and then calculate the pounds of chlorine in the effluent that you need the SO2 to react with. Hint: this is a simple pounds formula problem.

Estimate the pounds per day of SO2 necessary to dechlorinate the effluent if:

Initial chlorine dose = 9.5 mg/L. Chlorine demand = 7.0 mg/L. Flow = 10.2 MGD.