v>EPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F 99-074
September 1999
Decentralized Systems
Technology Fact Sheet
Mound Systems
DESCRIPTION
The mound system was originally developed in
North Dakota in the late 1940s and called the
NODAK disposal system. Some soil types are
unsuitable for conventional septic tank soil
absorption systems. As a result, alternative systems
such as the mound system can be used to overcome
certain soil and site conditions.
The mound design in predominate use today was
modified from the NODAK design by the University
of Wisconsin-Madison in the early 1970s. Although
there are now many different mound designs in use,
this fact sheet will focus on the Wisconsin design.
The Wisconsin mound has been widely accepted and
incorporated into many state regulations.
The three principle components of a mound system
are a pretreatment unit(s), dosing chamber and the
elevated mound. Figure 1 illustrates a Wisconsin
mound system.
APPLICABILITY
Mounds are pressure-dosed sand filters that
discharge directly to natural soil. They lie above the
soil surface and are designed to overcome site
restrictions such as:
Slow or fast permeability soils.
Shallow soil cover over creviced or porous
bedrock.
The main purpose of a mound system is to provide
sufficient treatment to the natural environment to
produce an effluent equivalent to, or better than, a
conventional onsite disposal system.
ADVANTAGES AND DISADVANTAGES
Listed below are some advantages and
disadvantages of mound systems when compared to
other alternative onsite systems.
Advantages
The mound system enables use of some sites
that would otherwise be unsuitable for
in-ground or at-grade onsite systems.
The natural soil utilized in a mound system is
the upper most horizon, which is typically the
most permeable.
A mound system does not have a direct
discharge to a ditch, stream, or other body of
water.
Construction damage is minimized since there
is little excavation required in the mound area.
Mounds can be utilized in most climates.
Disadvantages
Construction costs are typically much higher
than conventional systems.
A high water table.
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TOP SOIL
OBSERVATION TUBE
./DISTRIBUTION
/ LATERAL
SAND
'FILL
;..- "^
v-.'-j - - ... .----v >:'
.r^'
BASAL AREA -
PLOWED LAYER -J
-.. ABSORPTION
AREA
HIGH WATER.-
ALARM SWITCH
- PUMP SWITCH
SEPTIC TANK
DOSING CHAMBER
MOUND
Source: Converse and Tyler, Copyright© by the American Society of Agricultural Engineers, reprinted with permission, 1987.
FIGURE 1 SCHEMATIC OF A WISCONSIN MOUND SYSTEM
Since there is usually limited permeable
topsoil available at mound system sites.
Extreme care must be taken not to damage
this layer with construction equipment.
The location of the mound may affect
drainage patterns and limit land use options.
The mound may have to be partially rebuilt if
seepage or leakage occurs.
All systems require pumps or siphons.
Mounds may not be aesthetically pleasing in
unless properly landscaped.
DESIGN CRITERIA
Two factors that determine the size and
configuration of a mound are; how the effluent
moves away and the rate at which it moves away
from the system. The prediction of the movement
and rate of movement is done from studies of the
soil and site information obtained. To ensure proper
performance of the mound system, the following
concepts must be included in the design and
construction process:
1) Leaving the topsoil in place but plowing it
before placement of the fill.
2) Using a coarse sand fill meeting grain size
distribution specifications.
3) Using pressure to uniformly distribute the
effluent over the seepage area.
Soil Depth
A suitable depth of soil is required to treat the
effluent before it reaches the limiting condition, such
as bedrock, a high water table, or a slowly
permeable soil layer. Although the separation
distance varies, it is usually between 1 and 4 feet.
Site and Design
To date, siting and design experience at sites
suitable for mound systems indicates that absorption
systems should be long and narrow and should
follow the contour (i.e., level). The more restrictive
the site, the narrower and longer the system. Table
1 gives the soil criteria for a Wisconsin mound
based on research and field experience.
