Greywater is wastewater generated from domestic activities such as laundry, dishwashing, and bathing which can be recycled on-site for uses such as landscape irrigation and constructed wetlands. Greywater differs from water from the toilets which is designated sewage or blackwater to indicate it contains human waste.
Greywater composes 50–80% of residential wastewater generated from all of the house's sanitation equipment (except toilets).
Greywater gets its name from its cloudy appearance and from its status as being neither fresh (white water from groundwater or potable water), nor polluted (sewage). Wastewater containing significant food residues or high concentrations of toxic chemicals from household cleaners, etc., may be considered "dark grey".
In recent years, concerns over dwindling reserves of groundwater and overloaded or costly sewage treatment plants have generated much interest in the reuse or recycling of greywater, both domestically and for use in commercial irrigation.
A lack of knowledge and experience with modern greywater systems has led to some concern over potential health and environmental risks. If properly designed a greywater system will lower sewage costs, at the same time reducing water usage for irrigation and water quality can be increased to better than the government supplied initially. Less water enters the city sewage systems saving the government from building new treatment plants. Some jurisdictions demand highly intensive treatment systems for legal reuse of greywater whereas some encourage greywater use. Oregon, on June 12, 2009, passed a law which legalizes and encourages the use of greywater. Despite many obstacles, greywater is often reused for irrigation legally and illegally.
Anyone, including those in droughtzones or areas hit by hose pipe bans (irrigation restrictions), can harvest greywater by manual bucketing shower or sink water and using them to flush toilets. In the third world, reuse of greywater is often unregulated and is common. At present, the recycling of greywater is poorly understood by many compared with elimination.
Elimination of greywaterEdit
Domestic wastewater is usually combined at the sewer, so that grey- and blackwaters are removed together using a shared sewerage system in a process called elimination.
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. While all greywater contains micro-organisms[vague], the health hazards associated with greywater from a multiple-dwelling source should be considered differently from that of a single dwelling greywater source.
One may recycle greywater at home by putting a bucket in the shower or sink and using the water to flush the toilet.
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. 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 not to store it before use in irrigation purposes, unless it is properly treated first.
At present, several water recycling systems exist which can be used to:
- recycle the water without purifying it
- recycle the water while purifying or decontaminating it
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).
Water recycling systems without purificationEdit
Water diversion systemsEdit
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. Water diversion systems tend to be highly efficient, effective and safe for simple applications where potable water is not required.
Diversion systems can be as basic as running the outlet hose from a washing machine out a window to the garden, or can be designed as a permanent part of the home plumbing. Fully engineered systems incorporate a sump pump and surge tanks and deliver the water through sub-surface irrigation.
Greywater from the shower or bath is generally good quality water for the garden. The soap levels at the dilutions typical are actually good for the garden as they are a wetting agent. When laundry greywater is diverted to the garden, the laundry products used must be chosen carefully to ensure phosphate and salt levels are low, and that pH balance is neutral.
Basic guidelines are also available from system suppliers. It is essential that greywater is diverted to sewer when garden-unfriendly products are being used.
Water recycling with purificationEdit
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
- mechanical systems (sand filtration, lava filter systems and systems based on UV radiation)
- biological systems (plant systems as treatment ponds, constructed wetlands, living walls) and Bio reactors or compact systems as activated sludge systems, biorotors, aerobic and anaerobic biofilters, submerged aerated filters, biorolls[vague] 
Finally, "hard", direct processes, such as distillation (evaporation) or mechanical processes such as "Membrane Filtration", (typically "Ultra Filtration" and "Reverse Osmosis," which are capable of treating high volumes of grey water) can create potable, or near-potable water. 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.
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.
Application of recycled greywaterEdit
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.
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.
In any greywater system, it is essential to put nothing toxic down the drain—no bleaches, bath salts, artificial dyes, cleansers, and no products containing boron (which is toxic to plants at high levels).
It is crucial to use all-natural, biodegradable soaps whose ingredients do not harm plants. Most powdered detergents, and some liquid detergents, are sodium-based, which can inhibit seed-germination and destroy the structure of clay soils.
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 United States jurisdictions, prohibits greywater use indoors.
Extreme living conditionsEdit
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, and might be used on trips to Mars to reduce water consumption and increase oxygen generation.
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.
Lee Valley Ice Centre in Leyton, London, is the first example of the use of a greywater system in an ice arena context, a concept which reduces the otherwise considerable water use associated with such activities.
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 the goals of ecologically sustainable development.
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
- Increased plant growth
- 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.
Potential downsides of greywater recyclingEdit
The Southern Nevada Water Authority has expressed concern that greywater recycling for use in residential washing machines and for watering xeriscapes could encourage greater water use, and reduce the amount of water returned to Lake Mead, a reservoir currently experiencing a drought.
Greywater users and advocates dispute these claims.
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.
- Campus Center for Appropriate Technology (CCAT)
- Ecological sanitation
- List of waste water treatment technologies
- Organisms used in water purification
- Reclaimed water
- Water conservation
- Water purification
- ↑ Duttle, Marsha (January 1990). "NM State greywater advice". New Mexico State University. http://aces.nmsu.edu/pubs/_m/m-106.html. Retrieved 23 January 2010.
- ↑ "Greywater AUS". Sustainable Gardening Australia. http://www.sgaonline.org.au/info_greywater_update.html. Retrieved 23 January 2010.
- ↑ Overview of biological systems with pictures
- ↑ Lets Go Green Practical Alternatives to Sewer and Septic Systems
- ↑ Stephanie Tavares, "Legislature wades into water law", In Business Las Vegas, March 20–26, 2009.
- ↑ "SAVE works for passage of graywater legislation". Shelby Promoter. 2007-05-02. http://www.goldentrianglenews.com/articles/2007/05/02/shelby_promoter/news/news5.txt. Retrieved 2007-05-09.
- ↑ Gray water law is a good step forward, | The Montana Standard | 2009-04-01 | url =http://www.mtstandard.com/articles/2009/04/02/opinion/hjjajfjihgjfhd.txt |
- Greywater at the Open Directory Project
- NSW Government Greywater
- Center for the Study of the Built Environment's Graywater Reuse Project
- UK Environment Agency: Reusing greywater and harvesting rainwater
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