Greywater is any domestic wastewater produced, other than toilet water, that comes from sources such as the bathroom, kitchen, and laundry. It may contain traces of dirt, food, grease, hair, and certain household cleaning products.
If recycled properly, greywater can save approximately 70 litres of potable water per person per day in domestic households. Reusing it keeps it out of the sewer or septic system, thereby reducing the chance that it will pollute local water bodies. If it is released into rivers, lakes, or bays, its nutrients become pollutants, but to plants, they are valuable fertilizers. It, therefore, serves to substitute fresh water for toilet flushing, laundry purposes, and yard irrigation. Through the latter, it reconnects urban residents to the natural water cycle.
Treated greywater can be used to irrigate both food and non-food producing plants because its nutrients (such as phosphorus and nitrogen) provide an excellent source for these plants.
There are numerous benefits to using greywater:
  • Reduction of freshwater required by a household
  • Reduction of water wastage 
  • Reduction of energy used to pump the freshwater in the house
  • Reduction of chemicals used to treat sewage water
  • Beautification of landscapes that would otherwise see less irrigation
  • Reuse of nutrients
  • Organic filtering of water
  • Groundwater recharge (greywater filtered by the soil ensures waterways such as rivers sourcing their water from the underground aquifers are always fed)
Greywater can be collected from sinks, baths or washing machines by using basins or through piping. Then it is collected in a storage container although it must be for less than 24 hours to ensure bad smells do not accumulate. The water can be transported via the use of gravity or pumps to flush toilets and pour into the yards, but ideally, there should be a way to divert water into the usual sewage path if it is not needed.

Greywater recycling is advancing in water conscious countries around the world, in which it is regulated by law. These include 29 states in the US, Australia, Japan, Germany, Cyprus, Saudi Arabia, Oman, and Jordan.
Hotels are also considering the reuse of greywater; the Premier Inn at Abu Dhabi International Airport has collaborated with Waterscan (a water management company) to create a sustainable water filtration system. The latter sanitizes used shower water for flushing toilets and irrigation by conserving approximately 60 litres of water per guest.