What is Grey Water?
Benefits
Considerations
Plant Selection
Tips
Common Mistakes
Commercial Systems
References

What is grey water?

Grey water is considered all used water that is discharged from a house, excluding black (toilet) water.  This includes water from shwers, baths, sinks, dishwashers, laundry tubs, and washing machines.

Grey water commonly contains soap, shampoo, toothpaste, food scraps, cooking oils, detergents, and hair.

To learn more about grey water, visit websites:

• Greywater Re-Use: Hardware, Health, Environment and the Law
• Greywater: What is is... How to treat it...  How to use it
• Safe Use of Household Greywater
  

What are the benefits?

• Grey water is a valuable resource for landscaping, or encouraging plant growth in arid climates.
• In the presence of micro-organisms, grey water decays rapidly.  This is far more effective in active, live topsoil, than if it is discharged into a stream. 
• Grey water contains one-tenth the nitrogen content of black water (from toilets).  Half of this nitrogen is organic, which is more easily filtered and removed by biological uptake in plants. 
• On-site grey water treatment reduces the volume of wastewater that must be diverted to more costly sewage and septic treatments.
• Grey water can be reused for toilet flushing, or irrigation of trees, ornamentals, and even some food crops. 
• Grey water diversion is particularly well-suited for small-scale or decentralized wastewater systems, but can be implemented in either a rural or urban setting.
• Small-scale grey water treatment and reuse reduces construction and maintenance costs associated with sewer and septic system infrastructure, pumping, and storage.
• Localized grey water systems decrease freshwater use for transportation and treatment of wastewater.
• Valuable freshwater sources are protected, as diverting grey water from the stream of wastewater reduces nutrient loading and contamination of surface waters.
• It is rich in phosphorous, potassium, and nitrogen, making it a good nutrient or fertilizer source for irrigation.
• Grey water application as an irrigation source also capitalizes on a reliable and constant water source.
• The use of grey water for irrigation reincorporates nutrients from the waste stream into the land-based food chain, rather than contributing to surface and ground water pollution via sewers and septic systems. 

Considerations

• It is important to evaluate the local climatic conditions (available sunlight, precipitation, temperatures) in selecting appropriate plants to be grown in grey water.  Selecting plants that complement the soil and water chemistry is also beneficial in promoting more healthy, vibrant plants.  See plant selection for more information about complementary plant and soil types, as well as ideal growing conditions for individual plants. 
  

• The scale of the grey water use project depends on the available existing local infrastructure and treatment schemes for wastewater, as well as the population being served.   For smaller volumes of household grey water, it is simplest to collect the wastewater in a bucket and directly apply it to the plants.  On the other hand, it is possible to retrofit the plumbing system of a home to divert grey water on a larger scale.  A larger, collective grey water garden or treatment lagoon can be established to meet the wastewater disposal needs of a conglomeration of houses or a public facility.  For more information about commercial grey water use systems, see the commercial systems section.
  

• The source conditions of the wastewater can have an impact on the ability to use the resulting grey water for growing plants.   For example, if polluted waters are used as the water source, this pollution may adversely affect plant growth.  This would serve to limit the potential benefits of using grey water to reduce the waste stream and provide supplimental nutrients for plant growth.

• Local regulations regarding the quality of wastewater effluent may call for additional treatment measures, particularly in the cases of large volumes of wastewater or application on food crops.  The regulations target standard safety concerns, such as pathogen and pollutant levels, and proximity of grey water application source to ground and surface waters.

Bananas in Dominica, West Indies.  Photo care of Emily Stewart, Peace Corps Volunteer. 

What plants should you use?

Depending on the climate and soil conditions, certain plants are better adapted for use in grey water treatment schemes.  Here are a few examples of plants that grow well using grey water.  Click here to find more information about each plant, and to select the ideal plants for grey water treatment in your climate zone:

Food Crops and Oil Producers

• Australian Tea
• Banana trees
• Fan and Date Palms
• Olive
• Rosemary
• Stone Pine

Ornamentals

• Agapanthus
• Bermuda grass
• Bougainvillea
• Cottonwood
• Honeysuckle
• Juniper
• Oleander
• Redwoods
• Star Jasmine

Mulchable Species

• Reed beds

For an excellent database of these and many other plant species, visit the Floridata Encyclopedia.  Scroll to the bottom to search the site.

Tips

• Introduce plants that are tolerant of alkaline soils.  Grey water containing laundry detergents typically includes sodium, potassium, and calcium, all alkaline chemicals.  If the soil alkalinity is too high (pH>8.0) or if the desired plants prefer acid-soils, agricultural sulfur or acidifying fertilizers can be added, as well as organic matter.

