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

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, 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 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.[1] Treated greywater has many uses, for example toilet flushing or irrigation.[2]


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


Greywater is all wastewater that is generated in household or office building sources without fecal contamination. Therefore, by definition, greywater does not include the discharge of toilets or highly fecally contaminated wastewater, which is designated sewage or blackwater and contains human waste.

Any domestic wastewater can be defined as Greywater, but it does not including blackwater(sewage). The main difference between greywater and blackwater is the organic loading. Comparing to Greywater, blackwater has a much larger organic loading..[3]

Practical aspects[edit]

Misconnections of pipes can cause greywater tanks to contain a percentage of blackwater.[4]

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).

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 grey water tank. If this is difficult to apply, it could be directed to the black water system or to an existing sewer.[5]

The Greywater reuse in toilet flushing and garden irrigation, both producing aerosols. These may transmit Legionella and bring a potential health risk for people. However, the result of the research shows that the health risk due to reuse Greywater either for garden irrigation or toilet flushing was not significantly higher than the risk associated with using clear water for the same activities.[6]


Greywater usually contains some traces of excreta and is therefore not free of pathogens.[7] The excreta comes from washing a person's anal area in the bath and shower, or from the laundry (washing underwear and nappies). The quality of greywater can deteriorate rapidly during storage because it is often warm, 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.[citation needed]


In households with conventional flush toilets, greywater makes up about 65% of the total wastewater produced by that household.[7] 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.

Treatment processes[edit]

Greywater "towers" are used to treat and reuse greywater in Arba Minch
Underground greywater recycling tank

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.[7]

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.

Most greywater is easier to treat and recycle than blackwater (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:

In constructed wetlands, the plants use contaminants of greywater, such as food particles, as nutrients in their growth. However, 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.


The global water resources are worsening. According to a report from United Nations states that 2.7 billion people will face the problem of water shortage by 2025, which means almost 1/2-1/3 of the total population of the world will affect by this problem. Reusing the wastewater become a good way to solve this problem, and wastewater reuse is also called recycled or reclaimed water.[8]


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 gardens 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[9]

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

  • Can reduce the demand for fresh water, and when people reduce the use of fresh water, the cost of domestic water consumption is significantly reduced, while alleviating the pressure of global water resources.
  • Can reduce the amount of wastewater entering the sewer or on-site treatment system.</ref>"Sustainable Earth Technologies". Retrieved 28 November 2017. 


Greywater use for irrigation appears to be a safe practice. A 2015 epidemiological study found no additional burden of disease among greywater users irrigating arid regions.[10] The safety of reuse of greywater as potable water has also been studied. A few organic micropollutants including benzene were found in greywater in significant concentrations but most pollutants were in very low concentrations.[11] Faecal contamination, peripheral pathogens (eg, skin and mucous tissue), and food-derived pathogens are the three major sources of pathogens in grey water.[12]


Most greywater should be assumed to have some blackwater-type components, including pathogens. Greywater should be applied below the surface where possible (e.g., via drip line on top of the soil, under mulch; or in mulch-filled trenches) and not sprayed, as there is a danger of inhaling the water as an aerosol.

In any greywater system, it is important to avoid toxic materials such as bleaches, bath salts, artificial dyes, chlorine-based cleansers, strong acids/alkali, solvents, and products containing boron, which is toxic to plants at high levels. Most cleaning agents contain sodium salts, which can cause excessive soil alkalinity, inhibit seed germination, and destroy the structure of soils by dispersing clay. Soils watered with greywater systems can be amended with gypsum (calcium sulfate) to reduce pH. Cleaning products containing ammonia are safe to use, as plants can use it to obtain nitrogen.[13] A 2010 study of greywater irrigation found no major health effects on plants, and suggests sodium buildup is largely dependent on the degree to which greywater migrates vertically through the soil.[14]

Some greywater may be applied directly from the sink to the garden or container field, receiving further treatment from soil life and plant roots.

The use of non-toxic and low-sodium soap and personal care products is recommended to protect vegetation when reusing greywater for irrigation purposes.[15]

Indoor reuse[edit]

Recycled greywater from showers and bathtubs can be used for flushing toilets in most European and Australian jurisdictions and in United States jurisdictions that have adopted the International Plumbing Code.

Such a system could provide an estimated 30% reduction in water use for the average household. The danger of biological contamination is avoided by using:

  • A cleaning tank, to eliminate floating and sinking items
  • An intelligent control mechanism that flushes the collected water if it has been stored long enough to be hazardous; this completely avoids the problems of filtration and chemical treatment

The Uniform Plumbing Code, adopted in some U.S. jurisdictions, prohibits greywater use indoors.

