wtaer recycling is practical?

Yes, water is wasted in the average home by design, but designing a home to minimize water usage will not be easy. First, potable water will need to be piped through a water heater and then on to all the sinks and bathtubs/showers and kitchen appliances, most notably the dish washer. The drains from all these sinks and tubs and the dishwasher must then be routed to a central filtration/pretreatment tank before this gray water can be pumped through a second set of pipes to the toilets. The gray water must be filtered and pretreated before reuse, as it contains particulate matter and soap residues that will readily clog the water pipes, especially in areas with hard water. Finally, the toilets must be drained through a second set of drains to another collection point, where solids are separated for composting and any remaining wastewater must be either disposed of or collected for agricultural use, along with the solids.

As a contractor specializing in home construction for the last 24 years, I can easily state that the costs involved will be substantial. In new single family dwelling construction, plumbing costs will easily be three times the current cost, and that does not even include the costs of the various collection and treatment systems. Further, energy usage related to plumbing will probably double as well, since the gray water will not pump itself back through the house. Costs to retrofit these systems to existing homes will be far higher, and costs to retrofit multi-story and multi-family units, such as apartment buildings, will probably be prohibitive. In other words, these advanced ideas will only be usable on new construction homes.

These concepts are not new. Robert A. Heinlein discussed them in detail in his 1966 classic The Moon Is A Harsh Mistress. In this novel, lunar colonists revolt against their Earth based overlords, a revolution which, at the heart, is based on the misuse and wastage of water. However, the requirements of a capital intensive, energy intensive, resource constrained non-Terrestrial environment do not translate back to present day Earth except in the most extreme circumstances. It would be far better to invest a small amount in more efficient farming practices than plan on rebuilding the planets housing infrastructure.
Scientific American, August, 2008