• Well-drained, sandy soils work best for direct grey water application.  A more clay-rich soil should be supplemented with organic material (mulch or compost) to promote infiltration and prevent the degradation of clay minerals by the grey water.  Otherwise, the soil becomes sticky, and may act as a trap for sodium and other chemical constituents of the wastewater.  In regions with a rainy season, any build up of salts in the soils will be leached away by heavy rains.  Arid climates and those regions characterized by only light rains may require occasional application of fresh water to purge the salts from the soil.
  

• Too much water can lead to soil saturation, which can also cause problems in a grey water irrigation system.  The soil must dry out between irrigation applications, as excessive amounts of sodium, boron, and chlorine can result in leaf burn, chlorosis, premature leaf drop, branch and twig die back, as well as reduced growth.  Taking measures to improve soil drainage and introducing plants with a high tolerance for sodium and chloride will help reduce the risk of these symptoms of plant disease associated with grey water


• For further tips, visit the following website:

Common mistakes

Avoid making a system that is too complex for the situation.  If it involves a large amount of maintainence someone must be available to do the work.  Often transporting water by hand instead of pumping it is the best idea.

Grey water should not be stored or it will quickly turn into blackwater.  A maximum amount of time before application to biologically active soil should not exceed 24 hours.

Do not apply grey water directly to any part of a plant that you will eat.  Pathogen contamination is possible.  This is why grey water is best used for fruit trees and ornamental plants.

Some acid-loving plants cannot handle grey water.  Also be careful applying it to plants during the dry season that will not be able to handle the change.

Plants that  should be avoided include:

• Azaleas
• Begonias
• Bleeding Hearts
• Camellias
• Ferns
• Foxgloves
• Hydrangeas
• Impatiens
• Oxalis
• Philodendrons
• Primroses
• Rhododendrons
• Violets

For an extensive list of other common mistakes, visit the following website:

• Oasis Design: Common Grey Water Errors and Preferred Practices

Higher Volume Grey Water Treatment

• For treatment of larger volumes of wastewater from communal laundry stations, showers, and kitchen (only water with little-to-no organic kitchen waste) and hand-washing, some pre-treatment stages may improve the quality of the effluent water for irrigation.  Wastewater that has had no contact with black water (from toilets) can be dumped or piped into a settling tank, so that large particles can be removed.  The grey water must then be quickly transferred to well-aerated soil, however, so that macro- and microorganisms can begin to break down the pathogens and nutrients contained in the water.  Food particles trapped in the settling tank will begin to anaerobically decompose, producing undesirable odors.  It is therefore advisable to eliminate solid organic kitchen waste prior to its entry into the grey water system.  Otherwise, the pre-treatment tank would require more regular cleaning of the screens.
  

• In a more developed or urban setting with household plumbing, it is possible to divert grey water from sewer or septic systems with a separate piping systems for grey and black water.  Some commercial grey water treatment systems are currently on the market for household use in the developed world.  The grey water emanating from a kitchen sink would contain more organic waste and fat and oil residues, and a three-stage grey water treatment system is preferable.  The grease and food residue would remain in the three-stage septic tank, where it would undergo anaerobic digestion and be converted to sludge.  This sludge would need to be removed every four years, requiring more maintenance to the system.  The remaining grey water would pass through the septic tank, into a sand filter for further aerobic treatment, and finally into a planter or garden plot.  The planter containers can be an advantage in regions with poor soil quality (particularly fast or slow infiltration rates, shallow topsoil, nutrient-poor, etc.).  The raised box can also be beneficial in protecting the plants from pests and scavengers, as well as reducing back-strain to the gardeners.
  

• In a colder climate, biological activity slows down or stops during the winter.  The grey water treatment is less effective under these conditions; however, the temperature of the water is often sufficient to warm the soil surrounding the injection area, and continue some level of biological activity.  Another insulator can be created by filling the raised planters with leaves and yard waste.  The composting process also produces heat to further buffer the biological break down of grey water from the cold.           

 References

• The following sites were valuable resources in compiling this website about grey water treatment.  Please click on the links for further information about commercial grey water treatment guides, schemes, and systems.

• “Greywater: What it is…How to treat it…How to use it”, Carl Lindstrom, 2000, http://www.greywater.com/index.htm.
• “Smart Communities Network Success Stories: NutriCycle Systems LLC Composting Toilet/Greywater System”, U.S. Department of Energy, 2005, http://www.sustainable.doe.gov/success/hanson_assoc.shtml.
• “Grey Water Central”, Oasis Design, Art Ludwig, 2004, http://www.oasisdesign.net/greywater/index.htm.
• “Grey Water Treatment Systems: Nature Clear”, Nature Loo, 2004, http://www.nature-loo.com.au/greywater/natureclear/natureclear.html.
• “Safe Use of Household Greywater”, Marsha Duttle, New Mexico State University, 1994, http://cahe.nmsu.edu/pubs/_m/m-106.html. 
  
• "Grey Water Treatment Marsh", Humboldt State University Campus Center for aAppropriate Technology, 2005, http://www.humboldt.edu/~ccat/waterconservation/frames.html.