Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, EcoShield Environmental Systems, Inc.
December, 2002

Zinc Waste Treatment

Q. We have a microfilter unit with a 0.2micron filters. Prior to the microfilter the waste water is precipitated controlling pH and by adding mg sulfate and ferrous sulfate as needed. Are there any known chemicals in the plating processes of zinc, anodizing, chem film, electroless nickel, and passivation that prevents the normal zinc coagulation during the precipitation process?

A. Yes, there are typically organic chelating agents that are used in these plating baths. Their purpose is to keep the metals in solution in the plating bath. They can interfere with precipitation and coagulation steps as you described. Typically we depend on an ion exchange process with the chelating agents that is driven to equilibrium, to swap calcium, iron, aluminum, and / or magnesium for the target metals (such as zinc) tied up by the chelating agents, thus freeing the target metals for precipitation. However, equilibrium chelated metal concentrations may still be too high. There are more advanced treatment methods and combinations available that can address this problem. If you need assistance, I do consulting work in this area.