vvEPA
United States
Environmental Protection
Agency
Off ice of Water
Washington, D.C.
EPA 832-F-00-033
September 2000
Decentralized Systems
Technology Fact Sheet
Evapotranspiration
DESCRIPTION
Evapotranspiration (ET) is a method of onsite
wastewater treatment and disposal that offers an
alternative to conventional soil absorption systems
for sites where protection of the surface water and
groundwater is essential. An ET system disposes of
wastewater into the atmosphere through
evaporation from the soil surface and/or
transpiration by plants, without discharging
wastewater to the surface water or groundwater
reservoir. ET can offer flexibility by combining
seepage with evaporation when absolute protection
of the groundwater or surface water is not required.
An ET system is a feasible option in semi-arid
climates where the annual evaporation rate exceeds
the annual rate of precipitation. The amount that
evaporation exceeds precipitation is the wastewater
application capacity. The different design
configurations of ET are discussed in more detail in
the sections that follow.
Process
Evapotranspiration is the net water loss caused by
evaporation of moisture from the soil surface and
transpiration by vegetation. Three conditions must
be met for continuous evaporation. First, it requires
latent heat of approximately 590 cal/g of water
evaporated at 15 °C. Second, a vapor pressure
gradient between the evaporative surface and the
atmosphere must exist to remove vapor by
diffusion, convection, or a combination of the two.
Third, there must be a continuous supply of water
to the evaporative surface.
Evapotranspiration is also influenced by vegetation
on the disposal field. Theoretically, ET can remove
high volumes of effluent in the late spring, summer,
and early fall, especially if large silhouette and good
transpiring bushes are present.
There are three main types of evapotranspiration
systems; ET, evapotranspiration/absorption(ETA),
and mechanical.
The first type, an ET system, is the most common.
The main components are a pretreatment unit
(usually a septic tank or an aerobic unit) used to
remove settleable and floatable solids and an ET
sand bed with wastewater distribution piping, a bed
liner, fill material, monitoring wells, overflow
protection, and a surface cover. Vegetation must be
planted on the surface of the bed to enhance the
transpiration process.
The septic tank effluent flows into the lower portion
of a sealed ET bed equipped with continuous
impermeable liners and carefully selected sands.
Capillary action in the sand causes the wastewater
to rise to the surface and escape through
evaporation as water vapor. In addition, vegetation
transports the wastewater from the root zone to the
leaves, where it is transpired as a relatively clean
condensate. This design allows for complete
wastewater evaporation and transpiration with no
discharge to nearby soil.
Figure 1 shows a cross-sectional view of a typical
ET bed. Although this design may be acceptable in
certain sites, local and state regulations should be
checked to ensure approval.
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The second type of evapotranspiration system is
known as ETA. In addition to evaporation and
transpiration, percolation also occurs through an
unsealed bed. This design provides discharge to
both the atmosphere and to the subsurface.
NO? TO SCALi
Source: copyright © Water Environment Federation,
reprinted with permission, 1999.
FIGURE 1 CROSS SECTIONAL VIEW OF
A TYPICAL EVAPOTRANSPIRATION BED
The third type of evapotranspiration system, which
involves the use of mechanical devices, is still
under development. There are two types of
mechanical evaporation systems, both of which
require a septic tank for pretreatment and storage
tank. The first type consists of a rotating disk unit,
in which the disks rotate slowly, providing a large
surface area for the wastewater to evaporate.
The second type of mechanical ET system is a
concentric cylinder unit, where forced air enters the
center of the cylinder, moves outward through
wetted cloth wraps, and is discharged as vapor.
Mechanical systems use little electricity and
require minimal maintenance, which makes them
attractive options for individual home wastewater
disposal in regions where evaporation exceeds
precipitation.
APPLICABILITY
groundwater, relatively impermeable soils, absence
of fractured bedrock, or other conditions that put
the groundwater at risk. ET systems perform well
in semi-arid and arid locations. In certain parts of
the United States, ET systems are feasible for
homes, outdoor recreation areas, and highway rest
areas. It is important to note that assessment of the
reliability of the system requires micro-climatic
data.
