You're probably picturing desalination/filtration plants and pipelines here, but for millions of years this task has been amply and efficiently performed by plants, fungi, and soil organisms. This explains why mountain streams are so clean... unless clear-cutting begins.
Natural systems prove it's possible. The trick is to use those same principles to design and organize a human-serving ecosystems for us to inhabit (as opposed to the human-destroying[1] systems we currently employ to grow food, source water, dispose of waste, etc). Just the techniques of rainwater harvesting, contour earthworks, and guild planting are sufficiently powerful and flexible to create a zero-input food system (ie no inputs except for sunlight/air/bedrock).
[1] You can substitute the fashionable euphemism "unsustainable" here, but I feel this term is more honest.
I think the following http://raincatcher.org/what-we-do/ is a really interesting solution to the fresh water problem.
Here's a video of what they do. https://vimeo.com/29880712 skip to 1.38 if you are impatient.
1.1 x 10^14 cu m of water falls on land per year. [0]
5 x 10^14 cu m falls yearly on the earth [1]
80,000 cubic meters is said to be lost to this effect [2].
[0]http://voices.nationalgeographic.com/2012/03/14/are-we-runni... [1]http://hypertextbook.com/facts/2008/VernonWu.shtml [2]http://www.jordantimes.com/news/local/80000-cubic-metres-wat...
This type of analysis reveals that trees are responsible for about 80% of all terrestrial rainfall.[1] In contrast to man-made desalination facilities, this water falls freely as rain instead of being owned and controlled.
Global deforestation/desertification is perversely convenient for those interests vying to control water, since in a very real way it eliminates the competition.
[1] http://www.nature.com/nature/journal/v496/n7445/abs/nature11...
In the mid east, and other arid regions, fossil water is being used up faster than it can replenish itself.
Some places, the water table is falling by several meters per year.
That is not being helped at all by the now unsustainable population density, courtesy of the conjunction of modern pediatric medicine and traditional family planning.
This is happening all over, even in the USA. California's Central Valley's primary aquifer is so depleted that there are severe land subsidence problems even in some suburban areas.
CA politicians treat the aquifer like it's a canteen that can be emptied and refilled at will. Dry aquifers can collapse and never fully refill.
The only such use I see is small-scale, localized cloud seeding over watersheds.
Where I live, my city pols subsidized water, people used more, said pols cried about water shortages and passed restrictions, while we are awash in fresh water. The city across the bay restricted waste early, implemented recycling, now has one of the biggest waste-water recycling systems in the USA, raised the price of city water while providing financial aid to the poor, and finally, showed ruthless backbone in locking in water rights elsewhere, including stealthy purchases using strawmen.