Does 'residual dirt' or 'dirt residues' fit the context?

does the job IMO

try googling it together with e.g. "treatment" or "recycling"...

I would say, going by the definition provided in the source text:

"Schmutzflotte
Gebrauchtes Wasser nach dem Reinigungsverfahren."

Technology - produced challenge on earth. Grandkids gto survive!

The water used in washing cars doesn't need to be as clean as drinking water so Jeremy Thorne, owner of the Pit Stop Car Wash, has devised an innovative way to avoid using mains tap water. The run-off from washed cars is collected and processed through a series of settling tanks and a reed bed system which cleanses the water enough to reuse the water for more car washing. Jeremy also makes sure that the products he uses to clean the cars have no harmful chemicals in.
A car wash with a conscience!
http://www.sustainableswansea.net/water-conservation.html

Hand washing your car can have a harmful impact on the environment. Not only are you using more water than necessary, but you are adding to the toxic chemicals in the run-off that leads directly to our oceans and waterways.
http://www.justlivegreener.com/greener-transportation/143-an...

Run-off water from a car washed at home is unregulated unlike a full service car wash, which must take the wastewater to a water treatment facility. When washing a car at home, the run off water flows freely down the driveway to the storm drains and goes directly to our oceans, lakes and streams.
http://www.steamcarwash.com/steam-car-wash-system.aspx

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Note added at 56 mins (2012-05-15 11:27:06 GMT)
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Re Brigitte's suggestion 'wastewater'

Wastewater: What Is It?
Wastewater comes from ordinary living processes: bathing, toilet flushing, laundry, dishwashing, etc. It comes from residential and domestic sources.

Commercial wastewater comes from non-domestic sources, such as beauty salon, taxidermy, furniture refinishing, musical instrument cleaning, or auto body repair shops. This wastewater may contain hazardous materials and requires special treatment or disposal.

http://water.unl.edu/web/sewage/wastewater

So probably a bit too broad in its definition for your context. It should be noted that this option is written as one word, not two.

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Note added at 1 day23 hrs (2012-05-17 09:51:29 GMT) Post-grading
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"Schlussendlich hat mich die Erklärung überzeugt, dass run-off wohl nicht aufgefangen wird."
Actually, that was not an 'explanation' from hazmatgerman but rather his personal 'opinion'.
To native English ears, 'cleaning waste water' is a clumsy construction that would be better expressed as 'the waste water from cleaning', but that of course would be an 'explanation' of the concept rather than 'ein spezielles Wort'.

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Note added at 5 days (2012-05-20 23:49:16 GMT) Post-grading
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How strange. 24 hours ago, this answer had two 'agrees'. I can only think that they must have been deleted by moderators. For what possible reason, I can only speculate.