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TABLE 1 RECOMMENDED SOI LAND
SITE CRITERIA FOR THE WISCONSIN
MOUND SYSTEM BASED ON
RESEARCH AND FIELD EXPERIENCE
Parameter
Value
Depth of high water table
(permanent or seasonal)
Depth to crevice bedrock
Depth to non-crevice bedrock
Permeability of top 10 in.
Site slope
Filled site
Over old system
Flood plains
10 in.
2ft.
1 ft.
Moderately low
25%
Yesa
Yesb
No
a Suitable according to soil criteria (texture, structure,
consistence).
b The area and backfill must be treated as fill because it
is a disturbed site.
Source: Converse and Tyler, 1990.
High Water
The high water table is determined by direct
observation (soil boring), interpretation of soil
mottling, or other criteria. The bedrock should be
classified as crevice, non-crevice semi-permeable, or
non-crevice impermeable. This will determine the
depth of sand media required.
Percolation and Loading
Percolation tests are used in some jurisdictions to
estimate the soil permeability because they are
empirically related to the loading rate. Loading
rates should be based on the soil texture, structure,
and consistence, using the percolation test only to
confirm morphological interpretations.
Mounds
Mounds can be constructed on sites with slopes up
to 25%. The slope limitation is primarily for
construction safety, because it is difficult to operate
equipment on steep slopes, and they pose a
construction hazard. From a hydraulic perspective,
mounds can be positioned on steep slopes.
Sites
In the case of filled sites, fill material is placed on
top of the natural soil and may consist of soil
textures ranging from sand to clay. Sufficient time
must be allowed for the soil structure to stabilize
before constructing a system. Many more
observations are required for filled areas.
When evaluating the soil loading rate for a mound
over an old or failing in-ground system, the soil over
the system must be considered to be disturbed, and
thus, treated as a filled site. If a mound is to be
placed over a large in-ground system, a detailed
evaluation of the effluent movement should be done.
Mounds should not be installed in flood plains,
drainage ways, or depressions unless flood
protection is provided. Another siting consideration
is maintaining the horizontal separation distances
from water supply wells, surface waters, springs,
escarpments, cuts, the boundary of the property,
and the building foundation. Sites with trees and
large boulders can make it difficult in preparing the
site. Trees should be cut to the ground surface with
tilling around stumps. The size of the mound should
be increased to provide sufficient soil to accept the
effluent when trees and boulders occupy a
significant amount of the surface area.
The actual size of a mound system is determined by
estimating the sand fill loading rate, soil (basal)
loading rate, and the linear loading rate. Once these
values are established, the mound can be sized for
the site. The final step is to design the effluent
distribution network and the pumping system.
PERFORMANCE
One factor that determines good performance is the
type of sand fill material. A suitable sand is one that
can adequately treat the wastewater. Suitable sand
should contain 20% or less material greater than 2.0
mm and 5% or less finer than 0.053 mm. It should
also have a size distribution that meets certain sieve
analysis specifications, ASTMC-33 specifications,
or meets limits for effective diameter and coefficient
of uniformity.
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For design of residential mounds, the daily
wastewater volume is determined by the number of
bedrooms in a house. Typical design flow
requirements for individual homes are up to 150
gallons per day (gpd) per bedroom. Design
specifications for mound systems are usually the
same for both large and small flows for typical
domestic septic tank effluent. Higher strength
wastes must be pretreated to the levels of domestic
septic tank effluent, or lower hydraulic loading rates
may be applied.
IMPLEMENTATION
In Wisconsin, the success rate of the mound system
is over 95%, which is due to their emphasis on
siting, design, construction and maintenance.
Years of monitoring the performance of mound
systems have shown that mounds can consistently
and effectively treat and dispose of wastewater.
Studies have shown evidence that some nitrogen
removal does occur in mound systems when
approximately 2 feet of natural unsaturated soil is
below the fill material.
