Came across this story yesterday and I was absolutely intrigued: a billboard that produces potable water.
But just in case you didn't get the reference, Luke Skywalker first appears on Tatooine, a desert planet, in Episode IV living with his aunt and uncle who operate a moisture farm. Something something they harvest water vapour in the air because it's a desert planet with limited/no water bodies or something.
In regards to the actual billboard in Lima, Peru, it is only a pilot project or study (i.e. not mass produced) between a marketing agency and local university. The system reportedly provides potable water for residents of the city -- or however many people can be sustained on 96 L/day. Inside the system is an air filter, condenser, carbon filter, and a reverse osmosis filter.
The idea came about because Lima is apparently located in a very dry, desert region that receives very little rain (definition of a desert?) and there are limited sources of surface and underground sources of water. However, being a coastal city, the air happens to be very humid.
As for the components of the system itself, I haven't seen much detail on what the individual components do. In general, I'd guess the air filter removes some air pollutants, and the condenser operates like a dehumidier or air conditioner. Just cool a surface, probably coils, to a temperature below the dew point, and, voila, condensation. There you have your water, a carbon filter would help to remove some taste and odours, color, and some more minor issues. Finally, the reverse osmosis filter would purify the water further by removing pathogens and probably salt.
This type of filter system is the most effective water purification filter available and is often used for desalination, or just plain overkill filtering occasionally. I'm not that up to speed on desalination, but I have heard that cars in coastal cities die a young death due to rusting from salt in the air. Condensed water from air would likely contain salt, and probably too much of it for regular potable water consumption.
Obviously, this system does not work passively. The condensors and the reverse osmosis filter would require quite a bit of energy -- a perk of being the most powerful filter. Also, carbon filters and reverse osmosis filter membranes require regular replacement. Very interesting system, but not quite "sustainable".
