Steel car bodies are typically pre-treated by a tri-cationic phosphating process before receiving an electrophoretic paint coat. Historically the phosphated steel has received a hexavalent chromium seal although this type of process has been gradually phased out in accordance with the End-of-Life Vehicles Directive. Demineralised water is used for rinsing at various process stages to ensure a high quality of finish.

Waste water resulting from electrocoat lines is typically difficult to treat, involving the need for denaturing of the paint before it can be easily precipitated and settled. There is also a requirement to remove toxic heavy metals including Ni, Zn and Cr. Typical effluent treatment plant for dealing with waste rinse water from an automotive pre-treatment line with electrocoat includes:-
• Paint denaturing reaction stage including coagulant dosing.
• Cr reduction reaction stage***
• pH correction and precipitation reaction stage.
• Flocculant dosing.
• Settlement.
• Sludge dewatering.
• Filtration polishing.+++

*** Where the pre-treatment includes a hexavalent Cr process stage.
+++ Where discharge consent limits require further polishing of the effluent.

Although automotive body process lines are typically associated with high levels of process solution dragout, often relatively small volumes of rinse water are used in comparison. Due to the volumes involved pressure has been put on automotive manufacturing companies to reduce water usage and frequently water recovery may be part of the treatment process. Often ion exchange plant is used for recirculation of low contaminant rinses.

Other finishing processes are also allied to the automotive industry with decorative finishing including plating-on plastics (POP) using electroless Ni, Cr plating and bright anodising of Al. Often these processes require the use of demineralised water for rinsing and process solution make-up and the various waste streams produced can be treated in many different ways.

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