Boyd County Demonstration Project
A demonstration site was set up about five miles
from the Huntington Airport in Kentucky, in an
area with low population density and rough
topography. Approximately 60 families live in the
sanitary district. The demonstration project serves
47 families, with 36 individual home aeration
treatment plants and two multi-family aeration
plants which serve 11 families. Six manufacturers
provided 16 stream discharge units, two spray
irrigation units, one ET unit, and 19 subsurface
field discharge units. Four recycle units serving
five homes produced clear, odorless water.
The ET unit is 2,000 square feet (two 1,000 square
foot beds) designed for disposing effluent from a
Cromaglass model C-5 aeration plant. The beds are
sealed with plastic to keep the high ground water at
the site from flooding them. They contain 8 inches
of gravel, 18 inches of sand, and are covered with
topsoil and planted with grass and junipers. They
are crowned to shed rainwater.
The Kentucky test provided valuable data on how
the system handles variations in loading rates.
Although the ET beds were designed for a family of
four, seven people lived at the site which increased
water usage, yet the ET system continued to
perform well with only one small modification to
the distribution box. Before installation of the ET
beds, raw sewage pooled in the yard of this house
from a nonfunctioning septic tank and soil
absorption field. Despite high rainfall, the ET
system continues to perform satisfactorily.
Onsite systems with ET disposal are appropriate in
locations with a shallow soil mantle, high
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Leigh Marine Laboratory, University of
Auckland, New Zealand
Leigh Marine Laboratory, a research institution on
the New Zealand coastline about 62 miles north of
Auckland, has an ETA system which was installed
in 1982. It has a design load to support 35 persons
(including residents and day visitors) at 4,565 L/d
(1,180 gallons per day) total flow. Three septic
tanks feed a sump pump that discharges through a
400 m rising force main, to an ETA bed system on
an exposed grass ridge 70 m above the laboratory
complex.
There is a loading factor of 1.0, an ETA loading
rate of 10 mm per day for beds, and an areal rate
(including spaces between beds) of 3.75 mm per
day. This system includes extensive groundwater
and surface water drainage controls. The total bed
area is 450 m2 divided into 20 beds, each 15 m by
1.5 m, arranged in four groups of five beds, with
each group dose loaded for one week and rested for
three.
Since their commissioning, the ETA beds have
performed as predicted: in the summer, capillary
action in the sand draws effluent to support
vigorous grass growth; in the winter, the effluent
gradually accumulates for storage and disposal
during drier weather. The system is currently
loaded between 80 and 90 percent of its capacity
and is performing successfully.
ADVANTAGES AND DISADVANTAGES
Listed below are some advantages and
disadvantages of ET systems.
Advantages
• ET systems may overcome site, soil, and
geological limitations or physical constraints
of land that prevent the use of subsurface
wastewater disposal methods.
• The risk of groundwater contamination is
reduced with ET systems that have
impermeable liners.
Costs are competitive with other onsite
systems.
ET systems can be used to supplement soil
absorption for sites with slowly permeable
shallow soils with high water tables.
ET systems can be used for seasonal
application, especially for summer homes or
recreational parks in areas with high
evaporation and transpiration rates, such as in
the southwestern United States.
Landscaping enhances the aesthetics of anET
system as well as beautifies the area.
Disadvantages
ET systems are governed by climatic
conditions such as precipitation, wind speed,
humidity, solar radiation, and temperature.
ET systems are not suitable in areas where
the land is limited or where the surface is
irregular.
ET systems have a limited storage capacity
and thus cannot store much winter
wastewater for evaporation in the summer.
There is a potential for overloading from
infiltration of precipitation.
The bed liner must be watertight to prevent
groundwater contamination.
ET systems are generally limited to sites
where evaporation exceeds annual rainfall by
at least 24 inches (i.e., arid zones).
Transpiration and evaporation can be reduced
when the vegetation is dormant (i.e., winter
months).
Salt accumulation and other elements may
eventually eliminate vegetation and thus
transpiration.