Mound Systems in Wisconsin (State-Wide)
Using relatively conservative soil criteria, many
states have accepted the Wisconsin mound system
as an alternative when conventional in-ground
trenches and beds are not suitable. The Wisconsin
mound system has evolved into a viable onsite
system for the treatment of wastewater from
individual, commercial, and community systems by
overcoming some of the site limitations and meeting
code requirements and guidelines.
In 1978, an experimental study was initiated to
evaluate soil/site limitations for the Wisconsin
mound (see Converse and Tyler, 1987a). The
objectives of this research study were to determine
whether the existing soil/site limitations on mounds
were too restrictive and to determine the minimum
soil/site limitations under which the mounds would
perform without affecting public health and the
environment. The experimental approach was to
design, construct, and evaluate sites with mound
systems that currently did not meet code
requirements due to failing systems.
The sites selected for this study had to fit the
objectives of the research and generate a reasonable
amount of wastewater to be mound treated. The
sites selected had to have:
1. Fill soil placed over natural soil.
2. A high water table where the seasonal high
water table level was less than 60 cm below
the ground surface.
3. Slowly permeable soils that were rated
slower than moderately permeable soils.
4. Steep slopes greater than 12%.
5. Mounds over existing failing systems.
6. A combination of the above.
Over 40 experimental mounds were constructed
between 1979 and 1983 on sites that did not meet
the code requirements; 11 of these mounds are
described in detail in this study. Site evaluations
were done by certified soil scientists, plans prepared
by designers were reviewed and approved by the
state, and licensed contractors installed the systems
with inspections by county sanitarians during
construction.
The study concluded that the overall performance of
the mounds was very good. The systems functioned
satisfactory on filled sites, on sites with a high water
table (seasonal water table 25 to 30 cm from the
ground surface), on steep slope sites (up to 20 to
25%), on sites with slowly permeable soil, and on
top of failing systems. Leakage occurred at the base
of the mound on some sites during extremely wet
conditions, but the effluent quality was good, with
fecal counts generally less than 10 colonies per 100
ml in saturated toe effluent. It was found that
Wisconsin mound systems can be constructed on
difficult sites if the system is designed using linear
loading rates, which are established based on the
horizontal and vertical acceptance rates of the soil
for each system.
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Failure of Mound System in Wisconsin
Expansion of a Wisconsin firm's mound system in
1978, resulted in a clogging and seepage problem.
The system was originally built to handle 65
employees at 750 gpd and was now serving a staff
of 165. This expansion created a failure of the
mound system due to hydraulic overload. To solve
this problem, the mound system was expanded and
a water conservation program was initiated. The
expansion of the mound increased the hydraulic
capacity to 2,600 gpd (Otis, 1981.)
In November 1979, the mound system failed
againthis time due to a biological clogging mat.
The clogging mat was removed by using 450 gallons
of a 10% solution of hydrogen peroxide. The
mound system was operating successfully within 2
days. However, further research indicates that for
structured natural soils other than sand, hydrogen
peroxide may reduce the soil infiltration rate, and
thus, may not be an effective procedure to eliminate
soil clogging.
A third failure occurred in January 1980, again due
to hydraulic overload. The firm had expanded its
employee base to 215 employees, with an average
daily flow of 3,000 gpd. There was no room
available to expand the mound system itself, so the
firm redesigned the pumping chamber to avoid large
peak flows, allowing the mound system to receive
optimum dosing without failure.
OPERATION AND MAINTENANCE
The septic tank and dosing chamber should be
checked for sludge and scum buildup and pumped
as needed to avoid carryover of solids into the
mound. Screens or filters can be used to prevent
large solids from escaping the septic tank. The
dosing chamber, pump, and floats should be
checked annually and replaced or repaired as
necessary. It is critical that the septic tank and
dosing chamber be watertight. In addition,
electrical parts and conduits must be checked for
corrosion. Flushing of the laterals annually is
recommended.
When a mound system is properly installed and
maintained, it should last for a long period of time.
In general, the maintenance required for mounds is
minimal. However, as with any system, poor
maintenance could lead to early system failure.