Greywater recycling without treatment is used in certain dwellings for applications where potable water is not required (e.g., garden and land irrigation, toilet flushing). It may also be used in dwellings when the greywater (e.g., from rainwater) is already fairly clean to begin with and/or has not been polluted with non-degradable chemicals such as non-natural soaps (thus using natural cleaning products instead). It is not recommended to use water that has been in the greywater filtration system for more than 24 hours or bacteria builds up, affecting the water that is being reused.

Due to the limited purification technology, the treated grey water still contains some chemicals and bacteria, so some safety issues should be observed when using the treated grey water:

  • Untreated grey water can only be used for underground garden irrigation. In the prior art, sewer pipes were usually buried at least 1 meter below

ground, reducing the risk of human-animal contact.

  • Untreated grey water pipelines must have warning labels.
  • Use grey water immediately (or within 24 hours) and do not store untreated grey water at home as bacteria and other pathogens may be hazardous to human health.
  • If anyone at home has a gastrointestinal flu or other infectious disease, stop using grey water.
  • Do not use grey water containing bleach or dye.
  • Do not use untreated grey water to irrigate plants.
  • It is illegal to get grey water from home to home.</ref>"choice". Retrieved 28 November 2017. 

Heat reclamation[edit]

Devices are currently available that capture heat from residential and industrial greywater, through a process called drainwater heat recovery, greywater heat recovery, or hot water heat recycling.

Rather than flowing directly into a water heating device, incoming cold water flows first through a heat exchanger where it is pre-warmed by heat from greywater flowing out from such activities as dishwashing, or showering. Typical household devices receiving greywater from a shower can recover up to 60% of the heat that would otherwise go to waste.[citation needed]

Governmental regulation in the World[edit]

United States[edit]

Government regulation governing domestic greywater use for landscape irrigation (diversion for reuse) is still a developing area and continues to gain wider support as the actual risks and benefits are considered and put into clearer perspective.

"Greywater" (by pure legal definition) is considered in some jurisdictions to be "sewage" (all wastewater including greywater and toilet waste), but in the U.S. states that adopt the International Plumbing Code, it can be used for subsurface irrigation and for toilet flushing, and in states that adopt the Uniform Plumbing Code, it can be used in underground disposal fields that are akin to shallow sewage disposal fields.

Wyoming allows surface and subsurface irrigation and other non-specific use of greywater under a Department of Environmental Quality policy enacted in March 2010. California, Utah, New Mexico and some other states allow true subsurface drip irrigation with greywater. Where greywater is still considered sewage, it is bound by the same regulatory procedures enacted to ensure properly engineered septic tank and effluent disposal systems are installed for long system life and to control spread of disease and pollution. In such regulatory jurisdictions, this has commonly meant domestic greywater diversion for landscape irrigation was either not permitted or was discouraged by expensive and complex sewage system approval requirements. Wider legitimate community greywater diversion for landscape irrigation has subsequently been handicapped and resulted in greywater reuse continuing to still be widely undertaken by householders outside of and in preference to the legal avenues.

However, with water conservation becoming a necessity in a growing number of jurisdictions, business, political and community pressure has made regulators seriously reconsider the actual risks against actual benefits.

It is now recognized and accepted by an increasing number of regulators[citation needed] that the microbiological risks of greywater reuse at the single dwelling level where inhabitants already had intimate knowledge of that greywater are in reality an insignificant risk, when properly managed without the need for onerous approval processes. This is reflected in the New South Wales Government Department of Water and Energy's newly released greywater diversion rules, and the recent passage of greywater legislation in Montana.[16] In the 2009 Legislative Session, the state of Montana passed a bill expanding greywater use into multi-family and commercial buildings. The Department of Environmental Quality has already drafted rules and design guidelines for greywater re-use systems in all these applications. Existing staff would review systems proposed for new subdivisions in conjunction with review of all other wastewater system components.[17]

Strict permit requirements in Austin, Texas, led to issuance of only one residential graywater permit since 2010. A working group formed to streamline the permitting process, and in 2013, the city created new code that has eased the requirements, resulting in four more permits.[18]

In California, a push has been made in recent years to address greywater in connection with the State's greenhouse gas reduction goals (see AB 32). As a large amount of energy (electricity) is used for pumping, treating and transporting potable water within the state, water conservation has been identified as one of several ways California is seeking to reduce greenhouse gas emissions.[19]