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DESIGN CRITERIA
There are several variables that determine the size
requirement of an ET system. The flow rate of
domestic wastewater is site-specific. Accurate
estimates (daily, weekly, or monthly) of flow rates
must be calculated as part of the design process to
prevent overloading associated with undersizing or
the excessive cost of oversizing a system. The
design flow rate should also include a safety factor
to account for peak flows or increased site use in
the future.
Like other disposal methods that require
area-intensive construction, the use of ET systems
can be constrained by limited land availability and
site topography. For year-round, single-family
homes, ET systems generally require about 4,000 to
6,000 square feet of available land. However, the
use of water conservation plumbing devices could
reduce the bed area requirements.
The maximum slope that an ET system can be used
on has not yet been determined, although a slope
greater than 15 percent could be used if terracing,
serial distribution, and other necessary design
features are incorporated.
PERFORMANCE
By far the most important performance
consideration of any ET system is the rate of
evaporation. This is largely affected by climatic
conditions such as precipitation, wind speed,
humidity, solar radiation, and temperature. Since
these factors are variables, evaporation rates can
vary significantly, a factor which must be
considered in the design of an ET system.
Although most precipitation will be absorbed into
the ET bed, hydraulic overloading could occur if
more water enters the system than is evaporated.
Provisions for long-term storage of excess water
can be expensive. Thus, the evaporation rate must
exceed the precipitation rate. This makes an ET
system suitable for areas with relatively low
rainfall, such as the western and southwestern parts
of the United States. Climate requirements are not
as well defined for ETA systems, although the soils
must be able to accept all of the influent wastewater
if net evaporation is zero for a long period of time.
In addition to the climate, other factors influence
the performance of an ET system. These are
discussed below.
Hydraulic Loading
If the hydraulic loading is too high, wastewater
could seep out from the system. However, if a
loading rate is too low, it can result in a lower
gravity (standing) water level in the bed and
insufficient evaporation. This situation can be
solved by sectional construction in level areas to
maximize the water level in a particular section of
the bed.
Sand Capillary Rise Characteristics
The sand must be fine enough to draw the water up
from the saturated zone to the surface by capillary
action. The potential for capillary rising must be
slightly more than the depth of the bed. However,
if the sand is too fine, the bed can be clogged by
solids from the wastewater.
Cover Soil and Vegetation
The vegetation used in an ET system must be able
to handle the varying depths of free water surface in
the bed. Grasses, alfalfa, broad-leaf trees, and
evergreens are types of vegetation used in ET beds.
They have been known to increase the average
annual evaporation rate from an ET bed to a rate
higher than that for bare soil. However, grasses and
alfalfa also result in nearly identical or reduced
evaporation rates as compared to bare soil during
winter and spring, when evaporation rates are
normally at a minimum. Similarly, topsoil has been
shown to reduce evaporation rates. Some evergreen
shrubs have resulted in slightly higher evaporation
rates than bare soil throughout the year. Water
seekers with hair roots, such as berries, are not
recommended because they may clog the
distribution pipes.
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Construction Techniques
Installing additional beds as required.
Although ET system performance is generally
affected less by construction techniques than most
subsurface disposal methods, some aspects of ET
construction can affect performance. For ET
systems, main considerations are to ensure that the
impermeable liner is watertight and that the sand
has sufficient potential for capillary rise.
Salt Accumulation (for ET only)
As wastewater is evaporated during dry weather,
salt and other elements build up at the surface of the
ET bed. Precipitation distributes the salt
throughout the bed. For nonvegetated ET systems,
salt accumulation is generally not a problem, but
systems with vegetation may experience negative
effects over time.
Soil Permeability (for ETA only)
Soil permeability affects the performance of ETA
beds that use seepage into the soil in addition to
evaporation. A portion of pretreated wastewater is
absorbed and treated by the soil. As a general rule,
the wastewater must travel through two to four feet
of unsaturated soil for adequate treatment before
reaching the groundwater.