Possible problems that can occur in an improperly
designed or constructed mound system include:
Ponding in the absorption area of the mound.
Seepage out of the side or toe of the mound.
Spongy areas developing on the side, top, or
toe of the mound
Clogging of the distribution system.
Practices that can be used to reduce the possibility
of failure in a mound system include:
Installing water-saving devices to reduce the
hydraulic overload to the system.
Calibrating pumps and utilizing event
counters and running time meters.
Timed dosing to dose equally sized doses on
regular intervals throughout the day.
Diverting surface water and roof drainage
away from the mound.
Preventing traffic on the mound area.
Installing inspection tubes in the mound to
check for ponding.
Keeping deep-rooted plants (shrubs and trees)
off the mound.
Planting and maintaining grass or other
vegetative cover on the mound surface to
prevent erosion and to maximize water
uptake.
Stand-by power for the pump.
Follow all instructions recommended by the
manufacturer. All equipment must be tested and
calibrated as recommended by the equipment
manufacturer. A routine operation and maintenance
(O&M) schedule should be developed and followed
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for any mound system in addition to checking local
codes.
COSTS
The cost of a mound system is dependent on design
costs, energy costs, the contractor used, the
manufacturers, land, and the characteristics of the
wastewater. Table 2 lists some typical capital and
O&M costs for a mound system serving a
three-bedroom single home at a flow rate of 450
gpd (150 gallons per bedroom). Septic tank costs
were estimated at $1 per treated gallon. It should be
noted however, that costs will vary from site to site.
To keep construction costs to a minimum, use good
quality and local materials, when available.
TABLE 2 TYPICAL COST ESTIMATE FOR
A MOUND SYSTEM (SINGLE HOME)
Item
Cost ($)
Capital Costs
Construction Costs
Septic tank (1000 gallon
concrete tank)
Dosing chamber (includes
pump and controls)
Mound structure
1,000
2,000
6,000
Total Construction Costs
9.000
Non-Component Costs
Site evaluation
Permits
500
250
Total Costs
9,750
Annual O&M Costs
Labor @$20/hr.
Power @8 cents/kWh
Septic tank pumping
20 per year
35 per year
75 to 150 every 3
years
Source: Ayres Associates, Inc., 1997.
REFERENCES
1. Converse, J. C. and E. J. Tyler. 1987a.
On-Site Wastewater Treatment Using
Wisconsin Mounds on Difficult Sites.
Transactions of the ASAE. 1987. American
Society of Agricultural Engineers, vol. 30.
no. 2. pp. 362-368.
2. Converse, J. C. and E. J. Tyler. 1987b.
Inspecting and Trouble Shooting Wisconsin
Mounds. Small Scale Waste Management
Project. University of Wisconsin-Madison.
Madison, Wisconsin.
3. Converse, J. C. and E. J. Tyler. January
1990. Wisconsin Mound Soil Absorption
System Siting, Design, and Construction
Manual. Small Scale Waste Management
Project. University of Wisconsin-Madison.
Madison, Wisconsin.
4. Otis, R. J. 1981. Rehabilitation of a Mound
System. On-Site Sewage Treatment:
Proceedings of the Third National
Symposium on Individual and Small
Community Sewage Treatment. American
Society of Agricultural Engineers. St.
Joseph, Michigan.
5. U.S. Environmental Protection Agency
(EPA). 1980. Design Manual: Onsite
Wastewater Treatment and Disposal
Systems. EPA 625/1-80-012, EPA Office of
Water. EPA Municipal Environmental
Research Laboratory. Cincinnati, Ohio.
ADDITIONAL INFORMATION
Mr. Richard J. Otis, Ph.D., P.E., DEE
Ayres Associates
2445 Darwin Road
Madison, WI 53704-3186
National Small Flows Clearing House at
West Virginia University
P.O. Box 6064
Morgantown, WV 26506
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The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection
Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
IMTB
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