Water treatment of washing water in the automobile production with cavity wax.
Industrial Engineering.
Insight.

Water treatment of washing water in the automobile production with cavity wax.

At the end of the assembly of a vehicle in an automobile factory, the vehicle passes through a car wash for final cleaning before being prepared for transport to the end customer.
The cavity preservation applied at the end of the painting process has not yet dried to such an extent that rinsing off the excess cavity wax can be avoided.
Since the water of the respective processes is to be led to the reuse, a cleaning is to be provided, which frees the water in the present case to 90 % of dirt particles. Especially the elimination of the wax particles in the water posed a special challenge in this case.

Basisfiltration

All waste water in the plants is fed to the sedimentation tank via waste water pipes. The first stage of cleaning is done with the help of a hair filter. The water from the sedimentation tank is pumped through the hair filters by means of pumps. Coarse suspended matter is retained in the hair filters.
The next cleaning stage takes place in the flotation tank. The water insoluble substances as well as smaller suspended particles are transported to the surface with the aid of gas bubbles and a rotating rotor located near the bottom of the flotation tank, where they are removed from the water circuit with the aid of a separator.
Flocculants are added to the water before it flows into the flotation tank in order to accelerate the precipitation of detergents and waxes from the water.
The flotation tank is followed by a mechanical filter in which particles remaining in the water can be retained up to a size of approx. 0.1 mm.
As a rule, the subsequent sand filter represents the final stage of water treatment. In it, the suspended particles in the water are filtered out. The height of the sand creates an oxygen-free zone. This is where bacteria settle, which can completely decompose nitrogen, for example. The water passing through the sand filter has a residual contamination with particles smaller than 1 µm.

Ultrafiltration

However, the wax particles dissolved in water have a minimum size of 10 nm.
In order to filter these particles out of the water as well, the use of an ultrafiltration plant is necessary. Here the macromolecular wax particles can be separated from the water medium and concentrated.
90 % of the water obtained in this way is contaminated with particles smaller than 9 nm. Reuse for vehicle cleaning was thus sufficiently guaranteed to exclude wax stains on the paint after the washing process.
Finally, we would like to mention the use of a UV filter. Irradiation of the water with UV radiation can kill bacteria, viruses and many spores.

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