Recycled water is wastewater that has been recovered for potable or nonpotable use, such as reclaimed water. The source of the wastewater may be industrial use, human or animal waste, or any other source of water unusable in its existing state.

Water shortages throughout the world have many jurisdictions considering novel means to provide adequate water to communities. The reuse of treated water for a wide range of applications is becoming a more common practice. As an extreme example, in Singapore reuse water is being used as an indirect source of raw water for community drinking water and is being sold in bottled form under the label "NEWater".

The production of recycled water from wastewater involves the removal of impurities -- ranging from salts and chemicals to biological pathogens and particulate matter -- until the water meets certain standards appropriate for its intended use.

Recycled water for nonpotable uses is currently produced by some sewer treatment plants, through any one of several processes known as tertiary treatment . This water is suitable for any use except human consumption, although actual allowable uses are determined by local regulations.

Recycled water for potable use is more rare, although some would argue that current standards for nonpotable recycled water already exceed potable water quality standards. One example of an operating potable recycled water system is on the International Space Station, where all wastewater must be recovered for reuse due to the lack of fresh water supply.

Many large cities using water drawn from rivers are however inevitably re-using sewage effluent discharged upstream. London water had a reputation of having been drunk five times before it arrived at the tap but this is an exaggeration. However there are many large towns on the River Thames upstream of London (Oxford, Reading, Swindon Bracknell etc.) whose sewage discharge is subsequently abstracted to supply London with water. This phenomenon is also observed in the United States, where the Mississippi River serves both as the destination of sewage treatment plant effluent, and the source of potable water. Research in the 1960's (London Metropolitan Water Board ) demonstrated that the maximum extent of re-cycling water is about 11 times before the taste of water induces nausea in sensitive individuals. This is caused by the build up of inorganic ions such as Cl, SO4, K and Na in the water. Such ions are not removed or reduced by conventional sewage treatment

The cost of production for recycled water typically exceeds that of potable water in most regions of the world where a fresh water supply is plentiful. However, in areas where constrained supplies of potable sources drive up the cost of raw water, treating wastewater for reuse in certain applications is becoming an economically attractive option for many communities. The City of San Diego, California, and Sydney, Australia, are examples of communities which provide reuse water through dual pipelines to residences, businesses, and industries. Such water is called recycled water. Costs of reuse water quality treatment vary widely based on the standards of treatment, which in turn depend on the intended uses of the water.

Grey water, wastewater which does not contain sewage (such as flow from sinks, showers, and washing mashines), can also be reused. In this type of system, grey water is prevented from mixing with other wastewater sources and diverted away from flow to the treatment plant for local reuse in irrigation. This elimination of treatment plant inflow for reuse can be considered a form of pre-treatment.

The largest uses of reuse water internationally is for agricultural and landscape (including golf course) irrigation and deep well injection to replenish declining groundwater supplies; in most places reuse water is restricted from use for human consumption or uses where prolonged human exposure (such as to swimming pools) is likely.