SUBSURFACE SOIL ABSORPTION OF WASTEWATER
MOUND SYSTEMS
Richard J. Otts, P.E.
Rural Systems Engineering
Madison, Wisconsin
I. Introduction
A. Description
A mound system 1s a subsurface soil absorption system that 1s
elevated above the natural soil surface 1n a suitable fill material.
Trenches or beds are constructed 1n the fill maintaining one to two
feet of fill material between the bottom of the seepage area and the
natural soil. Septic tank effluent 1s pumped or siphoned Into the
gravel area through a pressure distribution network. The system 1s
covered with a finer textured soil material. See Figure I. A-l.
CLAY LOAM SOIL
AMNIIft MATIIIIAL
INLIT Hft
DISTRIBUTION IATIHAIS
DIVIMION POM
UNPACt WATIM
Figure I. A-l: Mound System Schematic
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B. History
1, NQDAK System
The mound system was originally developed 1n North Dakota
1n the late 1940's where 1t became known as the NODAK System
(Witz, et.al, 1974). It was conceived to overcome problems
with subsurface disposal on farmsteads located on slowly permeable
and high water table soils. The absorption bed was constructed
in a gravelly fill placed over the original soil after the topsoil
had been removed.
2. Wisconsin Mound
Monitoring of NODAK Systems revealed that the gravelly fill
was too coarse to provide adequate filtration to protect the
groundwater where shallow soil existed and that surface seepage
would occur during wet periods where the systems were constructed
over slowly permeable soils. Changes in design were made to
overcome these problems, the principal ones being: (1) to leave
the topsoil in place but plowing it prior to placement of the fill,
(2) to use a medium sand texture fill, and (3) to use pressure
distribution to uniformly apply the effluent over the seepage area
(Univ. Wis, 1978).
3. Other Mounds
Largely independent from the development in Wisconsin, other
states developed similar mounds, notably Pennsylvania ("elevated
sand mounds"), and Minnesota ("gopher mound"), but these designs
have been abandoned since in favor of the Wisconsin design.
II. A. Functions
1. Absorption
In slowly permeable soils, the primary function of the mound
is absorption of the wastewater into the natural soil. By
elevating the seepage area above the natural soil surface, several
advantages result:
Absorption into the topsoil
The topsoil is usually more permeable than the subsoil
because of greater soil flora and fauna activity. Conventional
systems are installed below the topsoil losing the benefit of
the greater permeability. Once into the topsoil, it can move
laterally until absorbed by the subsoil. See Figure II. A-l(a).
(Transpiration by plants may also remove significant amounts of
water during the growing season, but the mound is designed to
function solely by absorption.)
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TOPSOIL
lnrnrIrrrl T rrTf
I I I
PSIMIABLI SOIL
I\1 1 \ \ ) ___
WATIR TABLI OR CRIVICID SIDROCI
Figure II. Ai:
Schematic of water movement from a mound system.
(a) slowly permeable soils; (b) permeable soils
over high water tables or shallow porous bedrock.
5* 11 115 MATIRIAL
SAND PILL
DISTRIBUTION
LATIRAL
ABSORPTION
ARIA
$ I I I $1 I I I I
_____ __ --
- ROn--
(a)
SARRIIR
SAND PILL
TOPSOIL
CAP
DISTRIBUTION LATIRAL
S$ORPTION
ARIA
cY.I I I i. I
pLOW1DT0P*01(
(b)
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Less restrictive clogging mat
Pressure dtstrthutlon appears to reduce the severity
of the clogging mat in coarse, textured soils such as the
sandy fill material.
Construction damage minimized
Smearing and compaction of the wetter subsoil during
construction is avoided since excavation in the natural
soil is not necessary.
2. Treatment
In shallow permeable soils, the primary function of the
mound is to provide additional unsaturated soil material to ade-
quately treat the wastewater before It reachez the groundwater.
See Figure II. Al(b).
B. Treatment Effectiveness
Several years of monitoring of laboratory models and fullscale
field systems have demonstrated that mound systems consistently
remove all waste contaminates of concern except for nitrogen
(Univ. Wis. , 1978). There Is some evidence that some nitrogen removal
does occur in mound systems (Harkin, et.al, 1979). To maintain this
treatment level, approximately 2 ft. of natural unsaturated soil Is
required below the fill material.