In July 2009, the California Building Standards Commission (CBSC) approved the addition of Chapter 16A "Non-potable Water Reuse Systems" to the 2007 California Plumbing Code. Emergency regulations allowing greywater reuse systems were subsequently filed with the California Secretary of State August 2009 and became effective immediately upon filing. Assembly Bill 371 (Goldberg 2006) and Senate Bill 283 (DeSaulnier 2009) directed the California Department of Water Resources (DWR), in consultation with the State Department of Health Services, to adopt and submit to the CBSC regulations for a State version of Appendix J (renamed Chapter 16 Part 2) of the Uniform Plumbing Code to provide design standards to safely plumb buildings with both potable and recycled water systems. November 2009 the CBSC unanimously voted to approve the California Dual Plumbing Code that establishes statewide standards for potable and recycled water plumbing systems in commercial, retail and office buildings, theaters, auditoriums, condominiums, schools, hotels, apartments, barracks, dormitories, jails, prisons and reformatories. In addition, the California Department of Housing and Community Development has greywater standards and DWR has also proposed dual plumbing design standards.

In Arizona, Greywater is defined as water with a BOD5 less than 380 mg/L, TSS<430 and the Fats, Oil, and Grease (FOG) content should be less than 75 mg/L. The Arizona water has issued advice that people should avoid direct contact with grey water. Most Greywater use is by underground drip irrigation since surface irrigation is not permitted.There are three types of use in Arizona: up to a quota of 400 gpd per family (close to 1500 L per day) no permission is required for Greywater use, between 400 and 3000 gpd (1500 and 11,355 L per day, respectively) permission is required and above 3000 gpd (>11,355 L per day) it is considered as conventional wastewater venture. Other limitations include restrictions on contact, restrictions on use on herbaceous food plants, exclusion of hazardous materials and effective separation from surface water run-off. [20]

United Kingdom[edit]

Greywater recycling is relatively uncommon in the UK, largely because the financial cost and environmental impact of mains water is very low. Greywater systems should comply with BS8525 and the Water Supply (Water Fittings) Regulations in order to avoid risks to health.[21]

Greywater from single sewered premises has the potential to be reused on site for ornamental, garden and lawn irrigation, toilet flushing. The reuse options include Horizontal flow reed bed (HFRB), Vertical flow reed bed (VFRB), Green roof water recycling system (GROW), Membrane bioreactor (MBR) and Membrane chemical reactor (MCR).[22]


Although Canada is a water-rich country, they also plan to reuse Grey water. This may be due to the large costs that often accrue to traditional large-scale waste treatment systems in big cities. However, at present, the standards for Greywater reuse are not strict compared with other countries in arid regions.[23]


Household Grey water from a single contaminated site may be reused on-site at the ornamental garden and lawn watering, toilet flushing and laundry uses, depending on the type of Grey water and treatment level. Some people wisely re-use the gross weight, but others use it even worse (without any treatment), such as bathing in the bath or simply transferring laundry water to the lawn where children and pets may be exposed directly. The Department of Health and Community Services (DHCS) focuses on protecting public health and then takes action to control and minimize the public health risks associated with Grey water reuse. Therefore, they issued a few guidelines. Grey water converter must be designed and installed according to the following standards: (1) Greywater treatment must meet the requirements of the health department and remove all clogging and health risk factors (2) Each installation must only be applicable to single family homes (3) Only direct reuse is permitted (4) Minimum maintenance requirements must be specified (5) Greywater must pass through the filtration or sedimentation process (6) Greywater must be applied through underground systems (7) Disinfection Greywater can be applied by the ground system (8) The application should color and mark the area in the area (9) Greywater system must have an automatic overflow device (10) Ensure the soil is easily decomposed throughout the year (11) Greywater system installation, connections must be designed by authorized experts to prevent sewer gases entering from the property (12) All plumbing works must be carried out by plumbers licensed by the Water Co-ordination (Plumber's License) (13) The system of pumps and tanks (containers acting as reservoirs) must be connected according to all safety factors and it is acceptable to manually irrigate the absorbent soil with a bucket (eg: by dividing the wash bowl Greywater into a wash basin), as long as the relevant The health and safety conditions are respected[24]


The government of Cyprus has implemented four water-saving subsidies: drilling installations, drilling with lavatories, installation of hot water circulation systems and installation of Grey water recycling systems.[25]


