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Greywater

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Left: greywater sample from an office building. Right: Same greywater after treatment in membrane bioreactor

Greywater or sullage (also spelled graywater in the United States and sometimes written as two words: grey water, gray water) is domestic wastewater generated in households or office buildings from streams without substantial fecal contamination, i.e. all streams except for the wastewater from toilets. Sources of greywater include sinks, showers, baths, washing machines or dishwashers. As greywater contains fewer pathogens than blackwater, it is generally safer to handle and easier to treat and reuse onsite for toilet flushing, landscape or crop irrigation, and other non-potable uses. Greywater may still have some pathogen content from laundering soiled clothing or cleaning the anal area in the shower or bath.

The application of greywater reuse in urban water systems provides substantial benefits for both the water supply subsystem, by reducing the demand for fresh clean water, and the wastewater subsystems by reducing the amount of conveyed and treated wastewater.[1] Treated greywater has many uses, such as toilet flushing or irrigation.[2]

Overview

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Example of a source of greywater in the household: dirty water from cleaning the floor
Urban decentralized greywater treatment with constructed wetland in Oslo
Greywater treatment plant with membrane bioreactor in the basement of an office building in Frankfurt

Quality

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Greywater usually contains some traces of human waste and is therefore not free of pathogens.[3] The excreta come from washing the anal area in the bath and shower or from the laundry (washing underwear and diapers). The quality of greywater can deteriorate rapidly during storage because it is often warm and contains some nutrients and organic matter (e.g. dead skin cells), as well as pathogens. Stored greywater also leads to odour nuisances for the same reason.[4]

Synthetic personal care products (e.g. toothpaste, face wash, and shower gel) commonly rinsed into greywater may contain microbeads, a form of microplastics.[5] Greywater originating from