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Unexplained fluctuations in ORP values for chromium reduction

We have a hex-chrome bearing waste stream from chem film on aluminum, EP, and chromate on zinc processes. We reduce the chromium by dropping the pH to <2.10 and adding sodium metabisulfite in a rection tank. We have noticed that when the stir motor is on, the ORP values are up to 30 mv higher than when the tank is not in use. When the stir motor is running and the tank is in standby (no solution flow) there is variation of about 5-8 mv, but the average value is constant. If we then turn the tank (and stir motor)off, the variation diminishes as the average value drops, and then stabilizes at a fixed value.

The tank is fiberglass construction; we have tried using a ground lead to the solution to no avail. Any ideas on the cause, and if prudent, a solution?

Thanks in advance.

Willie Alexander
MF Supervisor - Colorado Springs, Colorado
May 27, 2010