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Greywater, sometimes spelled graywater, grey water or gray water and also known as sullage, is non-industrial wastewater generated from domestic processes such as washing dishes, laundry and bathing. Greywater comprises 50-80% of residential wastewater. Greywater is distinct from blackwater in the amount and composition of its chemical and biological contaminants (from feces or toxic chemicals). Greywater gets its name from its cloudy appearance and from its status as being neither fresh (white water from groundwater or potable water), nor heavily polluted (blackwater). According to this definition wastewater containing significant food residues or high concentrations of toxic chemicals from household cleaners etc. may be considered "dark grey" or blackwater.

In recent years concerns over dwindling reserves of groundwater and overloaded or costly sewage treatment plants has generated much interest in the reuse or recycling of greywater, both domestically and for use in commercial irrigation. However, concerns over potential health and environmental risks means that many jurisdictions demand such intensive treatment systems for legal reuse of greywater that the commercial cost is higher than for fresh water. Despite these obstacles, greywater is often reused for irrigation, illegally or not, in older rural construction, simple construction old and new, often consisting of nothing more than a "drain out back" (pipe pointed down the nearest hill). In droughtzones or areas hit by hose pipe bans (irrigation restrictions) greywater can be harvested informally by manual bucketing. In the third world, reuse of greywater is often unregulated and is common. At present, the recycling of greywater is poorly understood compared with elimination.


Elimination of greywater

Domestic wastewater is usually combined at the sewer, so that grey and black waters are removed together using a shared sewerage system. Sewage water can then be treated to limit pollution and health risks, before being returned to the environment at large. The majority of greywater ends up as effluent in rivers and oceans in this way. Despite treatment, this arguably results in greater contamination of natural waters, as the natural purification capacity of surface water is millions of times less than that of soil. Simply dumping greywater on the soil, from an ecological standpoint, is less damaging than sending highly treated greywater directly into natural waters.

There has not been one documented case of greywater transmitted illness in the U.S.[1] This suggests that the reuse of greywater could represent a safe way to conserve natural water supplies and keep natural freshwater free of contamination, and shows that the elimination of greywater is not the most efficient way to deal with it. Pouring greywater onto the soil is a better alternative to deal with it rather than pouring it down the drain because the soil acts as a natural filtration system. There are other alternatives to eliminating greywater that allow for efficient use; using it to irrigate plants is a common practice. The plants use the alleged contaminants of greywater, such as food particles, as nutrients in their growth.[1] Treating greywater before using it to irrigate plants is like treating water then pouring it into the drain; it's a pointless practice. The elimination of greywater in sewage treatment plants is a low priority to many environmental conservationists because of its many possible uses and alternatives to elimination are highly recommended by online sources.[2]

Recycling of greywater

Most greywaters are much easier to treat and recycle than blackwaters, due to their lower levels of contamination. However, entirely untreated greywater is still considered to be a potential health and pollution hazard, because studies have established the presence of the same micro-organisms within greywater as found in sewage (albeit in much lower concentrations). Nevertheless, whilst all greywater will contain micro-organisms the health hazards associated with greywater from a multiple dwelling source should be considered different from that of a single dwelling greywater source. Within single dwellings inhabitants and their clothing are mutually exposed to each other's greywater and their shared living arrangements will likewise expose them to the existing reservoir of micro-organisms within the dwelling, whereas greywater from multiple dwelling sources provides scope for exposure to a broader reservoir of micro-organisms thus increasing the risk of disease spread between dwelling unit inhabitants.

If collected using a separate plumbing system to blackwater, domestic greywater can be recycled directly within the home and garden and used either immediately or processed and stored. Recycled greywater of this kind is never clean enough to drink, but a number of stages of filtration and microbial digestion can be used to provide water for washing or flushing toilets; relatively clean greywater may be applied directly from the sink to the garden, as it receives high level treatment from soil and plant roots. Given that greywater may contain nutrients (e.g. from food), pathogens (e.g. from your skin), and is often discharged warm, it is very important not to store it before using it for irrigation purposes, unless it is treated first.

The simplest greywater system is to simply divert the water directly to the garden. Regulations change by country and region, but common guidelines for safe usage include not storing the greywater for more than 24 hours, ensuring it cannot pool or run off, and depositing it with subsurface irrigation. Greywater diversion systems can be both designed-in to new homes, or retrofitted to many existing homes. When systems are fully designed, manufactured and installed to relevant standards such as the Australian Watermark standards, like the Waterwise system, they are highly efficient, effective and safe.

