Wastewater electroplating comes from surface plating operations where the metal is immersed in a plating solution of various types of metals and then rinsed.
Typical plating includes brass, nickel, cadmium, zinc, silver, copper, and gold. Wastewater electroplating is typically washing, rinsing and batch weirs and is at a low pH of ~ 3-5 and contains soluble forms of various metals. To remove soluble metals from wastewater, it must first be made insoluble. The insoluble metal is then coagulated, flocculated and clarified by sedimentation.
The typical method for reducing and eliminating soluble electrodeposition metals from wastewater is as follows:
Stage 1 - Precipitation and coagulation:
The pH is raised from ~ 3 to 8.5 with the pH controller using caustic soda while adding a coagulant such as ferric chloride. The wastewater test can confirm that a coagulant is not needed. It develops a "pin floc" that indicates that the metal is insoluble. Some applications have enhancing coating chemicals, emulsifiers and such that they may require more sophisticated high performance coagulants to break the bonds and allow the metal to precipitate.
Step 2 - Flash Mix:
The waste water with its precipitated pin floc is introduced into the flash mixing zone where a polymer flocculant is added. This stage maximizes the flocculant dispersion through the coagulated wastewater.
Stage 3 - Flocculation:
The waste water is now introduced into the slow mixing zone to agglomerate the floc flock into larger, faster settling particles.
Clarifier, inclined plate:
The flocculated wastewater is introduced into the clarifier where the sedimentation particles fall on the inclined plates and are directed downwards and into the sludge chamber. The rinsed treated water then exits the top of the clarifier and flows downstream to the sewer or additional treatment if necessary.
Clarifier Sludge Handling:
The residual sludge from the resulting clarifier is periodically removed from the clarifier at a slow rate and sent to the sludge containment tank where a batch is thickened and accumulated for disposal or processing in a filter press.
Dehydration of sludge:
The thickened clarifier slurry is allowed to accumulate sufficiently to provide a complete batch for the filter press. The filter press is completely pumped out of the slurry until it is full. The filter press is then emptied from the "cake" which is a semi-solid of about 20-35% solids. The mud cake is rich in phosphate and should be disposed of in accordance with environmental regulations.