The term waste water refers to any water that has been used and that contains dissolved or suspended solids. Waste water is discharged from homes, businesses and factories, and farms. One of the primary concerns with waste water is that it typically needs to be processed before being released back into waterways so as to avoid contamination from harmful pollutants. However, pollutants within the waste water are not the only concern; the temperature of waste water can also be a factor. For example, if hot water from factory boilers is dumped directly into a river or lake, it can kill the indigenous flora and fauna, thereby harming the ecosystem just as surely as if pollutants had been released instead. 

In order to protect the US waterways and the quality of US water supplies, the United States Environmental Protection Agency has established a strict set of regulations for the management of waste water. These regulations are designed to restore and maintain the chemical, physical, and biological integrity of US waters. In particular, the Clean Water Act regulates the direct and indirect discharge of priority pollutants (i.e., toxins), conventional pollutants (e.g., biochemical oxygen demand, total suspended solids, fecal coliform, oil and grease, pH) and non-conventional pollutants (i.e., pollutants that are not classified as either priority or conventional pollutants). In addition, the EPA recommends that facilities required to develop and implement environmental management plans also comply with the guidelines of ISO 14001. These voluntary guidelines can provide a framework that can be useful in meeting federal and local guidelines for the management of environmental systems, including the recycling of waste water.

However, the recycling of waste water can include much more than simply treating it and releasing it back into the water ways. For example, heat from waste water that typically goes down the drain can be recovered and harnessed for other purposes. Storm water (that often contains oil and other contaminants) can be captured in cisterns and recycled for use in toilets. Waste water can also be recovered, treated, and used for other purposes such as irrigation, watering of lawns and golf courses, cooling of industrial equipment, fire fighting, washing cars, cleaning streets, and controlling dust. New technologies are being developed that can recycle waste water for other purposes. These technologies offer opportunities not only to save money to the business that uses them, but also to make money for the business that manufactures or sells them.