C. Application
Mounds are used in slowly permeable soils and permeable soils
with shallow water tables or shallow creviced or porous bedrock where
conventional trench or bed systems are unsuitable. Site criteria for
mounds are sunriarized in Table I!. Ci. These represent current
practice for small systems and can be expected to become broader
as experience is gained. Larger systems require more detailed
hydrogeologic site investigations.
III. Design
Mound design is based on: (1) the estimated peak daily wastewater
volume, (2) the fill characteristics, and (3) the natural soil characteris-
tics. Mounds must be designed to accept the peak daily wastewater flow
without surface seepage or encroachment of the zone of saturation into
the fill material.
A. Wastewater Characteristics
1. Volume
The peak daily wastewater volume is used to size the system.
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TABLE II. C-i
SITE CRITERIA FOR MOUND SYSTEMS
It o m Criteria
Landscape Position Welldrained areas, level or sloping. Crests
of slopes or convex slopes most desirable.
Avoid depressions, bases of slopes and concave
slopes unless suitable drainage is provided.
Slope 0 to 6% for soils with percolation rates slower
than 60 mm/In.
o to 12% for soils with percolation rates faster
than 60 mm/In.
Typical Horizontal Separation
Distances from Edge of Basal Area
Water Supply Wells 50 to 100 ft
Surface Waters, Springs 50 to 100 ft
Escarpments 10 to 20 ft
Boundary of Property 5 to 10 ft
Building Founditloni 10 to 20 ft (30 ft when located upsiope from
a bu11dln in slowly penneable soils).
Soil
Profile Description Soils with a welldeveloped and relatively
undistrubed A horizon (topsoil) are preferable. Old
filled areas should be carefully investigated for
abrupt textural changes that would affect water
movement. Newly filled areas should be avoided until
proper settlenent occurs.
Unsaturated Depth 20 to 24 in. of unsaturated soil should exist bebveen
the original soil surface and seasonally saturated
horizons or pervious or creviced bedrock.
Depth to Impermeable BarrIer 3 to 5 ftb
Percolation Rate 0 to 120 sin/In, measured at 12 to 20 1 c
a These are present limits used In Wisconsin, established to coincide with slope classes
used by the Soil Conservation Service In soil mapping. Mounds have been sited on slopes
greater than these, but experience is limited.
b Acceptable depth is site dependent.
C Tests are run at 20 in. unless water table is at 20 in., In which case test Is run at 16 in.
In shallow soils over pervious or creviced bedrock, tests are run at 12 in.
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For systems to serve single family residences the peak volume
is estimated from the size of the home, typically by the number
of bedrooms. Common practice is to use 150 gpd per bedroom.
For large cluster systems serving five houses or more, the peak
volume is based on the maximum population to be served. Fifty
to 75 gal per capita per day plus a factor for infiltration for
extensive collection systems is used.
2. Pretreatment
Septic tank treatment is sufficient for mound systems.
Further treatment rs not beneficial.
B. Fill Selection
The fill material must be selected before sizing of the mound can
be done because the materials Infiltrative capacity determines the
required absorption bed area. Medium textured sands, sandy barns, soil
mixtures, bottom ash, strip mine spoil and slags are being used or
are being tested (Converse, et.al, 1978). To keep costs of construction
to a minimum, the fill should be selected from locally available
materials. Coninonly used fill materials and their respective design
infiltration rates are presented in Table III. B-i.
TABLE III. Bi
COMMONLY USED FILL MATERIALS AND THEIR DESIGN INFILTRATION RATES
Design
Infiltration
Fill Material Characteristics Rate
gpd/ft 2
>255 O.25-2. mn
Medium sand* <30-35% 0.05-0.25mm 1.2
< 5-10% 0.002-0.05mm
Sandy loam 5-15% clay content 0.6
Sand/Sandy loam 8893% Sand 1 2
mixture 712% Finer grained material
Bottom ash 1.2
* Equivalent to 85% by weight between 10 and 60 mesh
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C. Location
1. Area Required
Mounds designed for 3-bedroom homes requIre 2200 ft 2
to 3500 ft 2 of area depending on the penneebility of the
natural soil. Therefore, lots no smaller than one-half
acre are necessary to maintain sufficient set back distances.
Larger lots are preferable.
2. Slope
Gently sloping sites are preferred sites for mounds to
promote lateral movement of liquid away from the mound.
