The Paris city government spends a lot of time and effort to keep the streets clean.  I'm not sure if it's due to the fact that quite a few Parisians don't pick up dog poop, but the street cleaning really makes a difference compared to U.S. cities.  Sidewalks and plazas are hosed down by trucks, but many streets have water sources built in to flush debris into the sewer system.

I wondered how environmentally friendly this is, but I read that it's a closed system for recycling non drinkable water.  Paris has two water lines, one for drinkable water and one not.