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Greywater or sullage is defined as wastewater generated from wash hand basins, showers and baths, which can be recycled on-site for uses such as toilet flushing, landscape irrigation and constructed wetlands. Greywater often includes discharge from laundry, dishwashers and kitchen sinks. It differs from the discharge of toilets which is designated sewage or blackwater to indicate it contains human waste.
There are other alternatives to eliminating greywater that allow for efficient use; using it to irrigate plants is a common practice. 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.
Most greywater is easier to treat and recycle than blackwater, 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 stages of filtration and microbial digestion can be used to provide water for washing or flushing toilets. Some greywater may be applied directly from the sink to the garden or container field, receiving further treatment from soil life and plant roots. Given that greywater may contain nutrients, pathogens, and is often discharged warm, it is very important to store it before use for irrigation purposes, unless it is properly treated first.
At present, several water recycling systems exist which can be used to:
Water recycling without purification is used in certain agricultural companies (e.g., tree nurseries) and 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). This water system also needs a supply of water to recycle and reuses water as well. It is also 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. Water purification/decontamination systems are used for applications where potable water is required (e.g., to allow drinking, and/or for other domestic tasks as washing, showering).
For filtering the water to become potable (or near-potable), there are numerous systems based on soft processes. These include natural biological principles such as
Finally, "hard", direct processes, such as distillation (evaporation) or mechanical processes such as membrane filtration, (typically ultrafiltration and reverse osmosis, which are capable of treating high volumes of grey water) can create potable, or near-potable water.
In order to purify the potable water adequately, several of these systems are usually combined to work as a whole. Combination of the systems is done in two to three stages, using a primary and a secondary purification. Sometimes a tertiary purification is also added.
Some municipal sewage systems recycle a certain amount of grey and blackwater using a high standard of treatment, providing reclaimed water for irrigation and other uses.
Greywater typically breaks down faster than blackwater and has lower levels of nitrogen and phosphorus. However, all greywater must be assumed to have some blackwater-type components, including pathogens of various sorts. 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 essential to put nothing toxic down the drain—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. 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.
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:
The Uniform Plumbing Code, adopted in some United States jurisdictions, prohibits greywater use indoors.
Greywater use promotes the ability to build in areas unsuitable for conventional treatment, or where conventional treatment is costly. The Mars Desert Research Station uses greywater recycling, which might be used on trips to Mars as a source of potable water and as a feedstock for oxygen regeneration.
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.
Because greywater use, especially domestically, reduces demand on conventional water supplies and pressure on sewage treatment systems, its use is very beneficial to local waterways. 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
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.
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 sub surface 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 simply 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 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 complex, expensive and onerous red tape approval processes. This is reflected in the NSW Government Department of Water and Energy's newly released greywater diversion rules, and the recent passage of greywater legislation in Montana. 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.
Strict permits 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. 
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.
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 installing both 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.