Crests of slopes are Ideal because flow will occur in both
directions from the divide. Slopes greater than 15 percent
should not be used because of the danger of surfacing at
the downslope perimeter or toe of the mound. The trenches or
beds should be curved to follow the contour. However, concave
slopes should be avoided because the flow from the mound may
converge downslope and result In surface seepage,
/ CONCAVI $LOPI
/ (AVOID)
, ____ %
/
CONVIX SLOPI
DISTSISUTION
LATS
FIGURE III. C-i: Mound Orientation on a Complex Slope
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3. Trees
If desired, large trees should be preserved. The mound
need not have exposure to the sun or wind to function properly.
It is designed to function by subsurface absorption only.
However, trees that are within the mound perimeter may die if
their trunks are partially buried. If it is necessary to remove
the trees, they should be cut off at ground level and the stumps
left in place so damage is not done to the natural soil.
D. Sizing
1. Geometry
The shape of the mound is largely dictated by the characteris-
tics of the site. It is important that the system be laid out
such that the water table or zone of saturation does not rise
up into the fill material during wastewater application. There-
fore, the following must be considered:
Soil permeability
If the soil is slowly permeable (slower than 60 to 90 mm/in),
the absorption area within the mound should be a narrow trench
5 ft or less in width oriented with its long axis perpendicular
to the natural ground slope. If the percolation rate is faster
than 40 to 60 mm/in, beds may be used instead of trenches to
reduce the mounds length. However, elongated beds with the
long axis perpendicular to the natural ground slope are pre-
ferred to square beds.
Unsaturated depth
In permeable soils with water tables at 1 to 2 ft, beds no
wider than 10 to 15 ft should be used within the mound. If
the water table is greater than 3 ft below the surface, square
beds are acceptable. In slowly permeable soils, perched water
table or saturated soil conditions may occur during wet periods.
The soil must be carefully examined for any evidence of this.
(See Site Evaluation for Onsite Treatement and Disposal Systems.)
Layering within the soil profile
The soil profile must be carefully examined for layers
which may impede the vertical movement of liquid. If found,
long narrow trenches oriented perpendicular to the natural
ground slope rather than beds should be used.
Bedrock or very slowly permeable barriers
Usually the natural surface topography conforms to the
topography of the bedrock surface. For large mound systems
(>1000 gpd), this should be confirmed. If they do not conform,
the mound should be oriented relative to the bedrock surface
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rather than the ground surface. However, plowing of the
natural soil should still follow the surface contours.
Example: A larqe mound is to be used to dispose of sanitary wastes
from an industrial firm. The available site has a shallow depth to
bedrock. Borings Indicate that the surface and bedrock topoqraphy
differ. The primary concern is movement of the liquid away from the
site which will occur In the saturated zone at the bedrock surface.
The secondary concern Is seepage at the based of the mound due to
movement along the fill/soil interface. Since this movement will be
parallel to the surface slope, the natural soil will be plowed alonq
the surface contours (See Figure II!. Dl).
maci cowto*.
SSSOCk CONTO S$
SlOUSIS 1 *V*TIOU
( ) SS OSGK 4V*T1ON
U .S
SAcK
FIGURE III. D-l: Mound Orientation Conforming to Shallow Barriers to Flow
S Small lots
If the site is too small to fit a single trench or bed
along the contour, multiple trenches or beds may be used.
The absorption areas may be tiered down on more steeply
sloping sites to reduce the amount of fill necessary (See
Figure III. D2). In areas with slowly permeable soils,
tiering may not be possible because the zone of saturation
may rise up into the fill material.
2. Absorption Area
The absorption trench or bed within the mound is sized on
bottom area only using the estimate peak daily wastewater flow
and the design infiltration rate of the selected fill material.
MOUN. PUNINSTIN
PLOWN WITH
$VSc a CONTOURS
PAy..
LOT
PUOPSRTY L I I I
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FIGURE III. D2: Tiered Mound on a Sloping Site
3. Mound Basal Area
The basal area or the fill-natural soil interface of the mound
must be sufficiently large to absorb all of the applied wastewater.
Once into the topsoil, the liquid can move laterally out beyond
the perimeter of the mound until absorbed by the subsoil. On
level sites, the entire fill-natural soil interface can be used
In determining the necessary area since lateral flow can occur
In all directions. On sloping sites, only the area imediately
below and downslope from the absorption area is considered.
