Some commercial builders take water conservation a step further than the federally-mandated low-flow toilets and use water that's not safe for drinking, but is safe for things like landscape irrigation or flushing toilets. One way is a type of plumbing system designed specifically to conserve water, known as a dual plumbing or dual piping system.
This system separates water into two types: potable and reclaimed. Potable water is safe for drinking, cooking and washing. Reclaimed water was formerly waste or sewage. Sounds nasty, but the water is treated to remove any solid waste and cleaned to be free of harmful bacteria. You just wouldn't want to drink it. It makes sense that you don't need to use the same water to flush a toilet as you do to fill your glass, right? There are some disadvantages, though. It costs more to have two plumbing systems. Plus, not all cities are set up for reclaimed water use. In the United States, this system was pioneered in the Irvine Ranch Water District in California, which also set the standard of using purple piping to distinguish reclaimed from potable water.
A cheaper alternative is a system that uses greywater generated within the building itself. There are lots of different ways to go about it, too. It can require two different plumbing systems, include collecting rainwater to fill toilets or irrigate plants, or be as simple as installing special sink/toilet combinations in commercial buildings that allow the water from hand-washing to fill the toilet for flushing.