Greywater

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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 greywater may exclude wastewater containing significant food residues or high concentrations of toxic chemicals from household cleaners etc.

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 greywater that the commercial cost is higher than for fresh water. Despite these obstacles, greywater is often reused for irrigation, illegally or not, in drought zones or areas hit by hose pipe bans, typically by manual bucketting. 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.

1 Elimination of greywater
2 Recycling of greywater
3 Application of recycled greywater
4 Greywater and the environment
5 Move towards Ecologically Sustainable Development
6 See also
7 External links

[edit] Elimination of greywater

Domestic wastewater plumbing 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.

[edit] 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.

If collected using a separate plumbing system to blackwater, domestic greywater can be recycled directly within the home and garden. 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.

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.

[edit] Application of recycled greywater

[edit] Irrigation

Greywater typically breaks down faster than blackwater and has much less nitrogen and phosphorus -- see http://www.greywater.com. 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.

[edit] Domestic use

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 (BOD), 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. However, a Quebec company, Brac Systems, has recently patented a cost-effective system that uses a chlorine treatment, and is selling the system in the Americas, Europe, and Australia.

[edit] 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.

[edit] Heat reclamation

Devices are being developed which 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 is first pumped through a heat exchange in which it is pre-warmed by heat from greywater flowing out from such activities as dishwashing, or showering.

[edit] 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. See the Greywater Policy Center, http://oasisdesign.net/greywater/law/index.htm .

[edit] 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.

[edit] See also

[edit] External links

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