Infiltration rates for the natural soil used in design are pre-
sented in Table III. D-l. The soil horizon with the lowest
permeability within the upper 24 inches should be used in this
sizing. Dimensions of other mound comoonents are shown in Fiaure
III. D3.
TABLE III. D-l
INFILTRATION RATES FOR DETERMINING MOUND BASAL AREA (IJ.S. EPA, 1980)
Percolation Infiltration
Natural Soil Texture Rate* Rate
mm/in gpd/ft
Sand, Sandy loam 0-30 1.2
Loams, Silt barns 3145 0.75
Silt barns, Silty clay loams 46-60 0.5
Clay barns, Clay 61120 0.25
CLAY LOAM SOIL.
NANRIER MATERIAL
- s-AssoRp:
Au4
uuossrn SAND
:.
DI$TNISUTION
LATERAL
* Measured at 12 to 20 in
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11
Example: A mound is to be built for a three-bedroom home on a lot
with a shallow water table. The soil is a silty clay loam with a
percolation rate of 60 mm/in. A level and a sloping site may be
used for the mound site. Compare the dimensions of the perimeter to
mounds designed for each site.
Design flow
Q = 3 bedrooms x 150 gpd/bdrm 450 gpd
Absorption area required (medium sand fill to be used: 1.2 qpd/ft 2
Infiltration rate)
Area = 450 gpd 375 ft 2
1.2 gpd/ft 2
Absorption area dimensions (10 ft max trench width due to high water table)
Length 375 ft 2 37 5 ft => 38 ft
lOft
FIGURE III. D.-3: Mound Dimensions
Dimensions: 10 ft x 38 ft
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12
Mound perimeter dimensions (From Table III. Dl select 0.5 gpd/ft 2 for
natural soil Infiltration rate)
Level site: Assume lateral flow is equal in all directions.
Therefore, entire basal area can be used for absorption.
Absorption area perimeter 2 x 10 ft + 2 x 38 ft = 96 ft
Lateral flow/ft of bed perimeter = 450 gpd = 4 7 d ft
6ft gp
Distance to mound perimeter = 4.7 qpd/ft 9 4 ft
0.5 gpd/ft
In this case, this distance is measured from the center line
of the bed since the soil Imediately below the bed is also
used for absorption. However, a minimum distance from the
perimeter to the absorption area sidewall is 10 ft to maintain
at least a 3:1 slope (See Figure III. D3).
Mound dimensions:
Width = 10 ft + 10 ft + 10 ft = 30 ft => 30 ft x 58 ft
Length = 10 ft + 38 ft + 10 ft = 58 ft
Sloping site: Lateral flow is assumed to be parallel to the
slope only. Therefore, only the area below the absorption
area and downslope from tt is considered.
Lateral flow/ft of bed = 450 gpd = 11 8 d
38ft gp
Distance to mound perimeter = 11.8 gpd 23 6 ft => 24 ft
0.5 gpd/ft
In this case, this distance is measured from the upsiope sidewall
of the absorption area. A 10 ft mm distance to the upsiope
and sideslope mound perimeters is required to maintain at least
a 3:1 slope (See Figure III. D3).
Mound dimensions:
Width = 10 ft + 24 ft 34 ft 34 ft x 58 ft
Length = 10 ft + 38 ft + 10 ft = 58 ft
Although the mound on the level site is smaller, the slopinq
site is preferred because of better subsoil drainage.
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4. Mound Height
Fill depth
The depth of fill under the absorption area is determined
by the degree of treatment required. The conthined depth of
fill and natural soil should be a minimum of 3 ft. In areas
of permeable soils with shallow creviced bedrock, a combined
minimum depth of 4 ft.is coninon because of the greater risk of
contaminating ground water used for water supply. On sloping
sites, the depth of fill below the absorption area increases
downslope. To prevent uneven settlingof the absorption area,
the mound should be designed to limit the depth of fill under
the downslope portion of the bed to 3 1/2 ft. To meet this
requirement, tiering of the absorption area may be necessary.
Trench or bed depth
A minimum depth of 6 in, of aggregate is used below the
distribution pipe to provide some water storage during peak
flows, An additional 2 in. of aggregate is placed above the
pipe to protect and insulate the pipe.
Mound cap
The cap provides frost protection and promotes runoff. It
is a finer textured material such as a silt loam or clay loam
to promote runoff and at the same time, retain more water for
a good vegetative cover. The cap depth should be 1 1/2 to 2 ft.
above the top of the aggregate in the center of the mound and
a minimum of 1 ft. at the edge of the absorption area in areas
of severe winters. An additional 4 in. to 6 In, of good toD soil
should cover the entire mound.
