paint boothsReading time:
In all cases, an ultrafiltration unit will be used to recover resins and paint pigments from the rinsing water and to recycle demineralised water.
In the past, the main problem was caused by the presence of lead that had to be reduced to levels of one mg · L–1 by precipitating a basic lead carbonate.
Sludge separation was carried out using dissolved air flotation and the sludge dry solids content fluctuated between 6 and 12% depending on the type of paint used; even when very thoroughly clarified, the water still had a residual COD of approximately 2 to 5 g · L–1 (mainly solvent).
New bath formulations (eliminating lead and the more toxic solvents) in order to optimise their recovery and UF developments (less frequent washes), limit discharges which can then be sent directly to the main treatment plant.
In the past, paint booth waste streams were treated by “breaking” the paints directly in the booth recovery sump using alkaline reagents; the supernatant was then treated using mineral or organic coagulation followed by dissolved air flotation.
The floated sludge was sticky but had a dry solids content of approximately 6%; the supernatant was clear but still contained COD (several g · L–1).
Current developments are evolving towards hydrosoluble products for paints and lacquers, solvents only being used with varnishes. The physical-chemical treatment has been replaced by biological treatment carried out directly in the sumps beneath the booths; the concentrated sludge is extracted at regular intervals and sent to a class 1 technical landfill. Blowdown can then be sent directly with the general wastewater to the treatment plant.
The basic principles referred to under plant design - general apply to all workshops in these different industries:
- reducing pollution and toxic products at source;
- recycling effluent whenever possible;
- minimising chemical consumption.