trees in Dominica,
West Indies. Photo
taken by PCV
is grey water?
Grey water is considered all used
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:
What are the
- 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
- 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
- Grey water application as an
irrigation source also capitalizes on a reliable and constant water
- 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.
- 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.
Dominica, West Indies. Photo care of Emily Stewart, Peace Corps
What plants should
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
- Australian Tea
- Banana trees
- Fan and Date Palms
- Stone Pine
- Bermuda grass
- Star Jasmine
For an excellent database of these and many other plant species, visit
. Scroll to the bottom to search the site.
- 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
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:
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
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:
For an extensive list of other common mistakes, visit the
- Bleeding Hearts
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.
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
- 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.
- 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
Water Treatment Marsh", Humboldt State University Campus Center for
aAppropriate Technology, 2005, http://www.humboldt.edu/~ccat/waterconservation/frames.html.
MS Candidates in Environmental Engineering at Michigan Technological
Master's International Peace Corps volunteers in Jamaica and East
Date created: 15 March 2005
Last updated: 28 April 2005