5. Side slopes
Side slopes should be no greater than 3:1 for stability and
maintenance. A minimum distance from the sidewalls of the absorp-
tion area to the mound perimeter of 10 ft. is coninon practice.
The length of the downslope may be greater due to basal absorption
area requirements.
0. Distribution
1. Method
Although both gravity and pressure distribution networks have
been used in mound systems, the pressure networks have been shown
to be superior (Converse, et. al, 1978, Univ. Wis, 1978). They
ensure unsaturated flow is maintained and prevent short circuiting
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through the fill material by applying the wastewater uniformly over
the entire absorption area. It is important that the network is
designed such. that the manifold drains between dosings.
Drainage can be either out the laterals or back into the lift
station. The method used depends on the relative elevations of
the dosing tank and the distribution laterals. Pressure networks
can be designed for simultaneous loading of each absorption area
but dual systems pressurized by duplex pumping units or alternat-
ing siphons are preferred to ensure equal division of flow. For
design of these networks, see Distribution Networks for Subsurface
Soil Absorption Systems. Also, see Converse (1978), EPA (1980)
and Otis (1981).
2. Dose volume
Frequent small doses of wastewater ensure unsaturated flow
through. the fill but too frequent dosing can lead to more severe
clogging. Dosing frequencies of 2 to 4 times daily should be
employed.
E. Porous Media
Any durable material that performs the necessary functions of
providing access to the Infiltrative surface, water storage, support
of the absorption area sidewalls and dissipating the energy In the
influent can be used. Gravel or crushed rock Is most coninon.
Recomended sizes range from 3/4 to 2-1/2 in. The smaller size Is
preferred. The rock should be free from fines, durable and resistant
to slaking and dissolution. A hardness of 3 or greater on Mohs
scale of hardness is suggested. Rock that can scratch a copper penny
without leaving any residual rock meets this criterion.
F. Barrier Material
To maintain the porous nature of the media, a barrier material
must be placed over the media to prevent backfilled material from
filling the voids. The material should be porous to water vapor and
resistant to rapid decay. A 6 to 9 in. layer of uncompacted marsh
hay or synthetic drainage fabric is reconinended.
G. Inspection Vents
Inspection vents provide limited access into the absorption area
to observe the depth of ponding which Is a measure of the systems
performance. They extend vertically from the gravel-fill interface
to 6 to 12 in. above finished grade. The pipe is open at the bottom
and perforated within the gravel layer. A vent cap or threaded plug
Is used to cover the open end.
IV. Construction
Proper construction is extremely important if the mound is to function
as designed. Detailed construction procedures are outlined by Converse
(1978).
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15
A. Machinery
Small track type tractors are recomended for working the
fill over the site. Wheeled tractors are difficult to maneuver
In the uncompacted fill. The narrow tires also have the tendency
to spin, penetrating the fill and disturbing the plowed soil
B. Site Preparation
1. Vegetative Cover
Excess vegetation should be cut and removed. Trees should be
cut at ground surface and the stimips left in place.
2. Staking
The mound is staked out in the proper orientation and the
ground elevation shot along the upslope edge of the absorption
area. This elevation is necessary to establish the elevation
of the bottom of the absorption area. It should be tied to a
bench mark established at the site.
3. Distribution Network Delivery Pipe
The delivery pipe from the dosing tank to the mound should be
installed and stubbed off 12 in. below the ground surface. The
backfill around the pipe should be compacted to prevent seepage
along the trench. Extend the pipe several feet above the ground
surface after the plowing is completed, but before the fill material
is spread.
4. Plowing
The area within the perimeter 0 f the mound is plowed to a
depth of about 8 tn. Plowing is done along the contour, throwing
the soil upsiope. Moldboard or chisel plows may be used. Roto-
tilling is too damaging to the soil. Soil moisture during plowing
must be below the plastic limit. Check by rolling a sample of soil
taken from the plow depth between the hands. If it crumbles,
plowing may proceed.
C. Fill Placement
1. Delivery
The fill should be spread over the area ininediately after plowing.
The delivered fill should be dumped around the upslope and side
edges of the plowed area. Traffic on the plowed area and the
downslope side should be avoided to prevent compaction of the topsoil.
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l 6
2. Sp reading
The fill is moved into place using a small track type tractor
with a blade. A minimum of 6 In. of fill should always be under
the tracks to prevent compaction of the natural soil.
