Page 293 - Membranes for Industrial Wastewater Recovery and Re-Use

262 Membranes for Industrial Wastewater Recovery and Re-use

5.1 0 Automotive water recycling (Germany)

5.70.7 Background
The automotive industry requires large volumes of water and chemicals in the
production of finished cars and trucks. The majority of the water is associated
with the pretreatment and electrocoating stages where the car bodies need to be
cleaned prior to the different stages of production, and includes a rinse between
each step.
The application of membrane technology within the electrocoating process is
widespread. In fact, the process would not be viable without ultrafiltration which
has been used for more than 20 years to extract rinsing solutions from paint. The
integration of the technology has numerous drivers such as a need to remove
drag out paint before stoving, almost total recovery of the paint and the
avoidance of effluent problems. In part this has been integral to the development
of electropainting techniques and so has become part of the core process rather
than additional technology that needs to be justified. This places the industry in a
different position from many of the others which are currently considering
membrane technology. However, uptake of membranes to other parts of the
pretreatment and electrocoating stages has been considerably slower. The result
is that still heavy water demands are placed on production with up to 500 m3 of
process water being required per car. In more recent times economic drivers have
required the industry to examine the potential to reduce costs of both effluent
treatment and chemicals consumption. Two applications where this is becoming
more established is in removal of oil from the pretreatment cleaners at the start
of the production and recycling and recovery of paint from the final rinse water
at the end. The economic benefits are to a large extent country-specific due to
differences in the available water and the disposal options for the effluent.
However, the post-paint rinse water offers the most obvious benefits due to the
recovery of high-value paint and so will be examined further.
Once the electrophoretic painting is complete drag out is removed from the car
bodies by rinsing with ultrafiltration permeate extracted from the paint. A final
deionised water rinse is then applied to remove final paint traces and salts which
may otherwise reduce product quality (Fig. 5.22). The wastewater produced
from this process contains paint solids which have to be treated in the liquid
effluent line prior to discharge and the solids disposed as a contaminated waste.
A number of techniques have been tried to reduce demand on water and effluent
and to recover lost paint:

1. Extension of UF rinsing
2. Extraction of RO permeate from UF
3. Treatment of post-paint rinse water by UF
Extension of using UF permeate for the entire rinse period and thus replacing
the deionised water (option 1) potentially has shortcomings as the permeate still