Due to some TRC effluent limits a dechlorinator using sodium bisulfite was added.  Original estimate for dosing was 1.5 gallons/day.  Flow out effluent (direct discharge) is about .2 to .3 MGD.  The levels have been running about .1 mg/l and reduced to .01-.03mg/l.

Operator has bumped up to 5 gallons per day despite my advice because he wants 0.0mg/l.  Can anyone provide guidance on why he would need to bump it up that much?  Does he have risk of exceeding the BOD limit?  Injection point is just prior to GAC filters if that matters.

Based on what I have researched I would agree that 1.5-2 gal/day should be adequate.

Appreciate any guidance.

I think that TRC is determined by grab samples unless you are permitted for continuous monitoring. If you are running TRC on grab samples, then flow and concentration variability may become an issue. Now, the maximum concentration limit was for direct discharge (if I remember rightly) is 0.05 mg/L and the GAC may even reduce the concentration lower.

Bisulfite is also an oxygen scavenger so that an excess may have an impact on BOD results and chemists are permitted to react the BOD samples with something (I forget what) to react with the sulfite before setting up the BOD samples.

Now if it is the cost that you are worried about; you and the operator are less concerned about the cost than about the effect on discharge limits. Why don't you suggest that the operator not change the pumping rate and start a statistical process control chart for both BOD and TRC. This would be a better solution tha arguing over calculations that may not considered all the significant variables. Then you may or may not be able to justify a change in feed rates.

grrun