It doesn't necessarily have to be on a scale to supply water for mega populated cities.
As the below clip demonstrates, a lot of 'let's feed the poor' is not demonstrating lateral thinking. Great invention on the roof top. 10L/day doesn't sound much but it's clean and pure.
http://www.youtube.com/watch?v=sJJh3AQ_u6U&feature=youtube_gdata_player