D. Absorption Area
1. Forming
The absorption trench or bed is formed in the fill after all
the fill is in place. The bottom of the area is handleveled
to the elevation established prior to plowing.
2. Aggregate
The aggregate Is laid in the absorption area to a minimum depth
of 6 in. Care must be taken to avoid rutting of the bottom surface.
3. DistrIbution Network
The distribution network is assembled in place setting the
manifold to ensure draining between doses. Additional rock is
placed over the pipe and the aggregate covered with a suitable
barrier material,
E. Cover Material
l.p-
The finer textured cover material should be dumped on the
upsiope edge of the mound for spreading with the track type
tractor. An additional 4 in, to 6 in. layer of good quality
topsoil is placed over the entlr mound.-
2. Vegetation
The mound should be seeded and mulched or sodded inriediately
after construction.to stabilize the side slopes. Moisture-tolerant
shrubs can be planted around the mound perimeter and up the side slopes.
V. Operation and Maintenance
A. Routine Maintenance
A properly designed and constructed mound requires no more attention
than a conventional trench or bed system.
B. Rehabilitation
Two different failure conditions may occur within the mound. Each
usually can be corrected.
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17
1. Absorption Area Clogging
Surfacing of septic tank effluent high on the sldeslo es
when the base of the mound remains dry Indicates severe clogging
of the absorption area within the mound. This may be caused
by: (1) improper maintenance of the pretreatment unit, (2) hydraulic
overloading, or (3) unusual wastewater characteristics. The cause
must first be determined and corrected. Hydrogen peroxide then
can be used to oxidize the clogging mat. If it Is necessary to
enlarge the mound, the cap can be removed, the absorption bed
stripped out and additional fill added,
2. Natural Soil Clogging
If the fillnatural soil interface fails to accept all the
wastewater, surfacing of the effluent will occur along the
base of the mounds, Additional fill may have to be added down-
slope, If this does not correct the situation, the mound should
be elongated along the contour. If neither corrects the problem,
the site may have to be abandoned.
VI. Questions
1. Which type of site would be best suited for a mound system: level,
crested or sloping? Would it vary with soil permeability?
2. What additional considerations should be included in the site
evaluation and design for large mound systems?
3. How might frozen soil conditions affect mound operation? Would this
affect the destgn?
4. If mound failure were to occur, how would the cause of failure be
diagnosed?
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VII. References
Converse, J.C. 1978. Design and construction manual for Wisconsin mounds.
Small Scale Waste Management Project, Uniyerslty of Wisconsin, Madison.
Converse, J.C,, B.L. Caritle and G ,W. Peterson, 1978, Mounds for th.e
treatment and disposal of septic tank effluent, In: Home Sewage Treatment .
Proceedings of the second national home sewage treatment symposium.
Mierican Society of Agricultural Engineers Publication Number 5-77, St.
Joseph, Michigan.
Harkin, J.M., C.J. Fitzgerald, C.P. Duffy and D.G. Kroll. 1979. Evaluation
of mound systems for purification of septic tank effluent. Technical
report. Wis. WRC 7905. Water Resources Center. University of Wisconsin,
Madison.
Otis, R.J. 1981. Design of pressure distribution networks for septic tank
tank-soil absorption systems. Small Scale Waste Management Project.
University of Wisconsin, Madison.
University of WisconsIn, 1978. Management of Smell waste flows. U.S.
Environmental Protection Agency. EPA600/278173. Cincinnati, Ohio.
U.S. Environmental Protection Agency. 1980. Onsite wastewater treatment
and disposal systems design manual. Technology Transfer. Office of
Water Program Operations and Office of Research and Development,
Cincinnati, Ohio.
Wltz, R.L. 1975. Twenty-five years with the North Dakota waste disposal
system. In: Home Sewage Disposal . Proceedings of a national home sewage
disposal symposium. American Society of Agricultural Engineers, Publication
Number 175. St. Joseph, Michigan.
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VIII. Problems
A. Problem Statements
1. Sin 1eFamily ROme
A 1acre lot in an older subdivision has slowly permeable
clay loam soils with distinct mottling at 28 in. Percolation
rates are 90 mm/in. The natural ground slope is 4 percent.
Design a mound for a 4-bedroom home
Sketch a plan and section of the mound showing all dimensions
Would your design differ If the natural soil were a sandy
loam? If the site were level?
