My idea is focused on saving clean drinking water, as we will be running short of drinking water in the near future.
As in the usual construction of buildings, we have a single source of water connection with a tank on the ground and a tank on the roof which is used for all purposes at home. With this system, a lot of water is wasted by the toilet for flushing. If we accumulated this flushing water, it would total in the millions of gallons or possibly even the trillions of gallons.
Now, if we construct two tanks in the ground and two on the roof, we could use one for normal drinking water while another will contain sea water which is either salted or without desalination. This water would be used for flushing the toilets and this way we can save millions or trillions of gallons per year.
Additionally, this sewage water could be used for watering parks, grass and plants on the road islands or side walks, after filtering the waste out the water, naturally. We can also save water with the sewage water, which is more beneficial for plantation as it contains natural fertilizers. Plants could grow more healthily and the cost of fertilizer would also be cut down to some extent.
Photo credit to Takomabibelot.