Greywater or sullage (also spelled graywater in the United States and sometimes written as two words: grey water, gray water) refers to 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. Treated greywater has many uses, such as toilet flushing or irrigation.
Overview
Quality Greywater usually contains some traces of human waste and is therefore not free of pathogens. 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. Synthetic personal care products (e.g. toothpaste, face wash, and shower gel) commonly rinsed into greywater may contain microbeads, a form of microplastics. Greywater originating from washing clothes made from synthetic fabrics (e.g. nylon) is also likely to contain microfibers.
Quantity In households with conventional flush toilets, greywater makes up about 65% of the total wastewater produced by that household. It may be a good source of water for reuse because there is a close relationship between the production of greywater and the potential demand for toilet flushing water.
Practical aspects Misconnections of pipes can cause greywater tanks to contain a percentage of blackwater. The small traces of feces that enter the greywater stream via effluent from the shower, sink, or washing machine do not pose practical hazards under normal conditions, as long as the greywater is used correctly (for example, percolated from a dry well or used correctly in farming irrigation).
Treatment processes
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 greatly reduced, and the greywater is therefore easier to treat and reuse. When greywater is mixed with toilet wastewater, it is called sewage or blackwater and should be treated in sewage treatment plants or an onsite sewage facility, which is often a septic system. Greywater from kitchen sinks contains fats, oils and grease, and high loads of organic matter. It should undergo preliminary treatment to remove these substances before discharge into a greywater tank. If this is difficult to apply, it could be directed to the sewage system or to an existing sewer. Most greywater is easier to treat and recycle than sewage because of lower levels of contaminants. If collected using a separate plumbing system from blackwater, domestic greywater can be recycled directly within the home, garden or company and used either immediately or processed and stored. If stored, it must be used within a very short time or it will begin to putrefy due to the organic solids in the water. Recycled greywater of this kind is never safe to drink, but a number of treatment steps can be used to provide water for washing or flushing toilets. The treatment processes that can be used are in principle the same as those used for sewage treatment, except that they are usually installed on a smaller scale (decentralized level), often at household or building level:
Biological systems such as constructed wetlands or living walls and more natural 'backyard' small scale systems, such as small ponds or biodiverse landscapes that naturally purify greywater. Bioreactors or more compact systems such as membrane bioreactors which are a variation of the activated sludge process and is also used to treat sewage. Mechanical systems (sand filtration, lava filter systems and systems based on UV radiation) In constructed wetlands, the plants use contaminants of greywater, such as food particles, as nutrients in their growth. Salt and soap residues can be toxic to microbial and plant life alike, but can be absorbed and degraded through constructed wetlands and aquatic plants such as sedges, rushes, and grasses.
Reuse
Global water resource supplies are shrinking. According to a report from the United Nations, water shortages will affect 2.7 billion people by 2025, which means 1 out of every 3 people in the world will be affected by this problem. Reusing greywater has become a good way to solve this problem, and wastewater reuse is also called recycled or reclaimed water.
Benefits Demand on conventional water supplies and pressure on sewage treatment systems is reduced by the use of greywater. Re-using greywater also reduces the volume of sewage effluent entering watercourses which can be ecologically beneficial. In times of drought, especially in urban areas, greywater use in irrigation or toilet systems helps to achieve some of the goals of ecologically sustainable development. The potential ecological benefits of greywater recycling include:
Reduced freshwater extraction from rivers and aquifers Less impact from septic tank and treatment plant infrastructure Reduced energy use and chemical pollution from treatment Groundwater recharge Reclamation of nutrients Greater quality of surface and ground water when preserved by the natural purification in the top layers of soil than generated water treatment processes In the U.S. Southwest and the Middle East where available water supplies are limited, especially in view of a rapidly growing population, a strong imperative exists for adoption of alternative water technologies. The potential economic benefits of greywater recycling include:
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