2, Elementary School
A ff00student elementary school is to be built on a
sloping property with limited space for a subsurface soil
absorption system. The available area is shown with the
natural topography on the attached plot plan. The soil is
loam with creviced bedrock at 24 to 36 inches. Percolation
rates are 35 mm/in,
Design a subsurface soil absorption system and lay it
out on the plot plan, The school will not have a
gymnasium and meals will be brought In.
What are your concerns about the site? Do you desire any
additional information?
Indicate on your drawing the elevation of the bottom of
the absorption area within the mound. What is the maximum
depth of fill required below the area?
Could you use dosing siphons? If so, does this affect the
location of the system components? How?
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B, Solutions
1. single-Family Home
Design flow
Q = 4 bdrms x 150 gpd/bdrm = 600 gpd
Absorption area required (use medium sand fill)
Area = 600 9pd
1.2 gpd/ft 2
Absorption area dimensions
A 5 ft maximum trench width is reconinended for slowly
permeable soils.
Width = 5 ft
Length 5OOft 1 100 ft
5 ft
Fill depth
Upsiope edge of absorption area = 1 ft mm
Downslope edge of absorption area = 1 ft + 5 ft x 1.04 = 1 .2 ft
Bed depth
A minimum depth of 6 in of rock below the pipe is
reconmiended with additional 2 in covering the pipe. If
1-in distribution laterals are used, total bed depth
(rock depth) will be 9 in.
Mound cap depth
Over the center of the bed a minimum depth of cover
reconinended is 1,5 ft of loamy soil and 4 to 6 In of topsoil.
At least 1 ft of cover should be maintained at the edge of the
absorption bed,
Basal absorption area
From Table III. D.l, a natural soil infiltration rate of
0.25 gpdfft 2 is determined.
Basal area required 600 gpd 2400 2
0.25 qpd/ft 2
Area length (bed length) = 100 ft
Area width 2400 ft 2 _
lOOft ft
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Mound for a 4-Bedroom Home in Clay Loam Soil
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23
Downslope length
Basal area width is measured from the upslope edge of
the bed.
Length of downslope = 24 ft 5 ft = 19 ft
Mound dimensions
Length = 12 ft + 100 ft+12 ft 124 ft
Width=l2ft+ 5ft+l9ft= 36ft
Sideslopes/Upsiope
A maximum side slope of 3:1 is recomended.
Slope length 3.75 ft x 3 11.25=> 12 ft
2. Elementary School
Design flow
Assume 10 gpd per student (Table TI!. A2 Onsite Waste-
water Treatment: Septic Tanks TM )
Q = 500 students x 10 gpd/student = 5lO0 qpd
Absorpttofl area required (use medium sand fill)
Area 5000 gpd - 4167 ft 2
1.2 qpd/ft 2
Absorption area dimensions
The length of the absorption area in the mound will be
limited by the site, Because of the steep slope, it is
desirable to elongate the absorption area as much as
possible. Maximum width of the slope is about 280 ft.
Subtracting approximately 40 ft for sideslopes and property
line setbacks, 240 ft remain for the bed.
Bed width = 4170 ft 2 17.4 ft => 18 ft
Depth of fill
Minimum depth of fill is 2 ft since creviced bedrock is
within 2 ft of the ground surface,
Depth under downslope edge of bed = 2 ft + 18 ft x 0.15 (% slope)
= 4.7 ft
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24-
This difference In depths across the bed bottom is too
great. Uneven settling may occur. Therefore, the bed will
be divided into two 9 x 240 beds and tiered down the slope.
Depth under downslope = 2 ft + 9 ft x 0.15 (% slope)
= 3.35 ft <= OK
Bed spacing
To limit hydraulic interference between the beds, the
beds will be spaced sufficiently far apart so that the
liquid from the upper trench is absorbed Into the natural
soil before the flow reaches the lower trench. A natural
soil Infiltration rate of 075 gpd/ft will be used (Table
rn. 01 )
Flow/linear ft of bed 9 ft x 1.2 gpd/ft 2 = 10.8 qpd/ft
Spacing = 108 gpdtft ft = 5 4 ft => 6 ft
0.75 gpd/ft
The remainder of the mound design follows the same procedures
used in conventional mound design.
Dosing siphons can be used if the septic tank and dosing tank
are constructed at a sufficent elevation above the dlstrlPwtion
lateral inverts. Two siphons would be used 1 one for each bed, They
will automatically alternate dosinq,
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