OPERATION AND MAINTENANCE
Regular operation and maintenance (O&M) of ET
and ETA systems is usually minimal, involving
typical yard maintenance such as trimming the
vegetation. If a septic tank is used for pretreatment,
it should be checked for sludge and scum buildup
and periodically pumped to avoid carryover of
solids into the bed. Recommended maintenance
practices include:
Ensuring that all stormwater drainage
paths/pipes are not blocked and that
stormwater drains away from the system.
Using high transpiration plants suitable for
the wetness at ground level.
• If there is more than one bed, alternating the
bed loading as necessary.
If an ET or ETA system is properly installed on a
suitable site, maintenance is rarely needed.
COSTS
The cost of an ET system depends on the type of
system, site, and wastewater characteristics. The
construction cost of an ET bed is determined by its
surface area, which is a function of the design
loading rate. (For non-discharging, permanent
home ET units located in suitable areas, the loading
rate ranges from approximately 1.0 mm per day to
3.0 mm per day.) Other cost considerations include
the availability of suitable sand, the type and
thickness of the liner, use of a retaining wall (if
needed), and vegetation (usually native to the area).
Typical costs for a three-bedroom residence with a
septic tank and ET system run about $10,000
(minimum) yet may be higher depending on site
conditions.
REFERENCES
Other Related Fact Sheets
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm
1. Bennett, E. R. and K. D. Linstedt. 1978.
Sewage Disposal by
E v ap oration-Transpiration.
EPA-600-2-78-163, U.S. Environmental
Protection Agency (EPA) Municipal
Environmental Research Laboratory. Office
of Research and Development. Cincinnati,
Ohio.
2. Bernhart, A. P. 1978.
Evapotranspiration—A Viable Method of
Reuse (or Disposal) of Wastewater in North
America, South of 52nd or 55th Parallel.
Individual Onsite Wastewater Systems:
Proceedings of the Fifth National
Conference. Ann Arbor Science Publishers,
Inc., pp. 185-195.
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3. Frank, W. L. July 1996. The
Evapotranspiration Bed Alternative. Water
Environment & Technology. Vol. 8. No. 7.
4. Gunn, I. W. 1989. Evapo-Tr(inspirationfor
On-Site Residential Wastewater
Disposal—The New Zealand Experience.
Alternative Waste Treatment Systems.
Edited by R. Bhamidmarri. pp. 197-208.
Massey University. Palmerston North, New
Zealand. Elsevier Applied Science. London
and New York.
5. U.S. Environmental Protection Agency.
1980. Design Manual: Onsite Wastewater
Treatment and Disposal Systems. EPA
Office of Water Program. EPA Office of
Research and Development. Washington,
D.C.
6. . Feb. 1980. Evapotranspiration
Systems Fact Sheet 7.1.5. Innovative and
Alternative Technology Assessment
Manual. EPA-430/9-78-009. EPA Office
of Water Program Operations. Washington,
D.C.
7. Waldorf, L. E. 1977. Boyd County
Demonstration Project. National
Conference on Less Costly Wastewater
Treatment Systems for Small Communities.
EPA-600/9-79-010. National Technical
Information Services Report No. PB 293
254/AS. Washington, D.C.
ADDITIONAL INFORMATION
Ram Oren
School of the Environment
Duke University
Durham, NC 27708-0328
David Sumner
Hydrologist
U.S. Geological Survey
224 W. Central Pkwy., Suite 1006
Altamonte Springs, FL 32714
Anthony Tarquin
Civil Engineering Department
University of Texas at El Paso
El Paso, TX 79968
National Small Flows Clearing House at
West Virginia University
P.O. Box 6064
Morgantown, WV 26506
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for use by the U. S. Environmental
Protection Agency.
The technical content of this fact sheet was
provided by the National Small Flows
Clearinghouse and is greatly acknowledged.
Gabriel Katul
School of the Environment, Box 90328
Duke University
Durham, NC 27708-0328
Dr. Bruce J. Lesikar
Associate Professor
Agricultural Engineering Department
Texas A&M University System
201 ScoatesHall
College Station, TX 77843-2117
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Ave., NW
Washington, D.C., 20460
1MTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY BRANCH
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