Grey water

Greywater (also spelled graywater, grey water, gray water) or sullage is all wastewater generated in households or office buildings from streams without fecal contamination, i.e. all streams except for the wastewater from toilets. Sources of greywater include, e.g. sinks, showers, baths, clothes washing machines or dish washers. As greywater contains fewer pathogens than domestic wastewater, 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.

The use of non-toxic and low-sodium soap and personal care products is recommended to protect vegetation when reusing greywater for irrigation purposes. 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 as well as the wastewater subsystems by reducing the amount of wastewater required to be conveyed and treated.

Greywater, by definition, does not include the discharge of toilets or highly fecally contaminated wastewater, which is designated sewage or blackwater to indicate it contains human waste. However, under certain conditions traces of feces, and therefore pathogens, might enter the greywater stream via effluent from the shower or washing machine.

When greywater is mixed with toilet wastewater, it is called sewage or blackwater and should be treated in sewage treatment plants or onsite sewage facility, which often is a septic system. When it is kept separate, it may open up interesting decentralized treatment and reuse options. The separate treatment of greywater falls under the concept of source separation which is one principle commonly applied in ecological sanitation approaches. The main advantage of keeping greywater separate from toilet wastewater is that the pathogen load is much reduced and the greywater is therefore easier to treat and reuse.

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