There are numerous "soft" processes based on natural biological principles such as using reedbed filter systems, the wetpark systems or the living wall that can be used to clean up greywater.

There are also "hard", direct processes, such as distillation (evaporation) which need not necessarily be as energy intensive as they might initially appear. There seem to be no commercially available "hard" greywater recovery devices suitable for on-site use in the individual household, even though a number of such technologies exist.

Some municipal sewerage systems recycle a certain amount of grey and black waters using a high standard of treatment, thus providing reclaimed water for irrigation and other uses.

Application of recycled greywater


Greywater typically breaks down faster than blackwater and has much less 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 soil surface where possible (e.g. in mulch filled trenches) and not sprayed, as there is a danger of inhaling the water as an aerosol.

However, long term research on greywater use on soil has not yet been done and it is possible that there may be negative impacts on soil productivity. If you are concerned about this, avoid using laundry powders; these often contain high levels of salt as a bulking agent, and this has the same effect on your soil as a drought.

Indoor reuse

Recycled greywater from showers and bathtubs can be used for flushing toilets, which saves great amounts of water. Many attempts at this have been made in Germany. However, untreated greywater cannot be used as flush-water as it will start to smell and discolor the flush toilet fixture if left for a day or more.

The level of treatment required in this case requires the water to have low or nil biochemical oxygen demand, but it is not necessary for it to be treated to the same standards as potable water. Greywater recycling for toilet flushing is currently considered to be uneconomical or environmentally unfriendly at most domestic levels. As an alternative to treatment, a South African Company is manufacturing and distributing the GardenResQ product in both South Africa and Australia - a grey water diversion systems that allows all household grey water to be automatically diverted to the garden for irrigation purposes.

Extreme living conditions

Greywater use promotes the ability to build in areas unsuitable for conventional treatment, or where conventional treatment is costly. The Mars Desert Research Station utilizes greywater recycling for this use, and might be used on trips to Mars to reduce water consumption and increase oxygen generation.

Heat reclamation

Devices are currently available that capture heat from residential and industrial greywater, through a process called "Drainwater Heat Recovery" or "Greywater Heat Recovery." 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.

Greywater and the environment

The potential ecological benefits of greywater recycling include:

  • Lower fresh water extraction from rivers and aquifers
  • Less impact from septic tank and treatment plant infrastructure
  • Topsoil nutrification
  • Reduced energy use and chemical pollution from treatment
  • Groundwater recharge
  • Plant growth
  • Reclamation of otherwise misdirected nutrients

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.

Move towards ecologically sustainable development

Because greywater use, especially domestically, reduces demand on conventional water supplies and pressure on sewage treatment systems, its use is very beneficial. In times of drought, especially in urban areas, greywater use on gardens or in toilet systems helps to achieve Ecologically Sustainable Development by helping to meet its principles.

Governmental Regulation

Government regulation governing domestic greywater use for garden 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 most jurisdictions to be 'Sewage’ (all wastewater including greywater and toilet waste) and under that definition greywater is commonly 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 garden irrigation was either simply not permitted or was discouraged by expensive and complex governmental sewage system approval requirements. Wider legitimate community greywater diversion for garden 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, even when generous cash rebates have been offered by governments.

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 potential 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 contact with that greywater are in reality an insignificant risk, if properly managed without the need for complex, expensive and onerous red tape approval processes. The most recent examples are reflected in the NSW Government Department of Water and Energy's newly released greywater diversion rules, and the recent passage of greywater legislation in the state of Montana.[3]

See also


  1. ^
  2. ^
  3. ^ "SAVE works for passage of graywater legislation", Shelby Promoter, 2007-05-02. Retrieved on 2007-05-09. 

External links

  • Greywater Central
  • Alternative Technology Association System Study Results 1.38MB
  • Greywater Diversion
  • Grey/Drainwater Heat recovery
  • Greywater Guerrillas: Do-it-yourself greywater sinks, showers, and toilets for the home
  • Waterwise Systems Australia: Complete Greywater diversion solutions either fully installed or DIY
  • Dam Nation: Dispatches from the Water Underground
  • NSW Government Greywater
  • Greywater information, projects and how-tos
  • Center for the Study of the Built Environment's Graywater Reuse Project
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Greywater". A list of authors is available in Wikipedia.
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