It may be something we don't like to think about, but one of the things the astronauts have to do while in orbit will be coming more and more to Earth. Fresh water is limited, and getting more so with time. Water conservation helps, but it may not be enough in the future.
Whether they are dependent on well water or surface water, many cities have to worry about having enough water to last through the dry season as the water levels drop. Water restrictions are common in some areas; where I grew up, part of the summer routine was that you couldn't water your lawn whenever you wanted, but only on certain days. Sometimes there was an outright ban on watering lawns if the river level was too low.
As wastewater treatment technology improves, the sewage plant's discharge gets cleaner and cleaner, so why not use it as feed for our clean water treatment system?
We already do recycle a lot of our water, and again, most people don't like to think about this particular aspect of it. But as the good folks at the american National Academy of Sciences say right in the summary of their study, reusing coastal discharges would augment available water resources.
You read that right: coastal discharges.
Unless you live at the headwaters, if your city's water supply is from a river, you're drinking somebody else's pee, from some city upstream. This is the water recycling most people don't like to think about.
But stop and think about it a moment: would you rather drink water from a source you don't control, or a source you do control? At least if you control it, you can improve it. If it's somebody else's city discharging into your drinking water, what can you do?
There are already quite a few cities that are already using what is usually called "reclaimed water", often for watering parks and golf courses and other areas that need irrigation water but not potable water. It's treated, it just isn't treated all the way to drinking water standards. This sort of thing can have a huge impact on how much water the city has to pull from their dwindling freshwater resources for non-potable use. It can also have a large impact on whether or not they can water the parks at all through the dry season. After all, people don't stop using the toilet when it's dry.
Most cities were built before fresh water conservation was a concern, and so built only one water system to serve the city. Treating all water to drinking water standards then using less than 10% of it for actual drinking is rather silly. Some cities are installing parallel water systems, so potable water isn't used for watering lawns and flushing toilets; for a slightly higher installation cost, you reduce the load on the drinking water treatment plant, meaning more people can get drinking water from the same plant, saving a lot in operating costs over the years.
Wastewater treatment isn't cheap or easy, but it has to be done. With the population of the world still growing and with it the demand for fresh water, water that's easy to access from rivers or wells is also getting more expensive.
Recycling wastewater right back to drinking water, though?
I think it's brilliant.