The emphasis on the use of Grey water in the Hashemite Kingdom of Jordan has two main purposes: water conservation and socio-economic aspects. The Amman Islamic Water Development and Management Network (INWRDAM) in Jordan promoted research on gray water reuse in Jordan. At present, Grey water research in Jordan is funded mainly by the International Development Research Center (IDRC) in Ottawa, Canada, to install and use Grey water systems based on the establishment of small wetland systems in private households. The cost of this system is about 500 US dollars per household. [26]


Grey water Reuse System In Israel, private owners started installing their own private systems at the very outset without any rules or guidelines. They installed a Grey water reuse system to save on water bills. The new Grey water standard in Israel is now the standard document SI-6147 that was finally released after many meetings and discussions after the British standards (BS-8525-1, 2010 and BS-8525-2, 2011).

Grey water reuse is a natural process occurring in the early stages. The Grey water system consists of the following components: (1) Collection system There are many ways for Israeli families to collect gray water. In general, there must be a clear separation between conventional wastewater treatment and Grey water collection systems. To ensure a clear separation, a check valve is installed in the Grey water collection system to allow excess gross weight to pass through the central sewerage system.

(2) Treatment system Treatment systems typically consist of sedimentation elements, screening / filtering components and biological processes, which may also include membrane technology components or constructed wetland components. All of these are intended to treat Grey water and allow them to be used for hospital irrigation and / or toilet flushing. Disinfection elements (small UV lamps and / or chlorinated) are often included in the process.

(3) Storage of processed Grey water Before processing and before processing the Grey water, the original Grey water is stored and then "applied". Treated gross weight should not be detained for more than a few days. Tanks should be located on the ground or underground, but not at risk to local residents. Storage should be made from prefabricated materials on site.

(4) Pumping parts (collection system and distribution system) The size of the pumping equipment should be determined by the family size and flow rate. It should be located in a safe area to ensure that the health of residents is not compromised in the event of a spill and spill.

(5) System maintenance Maintenance is regulated by local authorities. It mainly refers to the normal operation of the reuse system without interfering with the private property of a single resident. Maintenance refers to the annual normal operation of the system. The disinfection system runs continuously and is monitored twice a year for proper system operation.

(6) Water quality The general stage of Grey water treatment is based on the following principles:

1) Filtering and precipitation.

2) Requires concentrations of BOD5 ( Biochemical Oxygen Demand ) and TSS not to exceed 20 mg / L and 30 mg / L, respectively.

3) Treated Grey water distribution system outside the house must be marked with a special symbol that the Grey water is flowing through the network (in Hebrew, Arabic and English).

4) Although it is highly recommended not to exceed 12 hours, the original gross weight can be kept for 24 hours. This is to prevent odor development and germ regeneration.

5) Treated Greywater can be stored for 7 days to prevent additional growth and development of bacteria under anaerobic conditions.

6) Care must be taken not to connect the Greywater to the main water supply.

7) The front of the system must be connected to the one-way flow valve to prevent mixing Greywater and clean tap water.

8) When chlorine disinfection (or equivalent) is used, the residual chlorine content at the remote outlet of the system must be 0.5 mg / L after a residence time of at least half an hour.[27]


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  3. ^ "Sustainable Earth". 28 November 2017. 
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  7. ^ a b c Tilley, Elizabeth; Ulrich, Lukas; Lüthi, Christoph; Reymond, Philippe; Zurbrügg, Chris. Compendium of Sanitation Systems and Technologies (2nd ed.). Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag). ISBN 978-3-906484-57-0. Archived from the original on 2017-09-01. 
  8. ^ Juan, Yi-Kai; Chen, Yi; Lin, Jing-Ming (19 November 2016). "Greywater Reuse System Design and Economic Analysis for Residential Buildings in Taiwan". Retrieved 28 November 2017. 
  9. ^ Lets Go Green Practical Alternatives to Sewer and Septic Systems Archived 2009-01-30 at the Wayback Machine.
  10. ^ Busgang A, Friedler E, Ovadia O, Gross A. Epidemiological study for the assessment of health risks associated with greywater reuse for irrigation in arid regions. Science Of The Total Environment [serial online]. December 25, 2015;538:230-239.
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  14. ^ S. Sharvelle; L.A. Roesner; Y. Qian; M. Stromberger (2010). "Long-Term Study on Landscape Irrigation Using Household Graywater-Experimental Study" (PDF) (Interim Report). Colorado State University. Archived (PDF) from the original on 2013-04-09. 
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  19. ^ California Air Resources Board. AB 32 Scoping Plan. 2008.
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