Greywater systems recycle water by collecting water that has been used for one purpose, and then using it for another. Depending on local regulations, greywater may be collected from sinks, bathtubs, showers, laundry machines, etc., and used for flushing toilets and irrigation.Much like rainwater harvesting systems, a special plumbing system is used to collect, store and distribute the greywater. It is collected in a tank at a lower level and then pumped to a high level holding tank from which it flows for use. A treatment system is usually required to prevent the growth of microbes in the stored water, and a connection to municipal water is provided to ensure the system is never empty.
In some cases, rainwater and greywater systems may be combined, but the use of greywater is more strictly regulated by building codes. Neither is safe for drinking. Using greywater for flushing toilets alone will save large amounts of water, particularly in public buildings, in which the greywater system may be large and complex.
Greywater systems reduce the amount of fresh water required to operate a building, and therefore reduce the volume of wastewater produced by the building – also an important strategy in reducing the true cost of water consumption, which includes the energy-consuming treatment of wastewater.