The principle is 'dilution': with the first addition of clean water to dirty water, the dirty water becomes less dirty because it is diluted by the clean water. With the continued addition of clean water to dirty water, eventually the dirty water will be completely diluted and there will be no dirty water left; it will all be replaced by clean water.

dirty water is recycled in the water cycle

because they want dirty water

Dirty Water was created in 1966.

dirty water