Pump from the bottom of the tank, or would an overflow system work?

We have a customer that is an anodizing facility who is designing a new wastewater system that will tie into a new PLC that will run the manufacturing system as well. He is titrating for aluminum and using that result to maintain his etch. The question is: when he is too high on the aluminum content and needs to drop some of the tank to replenish with fresh etch, is it better to pump from the bottom of the tank or would an overflow system work?

If I read you right, you are asking whether you should decant by pumping out first, then adding water, or just add water and let it overflow. If that is correct, please read on. If not please clarify.

You should always be pumping out when you decant. When you pump solution directly from the tank, you remove and replace only the amount of solution needed to reach the correct concentration. For example to reduce the bath's concentration by 50%, you remove/replace half of the bath. By using an overflow, you are constantly diluting your solution with very gallon you add. This means the long your overflow runs, the less effect it has on the bath, which will this require much more water than pumping out.

As far as whether you should pump from the bottom or top, it doesn't matter if your bath is well agitated. You should not decant a still bath because the chemistry will tend to stratify with heavy molecules sinking to the bottom.
Kevin